Global warming - Earth's climate is changing
The climate crisis is 'unequivocally' caused by human activities. We are already experiencing the impacts of climate change, but changes will continue to escalate - with effects we cannot fully anticipate.
Our researchers pioneered the method that determines the relationships between greenhouse gas emissions and climate change.
- Humans are responsible for the observed warming of our planets atmosphere, land and oceans.
- The present state of our climate system is 'unprecedented over many centuries to many thousands of years'.
- Present-day global concentrations of atmospheric carbon dioxide (CO₂) are higher and rising faster than at any time in at least the past two million years.
- By the 2030's, the world's global temperature will reach or exceed the 1.5C danger limit agreed in the Paris Agreement.
- Only drastic cuts in greenhouse gas emissions this decade can prevent us from rising global temperatures to a disastrous extent.
So, what is happening?
You can find out key information by clicking on each heading below:
Our work analysed the causes of climate change over the last millennium, with several figures used by the AR6 report.
For the first time, the IPCC states unequivocally (leaving no room for doubt) - that humans are responsible for the observed warming of our planets atmosphere, land and oceans. Human activity is changing the Earth's climate in ways 'unprecedented' in thousands and even hundreds of thousands of years.
All regions of the world are already experiencing impacts of climate change. In addition, some of these changes are now inevitable and 'irreversible for centuries to millennia'.
The assessment is based on improved data on historical warming and progress in scientific understanding of the response of the climate system to human-caused emissions. The amount of observed global temperature rise has also been updated.
Here are a couple of notable differences:
Topic |
Fifth Assessment Report (AR5) 2013/14 |
Sixth Assessment Report (AR6) 2021 |
| Global warming |
It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century. (According to IPCC language, 'extremely likely' means scientists were between 95% and 100% sure that humans were warming the planet.) |
It is unequivocal that human influence has warmed the atmosphere, ocean and land. Widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred. (According to IPCC language, 'unequivocal' means that the science is clear, and that there is absolutely no doubt.) |
| Global warming | Each of the past three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850. | Each of the last four decades has been successively warmer than any decade that preceded it since 1850. |
| Global warming |
The total increase in global surface temperature between the average of the 1850–1900 period and the 2003–12 period is 0.78 [0.72 to 0.85] C |
The total increase in global surface temperature was 1.09 [0.95 to 1.20] C higher in 2011–2020 than 1850–1900. |
| Greenhouse gas emissions | Concentrations of CO2, CH4, and N2O now substantially exceed the highest concentrations recorded in ice cores during the past 800,000 years. | In 2019, atmospheric CO2 concentrations were higher than at any time in at least 2million years (high confidence), and concentrations of CH4 and N2O were higher than at any time in at least 800,000 years (very high confidence). |
| Greenhouse gas emissions | The atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have all increased since 1750 due to human activity. In 2011, the concentrations of these greenhouse gases were 391ppm, 1,803ppb, and 324ppb respectively. | Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities. Since 2011 (measurements reported in AR5), concentrations have continued to increase in the atmosphere, reaching annual averages of 410ppm for carbon dioxide (CO2), 1,866ppb for methane (CH4), and 332ppb for nitrous oxide (N2O) in 2019. |
What is the latest science on climate sensitivity?
One of the most significant differences in AR6 is narrowing the range for equilibrium climate sensitivity (ECS).
The tighter range broadly reflects the findings of a major study published in 2020 by a team of scientists from multiple universities, including Professor Gabi Hegerl from our School.
Fifth Assessment Report (AR5) 2013/14 |
Sixth Assessment Report (AR6) 2021 |
|
| ECS |
ECS was 'likely in the range 1.5C to 4.5C'. It was 'extremely unlikely to be less than 1C and very unlikely to be greater than 6C'. |
ECS 'likely in the range 2.5C to 4C'. In addition the report provides a “best estimate” of 3C – something not given in AR5. |
AR6 used a combination of historical observations, climate models and an update of climate sensitivity to provide the best estimate that the world might pass the 1.5C and 2C global warming levels.
For the coming decades, the report sets out five different pathways emissions could take, with 'climate future' scenarios attached to them.
Of all the troubling news in the IPCC report, one warning has generated the most headlines:
- under all the scenarios examined, the best estimate is that the world will reach the crucial 1.5C in the 2030s
- this will occur even under the rapid mitigation scenarios outlined in the report.
