IPCC 2021 – Evidence for Past Climate Change

According to the IPCC report for Policymakers “In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2 million years, and concentrations of CH4 and N2O were higher than at any time in at least 800,000 years. Since 1750, increases in CO2 (47%) and CH4 (156%) concentrations far exceed, and increases in N2O (23%) are similar to, the natural multi-millennial changes between glacial and interglacial periods over at least the past 800,000 years.”1

IPCC AR6 data sources


To gather information about the climate scientists need to use a wide range of sources. As can be seen on the graphic opposite, from 1800 onwards scientists can rely upon observations collected by various instruments. However to really understand climate change we need to examine longer term patterns going back thousands two hundreds of thousands of years. The evidence that we have for these can be taken for various sources as can be seen on graph B showing paleoclimatic sources of evidence. Paleoclimate is just a way of saying climate from the geological past.


Select one of the sources of instrumental observations and one of the paleoclimatic evidence and conduct some research into it.

Complete the tables to evaluate the methods of showing climate change.

Instrumental Evidence

instrumental evidence table

Paleoclimate Evidence

  1. IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press. P10. Accessed 28th November 2021 at Sixth Assessment Report (ipcc.ch)
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IPCC 2021 – The Evidence for Climate Change

climate stripes

Climate stripes Image source: Show your stripes2

The text below is simplified from FAQ3.1 of the IPCC’s 2021 WG1 report1.

How do we Know Humans are Responsible for Climate Change?

The dominant role of humans in driving recent climate change is clear. This conclusion is based on a synthesis (mixture) of information from multiple lines of evidence, including direct observations of recent changes in Earth’s climate; analyses of tree rings, ice cores, and other long-term records documenting how the climate has changed in the past; and computer simulations based on the fundamental physics that govern the climate system.

Climate is influenced by a range of factors

There are two main natural drivers of variations in climate on timescales of decades to centuries.

  1. The first is variations in the Sun’s activity, which alter the amount of incoming energy from the sun.
  2. The second is large volcanic eruptions, which increase the number of small particles (aerosols) in the upper atmosphere that reflect sunlight and cool the surface—an effect that can last for several years.

The main human drivers of climate change are increases in the atmospheric concentrations of greenhouse gases and of aerosols from burning fossil fuels, land use change (e.g. deforestation) and other sources. The greenhouse gases absorb infrared radiation (heat) near the surface, warming the climate. Aerosols, like those produced naturally by volcanoes, on average cool the climate by increasing the reflection of sunlight.

Evidence for human activity causing recent change.

Multiple lines of evidence demonstrate that human drivers are the main cause of recent climate change. The current rates of increase of the concentration of the major greenhouse gases (carbon dioxide, methane and nitrous oxide) are unprecedented over at least the last 800,000 years.

Climate models

The basic physics underlying the warming effect of greenhouse gases on the climate has been understood for more than a century, and our current understanding has been used to develop the latest generation climate models.

They include a representation of the ocean, atmosphere, sea ice, land and vegetation and the main processes important in driving climate and climate change.

IPCC climate change graphs FAQ3.1


Results consistently show that such climate models can only reproduce the observed warming  when including the effects of human activities, in particular the increasing concentrations of greenhouse gases.

These climate models show a dominant warming effect of greenhouse gas increases, which has been partly offset by the cooling effect of increases in atmospheric aerosols.

By contrast, simulations that include only natural processes such as variations in the activity of the Sun and emissions from large volcanoes are not able to reproduce the observed warming.

The fact that simulations including only natural processes show much smaller temperature increases indicates that natural processes alone cannot explain the strong rate of warming observed.

Extra evidence

An additional line of evidence for the role of humans in driving climate change comes from comparing the rate of warming observed over recent decades with that which occurred prior to human influence on climate. Evidence from tree rings and other paleoclimate records shows that the rate of increase of global surface temperature observed over the past fifty years exceeded that which occurred in any previous 50-year period over the past 2000 years.

Taken together, this evidence shows that humans are the dominant cause of observed global warming over recent decades.

Humans and climate change – comprehension exercise

Using the article make a list of all of the evidence that is available for proving what is responsible for climate change.

