In Depth – Rainfall and Relief

Precipitation has been recorded in the British Isles for over 200 years. Through the dedication and enthusiasm of W R Symons, the mid-19th century saw the formation of the British Rainfall Organisation whose main objective was to establish a countrywide network of rain gauges where daily measurements were made. This network has grown to over 7,000 gauges today, including many that are automatic. 

As most people in the British Isles know, precipitation can be extremely variable, both in intensity and duration. The spatial distribution of precipitation during an individual month is very uneven, just as it is on an individual day. Rainfall in Britain is associated with several distinct synoptic situations; all places may get rain from most such types, but some areas get more from some types than others. Therefore, it is not surprising that patterns of temporal variation of rainfall are complex.

Precipitation over the British Isles is the result of one or more, of three basic mechanisms.

1. Cyclonic, or frontal, rain associated with the passage of low-pressure systems. Bands of rain are associated with the passage of warm and cold fronts across the UK. These rain events are caused by the uplift and cooling of moist air parcels.

2. Convectional, with local showers and thunderstorms, caused by the localised thermal heating and overheating of the ground surface. Large towering cumulonimbus clouds may be generated, producing heavy rain.

3. Orographic, or relief rain, with precipitation increasing with altitude over upland areas. The mechanism for relief rain is the uplift and cooling of moist air over upland areas. The normal rate of cooling (environmental lapse rate) is 6.5 °C per 1,000 metres. Therefore, near the summit on the windward side of the hill or mountain, the air will have cooled sufficiently for thick cloud, rain and possibly snow to fall.

Air will descend and warm on the leeward side, so there is little or no rain on this leeward side of the hill or mountain. This is called a rain shadow, and sometimes there are warm winds in these sheltered areas as the air, now much drier than during its ascent, descends quickly and warms up. This is called a föhn effect, and the warm winds are called föhn winds.

Rain forms when air cools, the rate of condensation becomes faster than the rate at which water is evaporating and cloud droplets form. If these get big enough, they form as rain – or snow, sleet or hail.

There are various ways in which the air can cool to form rain – three common types which are often talked about are frontal, orographic or relief and convective rain. Frontal Rain
This is found where warm air meets cold at the cold and warm fronts in a depression.


Convective Rain

Convection is the term given to warm air rising. Convection is normally marked by cumulus clouds which billow upwards as the air rises. The base of such clouds is usually flat, marking the level where temperatures are cold enough for more condensation to be going on than evaporation.


Image copyright RMetS

Extreme convection can be found in thunder clouds – towering cumulonimbus, which can reach all the way up to the top of the troposphere – the lowest 10km or so of the atmosphere in which our weather is found. As air can’t rise into the stratosphere above, the top of the cumulonimbus cloud spreads out, giving it a characteristic ‘anvil’ shaped top. Such convection can occur where the ground has become particularly warm, heating the air above it. It is particularly associated with the Tropics, characteristically giving heavy rain in the late afternoon.

Cumulonimbus clouds can give heavy rain, hail, thunder lightning and sometimes tornadoes.


Cumulonimbus cloud over Kettering, UK, August 2014
Image copyright Sylvia Knight

Orographic or Relief Rain
When air is forced to rise over land, particularly higher ground such as hills and mountains, it cools as the pressure falls. Dry air cools at 9.8°C per 1000m it rises. Eventually, the air can cool enough for cloud to form.


Orographic cloud forming upstream of the Matterhorn
Image copyright RMetS

The cloud droplets may get big enough to fall as rain on the upstream side of the mountain. If that happens, then, when the air has passed over the top of the mountain and starts to descend, and warm, on the far side, there will be less water to evaporate back into the air. The air will end up drier than it was on the upstream side of the mountain.

This can produce ‘rain shadow’ – an area of land downstream from some mountains (for the prevailing wind direction, in the UK, this would be to the east) where there is noticeably less rainfall. The Gobi desert in Mongolia and China is so dry because it is in the rain shadow of the Himalayas.

Orography can enhance frontal or convective rain; for example, we have explored how polar maritime air, our prevailing air mass, brings convective rain to the Atlantic. As the air reaches the UK and rises over the land, the precipitation is increased.


A precipitation map showing the rainfall ‘climate’ (averaged over 30 years) of the UK. With prevailing westerly winds, there is clearly more rain on the western side of the country, enhanced by the mountains of Wales and Scotland and the English Pennines.
As well as being drier on the downwind side of the mountains, it can also be warmer. Remember that water releases heat into the atmosphere as it condenses and takes it up as it evaporates. If there is less water in the air on the downwind side, then there is less to evaporate and not all the heat that was released on the upwind side will be taken up again. This is known as the Föhn Effect.

The onset of a Föhn is generally sudden. For example, the temperature may rise more than 10°C in five minutes and the wind strength increase from almost calm to gale force just as quickly. Föhn winds occur quite often in the Alps (where the name originated) and in the Rockies (where the name chinook is used). They also occur in the Moray Firth and over eastern parts of New Zealand’s South Island. In addition, they occur over eastern Sri Lanka during the south-west monsoon.

Where there are steep snow-covered slopes, a Föhn  may cause avalanches from the sudden warming and blustery conditions. In Föhn conditions, the relative humidity may fall to less than 30%, causing vegetation and wooden buildings to dry out. This is a long-standing problem in Switzerland, where so many fires have occurred during Föhn conditions that fire-watching is obligatory when a Föhn is blowing.

 
 

Teaching Resources

A case study of orographic rainfall and Foehn winds in Scotland with images for students Image 1, Image 2Image 3Image 4Image 5.

Water in the Atmosphere from Weather and Climate: a Teachers’ Guide

Make a cloud in a bottle

Data and Image Sources

UK climate data from the Met Office

Rain, Wind, Snow and Fog

We were delighted to support BBC Bitesize Scotland in the creation of these 21 videos for primary schools. Here are some of our best resources to support the teaching of these topics:

 

 

[all images on this page are copyright of the BBC]

How will water circulation and flooding change?

Royal Geographical Society

This resource links to Figure 11.12 in the IPCC report of 2021. The aim of this resource is to answer the question how will the flow of water around the world be altered with climate change?

It was written with the Royal Geographical Society with IBG

Circulation

The global atmospheric circulation is described by the Met Office as ‘the world-wide system of winds by which the necessary transport of heat from tropical to polar latitudes is accomplished’. Figure 1 shows the different cells of this global system, in Idealized Earth and Actual Earth projections.

global atmospheric circulation

Figure 1 the different cells of the global atmospheric circulation © 2010 Encyclopædia Britannica Inc

Due to changes in our climate, there will be both small-scale and large-scale changes to the flow of water around the world in the twenty-first century.

  1. The central world map of Figure 8.21 in chapter 8 of the IPCC report shows the effect of 3°C of global warming on mean P-E (precipitation minus evaporation) compared to pre-industrial levels (1850-1900). Describe the anticipated changes to mean P-E across the globe with such projected change. Reference specific regions in your answer.
  1. Using Figure 2 in Appendix A, copy the choropleth colour coding to show the changes to global precipitation for:

a) +0.6 mm to 1 mm increase.

b) -0.6 mm to -1 mm decrease.

