Amidst a storm of climate records here at Bits of Science we’re searching for ‘The Real Global Temperature Trend’. Today is part three of the series – the second episode about ‘Global Dimming‘. Although worldwide aerosol pollution is slowly declining (see the first graph of this article, below), it could still mask about half of the actual global warming temperature trend, a publication in Nature from 2007 suggests – that looked at the 3D temperature effects of the ‘Asian Brown Cloud‘.

In response to air quality measures in the US and Europe, and more recently in China, the total amount of global dimming seems to be slowly declining. (The two peaks are tropical volcanic eruptions, the El Chichón eruption in Mexico in 1982 and the explosive eruption of Mount Pinatubo in the Philippines in 1991. The global dimming caused by both eruptions is clearly visible in world temperature graphs.) Credit: Michael Mishchenko, NASA

We took a proper look at aerosols in our previous article, that focused on the role of sulpur dioxide pollution, SO2. SO2 is a general climate cooler, because it is light-coloured, therefore reflects sunlight. It is also acts as a condensation nucleus helping to create or alter clouds. Again SO2’s effects are cooling: water droplets in clouds become smaller, therefore the clouds become ‘whiter’ and also more reflective. Also increased water condensation lowers air humidity, the concentration of water as a gas, which is a powerful greenhouse gas.

But if we look at the reality of aerosol emissions sulphur particles are only part of the story. Equally interesting is plane old soot – airborne black carbon. Black carbon aerosols have a rather more complex climatic influence: being dark-coloured they reflect very little sunlight, and are net energy absorbers.

So what is the general influence of an atmosphere with a mix of sulphur and carbon aerosols? Like the one in South Asia, that covers much of the Indian subcontinent, a result of a combination of (incomplete) combustion of fossil fuels (oil and coal) and biomass (wood, for cooking), and generally referred to as the Asian Brown Cloud – or in climatology, the ‘Atmospheric Brown Cloud’ (ABC).

The Asian Brown Cloud may be cooling the Indian surface, but in its atmospheric core the black carbon aerosols act as potent climate warmers – especially in the summer season, as the above climate model runs indicate

Well, it’s an interesting picture. The Nature authors write that their climate models show a net warming effect of the Asian Brown Cloud, at the atmospheric altitude where aerosol concentration is highest (approximately 3 kilometers up in the troposphere). Although both reflective and absorbing aerosols have a dimming effect – effectively lowering solar radiation at Earth’s surface – higher up in the atmosphere the brown cloud amplifies the greenhouse gas warming:

Asian Brown Cloud aerosols warm the atmosphere through increased heat absorption, and cool the surface through dimming solar radiation.

Here it is. An image based on the work of Veerabhadran Ramanthan of the University of California’s Scripps Institute and others, like atmospheric chemist Paul Crutzen. You might want to spend a minute on it, to grasp the numbers. If they hold true, global dimming would mask 47% of greenhouse gas climate warming, and as aerosols are short lived and declining and atmospheric CO2 only goes up, that the actual global temperature trend is significantly higher than most would think.

To help you out with the above image: If you add up direct and indirect temperature effects of aerosols, you would get a net cooling effect of 4.4 W/m2 at the Earth’s surface – and a net warming effect of aerosols (which is only direct, heat absorbtion) of 3 W/m2 higher up in the atmosphere. If we add greenhouse gas climate forcing to that picture, we see GHG warming is amplified by aerosols (3+1.4=4.4W/m2) higher up in the atmosphere, yet overcompensated to (rather strong) net cooling at the Earth’s surface directly under the Asian Brown Cloud: +1.6 -4.4 = -2.8 W/m2.

For the planet as a whole we need to sum the direct and indirect forcing effects of aerosols for both the atmosphere and the surface, to get to a negative forcing of +3 -4.4 = -1.4 W/m2.

It is this number that leads the authors to conclude that aerosol pollution masks almost half (1.4*100/3=47%) of greenhouse gas warming (1.4+1.6 = 3.0 W/m2).

Now let’s find out if other climate scientists agree with that number – or at least the magnitude of it.

If half of warming is masked, ‘real’ temperature trend is twice(!) as high – because aerosols settle, CO2 does not

If they do, that would lead us to conclude that CO2-induced climate warming is still strongly underestimated, and as aerosol pollution is shorter-lived and declining (and CO2 concentration is only going up) ‘The Real Global Temperature Trend’ would in fact be higher than that ‘insane peak’ on this graph.

© Rolf Schuttenhelm | www.bitsofscience.org