Unprecedented flooding, searing temperatures, and raging fires across Europe, Asia, and North America this summer have created a stark backdrop for this week’s release of the sixth physical science assessment report (AR6) of the Intergovernmental Panel on Climate Change (IPCC). These reports, initiated in 1990, arrive about every 7 years at the request of the countries of the United Nations Framework Convention on Climate Change. They form the basis for UN discussions and have become a crucial means to take stock of the latest scientific developments. The reports’ future projections about climate change have remained fairly stable over the years and have, sadly, proven quite accurate. So, what does the new report add?
Above all, AR6 expresses greater confidence in familiar findings, owing to stronger evidence. A notable example concerns “equilibrium climate sensitivity,” a measure of how much global warming ultimately occurs if the atmospheric carbon dioxide (CO2) concentration doubles. Based on improved understanding of cloud processes and climate changes that have already occurred, AR6 concludes that this figure is “likely” (a two-thirds chance or greater) to lie between 2.5° and 4°C—halving the spread of 1.5° to 4.5°C in previous reports. Global temperatures had stalled in the period before the 2013 assessment (AR5) but have since surged, reaching 1.1°C above that of preindustrial times. Atmospheric CO2 has reached concentrations not seen for at least 2 million years, and the new report expresses high confidence that oceans, plants, and soils will become less efficient at absorbing future carbon emissions.
As always, uncertainty remains. The latest climate models predict a wider range for climate sensitivity, with projected values implausibly weak in some cases but implausibly strong in others. This disagreement is largely a result of increased complexity in model representations of cloud feedbacks in the midlatitude storm-track regions. AR6 shrewdly deals with this inconsistency by focusing on what happens at a given level of global warming (say, 2°C), separating this from the question of when that warming level would be reached.
The report also provides new clarity on aspects like changes in extreme rainfall and drought. Almost all robustly observed regional trends in these events are upward and are projected to continue. One sobering finding is that even if global warming is limited to 2°C, heat events that once occurred twice per century will happen every 3 to 4 years—and will tend to coincide with droughts, compounding the impacts. Much better regional information is provided than in previous reports. However, the lack of adequate data in many regions, including most of Africa, is apparent and should be addressed.
The report dives into important new territory by emphasizing “low-probability, high-impact events” that are hard to quantify but unwise to ignore. For example, although the expected range of future sea level is similar to previous predictions, AR6 indicates that rises of 2 m or more by the end of the century cannot be ruled out. Nor can the possibility of abrupt responses and “tipping points” in the climate system. These are stark warnings compared with previous reports. As the authors note, the probabilities of forest dieback, ocean-circulation changes, and other disturbing scenarios increase with global temperature.
Although the IPCC reports provide an invaluable resource and periodic wake-up call, they come at a price. This report was written by 234 authors over 3 years, with similar effort invested in two more reports on adaptation and mitigation due next year. The process is arduous: Over 75,000 review comments were individually addressed. The world’s climate modeling centers invest heavily in simulations following common protocols, which is growing steadily more taxing for them.
If another assessment is commissioned on schedule, it will arrive not much before 2030. By then, if emissions persist at current rates—that is, even if emissions growth is halted—nearly all the remaining “global carbon budget,” which gives a 50-50 chance of keeping global warming below 1.5°C, will have been exhausted. So, this may be the last report that can meaningfully influence policy to keep the climate targets of the 2015 Paris Agreement within reach. AR6 is intended to inform discussions at the UN Climate Change Conference of the Parties (COP26) meeting in November. Our children and grandchildren are waiting to see what comes out of it.