Fig. 1. Global temperature relative to 1880-1920 based on the GISS analysis.¹ ²

Comments on Global Warming Acceleration, Sulfur Emissions, Observations

16 May 2024

James Hansen, Pushker Kharecha, Makiko Sato

Global temperature (12-month mean) is still rising at 1.56°C relative to 1880-1920 in the GISS analysis through April (Fig. 1). [Robert Rohde reports that it is 1.65°C relative to 1850-1900 in the BerkeleyEarth analysis.³ ] Global temperature is likely to continue to rise a bit for at least a month, peak this summer, and then decline as the El Nino fades toward La Nina.

Acceleration of global warming is now hard to deny. The GISS 12-month temperature is now 0.36°C above the 0.18°C/decade trend line, which is 3.6 times the standard deviation (0.1°C). Confidence in global warming acceleration thus exceeds 99%, but we need to see how far temperature falls with the next La Nina before evaluating the post-2010 global warming rate.

Present extreme planetary energy imbalance will limit La Nina-driven temperature decline. Thus, El Nino/La Nina average global temperature likely is about 1.5°C, suggesting that, for all practical purposes, global temperature has already reached that milestone. Temperature is temporarily well above the 50-100 percent increase that we projected ⁴ (yellow region in Fig. 1) for the post-2010 warming rate. That projected increase is based on evidence that humanmade aerosols and their cooling effect are in decline. In other words, we are beginning to realize the consequences of the Faustian bargain, in which humanity partly offset greenhouse gas warming with aerosol (particulate air pollution) cooling.

A recent comment in the social media that a decline of global temperature will signify that we are “back to normal” is right only if one considers accelerating global warming to be normal. However, we see no reason to believe ⁵ that the jump in 2023-24 global temperature indicates we are missing some fundamental climate physics – other than good aerosol physics.

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¹ Lenssen NJL, Schmidt GA, Hansen JE et al. Improvements in the GISTEMP uncertainty model, J Geophys Res Atmos 124(12), 6307-26, 2019
² Hansen J, Ruedy R, Sato M et al. Global surface temperature change. Rev Geophys 48:RG4004, 2010
³ Rohde R, tweet on 15 May 2024
⁴ Hansen J, Sato M, Simons L et al. Global warming in the pipeline. Oxford Open Clim Chan 3(1), doi.org/10.1093/oxfclm/kgad008, 2023
⁵ Dance S, Scientists fear shift as record ocean heat enter second year . Washington Post, 20 March 2024.

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