IPCC Report 2021

10th August 2021

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:

• it is unequivocal that human influence has warmed the atmosphere, ocean and land; concentrations of CO2 have reached 410ppm
• global surface temperature in 2011-20 was 1.09C higher than in 1850-1900, with larger increases over the land (1.59C) than over the ocean (0.88C)
• greenhouse gases caused warming of 1 to 2C, while aerosols contributed cooling of up to 0.8C
• on average over the world, precipitation over land has increased since 1950, with a faster rate of increase since 1980
• human influence is very likely the main driver of the global retreat of glaciers since the 1990s
• it is virtually certain that human-caused CO2 is the main driver of acidification of the oceans
• the rate of sea level rise has been increasing, and is up to 3.7mm per year in 2006-18

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

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

• CO2 concentrations are higher than at any time in the last 2 million years
• temperatures have increased faster since 1970 than in any other 50-year period over at least the last 2,000 years
• late Summer Arctic sea ice area is smaller than at any time in the last 1,000 years at least. Later in the report, the prediction is made that it will be ice-free in September at least once before 2050
• heatwaves have become more frequent and more intense since the 1950s; marine heatwaves have doubled in frequency since the 1980s
• heavy rain has become more frequent and more intense since the 1950s, and human-induced climate change has contributed to droughts
• ocean warming accounts for 91% of the heating in the climate system; land warming, ice loss and atmospheric warming account for the rest
• previously, thermal expansion was the main cause of sea level rise; in 2006-18, ice sheet and glacier mass loss became the dominant contributors

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).

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:

• ice sheet collapse
• abrupt ocean circulation changes
• compound extreme events
• warming substantially larger than that assessed as very likely in the report

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.

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.