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Climate Change 2021: The Physical Science Basis
IPCC Sixth Assessment Report. Working Group I
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.
The IPCC (the Intergovernmental Panel on Climate Change) has issued reports on a regular basis since its founding in 1988. (The First Assessment reports were published in the year 1990.) These reports provide an overall assessment of the current state of knowledge about climate change.
The IPCC does not conduct its own research; instead, it compiles information from thousands of peer-reviewed published scientific papers. Teams of unpaid volunteers compile its reports. The quality and conservatism of the IPCC’s work makes it the most authoritative source of information to do with climate change. The latest set of reports (the final four in the list shown below) are part of the overall Sixth Assessment.
Climate Change 2021: The Physical Science Basis (this one);
Climate Change 2022: Impacts, Adaptation and Vulnerability;
Climate Change 2022: Mitigation of Climate Change; and
Sixth Assessment Fact Sheet.
The Physical Science Basis Report
This report is the first in the overall Sixth Assessment. It was prepared by Working Group I, which says of its report,
The Working Group I contribution to the Sixth Assessment Report addresses the most up-to-date physical understanding of the climate system and climate change, bringing together the latest advances in climate science.
The following are highlights from the report.
Scientists are confident that human activity is causing global warming — principally due to the release of carbon dioxide (CO2), methane and nitrous oxide emissions. “It is unequivocal that human influence has warmed the atmosphere, ocean and land.”
Nearly half of the temperature increase has occurred since the year 2000. Global warming is not something that we can blame on previous generations.
Increased temperatures will lead to rising sea levels, more storms and more droughts (something that we are seeing already). “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. 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.”
Global warming of at least 1.5°C (compared with 1.2°C currently) is likely by 2040 under even the most ambitious emissions reduction scenario. (We are not even attempting the “most ambitious scenarios”, so it is highly unlikely that we will meet the 1.5°C target by the year 2030.)
If global warming is to be brought under control we will have to stop adding CO2 to the atmosphere. This is a critical conclusion — we have passed the point where we can adapt gradually and organically. We need to reduce our use of fossil fuels.
Like all IPCC publications, this report is written in cautious, bureaucratic language. It is a scientific report, so it relies on careful analysis, not emotion. (The posts A Clunky Sentence and Another Clunky Sentence describes the communication difficulties that these reports present.) Yet the report’s overall message is stark.
The climate is changing fast and we are causing those changes.
The 1.5°C target from the landmark Paris Conference is not going to be met. We have done far too little, far too late.
To have any chance of bringing atmospheric temperatures under control, we have to stop using fossil fuels now. Doing so would, of course, lead to a catastrophic crash in the world’s economies.
This chart, taken from the report, summarizes its message. The green line shows that, without human actions, global temperatures would have not changed much over the course of the last 200 years. As it is, we are now 1.3°C above the pre-industrial baseline. Moreover, if the trend shown continues, we cross the 1.5°C threshold around the year 2040.
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