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

Global Warming Acceleration: Causes and Consequences
12 January 2024
James Hansen, Makiko Sato, Reto Ruedy

Abstract. Record global temperature in 2023 helps reveal acceleration of global warming on decadal time scales. The proximate cause of the acceleration is increase of Earth’s energy imbalance, specifically a substantial darkening of the planet (decreased albedo) equivalent to a CO₂ increase of more than 100 ppm, although it is difficult to apportion the albedo change between aerosol forcing and cloud feedbacks because of limited global measurements. Large 2023 warming is consistent with key findings in Global Warming in the Pipeline:³ reduced aerosol cooling and high climate sensitivity. We expect record monthly temperatures to continue into mid-2024 due to the present large planetary energy imbalance, with the 12- month running-mean global temperature reaching +1.6-1.7°C relative to 1880-1920 and falling to only +1.4 ± 0.1°C during the following La Nina. Considering the large planetary energy imbalance, it will be clear that the world is passing through the 1.5°C ceiling, and is headed much higher, unless steps are taken to affect Earth’s energy imbalance.

Global temperature in the GISS analysis increased 0.28°C in 2023, from 1.16°C to 1.44°C (Fig. 1), the largest annual increase in the 144-year record. This annual rise is largely due to the ongoing tropical El Nino warming, but no prior El Nino engendered as much warming, which points to an additional drive for global warming acceleration. We have argued³ that the imminent threat of human-made climate change is understated in IPCC⁴ assessments, which are based predominately on global climate models (GCMs). We suggest that the IPCC best estimate for climate sensitivity (3°C for 2×CO₂ or 0. 75°C per W/m² ) is an underestimate, as we find real-world (paleoclimate) evidence for a sensitivity of 4.8°C ±1.2°C for 2×CO2₂ (1.2°C per W/m² ). In addition, we suggest that IPCC underestimates (negative) aerosol climate forcing and global cooling by aerosols that partly counterbalances greenhouse gas (GHG) warming. These two errors compensate and allow GCMs with low sensitivity to match observed warming of the past century by using an unrealistically small aerosol effect. Compensation is not an accident; it is a result of overreliance on GCMs. With aerosol forcing unmeasured, it is natural for modelers to focus on an aerosol forcing that yields agreement with global warming of the past century. Some clarification will be possible in 2024.

Global surface temperature is well measured since about 1950,⁵ but there are large interannual fluctuations of temperature that make it difficult to confirm a change in the rate of global warming until the change is large. Given that the El Nino/La Nina cycle is the main cause of interannual

Fig. 2. Global temperature during El Nino origin years (left side) and El Nino peak years, compared with monthly record high temperatures prior to the last three strong El Ninos.

variability, comparison of global temperatures at the well-defined peaks of strong El Ninos may provide potential for early detection of global warming acceleration.⁶ The three most recent strong El Ninos are 1997-98, 2015-16, and 2023-24. The first of these was prior to reduction of human-made aerosols. The second occurred just after the January 2015 imposition of restrictions on sulfur content of ship fuels by the International Maritime Organization (IMO), and the third occurred after strengthening of the restrictions in January 2020. Fig. 2 compares global temperatures during these years (red lines) with the prior record monthly temperatures (gray area).

The strong 1997-98 “El Nino of the century” produced 8-9 clear global temperature records, the 2015-2016 El Nino produced 10 clear record months, and the 2023-24 El Nino has already produced 7 record months about half way through the period of Nino-elevated temperature. The rate of warming between the first two El Ninos, i.e., between 1998 and 2016, was 0.23°C/decade, moderately larger than the 0.18°C/decade warming rate during 1970-2010. Another eight months of temperature data are needed to assess the warming rate between the last two El Ninos.

Fig. 3. Nino3.4 SST (upper panel) and temperature of ocean upper 300m in equatorial region.⁷

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¹ Lenssen NJL, Schmidt GA, Hansen JE et al. Improvements in the GISTEMP uncertainty model, J Geophys Res Atmos 2019;124(12):6307-26
² Hansen J, Ruedy R, Sato M et al. Global surface temperature change. Rev Geophys 2010;48:RG4004
³ Hansen J, Sato M, Simons L et al. Global warming in the pipeline. Oxford Open Clim Chan 2023;3(1):kgad008, doi.org/10.1093/oxfclm/kgad008
⁴ IPCC. Climate Change 2021: The Physical Science Basis [Masson-Delmotte V, Zhai P, Pirani A et al. (eds)]. Cambridge and New York: Cambridge University Press, 2021
⁵ There were still significant flaws in the World War II period because of limited and changing sources for sea surface temperature data. See Hansen J, Sato M, Kharecha P et al. Young people's burden: requirement of negative CO₂ emissions. Earth Syst Dyn 2017;8:577-616
⁶ Grantham, J., The Race of Our Lives Revisited, GMO White Paper, August 2018
⁷ Nino3.4 data are ERSSTv5 (1991-2020 base period) for 5°S-5°N, 170-120°W, while the equatorial upper 300m temperature data use 1981-2010 base period.for 5°S-5°N, 180-100°W1744

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