  1. Add the Tropical rain belt onto Figure 2, which is shown in red on the original figure.
  1. There are 5 anticipated changes to large-scale water circulation. They are the poleward expansion of the Hadley cells, the poleward migration of storm tracks, the narrowing and strengthening of the Intertropical Convergence (ITCZ) core, a regional shift in the ITCZ, and a weaker Walker circulation (for reference watch the MetLink video An Introduction to Atmospheric Circulation and read https://en.wikipedia.org/wiki/Walker_circulation). Add these notes to Figure 2.

There are multiple atmospheric triggers for changes to the water cycle, termed climate drivers. Figure 3 in Appendix B shows how an increase in precipitation, solar radiation, temperature, wind, and carbon dioxide (CO₂) and a decrease in humidity can influence the water cycle. The diagram flows down to illustrate the outcome on water availability and drought.

Precipitation, one of the climatic drivers

Precipitation has increased steadily over Eurasia, most of North America, south-eastern South America, and north-western Australia. Whilst in Africa, eastern Australia, the Mediterranean region, the Middle East, and parts of East Asia, central South America, and the Canadian Pacific coast it has decreased. Records from 1910 onwards show Scandinavia, north-west Russia, the UK, and Iceland have all experienced increased precipitation trends. The amount of, frequency, and intensity of precipitation is forecast to continue to increase for these areas, which will worsen the severity of flooding. Across Europe there has been a reduction in snowfall, an important component in precipitation, in high latitude and mountain watersheds. Per decade, there has been a reduction of 0.52 million km² of annual mean potential snowfall over Northern Europe, with the greatest loss occurring in the Alps.

Runoff, streamflow, and flooding effect

There have been substantial changes to runoff, streamflow, and flooding around the world. Although there are no significant global trends many human-induced drivers of change have been identified and linked to changes in the flow of water. Examples include decreasing runoff in the dry season in the Peruvian Amazon, a decline in streamflow in the Colorado River, and earlier snowmelt in Northern Europe. As a result, in the UK, there will continue to be problems over increased water availability and streamflow during winter, and a worsening decrease in water availability and streamflow during the summer months. These changes are caused by the difference between winter flooding, which occurs from storm precipitation falling on already waterlogged ground, and summer flooding, when precipitation falls on ground that has been baked hard by the Sun. These scenarios have been compounded by dam construction and water withdrawal, land use and land cover change, all leading to alterations of seasonality, amount, and variability of river discharge, especially in human-dominated small catchments.

Climate change is increasing the risk of both flooding and drought in the UK with flooding now being the most common form of natural disaster. The risk of flooding is increasing due to the anthropogenic drivers of climate change. Quite simply this is because, as the atmosphere warms, there is more evaporation from the surface and more condensation of water vapour into cloud droplets in the atmosphere. Intense precipitation will remain the main cause of flooding. However, there are other factors (such as local topography and geology, for example). In 2017 research by the Met Office found that climate change means there is a high chance of exceeding the observed record monthly rainfall totals in many regions of the UK.  Further analysis in 2020 (again by the Met Office) shows that, on average, for the decade 2010 to 2019, UK summers were 13% wetter, and winters 12% wetter, than in the period 1961 to 1990. 7 of the 11 wettest years since records began (in 1862) in the UK have occurred since 1998. The five wettest winters have been from 1990 onwards. Overall, in the UK there is a trend towards wetter winters and drier summers.

Further work

Exam-style question 

Using all the work you have completed answer the final question below. 

Answer the question: assess whether global flooding will become more severe or more frequent as a result of climate change?  This means you must consider the different arguments, likelihoods, and levels of certainty, after weighing them up, to come to a conclusion. 

Appendix A

Large Scale Circulation projected changes and their effect on the water cycle

Figure 2 circulation projected change maps © freeusandworldmaps.com arrows © cliparts.co and getdrawing.com

Appendix B

IPCC AR6 climatic drivers

Figure 3 climate drivers © The IPCC report

Answers

  1. The intertropical convergence zone will predominately see an increase in precipitation with 0.8 to 1 mm/d increase across the Pacific Ocean (with some variability near the Central American coast). In the rest of the tropics, both north and south, there will be a reduction in P-E balance with less precipitation over all major oceans within the subtropical boundaries. This is described as a future ‘drying tendency’ on the edges of the ITCZ. In the upper latitude the Barents Sea will also become drier as the P-E balance changes in the Russian Arctic, between Novaya Zemlya and Svalbard. On land much of climate over the South American Amazon will also continue to dry. In contrast Alaska in North America and the Congo basin in Sub-Saharan Africa will see an increase in P-E.
  2. As instructed.
  3. As instructed.
  4. Under a climate change 3° warming scenario the Hadley cell will move northwards away from the current 0° to 30° latitude (N and S). This will lead to the expansion of the subtropical dry zones outwards and away from the tropics. Equally there will be a poleward migration of storm tracks which will lead to stronger storms as they will feed off extra latent heat. Abnormally high sea surface temperatures, in the Atlantic for example, will intensified storms throughout the twenty-first century with associated storm surges being exacerbated by rising sea levels. It is also believed extra water vapor in the atmosphere will make storms wetter. In the future, the ITCZ will narrow, particularly over the Pacific, causing lower latitude subtropical jets to become unstable baroclinically (in temperature and pressure). This will allow midlatitude eddies (circular movements of air) to spread further equatorward leading to more precipitation in the ITCZ core region. The Walker Circulation has undergone a strengthening in the Pacific, thought to be caused by either internal variability or a response to greenhouse gas emissions. The altered circulation pattern is associated with other global changes in the water cycle over regions like the Maritime Continent, South America and Africa.

Orographic (relief) rainfall and the Foehn Effect

This case study or ‘mystery’ is taken from the afternoon of the 2nd September 2013. It focusses on Scotland and N. England. It can be used for two different purposes – either to identify orographic/ relief rain (use images 2-6 below), or to go on to identify a case study of the Foehn Effect (use all images).

We recommend that teachers present students in groups with a series of images, sequentially, to allow them to work out what the weather is doing and why. 

Expected Knowledge

Students should be

  • Familiar with a map of the UK
  • Know which way winds blow around a pressure system, and be able to identify fronts and pressure systems on a synoptic weather map
  • Know about the 3 main ways in which rain can form (frontal, convective or relief/ orographic rain)

Suggested Lesson Opener

Make a cloud in a bottle or watch the video.

Notes for Teachers 

Students can be helped, where appropriate to identify some of these points

Image 1

Image 1 – temperatures in degrees Centigrade at 15Z (1500GMT) (copyright Met Office)

The temperatures show that it is considerably warmer on the East side of Scotland than on the West. Temperatures are up to 9°C warmer on the East.

Why?

Image 2

Image 2 – a synoptic chart at 12Z (1200GMT) (copyright Met Office)

  • There are no weather fronts over the UK, although the whole country is in the warm sector of a low pressure system over Iceland.
  • There is a High Pressure system to the SW of the UK
  • Winds blow clockwise around a High pressure system, and along the isobars
  • The wind is therefore coming from the west (westerly winds) over Scotland
  • Alternatively, you could consider the winds blowing anticlockwise around the Low pressure system to the North – this also indicates that the wind direction over Scotland is from the west.
  • [We would normally associate this with Polar maritime/ returning Polar maritime air, but, in this case, if you follow the isoline back, the air has come from further south, so is Tropical maritime in nature.]
  • We would generally expect clear skies over most of the UK in this situation.

