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About Climate Change


In this section you will find a step-by-step guide to climate change. The information is written by climate experts using known facts and the latest projections and is suitable for teachers and for students aged 11+.

You will find in-depth answers to key questions:

Climate Literate person;

  • Understands the essential principles of Earth’s climate system and knows how to assess scientifically credible information about climate,
  • Communicates about climate and climate change in a meaningful way,
  • Can make informed and responsible decisions with regard to actions that may affect climate.

What do we mean by climate change?

Climate change means any significant change in climate, like temperature or rainfall, over a 30 year period or more. If the climate is changing, then the 30 year average temperature, or rainfall, or number of sunny days, is changing.

It’s easy to mix up climate and weather.

Here’s a simple way to think about it: climate is what we expect (e.g. cold winters) and weather is what we get (e.g. rain).

Weather is what is happening in the atmosphere at any one time: how warm, windy, sunny or humid it is. Climate is the description of the average weather we might expect at a given time, usually taken for several decades or longer to average out year to year variability. Variability might be due to a particularly hot summer or very cold winter.

The world’s climate has been getting warmer since 1900. However, this overall warming has not occurred evenly across the world’s surface and different places, because of their location and geography, are affected in different ways.

Does the sun cause climate change?

It’s true that changes in solar activity does affect global temperatures.

Changes in the energy output of the Sun, and the Earth’s orbit around the Sun, do have an effect on the Earth’s climate.

Solar Irradiance Graph
Changes in the amount of energy the Earth has received from the Sun over the last 150 years.

Ice ages have come and gone in regular cycles for nearly three million years. There is strong evidence that these are linked to regular variations in the Earth’s orbit around the Sun, the so-called Milankovitch cycles. These cycles change the amount of the Sun’s energy received by different places on the Earth’s surface.

However, over the last 50 years, increased greenhouse gas concentrations have had a much greater effect than changes in the Sun’s energy.



How has the greenhouse effect changed?

The figure shows the amount of Carbon Dioxide in the atmosphere
The figure shows the amount of Carbon Dioxide in the atmosphere at Hawaii (light green line) and at the South Pole (dark green line). There is an annual cycle in carbon dioxide as vegetation takes up carbon in the spring and releases it in the Autumn. As more fossil fuels have been burnt in the Northern Hemisphere, the increase in atmospheric CO₂ has been greater in Hawaii than at the South Pole.

Naturally occurring gases in our atmosphere, such as carbon dioxide and methane, provide an insulating effect without which the earth would be a frozen planet. However, levels of greenhouses gases in the atmosphere have increased, preventing more heat escaping to Space and leading to ‘global warming’.

Any increases in the levels of greenhouse gases in the atmosphere mean that less heat escapes to Space and global temperatures increase – an effect known as ‘global warming’.

Over the past 150 years in the industrial era, human activities have increased the emissions of three principal green house gases: carbon dioxide, methane and nitrous oxide. These gases accumulate in the atmosphere, causing concentrations to increase with time.

Carbon dioxide (CO₂) has increased from our use of fossil fuels which we burn for use in transportation, energy generation, building heating and cooling. Deforestation also releases CO₂ and reduces its uptake by plants.

Methane (CH₄) has more than doubled as a result of human activities related to agriculture, natural gas distribution and landfills. However, increases in methane concentrations are slowing down because the growth of emissions has decreased over the last two decades.

Nitrous oxide (N₂0) is also emitted by human activities such as fertilizer use and fossil fuel burning.

Extra information
There are other, lesser, contributors such as CFCs (whose emissions have decreased substantially) and ozone in the lower atmosphere.

Water vapour is the most abundant and important greenhouse gas in the atmosphere. However, human activities have only a small direct influence on the amount of atmospheric water vapour. Indirectly, humans have the potential to affect water vapour substantially by changing climate as a warmer atmosphere contains more water vapour.

Aerosols are small particles present in the atmosphere with widely varying size, concentration and chemical composition. Fossil fuel and biomass burning have increased aerosols containing sulfur compounds, organic compounds and black carbon (soot).

What has caused the rise in temperatures over the past 100 years?

The causes of climate change over the past 100 years.
The causes of climate change over the past 100 years.

In the first half of the 20th century global temperatures have risen because of increases in the levels of greenhouse gases in the atmosphere as well as changes in the amount of energy emitted by the Sun. In the second half of the 20th century warming is mainly due to changing greenhouse gas concentrations.

