ClimateLetter #2133

My letter of three days ago, CL#2130, reviewed the result of a study of hundreds of different climate scenarios that other climate scientists around the world had developed, after screening out hundreds more for being clearly implausible. All of the accepted scenarios were largely based on different realistic expectations for future carbon emissions, covering a wide range of possibilities, with allowances partly dependent on how humans respond to demands for reduction. The outcome resulted in a range bounded by +2C over pre-industrial on the low end and +3C for the high at the end of this century, essentially squashing the dreams of those holding out for meeting the UN target of 1.5C while also rejecting claims that go over the top. The study is valuable because it reflected the best quality work of a massive amount of scientific research, as reported in reputable studies. Any personal opinions of the four authors, led by Roger Pielke Jr., were systematically excluded. The study also excluded, as unrealistic, serious consideration of claims made by mainstream climate science messaging that holding on to the 1.5C target is still within reach—if only we try hard enough and succeed in developing practical technologies for direct removal of carbon from the air.

Yesterday a new and unrelated study was published that took a similar approach to answering questions about the actual likelihood of human performance at a level that would make a real difference.  You will need to read the press release at Phys.org—https://phys.org/news/2022-02-politics-society-tech-path-climate.html—and parts of the the study itself—https://www.nature.com/articles/s41586-022-04423-8—to see what all was involved and how the authors handled the task.  In this case they set up a total of 100,000 possible scenarios for computers to work on, and then sorted out the results.  This procedure, which is difficult for an outsider to evaluate, apparently met with the approval of regular peer reviewers and the equally fastidious standards of the editors of the journal Nature who accepted the work for publication.  What gets my attention is the main conclusion, as stated in the press release:  “The pathways fell into five clusters, with warming in 2100 varying between 1.8 to 3.6 degrees Celsius above the 1880-1910 average, but with a strong probability of warming between 2 and 3 degrees Celsius at the end of the century.”  The overlap of these numbers with the conclusions of the Pielke study can only be described as amazing.

If the scenarios had used wider parameters to work with, other than 1.8 and 3.6C, would the result have been different? We’ll never know, but the ones chosen seem quite acceptable. Here is a quote from the study, describing many reasons behind the decision not to select 1.5C as the low-end parameter: “Evidence regarding the likely emissions path over the twenty-first century is mixed. On the one hand, although emissions growth may have decelerated in recent years, with some evidence of declining emissions in a few advanced economies, global emissions continue to grow—National commitments under the Paris Agreement remain inadequate to meet either the 1.5-°C or 2-°C temperature target—and it is unclear whether government policies are yet sufficient to deliver on these emissions pledges. Carbon dioxide emissions from energy infrastructure currently in place or under development will exceed the 1.5-°C carbon budget, and standard energy-system models struggle to simulate pathways that meet either temperature target without the widespread deployment of negative emissions technologies that are highly speculative. The pace of decarbonization that is required to meet the Paris temperature targets vastly exceeds anything in the historical record at the global scale.”

Assuming that civilized life will not be overcome by a global temperature increase likely to be greater than 2C, the study does provide some reasons to be hopeful, based on proven feedback processes that are able to ripple through humanity in what can be an exceptionally rapid response to adversity.  “Overall, we find that the socio-politico-technical feedback processes can be decisive determinants of climate policy and emissions futures. Our parameterized model implies a high likelihood of accelerating emissions reductions over the twenty-first century, moving the world decisively away from a no-policy, business-as-usual baseline.”  The full report describes eight key feedback processes that were incorporated into the final model, and how they become accelerated upon reaching their respective tipping points.

From the conclusion, “The vast majority of runs (98%) produce warming of more than half a degree lower {than 3.9C}, although these warming estimates are sensitive to uncertainties in the climate system, including the climate sensitivity and the representation of carbon-cycle feedback, as well as the treatment of non-CO₂ greenhouse gases….. we do estimate a substantial probability of meeting the 2 °C Paris Agreement target—28% of our Monte Carlo runs result in 2091–2100 warming below 2 °C above 1880–1910 levels.” Uncertainties in the climate system are still out there, and must be incorporated in future models upon reaching clarification.

Carl