It projects that in the coming decades, climate changes will increase in all regions.
We are already experiencing the impacts of climate change, but unless we curb emissions towards net-zero, the changes will exacerbate with effects we cannot fully anticipate.
The results of AR6 are clear
Global warming of 1.5c then 2C will be exceeded this century unless we make deep cuts to CO2 and other greenhouse gas emissions in coming decades.
The difference between 1.5 and 2 degrees is substantial: every increment of a degree translates into increased risks.
- For 1.5C warming, there will be increasing heat waves, longer warm seasons and shorter cold seasons.
- At 2C of global warming, heat extremes would more often reach critical tolerance thresholds for agriculture and health.
If emissions do not fall in the next couple of decades, then 3C of heating looks likely - which is devastating. And if they don't fall at all, then we are on track for 4C to 5C, which can be catastrophic.
For example:
- extreme heatwaves expected once every 50 years without any global heating are already happening every decade.
- with 1.5C warming, these will happen about every 5 years
- with 2C every 3.5 years
- with 4C once every 15 months
Here are a couple of notable differences:
Topic |
Fifth Assessment Report (AR5) 2013/14 |
Sixth Assessment Report (AR6) 2021 |
| Global warming & greenhouse gas emissions (GHG) |
Relative to the average from year 1850 to 1900, global surface temperature change by the end of the 21st century is projected to likely exceed 1.5C for RCP4.5, RCP6.0 and RCP8.5 (high confidence). Warming is likely to exceed 2C for RCP6.0 and RCP8.5 (high confidence), more likely than not to exceed 2C for RCP4.5 (high confidence), but unlikely to exceed 2C for RCP2.6 (medium confidence). |
Compared to 1850–1900, global surface temperature averaged over 2081–2100 is very likely to be higher by
|
| CO2-warming relationship | Cumulative total emissions of CO2 and global mean surface temperature response are approximately linearly related. | This report reaffirms with high confidence the AR5 finding that there is a near-linear relationship between cumulative anthropogenic CO2 emissions and the global warming they cause. |
| Mitigation | A lower warming target, or a higher likelihood of remaining below a specific warming target, will require lower cumulative CO2 emissions. Accounting for warming effects of increases in non-CO2 greenhouse gases, reductions in aerosols, or the release of greenhouse gases from permafrost will also lower the cumulative CO2 emissions for a specific warming target. | From a physical science perspective, limiting human-induced global warming to a specific level requires limiting cumulative CO2 emissions, reaching at least net-zero CO2 emissions, along with strong reductions in other greenhouse gas emissions. Strong, rapid and sustained reductions in CH4 emissions would also limit the warming effect resulting from declining aerosol pollution and would improve air quality. |
| Mitigation |
A large fraction of anthropogenic climate change resulting from CO2 emissions is irreversible on a multi-century to millennial time scale, except in the case of a large net removal of CO2 from the atmosphere over a sustained period. Surface temperatures will remain approximately constant at elevated levels for many centuries after a complete cessation of net anthropogenic CO2 emissions. |
If global net-negative CO2 emissions were to be achieved and be sustained, the global CO2-induced surface temperature increase would be gradually reversed but other climate changes would continue in their current direction for decades to millennia (high confidence). |
For the tipping points, it's clear that every extra tonne of CO2 emitted today is pushing us into a minefield of feedback effects tomorrow.
Professor Gabi Hegerl's seminal work has pioneered the method to detect the 'human fingerprint' in anthropogenic climate change through combining observations and climate model simulations.
Her method has become one of the central pillars of climate science.
Since then, our researchers have played a significant role in determining critical relationships between greenhouse gas emissions and climate warming. We made crucial contributions to estimating the 'human fingerprint' in the climate system from observed change. This also included methods for estimating climate sensitivity, which is how much greenhouse gas emissions warm the climate.
We achieved this through world-leading analysis synthesising multiple sources of evidence, including:
- observed warming in the pre-and post-industrial period paleoclimate
- instrumental records
- physical understanding derived from worldwide modelling approaches.
This led to the quantification of the Equilibrium Climate Sensitivity (ECS) – a key climate metric showing how much the world is expected to warm if CO2 levels double compared to pre-industrial levels.