Sulphate particles from industrial emissions reflect solar radiation and therefore act to cool climate. These particles helped mask the warming for a few decades from 1940, but then reductions in these pollutants together with ever increasing concentrations of greenhouse gases led to renewed warming from the 1970s.

What is the greenhouse effect?

The Greenhouse Effect DiagramIt is essential to human life! The natural greenhouse gas effect keeps Earth much warmer than it would otherwise be. Without the greenhouse effect, planet Earth would be too cold to support human life as we know it.

The temperature of the Earth is determined by the balance between energy coming in from the Sun in the form of visible radiation (sunlight) and energy constantly being emitted from the surface of the Earth to outer space in the form of invisible infrared radiation (heat).

The energy coming in from the Sun can pass through the clear atmosphere pretty much unchanged and therefore heat the surface of the Earth. But the infrared radiation emanating from the surface of the Earth is partly absorbed by some gases in the atmosphere, and some of it is re-emitted downwards. The effect of this is to warm the surface of the Earth and lower part of the atmosphere. This is called the greenhouse effect.

The absorbing gases in the atmosphere are primarily water vapour (responsible for about two-thirds of the effect) and carbon dioxide. Methane, nitrous oxide, ozone and several other gases present in the atmosphere in small amounts also contribute to the greenhouse effect. Without the greenhouse effect the Earth would be, on average, about 33°C colder than it presently is.

What other things can change the climate?


Well volcanic eruptions certainly play their part! There were three volcanic eruptions big enough to affect the climate in the 20th century.

There were 3 volcanic eruptions big enough to affect the climate in the 20th century – Agung in Indonesia (1963), El Chichon in Mexico (1982) and Pinatubo in the Philippines (1991).

Material (particles) from violent volcanic eruptions can be projected far above the highest cloud, and into the stratosphere where they can significantly increase how much incoming solar energy is reflected. Major volcanic eruptions can reduce average global surface temperature by about 0.5°C for months or even years.

However, volcanic eruptions are not the only factor that can influence the climate. There are three ways to change the radiation balance of the Earth:
by changing the incoming energy from the Sun, for example by changes in Earth’s orbit or in the Sun itself;
by changing the fraction of solar radiation that is reflected, for example by changes in cloud cover, vegetation or atmospheric particles such as volcanic material or sulphate aerosols;
by changing greenhouse gas concentrations, for instance, the amount of carbon dioxide in the atmosphere has increased by about 35% in the industrial era, and this increase is known to be due to human activities, primarily the burning of fossil fuels and removal of forests.

How do global changes relate to local changes?

Warming, particularly since the 1970s, has generally been greater over land than over the oceans. Seasonally, warming has been slightly greater in the winter hemisphere. A few areas have cooled but warming has been strongest over the continental interiors of Asia and northern North America. At the same time, eastern North and South America, northern Europe and northern and central Asia have been getting wetter but the Sahel, southern Africa, the Mediterranean and southern Asia have been getting drier.

Has the number of extreme events changed?

natural-disaster floodingAs the Earth’s climate gets warmer, the likelihood of some extreme events such as heat waves increases. Remember the European summer heat wave in 2003? Well, scientists believe, the risk of a similar summer has doubled due to human activities such as fossil-fuel burning.

Determining whether a specific, single extreme event is due to a specific cause, such as increasing greenhouse gases, is difficult for two reasons: 1) extreme events are usually caused by a combination of many different factors and 2) a wide range of extreme events is normal even in an unchanging climate.

However, we can talk about changes to the risk of extremes. The likelihood of some extreme events, such as heat waves, has increased with the changing climate, and the likelihood of others, such as extremely cold nights, has decreased. For example, a recent study estimates that human influences have more than doubled the risk of a very hot European summer like that of 2003.

In some regions there have been increases in droughts and floods. The number of days of very heavy rain have increased in some places. Tropical storm and hurricane frequencies vary considerably from year to year, but evidence suggests substantial increases in intensity and duration since the 1970s.

How has the climate changed in the past?

How has the climate changed in the past?

Northern hemisphere temperatures over the past 1000 years. Temperatures were warmer in the Medieval warm period (MCA) and colder in the Little Ice Age (LIA).

The Earth’s climate has always changed, long before we humans existed!

There have been warmer and colder periods. For example, in the last ice age, 20,000 years ago, it was about 9°C colder than it is now. The causes of most of these changes are very well understood.

How has the climate changed in the recent past?