Image 3

Image 3 – wind speed in knots at 15Z (1500GMT) (copyright Met Office)

Image 4

Image 4 – satellite image at 15Z (1500GMT) : (c) EUMETSAT / Met Office

This is a satellite image from 1500Z (1500GMT) showing visible radiation ie light. The white areas are where the Sun’s light is being reflected from clouds.

There is cloud over the west coast of Scotland and N. England

You can also see the cloud associated with the warm front to the East of the UK, and the cold front to the west.

  • There is no front over Scotland, so the rain is not frontal rain.

Image 5

Image 5: Rainfall on 2nd September 2013 (copyright Met Office)

It is raining over the west coast of Scotland. Why?

  • There is no front there, so it is not frontal rain
  • Is it orographic rain or convective rain?

Image 6

A relief map of Scotland clearly shows the high ground on the East coast.

Synopsis

As the easterly winds blow in from the west, the air is forced to rise. As it rises, it cools until the rate of condensation is faster than the rate of evaporation. Cloud droplets form, which eventually become large enough to fall as rain.

Therefore, this rain is orographic or relief rain.

As the air descends again downwind of the mountains, the air warms and the cloud droplets evaporate.

As the cloud droplets form, they emit latent energy (heat) into the air around. This heat is remains in the air if the rain reaches the ground. This means that, downwind of the mountains when the air sinks, warms, and any remaining cloud droplets evaporate, there is more heat in the air than there was upwind of the mountains.

This is why temperatures were so much warmer on the east coast than on the west on this day!

Image 1

Image 2

Image 3

Image 4

Image 5

Orographic Rainfall in Scotland

Image 6

Scottish Orography

A Level

Independent Investigation

A guide to collecting weather data from the RGS student guide to the A Level independent investigation (Non-examined Assessment – NEA) and some further ideas.

Video Link: https://www.youtube.com/embed/Y_UdPbThbtQ

Carbon and Water Cycles; Weather and Climate

Carbon, water, weather and climate a PowerPoint presentation focussing on recent changes to the carbon and water cycles, and how the two cycles interact.

Climate and Weather – an overview for A level, on the RGS website.

Climate change updates for A level geography – supporting the 2016 specifications.

Background information for teachers on the water cycle and the carbon cycle.

Video link: https://www.youtube.com/embed/LBe4LTLOLvU

Deforestation, the water cycle and the carbon cycle in the Amazon.

Extensive information from the Cool Geography site: Case study of a tropical rainforest setting to illustrate and analyse key themes in water and carbon cycles and their relationship to environmental change and human activity and more generally on the carbon and water cycles.

Depressions, Anticyclones and Synoptic Charts

Weather Charts

Weather Systems

Mid-latitude weather systems video (with downloadable resources)

Depression based exercise where students draw contours of temperature, pressure and precipitation to work out what the system looks like: Student worksheets and notes for teachers. Simpler versions of the same exercise can be found on the KS3/4 web pages.

Use WOW data to track a cold front across the UK and work out its speed.

track a cold front across the UK and work out its speed practical excercises.

weather forecasting exercise

Shipping Forecast weather system excercise teachers notes and worksheet.

There are some more teaching resources covering weather systems and weather maps on the GCSE resources page.

Tropical Weather

Monsoons – a resource looking at the link between rainfall and food production in India. Teachers notes and Excel data sheet.

Some useful links about Super typhoon Haiyan/ Yolanda

Extreme Weather

Extreme Weather

Extreme weather in the UK

Climate and Climate Change

Climate Change with sections on atmospheric structure, composition, solar radiation, climate feedback mechanisms and ozone depletion.

Other Weather

Clouds

A case study of orographic rainfall and Foehn winds in Scotland with images for students Image 1Image 2Image 3Image 4Image 5.

An exercise using height/ temperature graphs to investigate atmospheric stability, lapse rates and cloud formation with a worksheet for students and an introductory Powerpoint.

Investigate How big is a raindrop collect data and analyse mode, mean and median, range, interquartile range and standard deviation etc.

This lovely animation explores integration through Is it better to walk or run in the rain?.

A one hour tutorial on Climate variability, change and water resources from MetEd (requires free registration). The level is suitable for A level.

GCSE resources

9. Water in the Atmosphere

Weather and Climate: a Teachers’ Guide

Pathway: Basic weather 

Climate ZonesAir MassesPressure and Wind – Water in the Atmosphere

Lesson overview: In this lesson, we focus on cloud formation due to convection, orography (relief) and frontal uplift.

The atmosphere is one of the smallest reservoirs of water in the hydrosphere.  Clouds form when air is cooled. Air can cool due to convection, when air is heated from below and rises, or when air is forced to rise at a front between two air masses. When air is forced to rise over hills and mountains, cloud formation is enhanced. Climate change will intensify the water cycle, increasing the amount of water vapour in the atmosphere.  As water vapour is a greenhouse gas this creates a positive feedback loop, amplifying climate change.

Learning objectives:

  • To understand why clouds form in the atmosphere.

  • To be able to explain two ways in which clouds form

Key Teaching Resources

Water in the Atmosphere PowerPoint
Water in the Atmosphere PowerPoint (easier)
Water in the Atmosphere Worksheet
Water in the Atmosphere Worksheet (easier)
Back-to-back image

Teacher CPD/ Extended Reading

Water in the Atmosphere – More for Teachers

Alternative or Extension Resources

Global Atmospheric Circulation and Global Precipitation Patterns 

A ‘mystery’ – a case study of orographic rainfall in Scotland (with optional extension looking at the Foehn Effect)

 

Weather and Climate: a Teachers’ Guide

Weather Charts Teachers’ Notes

Understanding weather charts

Teachers’ notes to accompany Understanding Weather Charts

Resources required

Computers with Internet access would be desirable. Alternatively if Internet access is not available, printed copies of student sheets and worksheets should be made.

Prior knowledge required

A basic background of weather and climate.

Teaching activities

Students can visit the following pages to gain a basic background into the topics covered:

The information on the student sheets can be delivered by the teacher and activities completed individually. Alternatively students can work through the whole lesson themselves.

Part A – Isobars, pressure and wind

Part B – Identifying pressure systems and fronts

Part C – Plotted weather charts

Exercises

Three worksheets with exercises are provided to consolidate learning.

A series of additional exercises are provided for more able students, or those who have already studied pressure systems and fronts in more detail prior to this lesson.

Suggestions for homework

Any of the worksheet activities can be completed. Alternatively students can collect weather charts from the Internet or a newspaper and repeat the exercises using these.

Web page reproduced with the kind permission of the Met Office

Rainforest Deforestation the Carbon and Water Cycles

This news item from NASA relates to this animation, as does this Nature Communication from October 2020.

Suggested learning activities:

***NEW*** – data and GIS exercise for A Level students

Explore leaf area, evapotranspiration and temperature data using various statistical techniques to explore the relationship between deforestation and weather on this resource on the RGS website.

Activity 1:
Ask students to write a voiceover for the film, demonstrating their understanding of the concepts involved.