  1. What are the natural drivers of climate change?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
  2. What are the main human drivers of climate change?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
  3. What is a climate model and how do they work?                                                                                                                                                                                                                                                                                                                        
  4. Using graph a, make two statements about the recent warming in the context of:
    • i) the last 2000 years
    • ii) the last 100,000 years?
  5. Using graph b which situation from the climate models matches the observed temperature change?                                                                                                                         
  6. What do the results of the climate models show when trying to explain why temperatures have increased in recent decades?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
  7. What other evidence is there to support that shown by the climate models?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
  8. What should governments and policymakers do to respond to this evidence?                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                


    1. ipcc.ch. 2021. AR6 WGI Report Frequently Asked Questions. [online] Available at: https://www.ipcc.ch/report/ar6/wg1/downloads/faqs/IPCC_AR6_WGI_FAQs.pdf [Accessed 28 November 2021]. P. 14
    2. Analytics, M., 2021. Show Your Stripes. [online] Showyourstripes.info. Available at: https://showyourstripes.info/s/globe [Accessed 28 November 2021].
    3. IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press. P.8.  Accessed 28th November 2021 at Sixth Assessment Report (ipcc.ch)
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How do CO₂ emissions link to global temperatures?

Royal Geographical Society

This resource links to B.4.1, FAQ5.4 and to Figure SPM.10 in the IPCC report of 2021. The aim of this resource is to answer the question how do CO emissions link to global temperatures?

It was written with the Royal Geographical Society with IBG

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The carbon budget

A carbon budget is the cumulative amount of carbon dioxide (CO₂) emissions permitted over a period of time to keep within a certain temperature threshold e.g. a 1.5°C target limit for global temperature rise.

It is tricky to estimate because a budget is influenced by core assumptions, chosen characteristics, and different variables (for example the amount of other greenhouse gases in the atmosphere). Read about the difficulties in estimating a budget on the Carbon Tracker webpage Carbon Budgets Explained. Carbon budgets are particularly tricky, because there is so little left in the budget if we are to stay under a 1.5°C level of warming – there is very little room for error in calculating the budget.

Mark Maslin neatly represents the pressing need to reduce CO₂ emissions in this interactive temporal pie chart Using up the carbon budget.

If global warming is to be held to a 1.5°C temperature rise, the current estimate (from Carbon Brief for the 1.5°C target) is that we have a range of 230-440 billion tonnes of CO₂ left (GtCO₂), from 2020 onwards[1]. Since 1751 the world has emitted over 1.5 trillion tonnes of CO₂.

  1. Create a pie chart to illustrate the historic carbon budget and the estimated remaining amount of carbon in the budget for the 1.5°C target. To complete this use the following steps.

a) 1000 kilograms is a tonne. 1 billion metric tonnes equal a gigatonne. 1 trillion tonnes equal 1000 gigatonnes. Standardise the total amount of CO₂ in the carbon budget by converting 440 billion tonnes and 1.5 trillion tonnes into GtCO₂.

b) Calculate the estimated total carbon budget. Take the upper estimate of how much carbon we have left in the budget (to emit) and add it to the amount emitted since 1751.

c) What proportion of your circle will be drawn per GtCO₂ by dividing 360° by your total carbon budget figure?

d) Draw the pie chart.

The idea of a carbon budget and the notion that Earth has a remaining amount before a target is missed is based on the near-linear relationship between cumulative CO₂ emissions (the impact on atmospheric concentrations) and the warming of the planet. In other words, as one increases so does the other. The IPCC report of 2021 confirmed that global temperatures rise in direct relation to cumulative emissions.

CO emissions and global warming

Scientist have investigated the correlation between CO₂ emissions and global warming. Table 1 in Appendix A contains data for CO₂ emissions and historic annual temperature change for the planet.

  1. Draw a line graph to illustrate the relationship between cumulative CO₂ emissions and global temperature.

There are 5 projected ‘pathways’ for future cumulative CO₂ emissions and temperature change; SSP1, SSP2, SSP3, SSP4, and SSP5 (standing for Shared Socio-economic Pathways). Currently the Earth is following the SSP2-4.5 or SSP3-7.0 scenarios. These pathways are sometimes also referred to as RCPs, Representative Concentration Pathways of CO₂. The bullet points below clearly highlight the preferable future path and, according to the Paris Agreement (by which 198 countries agreed to try to keep the rise in mean global temperature to well below 2 °C above pre-industrial levels, and preferably limit the increase to 1.5 °C), the only legal trajectory. 

 SSP1 the ‘green road’, honouring the Paris Agreement, by limiting global warming to 1.5°C

SSP2 ‘middle of the road’, some progress but environmental degradation

SSP3 ‘regional rivalry’, food and energy security are prioritised, strong environmental decline  

SSP4 inequality ‘a road divided’, environmental policies only focus on high-income areas

SSP5 fossil-fuel development taking ‘the highway’ business-as-usual, no-mitigation

Figure 2 in Appendix B is taken from the IPCC report. It shows cumulative CO₂ emissions since 1850 and °C temperature change with the 5 future SSP (Shared Socio-economic Pathways).