Global surface temperatures over the last 150 years
Global surface temperatures over the last 150 years

Current global temperatures are warmer than they have been during at least the past five centuries, probably for more than 1000 years. The 17 warmest years on record have all occurred in the last 20 years.

During the 20th century there have been two ‘warming phases’: from the 1910s to the 1940s (0.35°C), and more strongly from the 1970s to the present (0.55°C).

Alongside the warming, there has been an almost worldwide reduction in the extent and mass of glaciers in the 20th century. We know that the Greenland Ice Sheet is melting, that the thickness and extent of sea ice in the Arctic have decreased in all seasons and that sea level is rising due to thermal expansion of the oceans and melting of land ice.

Instrumental observations over the past 150 years show that air temperatures at the Earth’s surface have risen globally.

Why can’t we be sure what happened in the past?

kestrel greenlandTo know what was happening before 1850, we have to rely on what things like tree rings, fossils, and the gases trapped in ice cores tell us about local temperatures. This information is much less precise, and much less global, than for example the satellite data we have nowadays.

There is no single thermometer measuring the global temperature. Instead, individual thermometer measurements taken every day at several thousand stations over the land areas of the world are combined with thousands more measurements of sea surface temperature taken from ships moving over the oceans. These produce an estimate of global average temperature every month.

It is now possible to use these measurements from 1850 to the present, and although coverage is much less than global in the second half of the 19th century, it is much better after 1957 when measurements began in Antarctica, and best after about 1980, when satellite measurements began.

How can we make a climate prediction when we can’t forecast the weather for the next month?

man with an umbrella The chaotic nature of weather makes it unpredictable beyond a few days. To predict the weather you need to know exactly what is happening in the atmosphere down to the smallest scale. Climate is the average weather pattern of a region over many years (usually a period of 30 years).

Weather forecasts are very dependent on knowing exactly what is going on in the atmosphere, down to the smallest scales (it is ‘chaotic’), climate forecasts do not to the same extent.

Climate is the long term average of weather, including its variability. Climate predictions tell us about how the trends and patterns will change: will it be generally wetter in winter? Will there be more heavy downpours?

Projecting changes in climate due to changes in atmospheric composition or other factors is a much more manageable task than predicting the weather. As an analogy, while it is impossible to predict the age at which any particular man will die, we can say with high confidence what the average age of death for men is.

Similarly, a climate prediction might say that average summer rainfall over London is predicted to be 50% less by the 2080s; it will not predict that it will be raining in London on the morning of 23rd August 2089.

How do we make climate predictions?

Reconstructions of past temperatures: measured temperatures
Reconstructions of past temperatures: measured temperatures are shown with the black line and computer model simulations with yellow and blue lines.

The only way we can project climate for the next 100 years, is to use very complex mathematical models. Some of the biggest models contain ten million lines of computer code and require some of the world’s largest super-computers to run them!

These complex mathematical models contain equations that describe the physical processes at work in the atmosphere, ocean, cryosphere (areas of ice and snow) and on land. We use changes in greenhouse gas, solar and volcanic emissions to drive the climate prediction models.

In the top graph the computer models only consider natural changes such as changes in the Sun and volcanoes, in the lower graph man-made changes such as greenhouse gas emissions are also considered. In that case the computer models do a good job of recreating past temperatures.

This gives us confidence for future simulations.

Scientists are confident that the models can provide useful predictions of future climate, partly because of their ability to reproduce observed features of current climate and past climate changes, such as the larger degree of warming in the Arctic and the small, short-term global cooling (and subsequent recovery) which has followed major volcanic eruptions, such as that of Mt. Pinatubo in 1991.

Why are some aspects of climate change harder to predict than others?

Believe it or not, it is much easier to predict global temperature than rainfall in Beijing, Jakarta, London or Mexico City! This is because, the smaller the scale of the physical processes involved, the harder something is to predict.

Climate models allow scientists to predict some aspects of climate change with much more confidence than others. For example:
averages over the whole Earth are easier to get right than very local changes;

Projected global 21st century temperatures
Projected global 21st century temperatures could be anywhere between the bottom of the blue shading and the top of the red

temperature is easier than rainfall, which depends on the very small scale physical processes going on in clouds;
predicting how the climate will change in the relatively near future (within the next 40 years, say) is easier than further ahead, as we have a better understanding of what the world and the climate system will look like.

Why aren’t climate predictions exact?