Activity 2:
Complete this sentence based on the film:
When rainforests are deforested, places downwind are left with more/ less/ the same amount of rainfall and greater/ less/ the same amount of flood risk.

Activity 3:
Look at www.globalforestwatch.org/map and identify a Tropical region which has experienced deforestation in the last decade.
Look at earth.nullschool.net. What is the prevailing wind direction in that region?
Using www.google.com/maps, write a paragraph explaining how you think the water cycle has been affected by deforestation for a place downwind from the rainforest region you identified.

Activity 4:
Having watched the animation, use https://www.globalforestwatch.org/map , http://earth.nullschool.net and https://www.google.com/maps to write a paragraph explaining how you think the water cycle has been affected by deforestation for a specific place downwind and/ or downriver from a rainforest region.

Activity 5:
Having watched the animation, read these articles from Nature and NASA (noting that this predates the Nature article), NASA (2019)Geography Review (p22 – 25) and Carbon Brief.
Summarise the impact of tropical deforestation on the carbon and water cycles.

More information about the water cycle and climate change and the water cycle and an excellent summary from Cool Geography.

In Depth – The Water Cycle

The water cycle

The water cycle, also known as the hydrological cycle, is the process by which water travels from the Earth’s surface to the atmosphere and then back to the ground again. The sun provides the energy for a continuous exchange of moisture between the oceans, the land and the atmosphere.

The Earth’s water

Nearly all (about 97%) of the Earth’s water is contained in the oceans. A smaller amount is locked away as ice sheets and glaciers. This leaves a very small amount which travels around in our water cycle, although it may not always seem this way on wet days.

Water enters the atmosphere as water vapour through evaporation, transpiration and sublimation. Water vapour high in the atmosphere forms into clouds through condensation. Water eventually returns to earth through precipitation as rain, snow, sleet and hail. When precipitation occurs over land, some water seeps into the ground as groundwater, a small amount is taken up by plants and animals, and the rest will return to rivers and streams as surface run-off to begin its journey back to the oceans.

Underwater scene

The processes:

Surface runoff
Surface runoff is the precipitation that falls on land and flows downhill towards stream channels which join rivers and eventually reach the oceans. Only about one third of precipitation falling on land will return to rivers and oceans. The rest will be soaked into the soil as groundwater, evaporated or transpired.

Groundwater
Some of the water from precipitation will soak into the soil and rocks as groundwater. A varying proportion of groundwater stays in the shallow soil layer, and will move slowly towards streams and rivers. When groundwater soaks deeper into the soil it refills the underground aquifers, where it can stay for long periods of time or be used by humans through drilling wells into aquifers.

Aquifer
An aquifer is a layer of water soaked sand, soil, stone, silt or clay underground. Aquifers act as huge underground water storage systems which people all over the world rely on for fresh water.

Reversible change of state
A change that can be undone or reversed. Energy is required for a material to change state and whilst it may change in appearance, it will still remain the same material. Melting, freezing, boiling, evaporating and condensing are always reversible changes and can be reversed by heating or cooling.

Deforestation, the water cycle and the carbon cycle in the Amazon.

Web page reproduced with the kind permission of the Met Office

The Changing Water Cycle

Questions to consider:
Explain two reasons why a warming climate results in a more intense water cycle.
How do changes in the water cycle impact i) the surface water of oceans in the subtropics ii) the surface water of oceans in tropical and polar regions.
Describe the potential global change in annual mean precipitation for the period 2081 -2100.
Identify three key differences between the carbon cycle and the water cycle.

PDF Download

water cycke diagramEstimates of the current global water budget and its annual flow using observations from 2002-2008 (1000 km3 for storage and 1000 km3 yr−1 for exchanges).
Based on K.E. Trenberth, J. Fasullo, and J Mackaro, 2011: Atmospheric Moisture Transports from Ocean to Land and Global Energy Flows in Reanalyses. J. Climate, 24, 4907–4924. doi: http://dx.doi.org/10.1175/2011JCLI4171.1

Summary:

  • About 10% of the water evaporated from the ocean is transported over land by the winds and finds its way back to the ocean following condensation into clouds, eventual precipitation as rain or snow and subsequent surface runoff and sub-surface flows
  • As the climate warms, the water cycle intensifies. This is driven by an increase in evapotranspiration at the ground but is controlled by the temperature of the troposphere, which determines how much condensation, and hence precipitation, occurs.
  • Over the last century, northern mid-latitude precipitation has increased and the number of heavy precipitation events over land has increased in more regions than it has decreased, particularly in Europe and North America.
  • Globally, water vapour concentration in the lower atmosphere has increased by 3-4% since the 1970s.
  • Water vapour is a strong and fast feedback that amplifies changes in surface temperature in response to other changes (for example increasing CO2) by about a factor of 2.
  • Many human and natural systems are highly sensitive to changes in precipitation, river flow, soil and groundwater.

Case Studies

Changes to Groundwater in Uganda

The mass budgets of Himalayan Glaciers

Return to main menu

 

Further Information

Carbon, water, weather and climate a PowerPoint presentation focussing on recent changes to the carbon and water cycles, and how the two cycles interact.

Background information on the water cycle.

Is There Evidence for Changes in the Earth’s Water Cycle?

How Important Is Water Vapour to Climate Change?

How Will the Earth’s Water Cycle Change?


How will climate change affect the frequency and severity of floods and droughts?


How will the availability of water resources be affected by climate change?


How is climate change affecting monsoons?

In general, precipitation is limited by the availability of water, energy, or both. The world’s oceans contain an effectively unlimited supply of water but locally, especially over land, a shortage of water can limit precipitation. As the climate warms, and there is more energy available to drive evaporation, the amount and intensity of precipitation is expected to increase. Evapotranspiration also increases over most land areas in a warmer climate, thereby accelerating the water cycle. However, changes in vegetation and soil moisture availability can also affect evapotranspiration rates.

The differences between the carbon and water cycles:

  • Human emissions of water have no effect on the concentration of water in the atmosphere; the concentration of water in the atmosphere is largely controlled by temperature. However, human emissions of carbon have increased the concentration of carbon in the atmosphere by over 40%.
  • The effective lifetime of water in the atmosphere is of order 10 days, whereas that of CO2 ranges between a few years to thousands of years.
  • The concentration of water in the atmosphere is extremely variable spatially, ranging from close to zero at high altitudes to about 20g per kg of dry air near the tropical ocean surface, whereas, because of its long effective lifetime, that of carbon dioxide has a range of around only 2% both seasonally and geographically.
  • The cryosphere (ice on land and sea) is an important part of the water cycle, but not of the carbon cycle. Humans have indirectly, through temperature change, caused impacts on the cryosphere.
WG1 Chapter 2, Figure 28.
WG1 Chapter 2, Figure 28.

Annual precipitation anomalies averaged over land areas for four latitudinal bands and the globe relative to 1981-2000. Globally, there has been no significant long term trend in precipitation. In the Tropics, (30°S-30°N) precipitation has increased over the last decade, reversing the drying trend from the mid 70s to the mid 90s. The Northern Hemisphere mid-latitudes show a significant increase in precipitation over the last century.

WG1 Summary for Policy Makers, Figure 8.
WG1 Summary for Policy Makers, Figure 8.