Our work was used as evidence in the IPCC Fifth Assessment (AR5) reports which underpinned the Paris Agreement. Professor Gabi Hegerl presented the keynote presentation on climate sensitivity at the approval plenary for the report involving all the United Nations country delegations. In addition, she wrote the guidance notes for attribution of change in both physical climate and impacts such as ecosystem or agricultural changes, which is still being used to this day.
Latest research
For the IPCC Sixth Assessment, our work analysed the causes of climate change over the last millennium. Various works from 19 of our researchers were cited throughout, plus several figures were used by the AR6 report.
- Dr Andrew Schurer and Professor Gabi Hergel provided analysis on what extent climate variability and model-simulated climate change explain observed records back to early industrialisation.
- Professor Gabi Hegerl was specifically selected as an Expert Reviewer for the report and was involved in discussions across the IPCC Working Groups on attributing causes to observed changes.
- Dr Andrew Schurer was specially selected as a Contributing Author to provide text and expert opinion on human influence on the climate system. The report includes his figure that compares climate model output to temperature reconstructions over the past millennium.
Our work with the IPCC
Find out more
The following pages on this website may interest you:
|
Transforming the energy industry We are leading the way towards developing and deploying carbon capture and underground storage (CCS) to reduce greenhouse gas emissions. Find out how we are helping to ensure that CCS technologies are applied not just in the UK but all over the world. |
Transforming the energy industry |
|
Harnessing satellites for carbon monitoring We have pioneered the interpretation of satellite measurements to estimate global terrestrial carbon fluxes. Our work is helping governments around the world to meet the UN Paris Agreement goals on carbon stocktakes. |
Carbon monitoring |
University impact
You can also read related stories on the University Impact website:
|
Adjusting for humanity’s fingerprints Six years ago, Professors Gabi Hegerl and Simon Tett’s work to prove human-caused greenhouse gasses are warming our planet underpinned the 2015 Paris Agreement. Today they argue we still aren’t doing enough to adapt to climate change. |
Adjusting for humanity's fingerprints |
|
Going underground For decades removing harmful carbon from the atmosphere and storing it safely under the sea was a mere pipe dream. Edinburgh researchers are helping make it a reality. |
Going underground |
|
Seeing the woods Getting a clear picture of how much living matter is in the world's forests and savannas - key data in managing these vital carbon stores - has always been tricky. Satellite technology is changing that. |
Seeing the woods |
Many of the findings in earlier reports have been strengthened and sharpened in this report. Importantly, the need for a limited carbon budget and transition to net-zero emissions has been confirmed, and the uncertainty around the climate sensitivity narrowed.
Want to know more?
We've provided some useful links for you. To see the information, simply click on each heading below:
Study with us, and join one of the largest and most successful groupings of geographers and geoscientists in the UK as we address the most compelling issues of our time.
The following degrees may be of interest to you:
Undergraduate study
Postgraduate study
Postgraduate research
Check out more of our diverse undergraduate, postgraduate and research degrees and help us change the world.
We've provided more information on our work and the role of the IPCC:
You can find more information on how our work supports global initiatives to mitigate the effects of climate change and accelerate the transition to a low-carbon economy.
The following pages on this website may interest you:
|
Transforming the energy industry We are leading the way towards developing and deploying carbon capture and underground storage (CCS) to reduce greenhouse gas emissions. Find out how we are helping to ensure that CCS technologies are applied not just in the UK but all over the world. |
Transforming the energy industry |
|
Harnessing satellites for carbon monitoring We have pioneered the interpretation of satellite measurements to estimate global terrestrial carbon fluxes. Our work is helping governments around the world to meet the UN Paris Agreement goals on carbon stocktakes. |
Carbon monitoring |
University impact
You can also read related stories on the University Impact website:
|
Adjusting for humanity’s fingerprints Six years ago, Professors Gabi Hegerl and Simon Tett’s work to prove human-caused greenhouse gasses are warming our planet underpinned the 2015 Paris Agreement. Today they argue we still aren’t doing enough to adapt to climate change. |
Adjusting for humanity's fingerprints |
|
Going underground For decades removing harmful carbon from the atmosphere and storing it safely under the sea was a mere pipe dream. Edinburgh researchers are helping make it a reality. |
Going underground |
|
Seeing the woods Getting a clear picture of how much living matter is in the world's forests and savannas - key data in managing these vital carbon stores - has always been tricky. Satellite technology is changing that. |