There are many stages involved in making climate predictions. These include:

  • Making estimates of the gases and particles that will be released into the atmosphere in the future. These are created by making assumptions about population growth, energy use, economic and technological developments;
  • Using carbon cycle models to convert emissions to concentrations of greenhouse gases in the atmosphere. More assumptions have to be made, based on our knowledge of things like how ecosystems respond to changing carbon dioxide availability etc;
  • Using full climate models to calculate the effects of increasing greenhouse gas concentrations on global climate. There are uncertainties in the models themselves, mainly due to the fact that very small scale processes have to be represented in a fairly coarse sort of way, as well as uncertainties in our knowledge of the climate system – are there feedback mechanisms that will come into operation that we don’t know about?
  • Translating global change into local impacts, a whole range of more uncertainties come into play, like how local land use change will impact on the chances of a particular river flooding.

Each stage involves an increasing amount of uncertainty.

What are “climate feedbacks”?

a glacierA climate feedback happens when an initial change in the climate system triggers a process that either intensifies or reduces the initial change.

Imagine snow and ice melting, exposing the darker land or water beneath. This land or water will now absorb more of the Sun’s energy, rather than reflecting it back into space. This causes warming. If it is warmer, there is more melting, more energy absorbed and then more warming and so on. This is an example of a positive feedback.

There are many feedback mechanisms in the climate system that can either amplify (‘positive feedback’) or diminish (‘negative feedback’) changes in the Earth’s climate. Here are two more examples:

Water vapour

The water vapour feedback in terms of the direct greenhouse effect is positive. As the atmosphere warms due to rising levels of greenhouse gases, its concentration of water vapour increases. As water vapour is a greenhouse gas, this in turn causes more warming. This feed back may be strong enough to approximately double the increase in the greenhouse effect due to the added CO2 alone.


Clouds can amplify (increase – positive feedback) or diminish (decrease – negative feedback) warming. Clouds are effective at absorbing infrared radiation emitted from the Earth, re-radiating that energy (heat) back to the ground and therefore exert a large greenhouse effect, warming the Earth. However, clouds can also reflect away incoming solar energy, cooling the Earth. A change in almost any aspect of clouds, such as their type, location, water content, cloud altitude, particle size and shape, or lifetimes, affects the degree to which clouds warm or cool the Earth. Some changes amplify warming while others diminish it. The feedback of clouds can therefore be positive or negative depending on the circumstances.

Those feedback examples presented here are just a few of the feedback mechanisms which exist within the climate system.


Low clouds (left) tend to cool the climate, high clouds (right) tend to warm the climate

How do we think extreme events will change?

They will vary from region to region. For example more flooding is expected in the Asian monsoon region and other tropical areas, future tropical cyclones could become more severe and there will be an increased risk of more intense, more frequent and longer-lasting heat waves in Europe.

Heat waves
The European heat wave of 2003 is an example of the type of extreme heat event lasting from several days to over a week that is likely to become more common in a warmer future climate. In pre-industrial times, the 2003 heat wave would have been a 1 in 1000 event. By the 2040s the average summer is predicted to be like the one we experienced in 2003; this in turn would be viewed as being relatively cold compared to the average summer temperature predicted for the 2060s.

It is also likely that a warmer future climate would have fewer frost days (i.e., nights where the temperature dips below freezing) and so the growing season is expected to get longer.

Rainfall and flooding
In a warmer future climate, most climate models project decreased summer rainfall and increased winter rainfall in most parts of the northern middle and high latitudes. Along with the risk of summer drought, there is an increased chance of episodes of intense rainfall and flooding. More flooding is also expected in the Asian monsoon region and other tropical areas and in a number of major river basins.

There is evidence from modelling studies that future tropical cyclones could become more severe, with greater wind speeds and more intense rainfall, although the actual number of cyclones may not change.

What about other aspects of climate, like rainfall, ice cover and sea level?

Projected rainfall chart
Projected rainfall changes by the end of the Century

Many aspects of climate will change, not just temperature. Sea level is predicted to rise as the oceans get warmer and as some ice on land melts.

Most climate models predict that globally averaged rainfall will increase over time, with the biggest increases closes to the poles and in the Indian monsoon, and the smallest changes in subtropical regions.

Sea level
The two major causes of global sea level rise are water expanding as it warms and loss of land-based ice due to increased melting. Currently thermal expansion of the oceans is the main contributor to rising sea levels. The increasing melting of land-based ice is unlikely to have a significant effect on sea levels until 2100.