Maps of modelled annual mean precipitation changes between 1986-2005 and 2081-2100 for a low (RCP2.6) and high (RCP8.5) emissions scenarios. Hatching indicates regions where there is low confidence in the projected change. Stippling indicates regions where there is more confidence in the projected precipitation change.

Global and annual averages of total atmospheric water vapour over ocean surfaces, shown relative to the average for the period 1988-2007.

WG1 Chapter 2, Figure 31.
WG1 Chapter 2, Figure 31.

Evidence of climate change is also seen in other variables. Since the last IPCC report, satellite evidence has shown an increase in the amount of water vapour in the troposphere, the lowest part of the atmosphere.  The year-to-year variability and long term trend in atmospheric water vapour content are closely linked to changes in global sea surface temperature, partly because warmer temperatures cause increased evaporation and partly because warmer air can carry more water vapour. Tropospheric water vapour is an important climate change feedback mechanism (through its powerful greenhouse effect) and is essential to the formation of clouds and precipitation. No significant change to the amount or type of clouds globally has been detected yet, although locally changes have been observed and linked to changing wind patterns.

Changes in the water cycle also have an impact on the world’s oceans, with surface waters in the evaporation dominated sub-tropics becoming more saline and surface waters in the rainfall-dominated tropical and polar regions becoming fresher.

WG2 Chapter 3, Figure 4.

Percentage change of mean annual streamflow for a global mean temperature rise of 2°C above 1980–2010 (2.7°C above pre-industrial). Colour hues show the amount of change and saturation shows the agreement on the sign of change (i.e. the darker the colour, the more confidence in the result).

Extreme Precipitation and Flooding

Since 1951 there have been statistically significant increases in the number of heavy precipitation events in more regions than there have been decreases, but there are many regional and seasonal variations in the trend. The most consistent trend is seen in North America, where there has been an increase in the frequency and intensity of extreme precipitation. In the future, the number of tropical cyclones globally may fall but their maximum wind speed and precipitation is expected to increase, in a warmer and more energetic atmosphere. Freshwater-related risks (e.g. river flooding) of climate change increase significantly with increasing greenhouse gas concentrations.

Drought Increased evapotranspiration over land can lead to more intense and frequent periods of agricultural drought. There has been an increase in the frequency and intensity of drought in the Mediterranean and West Africa, but a decrease in central North America and north-west Australia.  

ENSO
We cannot yet predict how the El Niño Southern Oscillation (ENSO), which has a significant impact on precipitation patterns around the world in both its El Niño and La Niño phases, may change in the 21st century.

The Cryosphere Frozen stores of water form an important component of the water cycle and are depended upon by societies and ecosystems. The Arctic and Antarctic sea-ice covers are projected to shrink in the 21st century. The Arctic may become almost entirely ice-free in late summer. Polar amplification occurs if the magnitude of the surface temperature change at high latitudes exceeds the globally averaged temperature change on time scales greater than the annual cycle. One of the ways this can happen is in response to rising CO2 levels in the atmosphere. As the air temperature warms, sea ice retreats and snow cover reduced, the surface albedo decreases, air temperatures increase and the ocean can absorb more heat – a positive feedback mechanism which result in a local amplification of the warming.

This polar amplification can have consequences for the melting of ice sheets, global sea level and on the carbon cycle through the melting of the permafrost.

Ice sheets also play an essential role in the Earth’s climate. They interact with the atmosphere, the ocean–sea ice system, the lithosphere and the surrounding vegetation, respond to greenhouse gas changes and affect global climate on a variety of time scales. Ice sheets grow when annual snow accumulation exceeds melting. Growing ice sheets expand on previously darker vegetated areas, thus leading to an increase of surface albedo, further cooling and a local drying of the air as the ice sheets grow upwards into colder air. Massive freshwater release from retreating ice sheets, can feed back to the climate system by altering sea level, oceanic deep convection, ocean circulation, heat transport, sea ice and the global atmospheric circulation. Whereas the initial response of ice sheets to external forcings (such as greenhouse gas changes) can be quite fast, involving for instance ice shelf processes and outlet glaciers (10 to 1000 years), their long-term adjustment can take much longer.

Is There Evidence for Changes in the Earth’s Water Cycle?

WG1 FAQ3.2
The Earth’s water cycle involves evaporation and precipitation of moisture at the Earth’s surface. Changes in the atmosphere’s water vapour content provide strong evidence that the water cycle is already responding to a warming climate. Further evidence comes from changes in the distribution of ocean salinity, which, due to a lack of long-term observations of rain and evaporation over the global oceans, has become an important proxy rain gauge.

The water cycle is expected to intensify in a warmer climate, because warmer air can be moister: the atmosphere can hold about 7% more water vapour for each degree Celsius of warming. Observations since the 1970s show increases in surface and lower atmospheric water vapour (Figure 1a), at a rate consistent with observed warming. Moreover, evaporation and precipitation are projected to intensify in a warmer climate.
Recorded changes in ocean salinity in the last 50 years support that projection. Seawater contains both salt and fresh water, and its salinity is a function of the weight of dissolved salts it contains. Because the total amount of salt—which comes from the weathering of rocks—does not change over human time scales, seawater’s salinity can only be altered—over days or centuries—by the addition or removal of fresh water.

The atmosphere connects the ocean’s regions of net fresh water loss to those of fresh water gain by moving evaporated water vapour from one place to another. The distribution of salinity at the ocean surface largely reflects the spatial pattern of evaporation minus precipitation, runoff from land, and sea ice processes. There is some shifting of the patterns relative to each other, because of the ocean’s currents.

Subtropical waters are highly saline, because evaporation exceeds rainfall, whereas seawater at high latitudes and in the tropics—where more rain falls than evaporates—is less so (Figure 1b, d). The Atlantic, the saltiest ocean basin, loses more freshwater through evaporation than it gains from precipitation, while the Pacific is nearly neutral (i.e., precipitation gain nearly balances evaporation loss), and the Southern Ocean (region around Antarctica) is dominated by precipitation.

Changes in surface salinity and in the upper ocean have reinforced the mean salinity pattern. The evaporation-dominated subtropical regions have become saltier, while the precipitation-dominated subpolar and tropical regions have become fresher. When changes over the top 500m are considered, the evaporation-dominated Atlantic has become saltier, while the nearly neutral Pacific and precipitation-dominated Southern Ocean have become fresher (Figure 1c).

Observing changes in precipitation and evaporation directly and globally is difficult, because most of the exchange of fresh water between the atmosphere and the surface happens over the 70% of the Earth’s surface covered by ocean. Long-term precipitation records are available only from over the land, and there are no long-term measurements of evaporation.

Land-based observations show precipitation increases in some regions, and decreases in others, making it difficult to construct a globally integrated picture. Land-based observations have shown more extreme rainfall events, and more flooding associated with earlier snow melt at high northern latitudes, but there is strong regionality in the trends. Land-based observations are so far insufficient to provide evidence of changes in drought.

Ocean salinity, on the other hand, acts as a sensitive and effective rain gauge over the ocean. It naturally reflects and smooths out the difference between water gained by the ocean from precipitation, and water lost by the ocean through evaporation, both of which are very patchy and episodic. Ocean salinity is also affected by water runoff from the continents, and by the melting and freezing of sea ice or floating glacial ice. Fresh water added by melting ice on land will change global-averaged salinity, but changes to date are too small to observe.

