Global Warming Has Accelerated. Why? What Are the Consequences?

12 February 2025
James Hansen and Pushker Kharecha

Once upon a time, Earth Sciences was blessed to have brilliant, articulate, scientific leaders, such as Jule Charney and Francis Bretherton,[1] whose knowledge and overview of climate science commanded respect. And there were many other scientists with deep understanding of the scientific method, who helped spur progress in the field and assure that progress was recognized. Top science writers, such as Walter Sullivan, could rely on such scientific researchers for perceptive descriptions of the major issues and progress in addressing them. We recall fondly learning from Charney’s colleague at MIT, Peter Stone, who served as the principal adviser for climate research at NASA Goddard Institute for Space Studies, back in the days when Charney was trying to decide whether global equilibrium climate sensitivity to doubled atmospheric CO₂ was more like 2°C or 4°C. The correct answer would have enormous practical implications.

The Intergovernmental Panel on Climate Change (IPCC),[2] set up by the World Meteorological Organization (WMO) and the United Nations Environment Program (UNEP) in 1988, and endorsed by the United Nations that year, produces comprehensive climate assessments about every six or seven years. The reports contain a large amount of useful information; the most recent report on the physical science basis of climate change, the Sixth Assessment Report (IPCC AR6),[3] was published in August 2021. IPCC’s approach to climate analysis came to be dominated by use of global climate models (GCMs) for climate simulations of the past 1-2 centuries. We have taken a complementary approach, placing comparable emphasis on paleoclimate data, GCM modeling, and modern observations of climate processes, as described in our three main papers published in the past decade: (1) “Ice melt, sea level rise and superstorms,”[4] (2) “Global warming in the pipeline,”[5] and (3) “Global warming has accelerated.”[6] The third of these, published last week, was long, as it tied all three together, especially via its Supplementary Material (SM),[7] which usually houses only secondary material. Here is a link to the Abstract + Paper + SM as a single document. Below, we first provide a plain language summary of the three principal conclusions of this paper and then address questions raised in the media by kibitzers.

1. The leap of global temperature in 2023-2024 is explained; no new physics is required.
The 0.4°C increase of global temperature in 2023-24 was caused equally by increase of absorbed solar radiation and a weak El Nino. Increase of absorbed sunlight was mainly spurred by reduction of aerosols (tiny particles), especially those emitted by ships, as the International Maritime Organization imposed a strict limit on the sulfur content of ship fuels beginning in 2020.[8] Aerosols serve as cloud formation nuclei; the induced clouds reflect sunlight and cause global cooling that offsets part of the global warming caused by increasing greenhouse gases. This cooling offset has long been described as a “Faustian bargain” because aerosols constitute particulate air pollution that kills millions of people every year. Our Faustian payments – an increase of global warming – come due when we reduce health-damaging air pollution and thus reduce aerosol cooling.

2. Climate sensitivity is 50 percent larger than the best estimate of IPCC.
We show that the climate sensitivity required to yield best agreement with observed global warming in the past century is 4.5°C for doubled CO₂, which is 50% larger than IPCC’s best estimate of 3°C. Together, conclusions 1 and 2 imply that near-term global temperature will decrease very little: thus, averaged over the El Nino/La Nina cycle, the 1.5°C limit has been reached. IPCC’s estimate of climate sensitivity depended on the assumption that aerosol climate forcing was unchanging during the period 1970-2005, but we show that aerosol forcing increased (became more negative) during that period as aerosols spread more globally, including over pristine ocean areas where their effect is greater. If aerosols were fixed, greenhouse gases are the only forcing and the climate sensitivity required to match observed warming would be about 3°C for doubled CO₂. But the net forcing was actually smaller during that period because the negative aerosol forcing was growing, so a larger climate sensitivity is required to match observed warming of the past century. Our estimated climate sensitivity coincides with the sensitivity derived from glacial-to-interglacial climate change, the portion of the paleoclimate record for which precise knowledge of greenhouse gases is available.

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