Climate: the acceleration is confirmed

In April 2024, the global average temperature was 1.32°C higher than that of April in the period 1951-1980, announces the NASA/Columbia University team in New York.

But, and above all, notes climatologist James Hansen in a commentary, the last 12 months, from May 2023 to April 2024, are 1.56°C above the 1880-1920 average, the closest reference to that noted. “before the industrial era” in international treaties such as the UN Climate Convention and serving as a basis for the climate objectives decided in Copenhagen in 2009 and Paris in 2015 (no more than 2°C of warming, then approaching the more possible by 1.5°C).

This is what we can read in the graph below, where the climatological reference is no longer 1951-1980, but 1880-1920. James Hansen noted two trends. The first, in green, indicates the linearized evolution over the period 1970 to 2010. It practically sticks to the average calculated over 132 months, or 11 years… but only until 2010. Since then, it has deviated from it, by at first very slowly, then, in recent years, more and more strongly. This can be seen in particular by the fact that Niña years, the opposite situation of El Niño, which are usually found under this medium-term trend because this phenomenon of the Tropical Pacific then produces the coldest years on a planetary scale. , are right on trend. Also, James Hansen has shown on this classic graph a trend indicated by the yellow zone which, according to him, corresponds to an acceleration of warming.

The very existence and causes of this acceleration are the subject of debate among specialists and a consensus has not yet emerged. Hansen brings some elements linked to the spatialization of this warming, because it is indeed characterized by very marked regional developments.

Here first is the evolution of ocean warming depending on latitude. The red areas at the equator, latitude 0, are the marks of El Niño events. We can clearly distinguish the formidable Niño of 1997-98, the super El Niño of 2015-2016, and the recent episode of 2023. But Hansen draws attention to the very strong warming of the Atlantic and the North Pacific, between latitudes 30°N and 60°N between 2020 and 2024 (the graph indicates temperatures deviating from the 1951-1980 climatological average over 12 rolling months).

If we look at oceanic and continental temperatures below, this warming in the mid and high latitudes of the northern hemisphere becomes even more spectacular, with the maximum warming recorded in the Arctic.

The causes of this acceleration, according to Hansen, come from the convergence of several factors moving in this direction. Some are well measured, such as the continuation of the intensification of the greenhouse effect through our greenhouse gas emissions, especially linked to the use of fossil fuels. This is obviously the main cause. Added to this is the fact that the 11-year solar cycle is at its maximum in 2023-2025, but for a very small part. The role of El Niño is major for the years 2015-2016 and for 2023-2024. But, according to Hansen, there is also a factor directly resulting from policies aimed at avoiding massive emissions of sulfate aerosols from ships. However, these policies led to major changes imposed by the IMO (International Maritime Organization) in 2020. These aerosols have a net cooling effect, by reflecting the solar energy received back into space. This phenomenon is very poorly measured because space agencies have not responded to the call from scientists to have precise measuring instruments for these aerosols. However, notes Hansen, satellites show a clear increase in the absorption of solar radiation precisely at mid-latitudes of the northern hemisphere over the past ten years, as shown in the graph below.

The El Niño phenomenon is easing. Even if it could raise the planetary average of 2024 above 2023, the end of the year will decide, it could be followed by a new Niña. We will then know, if it brings the average temperature back below the green bar in the third graph of this article, if the acceleration is a transient phenomenon. But it would take a formidable Niña, the most important in a century, to achieve it. Which is unlikely.

This latest bad news about climate change only reinforces the need for much stronger action.