Data from the past 50 years show widespread salinity changes in the upper ocean, which are indicative of systematic changes in precipitation and runoff minus evaporation, as illustrated below.

Figure 1: Changes in sea surface salinity are related to the atmospheric patterns of evaporation minus precipitation (E – P) and trends in total precipitable water: (a) Linear trend (1988–2010) in total precipitable water (water vapour integrated from the Earth’s surface up through the entire atmosphere) (kg m–2 per decade) from satellite observations (Special Sensor Microwave Imager) (blues: wetter; yellows: drier). (b) The 1979–2005 climatological mean net E –P (cm yr–1) from meteorological reanalysis (reds: net evaporation; blues: net precipitation). (c) Trend (1950–2000) in surface salinity (PSS78 per 50 years) (blues freshening; yellows-reds saltier). (d) The climatological-mean surface salinity (PSS78) (blues: <35; yellows–reds: >35).

How Important Is Water Vapour to Climate Change?
WG1 FAQ 8.1

As the largest contributor to the natural greenhouse effect, water vapour plays an essential role in the Earth’s climate. However, the amount of water vapour in the atmosphere is controlled mostly by air temperature, rather than by emissions. For that reason, scientists consider it a feedback agent, rather than a forcing to climate change. Anthropogenic emissions of water vapour through irrigation or power plant cooling have a negligible impact on the global climate.
Water vapour is the primary greenhouse gas in the Earth’s atmosphere. The contribution of water vapour to the natural greenhouse effect relative to that of carbon dioxide (CO2) depends on the accounting method, but can be considered to be approximately two to three times greater. Additional water vapour is injected into the atmosphere from anthropogenic activities, mostly through increased evaporation from irrigated crops, but also through power plant cooling, and marginally through the combustion of fossil fuel. One may therefore question why there is so much focus on CO2, and not on water vapour, as a forcing to climate change.

Water vapour behaves differently from CO2 in one fundamental way: it can condense and precipitate. When air with high humidity cools, some of the vapour condenses into water droplets or ice particles and precipitates. The typical residence time of water vapour in the atmosphere is ten days. The flux of water vapour into the atmosphere from anthropogenic sources is considerably less than from ‘natural’ evaporation. Therefore, it has a negligible impact on overall concentrations, and does not contribute significantly to the long-term greenhouse effect. This is the main reason why tropospheric water vapour (typically below 10km altitude) is not considered to be an anthropogenic gas contributing to radiative forcing.

Anthropogenic emissions do have a significant impact on water vapour in the stratosphere, which is the part of the atmosphere above about 10 km. Increased concentrations of methane (CH4) due to human activities lead to an additional source of water, through oxidation, which partly explains the observed changes in that atmospheric layer. That stratospheric water change has a radiative impact, is considered a forcing, and can be evaluated. Stratospheric concentrations of water have varied significantly in past decades. The full extent of these variations is not well understood and is probably less a forcing than a feedback process added to natural variability. The contribution of stratospheric water vapour to warming, both forcing and feedback, is much smaller than from CH4 or CO2.

The maximum amount of water vapour in the air is controlled by temperature. A typical column of air extending from the surface to the stratosphere (about 10km in height) in polar regions may contain only a few kilograms of water vapour per square metre, while a similar column of air in the tropics may contain up to 70 kg. With every extra degree of air temperature, the atmosphere can retain around 7% more water vapour (see upper-left insert in the figure). This increase in concentration amplifies the green­house effect, and therefore leads to more warming. This process, referred to as the water vapour feedback, is well understood and quantified. It occurs in all models used to estimate climate change, where its strength is consistent with observations. Although an increase in atmospheric water vapour has been observed, this change is recognized as a climate feedback (from increased atmospheric temperature) and should not be interpreted as a radiative forcing from anthropogenic emissions.

Currently, water vapour has the largest greenhouse effect in the Earth’s atmosphere. However, other greenhouse gases, primarily CO2, are necessary to sustain the presence of water vapour in the atmosphere. Indeed, if these other gases were removed from the atmosphere, its temperature would drop sufficiently to induce a decrease of water vapour, leading to a runaway drop of the greenhouse effect that would plunge the Earth into a frozen state. So greenhouse gases other than water vapour provide the temperature structure that sustains current levels of atmospheric water vapour. Therefore, although CO2 is the main anthropogenic control knob on climate, water vapour is a strong and fast feedback that amplifies any initial forcing by a typical factor between two and three. Water vapour is not a significant initial forcing, but is nevertheless a fundamental agent of climate change.

greenhouse effectIllustration of the water cycle and its interaction with the greenhouse effect. The upper-left insert indicates the relative increase of potential water vapour content in the air with an increase of temperature (roughly 7% per degree). The white curls illustrate evaporation, which is compensated by precipitation to close the water budget. The red arrows illustrate the outgoing infrared radiation that is partly absorbed by water vapour and other gases, a process that is one component of the greenhouse effect. The stratospheric processes are not included in this figure.

 

How Will the Earth’s Water Cycle Change?
WG1 FAQ 12.2

The flow and storage of water in the Earth’s climate system are highly variable, but changes beyond those due to natural variability are expected by the end of the current century. In a warmer world, there will be net increases in rainfall, surface evaporation and plant transpiration. However, there will be substantial differences in the changes between locations. Some places will experience more precipitation and an accumulation of water on land. In others, the amount of water will decrease, due to regional drying and loss of snow and ice cover.
The water cycle consists of water stored on the Earth in all its phases, along with the movement of water through the Earth’s climate system. In the atmosphere, water occurs primarily as a gas—water vapour—but it also occurs as ice and liquid water in clouds. The ocean, of course, is primarily liquid water, but the ocean is also partly covered by ice in polar regions. Terrestrial water in liquid form appears as surface water—such as lakes and rivers—soil moisture and groundwater. Solid terrestrial water occurs in ice sheets, glaciers, snow and ice on the surface and in permafrost and seasonally frozen soil.

Statements about future climate sometimes say that the water cycle will accelerate, but this can be misleading, for strictly speaking, it implies that the cycling of water will occur more and more quickly with time and at all locations. Parts of the world will indeed experience intensification of the water cycle, with larger transports of water and more rapid movement of water into and out of storage reservoirs. However, other parts of the climate system will experience substantial depletion of water, and thus less movement of water. Some stores of water may even vanish.

As the Earth warms, some general features of change will occur simply in response to a warmer climate. Those changes are governed by the amount of energy that global warming adds to the climate system. Ice in all forms will melt more rapidly, and be less pervasive. For example, for some simulations assessed in this report, summer Arctic sea ice disappears before the middle of this century. The atmosphere will have more water vapour, and observations and model results indicate that it already does. By the end of the 21st century, the average amount of water vapour in the atmosphere could increase by 5 to 25%, depending on the amount of human emissions of greenhouse gases and radiatively active particles, such as smoke. Water will evaporate more quickly from the surface. Sea level will rise due to expansion of warming ocean waters and melting land ice flowing into the ocean.