  1. Describe the relationship between cumulative CO₂ emissions and global warming. Be careful: emissions don’t necessarily determine the temperature of the Earth, read Carbon Dioxide in the Atmosphere – Balancing the Flow to learn more.
  1. Do cumulative CO₂ emissions cause annual mean global land-ocean temperatures to rise? Use data in your answer. 

Within your answer for question 2 there is variation in emissions by country. Some countries have historically contributed more than others to global warming. Table 2 in Appendix B gives data on CO₂ emissions in 1750 and 2019 for 6 countries.

  1. Create a line graph for cumulative CO₂ emissions for Canada, China, India, Kenya, the US, and the UK.
  1. Which country emitted the most CO₂ in 2019?
  1. Which country has had the greatest relative change between 1750 and 2019?

Further work

Exam-style question

Open the Global Carbon Atlas.

Using all the work you have completed answer the final question below. The instruction describe means you must give an account of the pattern you see in the world map, and how it changes.

  1. Press the play button at the bottom of the screen. Describe how the pattern of CO₂ emissions changes from 1960 to 2020.

[1] Carbon budgets are an estimate of the total quantity of CO₂equivalent emissions that can be allowed in order to maintain a 66% chance of staying within the Paris Agreement target of capping global warming at 1.5°C this century.

Appendix A


Appendix B

CO2 and temperature

Figure 2 is there a relationship between cumulative CO₂ emissions and the increase in global surface temperature? © The 2021 IPCC Working Group I report

CO2 emissions data


  1. The Washington Post explains that a gigaton is equivalent to a billion metric tonnes.

a) Standardise the total amount of CO₂ in the carbon budget into GtCO₂. 440bn tonnes and 1.5 trillion tonnes of CO₂ = 440 GtCO₂ and 1500 GtCO₂

b) 440 GtCO₂ and 1500 GtCO₂ = 1940 GtCO₂.

c) 360 ÷ 1940 = 0.18556701. Each GtCO₂ will be worth 0.18556701°.

2. As instructed.

  1. There is a strong relationship between CO₂ emissions and global warming. Both historical and future emission pathways show that as CO₂ increases as a gas in the atmosphere, global temperatures rise. When analysing the paleoclimate record this strong correspondence between temperature and the concentration of carbon dioxide in the atmosphere is equally evident over the past the past several hundred thousand years.
  2. Yes, cumulative CO₂ emissions cause annual mean global land-ocean temperature change. Figure 2 clearly shows the near linear relationship. If SSP1-1.9 (with a temperature increase under 2°C) is to be achieved, then world population will have to be held at 8.24 billion with CO₂ emissions being cut to net zero by 2050. 
  3. As instructed.
  4. Kenya emitted 449.09 million t in 2019.
  5. Column 5 from Table 2 is complete below.

8. As instructed.

Is our Weather Becoming More Extreme?

Royal Geographical Society
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This is a teaching resource linked to section 11.1.4 of the sixth assessment IPCC report of 2021, written with the Royal Geographical Society with IBG. The aim is to answer the question: are we experiencing more weather extremes as a result of climate change?

Weather and climate

Weather and climate are separate concepts. Weather describes the short-term conditions in the atmosphere, whilst climate refers to its long-term state. Weather measurements might be hourly or daily readings of temperature (°C), rainfall (mm) and wind speed (m/s). Climate is usually defined as the average of 30 years of weather measurements.

Effects of greenhouse gas emissions and other external forcings

There is now evidence that climatic extremes have changed since the mid-twentieth century — and some of these changes have been the result of anthropogenic influence (if you do not understand the term read What is the Anthropocene? or go to the Natural History Museum webpage on why it matters). Generally speaking, extreme weather events have increased in intensity and frequency since pre-industrial time. In particular, global temperature has increased, both by annual average globally and in localised spikes.

  1. Before you begin this resource, define what the words ‘extreme’ and ‘rare’ mean.

The IPCC report says that even with relatively small incremental increases in global warming (as per the SSP1 pathway which holds global warming to 1.5°C) there will be ‘significant changes in extremes’ on the global scale and for large regions. It is:

  • Globally, (it is very likely) temperature extremes will continue
  • Heavy precipitation will intensify (predicted with high confidence in particular for North America, Europe, and Asia according to the Met Office)
  • Tropical cyclones are getting more intense (medium confidence)

The local exchanges of heat and the heat from the evaporation and condensation of moisture, called thermodynamic changes, create temperature extremes. Unprecedented temperatures begin with the initial thermodynamic effect (of a warming troposphere) and lead to increased intensity and frequency of hot extremes. Recently, this has been coupled with decreases in the intensity and frequency of cold extremes.

