Searching for Real Global Temperature Trend, part 2 – Global Dimming: if aerosol cooling is underestimated, then warming trend is higher

Yes. It’s been a while, but we have an update in our Today’s Paradox series: If aerosol climate cooling is underestimated, that means the trend line of the global temperature graph would lie higher than the one you get by “evening-out La Niña and El Niño,” and global climatic warming would speed up (further) in decades to come. That would also mean current peaks in global temperature, like February 2016, might not be peaks – but rather ‘corrections’ towards that higher actual trend.

But is it? Lets skip another breakfast, and read another proper journal. Got coffee? We are investigating!

Global dimming aerosol cooling - CO2 vs SO2 emissions, Nature
This image comes from the Nature study, so we had to put it in. It shows two scenarios for emissions, one for CO2 and one for SO2 – indicating that the ratio of CO2/SO2 from fossil combustion is going down. It would have been better still to show a graph of CO2 and SO2 concentrations – because then you’d see these lines separate much stronger. This is because SO2 stays in the atmosphere for a very short time (years at most) whereas fossil CO2 is more or less cumulative. This is why CO2 warming becomes more and more dominant over SO2 (or any aerosol) cooling – with passing of time.

First of, this study is ten years old – so it’s not at the cutting edge of the current scientific climate debate. But it was published in Nature by researchers of the Potsdam Institute and the Hadley Centre (so let’s say it’s respectable) – and it does address a subject that interests us a lot: Aerosols. Or rather: sulphur aerosols – SO2 to be precise, sulphur dioxide.

About those aerosols. Reflection and dimming add up – that’s why they cool the climate, generally

There are several ways in which aerosols influence the climate. The first depends on their colour – or rather reflectivity/albedo: SO2 particles are light-coloured – and that means they reflect a relatively large amount of incoming solar radiation. This energy can therefore not enter system Earth as infra-red energy (heat), but is returned to the cosmos. Dark-coloured aerosols (like soot) do the opposite – they reflect very little of incoming solar radiation, and absorb that energy instead, transforming it to infra-red energy, that is then radiated to other surrounding air particles (‘the atmosphere’) and eventually Earth’s surface. (Yes, that’s comparable to what greenhouse gases do.)

Aerosols also influence cloud formation – and cloud alteration, by acting as condensation nuclei. Some studies suggest that “polluted clouds” – clouds with added SO2 aerosols – carry smaller water droplets and are therefore whiter, more reflective. This is shown in the image below:

Aerosol cooling (SO2, sulphur) here is how it works! Direct albedo & clouds
Image credit: Georgia State University.

All aerosols – no matter their colour – do something else: they block the Sun (by scattering, reflecting and absorbing). Unlike greenhouse gases that trap some energy, but still let (most) light run through, aerosols form a tiny sunshade. That means if you have a lot of aerosols up high, less solar radiation reaches the Earth’s surface. This is referred to as ‘global dimming’.

Global dimming has a separate climate effect. Let’s say the aerosol shade consist of both light-coloured sulphur particles and dark-coloured soot – something resembling the Asian Brown Cloud. Then, by this pollution haze, some of the solar radiation is reflected, but a big chunk is also absorbed. This is warming though that does not take place at Earth’s surface, but higher up in the atmosphere. Some of the heat may escape to the cosmos again – and locally (directly under that shade) Earth’s surface is likely to be a bit cooler than when solar radiation would have been higher.

You get the point: All in all the climatic influence of aerosols is quite complicated. A bit like CO2. Because let’s not forget, there is also still a relatively large band of uncertainty around the actual warming potency of carbon dioxide. It’s known – we keep repeating, since the 19th century – that CO2 has heat-absorbing properties, is therefore a greenhouse gas, and we also know that a rise in its concentration leads to substantial warming. But how much? Well, some use Earth’s thermometer measurements to define the most likely CO2 climate sensitivity:

Is the global temperature graph masked still?

When you look at a proper graph of global average temperatures from 1880 to present, you see unmistakable evidence of a warming trend – one that follows climate model forecasts of increased atmospheric heat absorption due to an equally unmistakable rise in atmospheric CO2.

But as climate sensitivity (the global climatic warming resulting from a doubling in CO2) remains within a margin of uncertainty (IPCC AR5 states ‘high confidence’ that it lies between 1.5 and 4.5 degrees Celsius) and both fast-indirect climate feedbacks (like sea-ice albedo and cloud influence) and slow-indirect feedbacks (like deep-sea methane, declining ocean CO2 uptake) still add levels of uncertainty it is challenging to determine where we should draw the precise warming trend.

So if somehow we could decrease the uncertainty around other factors, that might be helpful. Like aerosols. Much research goes into studying the climate impacts of volcanic eruptions. These have been suggested as a cause of Europe’s Little Ice Age. We tend to think, based on skipping through a couple such studies, that perhaps volcanic cooling is sometimes overestimated – but if we skip a little further we conclude this matter might actually be too complicated for our medium-sized brains.

And what if the opposite were true? What if aerosol cooling is underestimated? Let’s get back to the Nature study.

These researchers state ‘aerosol cooling is still uncertain, but potentially large.’ Although the scientists don’t decrease the uncertainty with their study, they do express their ‘educated gut feeling’ – by expressing the very large relevance:

“In the future, aerosol cooling is expected to decline relative to greenhouse gas forcing, because of the aerosols’ much shorter lifetime and the pursuit of a cleaner atmosphere. Strong aerosol cooling in the past and present would then imply that future global warming may proceed at or even above the upper extreme of the range projected by the Intergovernmental Panel on Climate Change.”

Afraid we’ll have to delve a little deeper for our series.

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© Rolf Schuttenhelm | www.bitsofscience.org

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