These general changes are modified by the complexity of the climate system, so that they should not be expected to occur equally in all locations or at the same pace. For example, circulation of water in the atmosphere, on land and in the ocean can change as climate changes, concentrating water in some locations and depleting it in others. The changes also may vary throughout the year: some seasons tend to be wetter than others. Thus, model simulations assessed in this report show that winter precipitation in northern Asia may increase by more than 50%, whereas summer precipitation there is projected to hardly change. Humans also intervene directly in the water cycle, through water management and changes in land use. Changing population distributions and water practices would produce further changes in the water cycle.

Water cycle processes can occur over minutes, hours, days and longer, and over distances from metres to kilometres and greater. Variability on these scales is typically greater than for temperature, so climate changes in precipitation are harder to discern. Despite this complexity, projections of future climate show changes that are common across many models and climate forcing scenarios. These results collectively suggest well understood mechanisms of change, even if magnitudes vary with model and forcing. We focus here on changes over land, where changes in the water cycle have their largest impact on human and natural systems.

Projected climate changes from simulations assessed in this report (shown schematically in Figure 1) generally show an increase in precipitation in parts of the deep tropics and polar latitudes that could exceed 50% by the end of the 21st century under the most extreme emissions scenario. In contrast, large areas of the subtropics could have decreases of 30% or more. In the tropics, these changes appear to be governed by increases in atmospheric water vapour and changes in atmospheric circulation that further concentrate water vapour in the tropics and thus promote more tropical rainfall. In the subtropics, these circulation changes simultaneously promote less rainfall despite warming in these regions. Because the subtropics are home to most of the world’s deserts, these changes imply increasing aridity in already dry areas, and possible expansion of deserts.

Increases at higher latitudes are governed by warmer temperatures, which allow more water in the atmosphere and thus, more water that can precipitate. The warmer climate also allows storm systems in the extratropics to transport more water vapour into the higher latitudes, without requiring substantial changes in typical wind strength. As indicated above, high latitude changes are more pronounced during the colder seasons.

Whether land becomes drier or wetter depends partly on precipitation changes, but also on changes in surface evaporation and transpiration from plants (together called evapotranspiration). Because a warmer atmosphere can have more water vapour, it can induce greater evapotranspiration, given sufficient terrestrial water. However, increased carbon dioxide in the atmosphere reduces a plant’s tendency to transpire into the atmosphere, partly counteracting the effect of warming.

In the tropics, increased evapotranspiration tends to counteract the effects of increased precipitation on soil moisture, whereas in the subtropics, already low amounts of soil moisture allow for little change in evapotranspiration. At higher latitudes, the increased precipitation generally outweighs increased evapotranspiration in projected climates, yielding increased annual mean runoff, but mixed changes in soil moisture. As implied by circulation changes in Figure 1, boundaries of high or low moisture regions may also shift.

A further complicating factor is the character of rainfall. Model projections show rainfall becoming more intense, in part because more moisture will be present in the atmosphere. Thus, for simulations assessed in this report, over much of the land, 1-day precipitation events that currently occur on average every 20 years could occur every 10 years or even more frequently by the end of the 21st century. At the same time, projections also show that precipitation events overall will tend to occur less frequently. These changes produce two seemingly contradictory effects: more intense downpours, leading to more floods, yet longer dry periods between rain events, leading to more drought.
At high latitudes and at high elevation, further changes occur due to the loss of frozen water. Some of these are resolved by the present generation of global climate models (GCMs), and some changes can only be inferred because they involve features such as glaciers, which typically are not resolved or included in the models. The warmer climate means that snow tends to start accumulating later in the fall, and melt earlier in the spring. Simulations assessed in this report show March to April snow cover in the Northern Hemisphere is projected to decrease by approximately 10 to 30% on average by the end of this century, depending on the greenhouse gas scenario. The earlier spring melt alters the timing of peak springtime flow in rivers receiving snowmelt. As a result, later flow rates will decrease, potentially affecting water resource management. These features appear in GCM simulations.

Loss of permafrost will allow moisture to seep more deeply into the ground, but it will also allow the ground to warm, which could enhance evapotranspiration. However, most current GCMs do not include all the processes needed to simulate well permafrost changes. Studies analysing soils freezing or using GCM output to drive more detailed land models suggest substantial permafrost loss by the end of this centuryChanges to Groundwater in Uganda . In addition, even though current GCMs do not explicitly include glacier evolution, we can expect that glaciers will continue to recede, and the volume of water they provide to rivers in the summer may dwindle in some locations as they disappear. Loss of glaciers will also contribute to a reduction in springtime river flow. However, if annual mean precipitation increases—either as snow or rain—then these results do not necessarily mean that annual mean river flow will decrease.

Schematic diagram of projected changes in major components of the water cycle.Figure 1: Schematic diagram of projected changes in major components of the water cycle. The blue arrows indicate major types of water movement changes through the Earth’s climate system: poleward water transport by extratropical winds, evaporation from the surface and runoff from the land to the oceans. The shaded regions denote areas more likely to become drier or wetter. Yellow arrows indicate an important atmospheric circulation change by the Hadley Circulation, whose upward motion promotes tropical rainfall, while suppressing subtropical rainfall. Model projections indicate that the Hadley Circulation will shift its downward branch poleward in both the Northern and Southern Hemispheres, with associated drying. Wetter conditions are projected at high latitudes, because a warmer atmosphere will allow greater precipitation, with greater movement of water into these regions.

How will climate change affect the frequency and severity of floods and droughts?

WG2 FAQ 3.1

Climate change is projected to alter the frequency and magnitude of both floods and droughts. The impact is expected to vary from region to region. The few available studies suggest that flood hazards will increase over more than half of the globe, in particular in central and eastern Siberia, parts of south-east Asia including India, tropical Africa, and northern South America, but decreases are projected in parts of northern and eastern Europe, Anatolia, central and east Asia, central North America, and southern South America. The frequency of floods in small river basins is very likely to increase, but that may not be true of larger watersheds because intense rain is usually confined to more limited areas. Spring snowmelt floods are likely to become smaller, both because less winter precipitation will fall as snow and because more snow will melt during thaws over the course of the entire winter. Worldwide, the damage from floods will increase because more people and more assets will be in harm’s way.

By the end of the 21st century meteorological droughts (less rainfall) and agricultural droughts (drier soil) are projected to become longer, or more frequent, or both, in some regions and some seasons, because of reduced rainfall or increased evaporation or both. But it is still uncertain what these rainfall and soil moisture deficits might mean for prolonged reductions of streamflow and lake and groundwater levels. Droughts are projected to intensify in southern Europe and the Mediterranean region, central Europe, central and southern North America, Central America, northeast Brazil and southern Africa. In dry regions, more intense droughts will stress water-supply systems. In wetter regions, more intense seasonal droughts can be managed by current water-supply systems and by adaptation; for example, demand can be reduced by using water more efficiently, or supply can be increased by increasing the storage capacity in reservoirs.

How will the availability of water resources be affected by climate change?
WG2 FAQ 3.2

Climate models project decreases of renewable water resources in some regions and increases in others, albeit with large uncertainty in many places. Broadly, water resources are projected to decrease in many mid-latitude and dry subtropical regions, and to increase at high latitudes and in many humid mid-latitude regions. Even where increases are projected, there can be short-term shortages due to more variable streamflow (because of greater variability of precipitation), and seasonal reductions of water supply due to reduced snow and ice storage. Availability of clean water can also be reduced by negative impacts of climate change on water quality; for instance the quality of lakes used for water supply could be impaired by the presence of algae producing toxins.

