Clouds, Earth's albedo, and accelerated warming
Sunday 12 January 2025 6:38 AM
The question of whether warming is accelerating, and if so, why, has become a serious research subject since the record-breaking heat of 2023, and the publication of Jim Hansen’s paper, the one we examined in Chapter 18. Hansen and his colleagues made a case that the warming rate had increased a lot since around 2010, from a 40 year mean of 0.18 ℃ per decade, to at least 0.32 ℃. They provided an analysis of CERES data showing an increase in absorbed solar radiation, and evidence of a reduction of atmospheric aerosol opacity consistent with the claim. But some specialists attributed the causes of 2023 differently; others thought it might turn out to be a temporary anomaly rather than a new trend, and still others wondered if there might be some yet undiscovered reason.
Now, a new study has determined, again from the CERES record, that most or all of the trend in Earth's energy ‘net income’ - the quantity of sunlight absorbed by the Earth and its atmosphere - can be accounted for by a change in the distribution of clouds, specially low clouds in the tropics and northern mid-latitudes. As we saw, anthropogenic aerosols have large effects on cloud formation and behaviour, and although this has been known for some time, we still don’t understand the details of those interactions well. That leaves room for doubt about the size of aerosol changes, and their relation with this newly observed trend. The authors of this study [Goessler et al 2024, Science, 387, 6729, 68-73] make no definite claims about the causes of the cloud change. Nevertheless, their accurate analysis will be very valuable as the problem is investigated further.
The finding is effectively a reduction in Earth’s albedo - the planet has become a little bit darker. Because the data are complicated, and interpretation is tricky, the authors used the Copernicus ERA5 reanalysis data in addition to those of the satellite program. The map below shows the anomaly in calculated albedo for 2023.
And this diagram summarises the forcings contributing to that unusual year. The left column shows four components: the underlying greenhouse gas forcing; the El Nino; the albedo reduction; and the small effect of the solar cycle. The right column shows the contributions to increased solar absorbtion in five latitude zones.
The authors acknowledge that, in spite of their results, and the close spatial correlation between the cloud changes and the pattern of absorbed radiation, this is still a difficult causal connection to make. And it’s not possible to attribute the role of aerosols. All the same, this finding appears to be an important contribution to understanding what is happening to Earth’s energy budget, the primary driver of warming. Below is a record of the longer trend in two key quantities in this budget: the top-of-atmosphgere net solar clear-sky radiation; and the planetary albedo. According to these records, the 2023 anomalies didn’t exactly come out of the blue. But of course, if that is the case, they will not simply disappear either. The time scale isn’t shown, but the albedo trend is visible since the last big El Nino in 2016. That, as far as it goes, would appear to support Hansen’s case.