In Chapter 17, we explored evidence that the rate of warming has increased in the last decade or so, by examining James Hansen’s long paper on this, published in October 2023. The study was not done in order to explain the record heat of that year - most of the work was complete before records began falling - yet it turmned out to tbe a very useful and timely contibution to the following enquiry into the anomalies of 2023 and 2024 - easily the two hottest years in the record, and the first calendar year above 1.5℃. Hansen has issued a new study designed to update the authors’ argument, and to reinforce his case that the single biggest factor is a reduction in the burden of man-made atmospheric aerosols - specifically, those from burning shipping fuel. The new study can be read [here]

First, the record … Can the 2023-4 ElNino explain this? Well, it doesn’t look like it - and now most other experts agree. Here is a rendering of the last three major El Ninos, showing how they evolved during the second half of one year, peak early in the next, then subside (2016 was a double one, so it shows a second peak). But 2023 is not double - and still there has been no expected cooling. Obviously, each successive event is hotter than the one before, because there is a background warming trend.

Next, the distribution of the anomalous warming is unique. It is straightforward, with the available data, to map the pattern of warming over the globe. But if you use this data to plot the sea surface warming by latitude, you get this:

You can see the big 2016 El Nino, and the smaller 2020 one, as well as the recent one, all in the tropics, as expected. You can see Arctic warming right at the top. And you see the big anomaly between 30°N and 55°N, beginning about 2020. What can explain that? Well, the most economical account concerns a big reduction in the emission of sulphur dioxide shipping exhaust, following the International Maritime Organisation mandate on the sulphur content of shipping fuel. This came into force in stages, but the main change was in 2020, as this diagram shows. Emission went down about 85%.

You can see on this map that emissions of these aerosols is mostly in the North Pacific and North Atlantic, where most maritime trade is. The second map shows shipping is a much bigger source than land based industry.

The climate effects of atmospheric shipping tracks have never been robustly quantified. They are complex, and the global observing system has not had the right instruments in orbit - so Hansen and his colleagues argue that if a reduction in aerosol negative forcing is largely responsible, then 2025 will not cool much, if at all. So far, January has been hot, but we shall have to wait and see. That red line in the first diagram will not dip as a resolving El Nino should do. Second, every year from now will be at or above 1.5℃, and they expect 2℃ toward the end of the 2030s. Hansen gives reasons for thinking that the effect of shipping aerosols on marine clouds has been underestimated - that the cloud-seeding effect is greater in the pristine ocean environment than it is over land already burdened with opaque aerosols. In other words, brightening of oceanic clouds has been a greater forcing than allowed in model calculations - and so the warming ‘debt’ due to removing them is bigger than we expected. He further claims that this fact has implications for the true value of the so-called ‘climate equilibrium sensitivity’ - the amount of warming due to doubling CO₂. The sensitivity, according to this, and other evidence is closer to 4.5℃ than the accepted 3℃. 

Hansen’s claim about the ‘darkening’ of the Earth - that is, a reduction of planetary albedo from this cause - appears to have been confirmed by a finding from Helge Goessling and her team at the Alfred Wegener Institute in Bremerhaven, published last December. Using the CERES satellite data they were able to show that there has been a big reduction in clouds in northern mid-latitudes and in the tropics - enough to explain the albedo reduction; and therefore the increased solar absorbed radiation. This map shows the distribution of the albedo anomaly.

We seem to be at an inflection of some importance. If Hansen and his co-authors are correct, the climate problem just got even more urgent, just when the capacity of the world’s people to act collectively has been given a severe political blow.