How is climate change affecting monsoons?
WG1 FAQ 14.1

Monsoons are the most important mode of seasonal climate variation in the tropics, and are responsible for a large fraction of the annual rainfall in many regions. Their strength and timing is related to atmospheric moisture content, land–sea temperature contrast, land cover and use, atmospheric aerosol loadings and other factors. Overall, monsoonal rainfall is projected to become more intense in future, and to affect larger areas, because atmospheric moisture content increases with temperature. However, the localized effects of climate change on regional monsoon strength and variability are complex and more uncertain.

Monsoon rains fall over all tropical continents: Asia, Australia, the Americas and Africa. The monsoon circulation is driven by the difference in temperature between land and sea, which varies seasonally with the distribution of solar heating. The duration and amount of rainfall depends on the moisture content of the air, and on the configuration and strength of the atmospheric circulation. The regional distribution of land and ocean also plays a role, as does topography. For example, the Tibetan Plateau—through variations in its snow cover and surface heating—modulates the strength of the complex Asian monsoon systems. Where moist on-shore winds rise over mountains, as they do in southwest India, monsoon rainfall is intensified. On the lee side of such mountains, it lessens.

Since the late 1970s, the East Asian summer monsoon has been weakening and not extending as far north as it used to in earlier times, as a result of changes in the atmospheric circulation. That in turn has led to increasing drought in northern China, but floods in the Yangtze River Valley farther south. In contrast, the Indo-Australian and Western Pacific monsoon systems show no coherent trends since the mid-20th century, but are strongly modulated by the El Niño-Southern Oscillation (ENSO). Similarly, changes observed in the South American monsoon system over the last few decades are strongly related to ENSO variability. Evidence of trends in the North American monsoon system is limited, but a tendency towards heavier rainfalls on the northern side of the main monsoon region has been observed. No systematic long-term trends have been observed in the behaviour of the Indian or the African monsoons.

The land surface warms more rapidly than the ocean surface, so that surface temperature contrast is increasing in most regions. The tropical atmospheric overturning circulation, however, slows down on average as the climate warms due to energy balance constraints in the tropical atmosphere. These changes in the atmospheric circulation lead to regional changes in monsoon intensity, area and timing. There are a number of other effects as to how climate change can influence monsoons. Surface heating varies with the intensity of solar radiation absorption, which is itself affected by any land use changes that alter the reflectivity (albedo) of the land surface. Also, changing atmospheric aerosol loadings, such as air pollution, affect how much solar radiation reaches the ground, which can change the monsoon circulation by altering summer solar heating of the land surface. Absorption of solar radiation by aerosols, on the other hand, warms the atmosphere, changing the atmospheric heating distribution.

The strongest effect of climate change on the monsoons is the increase in atmospheric moisture associated with warming of the atmosphere, resulting in an increase in total monsoon rainfall even if the strength of the monsoon circulation weakens or does not change.

Climate model projections through the 21st century show an increase in total monsoon rainfall, largely due to increasing atmospheric moisture content. The total surface area affected by the monsoons is projected to increase, along with the general poleward expansion of the tropical regions. Climate models project from 5% to an approximately 15% increase of global monsoon rainfall depending on scenarios. Though total tropical monsoon rainfall increases, some areas will receive less monsoon rainfall, due to weakening tropical wind circulations. Monsoon onset dates are likely to be early or not to change much and the monsoon retreat dates are likely to delay, resulting in lengthening of the monsoon season.

Future regional trends in monsoon intensity and timing remain uncertain in many parts of the world. Year-to-year variations in the monsoons in many tropical regions are affected by ENSO. How ENSO will change in future—and how its effects on monsoon will change—also remain uncertain. However, the projected overall increase in monsoon rainfall indicates a corresponding increase in the risk of extreme rain events in most regions.

Schematic diagram illustrating the main ways that human activity influences monsoon rainfall.Schematic diagram illustrating the main ways that human activity influences monsoon rainfall. As the climate warms, increasing water vapour transport from the ocean into land increases because warmer air contains more water vapour. This also increases the potential for heavy rainfalls. Warming-related changes in large-scale circulation influence the strength and extent of the overall monsoon circulation. Land use change and atmospheric aerosol loading can also affect the amount of solar radiation that is absorbed in the atmosphere and land, potentially moderating the land–sea temperature difference.

Changes to Groundwater in Uganda

WG2 Chapter 3, Figure 5.
WG2 Chapter 3, Figure 5.

Simulated change in mean monthly runoff of the Mitano River in Uganda across seven climate models, for a 2°C increase in global mean temperature above the 1961-1990 average.  One of the seven climate models is highlighted separately, showing changes for both a 2°C increase (dotted line) and a 4°C increase (solid line).

 

Simulated change in mean monthly runoff of the Mitano River in Uganda across seven climate models, for a 2°C increase in global mean temperature above the 1961-1990 average.  One of the seven climate models is highlighted separately, showing changes for both a 2°C increase (dotted line) and a 4°C increase (solid line).

Precipitation is the main driver of changes to river runoff and the seasonal changes in precipitation are usually reflected in seasonal streamflow variability. However, changes in groundwater recharge also affect streamflow. As global temperatures rise, evapotranspiration increases, reducing the amount of groundwater.  Human and natural systems in the Upper Nile region have historically been very vulnerable to changes in river flow. People are particularly reliant on the river for fishing and hydro-electric power production, so, although it can be hard for models to accurately recreate local rainfall patterns, it is important to try to project how they might change in the future.

The Mitano river is in the Upper Nile basin in Uganda, which has a humid, tropical climate.  Usually, the effect of the ‘short’ (March-May) and ‘long’ (September- November) rains determine the river’s discharge. However, mean global temperature increases of 4°C or more with respect to the 1961-1990 average are projected to decrease groundwater outflow to the river so much that the annual mean discharge will fall, even though annual mean precipitation will be higher. The spring discharge peak disappears and the river flow regime changes from having two seasonal peaks to only one – the effect of the ‘short rains’ could be lost. This is because, for the Mitano, groundwater flow is far more important than lateral (quickflow within the upper soil profile) or surface flow. Increasing evapotranspiration limits the amount of water penetrating the soil profile and replenishing the shallow groundwater store during the first, short, wet season.

Changing groundwater levels also affect land surface energy fluxes, including through evaporation, and thus feedback on the climate system, particularly in semi-arid areas.

Additional source:

Kingston, D. G. and Taylor, R. G.: Sources of uncertainty in climate change impacts on river discharge and groundwater in a headwater catchment of the Upper Nile Basin, Uganda, Hydrol. Earth Syst. Sci., 14, 1297-1308, doi:10.5194/hess-14-1297-2010, 2010.

The mass budgets of Himalayan Glaciers

The mass budgets of Himalayan glaciers in Bhutan, China, India, Nepal and Pakistan have been negative, on average, for the past 5 decades. Changes in temperature and precipitation affect the glaciers, as do changes in the amount of black carbon (soot) in the atmosphere, which, when deposited on the glaciers’ surface, can decrease the albedo and lead to melting. Many communities are both reliant on glacial meltwater and vulnerable to flooding from moraine-dammed ice marginal lakes.

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