Why Global Warming Has Accelerated: Hansen's Lay Explanation

Links for Friday, February 14

Programming note: I’m on travel next week, so posting will be light if at all. Thanks for your patience.

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ShipMap visual representation of calendar year 2012 global shipping activity data, as created by UCL and Kiln from the data used for the Third IMO GHG Study. Beyond showing the ubiquity of vessel traffic overall, the illustration reveals key patterns and concentrations in the major types of vessels used to carry cargo . The lines in yellow refer to container ships, which increasingly carry more and more cargo that previously would have been carried, for example, in a bulk carrier. Dry bulk ships are shown in blue. Tankers (e.g. for oil) are in red. Gas bulk tankers are in green, and vehicle carriers are in magenta. (Source: EPA site snapshot)

I’ve written before about James Hansen’s latest paper, “Global Warming Is Accelerating,” but only lightly. In it he makes a couple of main points, but the language is aimed at his colleagues, so to lay folks like us, it may seem a little obtuse. What is he saying really?

Hansen recently followed up this paper with a more lay-friendly piece explaining its main points, then went on to discuss why the new paper has been excluded from polite (read, IPCC) conversation. See below for that.

Here, I want to give you Hansen’s explanation in layperson terms of what the original paper reports.

A Note on ‘Climate Sensitivity’

“Climate sensitivity,” discussed below, is a determination of how the climate responds to increased CO2, how much more heat is retained for a given increase in emissions.

Consider a basketball thrown at a schoolyard wall. If thrown with force X, how far does the ball bounce back? A foot? Three feet? Ten?

Climate sensitivity measures the “bounciness” of the ball. Low sensitivity means not much new warming is created by a given injection of new CO2. High sensitivity and the planet warms up a lot more.

Measuring climate sensitivity is the key to knowing if we have 100 years to correct the imbalances, or just ten. Important topic, right?

James Hansen’s Latest on Global Warming

Now on to Hansen and his layperson’s explanation of his latest findings. Note, when you get to them, his bolded three points.

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

12 February 2025
James Hansen and Pushker Kharecha

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.

3. Accelerated warming increases ice melt and upper ocean warming, threatening to shut down North Atlantic overturning circulation by mid-century and cause large sea level rise.

We show that observed ice melt over the past 20 years was similar to assumed ice melt in climate simulations of “Ice Melt, Sea Level Rise, and Superstorms.”4 The rate of ice melt did not increase in the past decade, but, given the leap of global temperature to +1.5°C above preindustrial, we expect ice melt to accelerate, especially in regions such as southeast Greenland where ice melt is injected directly into the Irminger Sea, a region where deepwater forms. The North Atlantic is warming at depths beneath the surface wind-mixed ocean layer, with warmer water penetrating beneath the sea ice and ice shelves. Paleoclimate data suggest that such sub-ice warming can lead to sudden loss of regional sea ice and thus increased warming and summer rainfall on lower reaches of the Greenland ice sheet and increased freshwater injection into the ocean. Our climate simulations4 suggest that such increased ice melt and rapid surface warming can shut down the overturning ocean circulation by mid-century, which would be the “Point of No Return” because shutdown is irreversible in less than centuries. Large sea level rise would become inevitable, as heat normally transported into the North Atlantic would remain in the Southern Hemisphere and speed melting of the West Antarctic ice sheet. Global warming acceleration increases this danger because the increased heating both reduces the density of the upper layer of the ocean and increases the rate of ice melt.

Put simply, Hansen’s three main points are these:

1. Reduced air pollution (“aerosols”) from Northern Hemisphere shipping, (think Amazon, Exxon, et al) removes a big dampening agent on global warming. Pollution makes clouds; clouds reflect sunlight into space. Far fewer clouds makes global warming increase at an undampened pace.

2. If you correct for reduced pollution, climate sensitivity is 50% higher than everyone has comfortably thought. This new sensitivity number now accords with what we know of paleoclimate, earth before historical records. The real “bounce in the ball” is much higher.

3. Increased warming and ice melt could shut down the Gulf Stream (called the AMOC) by the middle of this century and cause massive sea level rise. What will it do to the world when England has Montreal’s weather, and Paris has Buffalo’s?

Bottom line: The crisis won’t just be faced by the yet-unborn. This generation will reap what it let be done … I’ll add, by the billionaires they handed their sovereignty to.

What’s Wrong with the IPCC?

The rest of Hansen’s paper discusses the rejection of its conclusions by the IPCC, and why, in scientific terms, that rejection is wrong. It’s worth a read.

Note that their reasons for rejection aren’t just science-type arguments, disagreements over data and interpretation. The IPCC is heavily and structurally invested in keeping the world safe for wealthy fossil fuel nations and investors. For my own discussion of that, see here:

To jump to the meat of the argument in my article, start here. Or just read this brief taste from critic and scientist David Wasdell:

The constitution of the IPCC WG1 [the working group that studies the science of warming] carries within its structure a conflict of interests which lays it open to the charge of collusion in the management of scientific analysis of climate change. It is hardly surprising that the resultant Summary for Policy Makers was immediately welcomed and affirmed as acceptable to the Washington Administration.

If you do read that section, pay attention to the IPCC document called the Summary for Policymakers, or SPM. It runs about 30 pages, an easy read on your corporate jet to Davos, an easy prep for your one-on-one interview with the Saudi Minister of making-themselves-richer-still, or America’s “climate czar,” when we still had one. Fossil fuel representatives have a seat in the room where it’s written.

Of the many thousands of pages produced by the IPCC in each Assessment Report, the SPM is the only thing anyone reads. The process by which it’s produced is a study in capture. Hansen says much the same thing.