Decreasing aerosols and clouds accelerate climate change

The price of the clean-up, and a surprising positive feedback (but 'positive' doesn't mean 'good' this time)

In a recent Note here at UV & You (viewable only from the Substack App), I linked to an excellent hour-long webinar discussing this must-read paper by climate-guru, James Hansen.

Hansen questions whether the messages we’ve been getting from the Intergovernmental Panel on Climate Change (the IPCC) have been overly optimistic. Here I’ll outline his reasoning, including just three of the paper’s many great figures.

Essentially, he’s saying that although greenhouse gases are the main driver for climate change, the recent acceleration in warming is because the atmosphere is becoming cleaner and clearer. Key drivers are new regulations about the maximum sulphur content of fuels used for shipping. In the longer term, he’s also concerned about future effects on climate due to a shut-down in deep-ocean circulation, but I won’t go into that part of it here. Perhaps another time …

Up to the second half of last century, the aerosol burden in the atmosphere was steadily increasing, but since then it stabilized, and around the turn of the century it began to decrease. The effect on climate forcing, compared with that from increasing greenhouse gases is shown below (where positive forcings lead to climate warming).

Climate forcing by Greenhouse gas and IPCC aerosols

During that earlier period, the effect of those increasing aerosols (blue line) counteracted the effect of increasing greenhouse gases (red line), so the net rate of increase (black line) was smaller. The increasing aerosols suppressed some of the warming that would have otherwise occurred due to increasing greenhouse gases. But since then, the slopes of the red and the blue curves have both been positive, leading to the more rapid overall climate forcing (heating).

But that’s only part of the story. Aerosols act as condensation nuclei for the formation of clouds, and with those reductions in aerosol concentrations, the amount of cloud cover has reduced. That’s a hugely important positive feedback. The plot below shows mean reflectivity of planet Earth, as derived from satellite data. It shows that about 29 percent of the incoming solar radiation is reflected back to space (mainly by clouds). But, because of the reduction in aerosols, it also shows that the reflectivity of planet Earth has reduced by nearly 1 percent in the last couple of decades. That means a lot of extra energy is now penetrating to Earth’s surface.

Earth’s albedo (mean reflectivity, in percent), seasonality removed. Corresponding changes from the pre 2010 mean are shown at right.

To put those two effects into context, the first plot shows that the effect of increasing green-house gases (GHGs) has been to add about 3 Watts per square metre of forcing, compared with a mean incoming energy of around 300 Watts per square meter. So, the effect of the increasing GHGs is equivalent to a net increase of about 1 percent in the incoming energy.

The change in albedo since the turn of the century is nearly as important by itself, which explains why the rate of warming has become so much steeper in the last couple of decades. The change is illustrated below, from the first plot in Hansen’s paper. The rate of temperature-increase for the period 1970 to 2010 was 0.18 ºC/decade (green dots), but since 2010 it’s been nearly twice that (purple dots). Last year (2024) was the easily warmest on record. It clearly surpassed the previous record that had been set the preceding year (that was also attributed in part to the record-low planetary albedo).

It remains to be seen whether these decreases in albedo continue, or stabilise, or even reverse. In any case, recent evidence shows they’ll clearly be important!

Just to add some UV & You relevance, I should mention that these reductions in aerosols and cloud reflectance both contribute to higher UV levels (as I discussed here), though in pristine locations like New Zealand, no significant increases in UV have been observed since the turn of the century.

The good news - I suppose (?☹)- is that the air can’t keep getting cleaner for ever. And I doubt if the albedo will continue to decrease because (simplistically) higher temperature means more water vapour means more cloud cover means higher albedo. With any luck, in the decades ahead, the rate of temperature increase will revert to something like that around the turn of the century, before eventually slowing down completely once the remaining fossil fuel stocks become too expensive to extract.

Finally, in the excellent webinar about his paper, Hansen argues for a new political movement in the USA that accepts no corporate donations. Hopefully that new party will materialise and succeed. Only then, he says, will the elected Government represent the views of the people rather than the views of the corporate interests they’re beholden to. It sounds like good idea for New Zealand, and every other democracy too! My other go-to oracle, Bill McKibben, has just added some salient points on those political angles.

Sadly, not everybody is on board with Hansen. Our old friend Qing-Bin Lu, who I’ve grumbled about before, has a different take on this aerosol story. Sigh …

He’s convinced that CFCs rather than GHGs are the main source of warming. And because they’re now decreasing, everything will be better soon. I wish I could agree, but even the figures in his own paper don’t support his thesis. Unfortunately, my money remains firmly in Hansen’s corner.