Sunday, November 30, 2008

ABCs and GHGs in Asia

A report recently released by the UN Environment Programme sheds further light (no pun intended) on the fact that cities across Asia are getting dimmer. The Atmospheric Brown Cloud (ABC) Report named 13 cities as ABC hotspots, with Beijing, Shanghai and Shenzhen all making the cut. The thing I found most interesting about the report is that it illustrates the incredibly complex nature of the climate system.

ABCs are made up of emissions and particulate matter released from the burning of fossil fuels and biomass. This particulate matter reduces the amount of sunlight hitting the earth’s surface in two ways. First, some particulates, such as sulfate, act as reflectors that bounce sunlight away from the earth. Second, other particulates, such as black carbon in soot, trap and absorb light before it hits the ground. The effect of these ABCs has been quite pronounced over the last quarter century: Guangzhou has seen a 20% reduction in sunlight since 1970.

Complicated Climate Effects
The effect of ABCs on the climate is quite complicated. The sulphates that reflect sunlight away from earth actually keep the climate cooler than it otherwise would be. In fact, the report estimates that ABCs may have reduced GHG-caused rises in global temperature by between 20 and 80%! This may help explain why eastern China has actually seen average temperatures decline over the past decade, globally the warmest on record.

The idea that pollution actually helps moderate climate change is pretty incredible. The scary thing is, though, as China continues to develop and eventually begins to aggressively reduce local pollution and the associated sulfates, this may actually increase the global warming effect of GHGs. As the report notes:
Simply tackling the pollution linked with brown cloud formation without simultaneously delivering big cuts in greenhouse gases could have a potentially disastrous effect.
But on the bright side (I’m really on today), if China were able to eliminate ABCs, presumably sunlight would return to pre-1970 levels and increase 10-25%. This would likely cause solar power generation to be more effective. SOM, designers of the Pearl River Tower, mentioned that the lack of strong sunlight somewhat impeded their energy generation from the PV systems. So, this brightening effect would seem to reduce GHG emissions by making solar generation more effective.

In other climate related news, the World Meteorological Organization (WMO) released their latest Annual Greenhouse Gas Bulletin, which shows that atmospheric CO2 concentrations have continued to rise, and are now at 383 parts per million (PPM). CO2 is by far the most important GHG, accounting for about 90% of radiative forcing (link to wiki or something) in recent years. This level of CO2 is particularly disturbing, as it is already higher than the 350 PPM level that NASA scientist James Hansen calls safe (PDF). Climate scientists are engaged in ongoing debate over what atmospheric CO2 level humanity ultimately needs to aim for (see Joe Romm's ClimateProgress blog for more), but the “safe” range will probably be somewhere between 350 and 450 PPM. The world added 2 PPM to the atmosphere last year, and this number is set to rise to 3 PPM annually as emissions continue to grow. This doesn’t give us much time, but I remain optimistic we can avoid the worst effects of climate change if we get our act together soon.

BTW Charlie- if you're reading, this title was not meant to be a rip-off of your recent post "The ABCs of SEDs"...


Charlie said...

I wish I had a trademark on the crowd-pleasing "ABCs" title starter, but alas I don't. Therefore anyone is free to use it, although I caution against overuse. Re: the substance of your post, fascinating issue! If the brown clouds are high enough they are not inhaled by humans, but help prevent global warming and presumably decrease the risk of skin cancers, are they really such a bad thing? Lower crop yields I suppose, unattractive, what are the other downsides?

Geoff said...


Crop yields are definitely lower thanks to ABCs. 3 direct effects: less sunlight means less photosynthesis; settling of aerosol particles on plants shields the leaves from solar radiation; aerosol can increase acidity in soil and rain harming plants.

the other big problem is the way ABCs redistribute natural rainfall patterns. ABCs have caused an overall reduction in mean precipitation in asia by about 1-2%, but this isnt distributed evenly. Northern China has gotten much drier, while equatorial regions have gotten more sporadic and intense rainfall events. intense rainfall events arent really usable agriculturally as they result in flooding. dryness in northern china is obviously a huge problem.

as far as how far above the earth these ABCs are, i cant tell. they make mention to the health impacts of local pollution in this report, so i think these clouds are the clouds that we can see hanging over the cities when we fly in.

fundamentally, i think the clouds are bad, but it's interesting that a "bad" thing like these clouds can have such a large mitigating effect on climate change.

Scott Moore said...

Hey Geoff,

Yeah, the brown cloud is pretty ominous. I've also read some earlier stuff about how northern hemisphere sulfate levels have caused a marked decrease in cloud formation (fewer cloud nuclei), and as a result solar insolation has increased at those higher latitudes. So yeah, basically just the inverse of what you've noted with the Asian brown cloud. So good point: talking about co-benefits between air quality and climate is all fine and well, but don't forget the serious complexity of the climate system.