IPCC Report 2021

10th August 2021

Patches of snow on a ski piste
A ski resort in a warming world

The IPCC published the first of a series of documents yesterday, as part of the Sixth Assessment Report on Climate Change. Yesterday's report was by Working Group I, and concerns the physical science basis of climate change. Key findings are presented in the Summary for Policymakers.

IPCC Report 2021: Introduction

The Introduction explains the nature of the document.

'This Summary for Policymakers provides a high-level summary of the understanding of the current state of the climate, including how it is changing and the role of human influence, the state of knowledge about possible climate futures, climate information relevant to regions and sectors, and limiting human-induced climate change.'
Introduction, Summary for Policymakers

IPCC Report 2021: the Current State of the Climate

These are some of the points made about the current state of the climate:

This graph shows the way temperatures have shot up recently, driven by human activities.

Change in global surface temperatures

These are some further points about the current state of the climate:

IPCC Report 2021: Possible Climate Futures

The report uses five illustrative scenarios to show climate futures in the near-, mid- and long-term. Temperatures will continue to increase in all scenarios until at least 2050, but the outcomes are very different in the mid- and long-term depending on future CO2 emissions.

This table below shows the differences in the five scenarios, from SSP1-1.9 (very low future emissions) to SSP5-8.5 (very high future emissions).

Temperature table in different emissions scenarios

More global warming leads to more extreme weather, including floods and droughts.

Natural land ocean carbon sinks will become less effective the more CO2 is emitted, and a higher proportion of CO2 will remain in the atmosphere.

Many changes are irreversible for centuries to millennia. Past greenhouse gas emissions mean oceans will continue to warm, acidify, and lose oxygen. Mountain and polar glaciers are committed to continue melting for decades or centuries. The Greenland Ice Sheet is at risk of total loss.

Sea level will continue to rise over the C21st.

IPCC Report 2021: Climate Information for Risk Assessment and Regional Adaptation

At 1.5C of warming, we can expect more floods and droughts. At 2C, it would be worse.

Cities intensify warming locally, and will suffer from more severe heatwaves. Cities can also generate heavier rainfall. Coastal cities may be caught by a combination of extreme rainfall and storm surges amplified by sea level rise.

Low-likelihood outcomes that cannot be ruled out include:

The Atlantic Meridional Overturning Circulation (which includes the Gulf Stream) is very likely to weaken over the C21st. There is medium confidence there won't be an abrupt collapse before 2100, but if there were, it would cause a lot of disruption to regional weather patterns.

IPCC Report 2021: Limiting Future Climate Change

The solution to all this is to reduce cumulative CO2 emissions. Rapid reductions in methane (CH4) would also limit warming and improve air quality.

The authors of the report have calculated remaining CO2 budgets - how much more can be emitted to result in various temperature increases.

Carbon budgets table

Anthropogenic CO2 removal could remove CO2 from the atmosphere. In theory this could reverse temperature increases, but other changes such as sea level rises would take centuries to millennia to undo.

Achieving global net zero CO2 emissions is a requirement for stabilising temperature increase.

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