  1. Do you know the different levels of the atmosphere? Sketch planet Earth with the text boxes below in the correct altitude sequence (add aeroplanes, satellites, the aurora borealis, the ozone layer, and the Kármán line to extend the activity).


layers of the atmosphere

As Greenhouse Gases (GHGs) increase there is an immediate impact on the temperature of the troposphere, stratosphere, and the surface of the Earth – land, sea and ice. The water vapour cycle intensifies as it becomes easier for water vapour to evaporate from the surface of the Earth and vegetation and for water vapour to condense into cloud droplets in a warmer troposphere. As water vapour is itself a greenhouse gas,  changes in atmospheric water vapour always amplify the initial temperature increases (a positive feedback) whilst the lapse rate[1] feedback also amplifies near-surface temperature increases (positive feedback) in mid- and high latitude countries, such as the UK. This means extreme weather from the larger cumulonimbus clouds and more severe thunderstorms.

[1] A lapse rate describes the rate at which temperature decreases with increasing altitude.

storm waves

Figure 1 extreme weather and huge waves crashing into Cornwall at high tide © Greg Martin

3. Figure 4 in Appendix A is a feedback diagram on the creation of extreme weather for a typical mid-latitude country. Complete the diagram using the explanation:

The greenhouse effect leads to the temperature of the troposphere and the surface of the Earth rising. With more heat available, evaporation and evapotranspiration rates from surface water and vegetation increases. With more water vapour available in a warmer atmosphere, more clouds can form. As the water vapour condenses, latent heat is released which further heats the atmosphere locally and promotes convection, warm air rising, and the further formation of cumulus clouds. (Latent heat is the energy absorbed by or released from a substance during the change from a gas to a liquid or a solid or vice versa). As a consequence, increased cumulonimbus clouds and thunderstorms creates ultimately causing extreme precipitation events.

4. Now add the processes to the blue connecting lines, using the explainers.

storm waves

Figure 2 extreme weather will increase both around the world, and specifically in the UK © Greg Martin

Extreme Weather

The IPCC report states that, for each additional 1°C, extreme rainfall will intensify by 7%. Currently annual rainfall on land is increasing and monsoons are changing in complex ways.

5. Analyse Figure 5 in Appendix B. Describe the overview for wet extremes and their potential for human contribution. Reference the areas: NWN and NEC, NEU, SEAF, and SAS.

Further work

Access the following resources for suggested further work on weather extremes.

Exam-style question

Using all the work you have completed answer the final question below. The instruction to assess means you should weigh up several opinions to conclude about their effectiveness or validity.

Use the State of the UK Climate 2020 special issue article (in the Further work list above) to inform your answer.

6. Assess whether UK weather is becoming more extreme in the twenty-first century.

train storm wave

Figure 3 big waves crash over the seafront in Penzance, Cornwall © Greg Martin

Appendix A

extreme weather positive feedback

Figure 4 a positive feedback loop for extreme weather

Appendix B

IPCC AR6 overview of assessed events

Figure 5 what has been observed in wet extremes around the world? © IPCC report


  1. An extreme weather event is defined as ‘an event that is rare at a particular place and time of year’ – a usual definition is an event which happens less than 5% of the time. For example, the warmest 5 July 1st days in London over the past 100 years would be defined as extremely warm. A short-term extreme climate event is ‘a pattern of extreme weather that persists for some time, such as a season.’ Some studies consider an event as extreme if it is unprecedented; on the other hand, other studies consider events that occur several times a year as moderate extreme events.
  2. The Earth has 5 major layers. In order from the surface of the planet upwards, they are Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.
extreme weather positive feedbacks

4. All sentences: amplifies temperature increase, stronger convection, more evaporation, and lapse rate feedback amplifies near-surface increases.

5. Overall, the pattern for observed wet extremes around the world is a general increase in the mid and high latitudes, in particular across the US, Europe and Eurasia. There has also been an increase in wet extremes in parts of South America, for example Argentina. The greatest risk region is by far northern Europe (NEU) with a high confidence level that these changes derive from human influence. India (SAS) has also experienced an increase in extreme precipitation with medium confidence from observed trends. Canada (NWN and NEC) and Kenya (SEAF) have a lack of evidence for extreme precipitation.

6. As instructed.

Leaves as Thermometers

Leaves as thermometers

Leaf shape changes with climate. Generally smoother leaves are found in warmer climates and more jagged leaves are found in cooler climates.

Because the shape of the leaves change with climate, fossilised leaves are used to help learn about past climates.

By studying different types of plant they can gather climate information, such as annual temperature range and water availability that corresponds to the time when the plant was living.

This graph shows the relationship between the temperature and the percentage of smooth leaves found together:

leaf graph

The main problem with this method is that lots of samples are needed to get a good picture of the past climate. 

Using the graph, work out the approximate mean annual temperature if the following leaves were found together:


smooth and jagged edged leaves

This resource was originally developed by the Climate Change Schools Project