Note that, although sea ice (ice floating on the sea) has already started melting, and is predicted to melt more rapidly in the future, this does not affect sea level, as it is already floating and displacing its own weight in water.

Ice sheets
The two major ice sheets are the Greenland ice sheet and the Antarctic ice sheet.

The Greenland ice sheet contains enough water to contribute about 7 m to sea level. A sustained rise in local temperatures of about 3 °C (that’s global warming of about 1.5 °C) is likely to be reached by the end of the century if human-made emissions are not controlled. This would melt the Greenland ice sheet, although it would not happen immediately and it is estimated that this would take a few thousand years.

The West Antarctic ice sheet is the part of the Antarctic ice sheet most vulnerable to climate change. It contains enough water to contribute about 6m to sea level.

What are “tipping points”?

calving glacierTipping points refer to abrupt climate change. An example of abrupt climate change would be the rapid loss of the Greenland ice sheet. However, abrupt changes like this are not likely to occur in the 21st century.

The potential for climate to change relatively rapidly does exist. Abrupt climate change has occurred naturally in the past. A gigantic release of methane from below the ocean bed 56 million years ago led to a sudden warming of 6°C in the climate at a time when global temperatures were much higher than now. During the last ice age, collapses in the ice sheet over North America led to the Gulf Stream switching direction and the temperature across the North Atlantic dropping some 10°C within decades.

An important concern is that the continued growth of greenhouse gas concentrations in the atmosphere may trigger abrupt changes, which become more likely as the concentration of greenhouse gases in the atmosphere increases.

However, abrupt changes such as the collapse of the West Ant arctic ice sheet, the rapid loss of the Greenland ice sheet or large-scale changes of ocean circulation systems, are not considered likely to occur in the 21st century. As the total melting of the Greenland ice sheet, which would raise global sea level by about seven metres, is a slow process, it would take many hundreds of years to complete.

What do we think will happen to the temperature of the Earth?

Projected temperature rise through the 21st century
Projected temperature rise through the 21st century

Global temperatures are likely to rise 2-4°C by the end of the 21st century. The actual temperature rise depends partly on how much greenhouse gas is emitted over the next 90 years.

The Intergovernmental Panel on Climate Change (IPCC) “best estimate” of global warming is 2-4°C by the end of the century. This may not seem like much but it is an average; it conceals a greater warming in some seasons and some areas (particularly at higher latitudes) and less in others, for example nearer the Equator.

In order to make projections about future climate change, scenarios that describe possible global emissions of greenhouse gases are used. These scenarios are based on different ‘storylines’ that illustrate how things may change in the future. They take into account different projected trends in population, economic and technological developments, as well as changes in the political environment. In the graph, the blue scenario is one where radical changes are made to greenhouse gas emissions immediately, whilst the red one is a scenario in which no emissions are reduced.

How do predictions for global temperature relate to what might happen locally?

One projection of how temperatures might change around the world by the end of the century
One projection of how temperatures might change around the world by the end of the century

Climate varies from region to region. This variation is driven by the uneven distribution of solar heating, the individual responses of the atmosphere, oceans and land surface, the interactions between these, and the physical characteristics of the regions. Some human-induced factors that affect climate are global in nature, while others differ from one region to another. For example, carbon dioxide, which causes warming, is distributed evenly around the globe, regardless of where the emissions originate, whereas sulphate aerosols (small particles) that offset some of the warming tend to be regional in their distribution. As a result, the projected changes in climate will vary from region to region. For example, temperatures over land are expected to increase about twice as rapidly as temperatures over the ocean and warming will be greatest at higher latitudes. Similarly some areas will get wetter, whereas other areas will get drier.

If we stopped emitting greenhouse gases now, would the climate stop warming?

Two important factors mean the climate would not stop warming immediately.

Firstly, the adjustment of greenhouse gas concentrations in the atmosphere to reductions in emissions depends on the chemical and physical processes that remove each gas from the atmosphere. Concentrations of some greenhouse gases decrease almost immediately in response to emission reduction, while others can actually continue to increase for centuries even with reduced emissions. While they remain in the atmosphere, the gases continue to have an enhanced warming effect. For example, complete elimination of CO2 emissions would lead to a slow decrease in atmospheric CO2 concentrations through the 21st century.

Secondly, the surface of the Earth and especially the deep oceans take a while to adjust to new conditions, and so, even if the concentration of greenhouse gases in the atmosphere stops rising, the Earth’s surface will continue to warm for many years.

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