“Many changes due to past and future greenhouse gas emissions are irreversible for centuries to millennia.” That’s just one of the headlines from a major new report published this week by the Intergovernmental Panel on Climate Change (IPCC). The report, Climate Change 2021: The Physical Science Basis, represents the consensus view of the more than 200 scientists from 66 countries who make up the IPCC’s Working Group I. Its nearly 4,000 pages will become part of the IPCC’s Sixth Assessment Report (AR6), which is due for release in September 2022.
It has been eight years since AR5 was published. This new report thus draws on additional years of observation data and published research, as well as significant advances in climate modeling. These have increased the working group’s confidence in just how sensitive Earth’s surface temperature is to greenhouse gas (GHG) emissions. Telling, perhaps, is that in the Summary for Policymakers of Working Group I’s contribution to AR5 they use the phrase “high [or very high] confidence” in the context of their findings just seven times. In this latest Summary, the phrase “high [or very high] confidence” appears more than 100 times.
Working Group I has not been altogether silent in the years since AR5. They issued two “special reports” in 2019, which I blogged about here. In the first of those reports, "Climate Change and Land," the authors found that mean land surface temperatures were estimated to have already risen by 1.53˚C since pre-industrial times. Fortunately, Earth’s oceans (and cryosphere—that is, glaciers, ice sheets, snow cover, and the like) exert a moderating influence. As a result, the group found a 1.0˚C rise in combined land and ocean mean surface temperature since 1850. The second special report, however, "The Ocean and Cryosphere in a Changing Climate,” questioned how long and to what degree that moderating influence might continue.
What’s New in IPCC’s Latest Report?
For one thing, in this latest report the estimate of the increase in global mean temperature has risen from 1˚C in the Special Report to about 1.1˚C. But much of what is different about this report compared to AR5 is the increase in confidence levels. No longer is it “extremely likely” that human influence has warmed the planet—it is now “unequivocal.” The working group has also narrowed its estimate of climate sensitivity, adopting a consensus view that a doubling of GHGs (since pre-industrial times) would produce a mean increase in surface temperatures of 3˚C, with a narrower range of uncertainty than used in previous reports.
Meanwhile, both GHG emissions and surface temperatures have continued to climb. Global mean sea levels, which were increasing by 1.3 mm/year between 1901 and 1971, are now increasing by an estimated 3.7 mm/year, and the rate of rise is accelerating. Sea ice and glaciers are retreating at unprecedented rates. Climate zones and, with them, mid-latitude storm tracks have shifted poleward (although there is higher confidence in such a shift in the North Pacific than in the North Atlantic).
The frequency and intensity of weather extremes are changing, and confidence in the attribution of extremes to human influence has strengthened since the last report. Heat waves are happening more often, as is heavy precipitation. Rainfall variability is increasing, with implications for both flooding and drought. The proportion of tropical cyclones that reach major hurricane status (Categories 3-5) is likely increasing and the change cannot be explained by natural variability alone. In addition, correlations between extreme events appear to be changing; for example, sequential droughts, wildfires, and floods are likely to increase.
From RCPs to SSPs
Previous Assessment Reports made use of what are called "Representative Concentration Pathways," or RCPs, which I’ve blogged about here and here. These were four potential pathways, or trajectories, of GHG concentrations that reflected (more or less strict) efforts to constrain emissions. What RCPs did not reflect, at least explicitly, were assumptions regarding changing population, economic growth, the rate of technological innovation, and other factors.
This latest report from Working Group I uses five Shared Socioeconomic Pathways (SSPs) rather than RCPs to examine possible future climate states. RCPs have not been abandoned, exactly. Rather, each SSP reflects how different RCPs might be achieved within the context of underlying socioeconomic conditions and climate policy (mitigation) assumptions.
When fed into the latest climate models, under even the most optimistic scenario (in which GHG emissions drop to net zero by 2050, followed by net negative CO2 emissions), global surface temperatures continue to rise at least until mid-century. All five scenarios project that we will reach or exceed the more aggressive Paris Agreement goal of 1.5˚C warming by just 2040, not 2100. Global warming of 2˚C (best estimate) is projected by mid-century (2041-2060) even under the intermediate SSP emissions scenario. And with every additional increment of warming, weather and climate extremes will become yet more extreme.
Is There Any Good News?
At warming of 1.5˚C, we are in for a rough ride. So, if under all SSP scenarios we are virtually certain to reach warming of at least 1.5°C in the next two decades, should we simply throw up our hands in surrender? Clearly not. "Climate Change 2021: The Physical Science Basis" makes clear that warming well beyond 1.5°C is quite possible. And for each additional increment in temperature, the probability of reaching “tipping points” increases—points beyond which the climate, and indeed entire species, cannot return to their initial state even if the drivers of the change are eliminated.
The good news is that it is still possible, under the lowest emissions scenario, to "settle" for the rough ride—to limit warming to 1.5°C. The way to do that—indeed the only way to do that, according to the authors—is to stop adding and potentially start removing atmospheric carbon dioxide by mid-century. That, in turn, requires a more aggressive shift away from fossil fuels than has, as yet, been contemplated by governments around the world.
Representatives of those governments will have a chance to come to meaningful agreement when they meet in Glasgow in November, at COP26. More on that in a future blog.
