Where is Climate Change Taking Us?
IPCC 6th Assessment Report
The Physical Science Base
released 9 August 2021
The Intergovernmental Panel on Climate Change (IPCC) has started to release its Sixth Assessment Report with the results of Working Group I on the Physical Science Base. The reports of other working groups will come out over the next year.
A press conference on 9 August 2021 for the launching of the report confirmed its main conclusion that the human causes of climate change are indisputable, it is happening now and affecting all parts of the world in multiple ways, and it will get much worse if we do not take urgent action to reduce all emissions of greenhouse gases immediately.
Have we lost hope? The answer is no and yes. Some changes will continue for thousands of years, but if we take extreme action immediately, some effects can be slowed and eventually reversed. The next decade is critical; 2050 is too late. We must raise ambition at COP26.
What is new in the report?
Climate science has advanced greatly since the last report in 2013, demonstrating unequivocally that human activities are causing the extreme events we are now experiencing. The range of uncertainty about climate sensitivity has narrowed. Past warming has been masked by aerosol cooling from pollution, and natural carbon sinks absorbing half of the greenhouse gases we have released, but this will decrease. We have already experienced 1.1°C of warming in the last forty years, unprecedented for the last 2,000 years. The atmospheric CO2 concentration is the highest in 2 million years. Sea level rise is the fastest in 3,000 years. Arctic sea ice is the lowest in a thousand years, and glacier melting highest in 2,000 years. The consequences with global warming are extreme heat, heavy rainfall and drought affecting nature and agriculture. There is more fire weather, and oceans are warming, acidifying and losing oxygen. Further warming is coming. Keeping below 1.5°C will be beyond reach in the near term, but if we eliminate global net greenhouse gas emissions by 2050 we could get a gradual decline by 2100 to within 0.1°C of the target. If not, we shall pass critical thresholds for agriculture and human health and change will get larger.
The better science allows more accurate projections of future climate change under various scenarios, from extreme action bringing warming down to 1.5°C, to business as usual with runaway global heating. One third of the report is devoted to regional information, making possible regional projections of the consequences from climate change adequate to guide regional planning of critical infrastructure for climate extremes.
The Working Group that prepared the 4,000 page report included 63% new authors from 66 countries, 28% women, who reviewed 14,000 peer-reviewed scientific papers and addressed 78,000 comments. This guarantees that it represents the best science presently available, and can be relied upon for policy-making. Launched on the UN Day for Indigenous Peoples, the IPCC respects indigenous knowledge and traditional agricultural practices that are often better adapted to a changing climate.
The report provides three levels of information: a summary for policy-makers, a technical summary of 60 pages, and the chapters of the full report intended for specialists. One innovation is the Interactive Atlas at https://interactive-atlas.ipcc.ch which allows anyone to explore and map what climate change might be like in any specific locality or country.
In summary, there is no going back from some changes like melting ice sheets, deep ocean warming and acidification, which will continue for thousands of years, but strong and sustained reductions in CO2 emissions can make a difference. There is a linear relationship, with every tonne of CO2 adding to warming, requiring zero net emissions at a global scale. Controlling other greenhouse gases like methane will also help. The report shows that it is still possible to limit warming in a few decades if we act now.
Headline Statements from the Summary for Policymakers
9 August 2021 (subject to final copy-editing)
A. The Current State of the Climate
A.1 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.
A.2 The scale of recent changes across the climate system as a whole and the present state of many aspects of the climate system are unprecedented over many centuries to many thousands of years.
A.3 Human-induced climate change is already affecting many weather and climate extremes in every region across the globe. Evidence of observed changes in extremes such as heatwaves, heavy precipitation, droughts, and tropical cyclones, and, in particular, their attribution to human influence, has strengthened since the Fifth Assessment Report (AR5).
A.4 Improved knowledge of climate processes, paleoclimate evidence and the response of the climate system to increasing radiative forcing gives a best estimate of equilibrium climate sensitivity of 3°C, with a narrower range compared to AR5.
B. Possible Climate Futures
B.1 Global surface temperature will continue to increase until at least the mid-century under all emissions scenarios considered. Global warming of 1.5°C and 2°C will be exceeded during the 21st century unless deep reductions in carbon dioxide (CO2) and other greenhouse gas emissions occur in the coming decades.
B.2 Many changes in the climate system become larger in direct relation to increasing global warming. They include increases in the frequency and intensity of hot extremes, marine heatwaves, and heavy precipitation, agricultural and ecological droughts in some regions, and proportion of intense tropical cyclones, as well as reductions in Arctic sea ice, snow cover and permafrost.
B.3 Continued global warming is projected to further intensify the global water cycle, including its variability, global monsoon precipitation and the severity of wet and dry events.
B.4 Under scenarios with increasing CO2 emissions, the ocean and land carbon sinks are projected to be less effective at slowing the accumulation of CO2 in the atmosphere.
B.5 Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia, especially changes in the ocean, ice sheets and global sea level.
C. Climate Information for Risk Assessment and Regional Adaptation
C.1 Natural drivers and internal variability will modulate human-caused changes, especially at regional scales and in the near term, with little effect on centennial global warming. These modulations are important to consider in planning for the full range of possible changes.
C.2 With further global warming, every region is projected to increasingly experience concurrent and multiple changes in climatic impact-drivers. Changes in several climatic impact-drivers would be more widespread at 2°C compared to 1.5°C global warming and even more widespread and/or pronounced for higher warming levels.
C.3 Low-likelihood outcomes, such as ice sheet collapse, abrupt ocean circulation changes, some compound extreme events and warming substantially larger than the assessed very likely range of future warming cannot be ruled out and are part of risk assessment.
D. Limiting Future Climate Change
D.1 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.
D.2 Scenarios with low or very low greenhouse gas (GHG) emissions (SSP1-1.9 and SSP1- 2.6) lead within years to discernible effects on greenhouse gas and aerosol concentrations, and air quality, relative to high and very high GHG emissions scenarios (SSP3-7.0 or SSP5-8.5). Under these contrasting scenarios, discernible differences in trends of global surface temperature would begin to emerge from natural variability within around 20 years, and over longer time periods for many other climatic impact-drivers (high confidence).
Last updated 9 August 2021