NewScientist’s leader (6 January 2016) covers another problem tied to climate change: global browning:
Now there is a new pollution problem on the horizon: global browning. Like global dimming, it might at first sound almost comical – but like global dimming, it is anything but. All over the world, increased inflows of dead organic matter are making lakes and rivers murkier. The full extent of the problem is still being assessed, but it is likely to be bad news for wildlife. It is also bad news for humans, because it makes water purification more difficult and expensive.
Somewhat predictably, the cause of global browning is also industrial pollution, although not in the form of smog. Instead, it is largely the unanticipated consequence of another clean-up success story: the reduction of acid rain (see “Global browning: Why the world’s fresh water is getting murkier“).
That hangover will eventually dwindle, but browning also seems to be driven by climate change itself. If so, it will be harder to tackle than global dimming. And if we do tackle it successfully, there may be unexpected knock-on effects – as there were with dimming. The pollutants in that case, while generally undesirable, alleviated climate change by reflecting sunlight back into space.
The referenced article is in the same issue, and explores in more detail.
Acid rain began increasing in the mid-1800s as the Industrial Revolution took off, powered by fossil fuels. Burning these hydrocarbons, especially coal, produces sulphur dioxide and nitrogen oxides, which react with water in the atmosphere to produce acids. By the 1970s, it was apparent that this was damaging trees and aquatic ecosystems, and governments started enacting legislation to clean up smokestacks. Acid rain began to decrease. But there was an unforeseen consequence. In many temperate and subarctic areas, deposits of sulphur had changed the chemistry of soils, making them “stickier”, says Chris Evans, a biogeochemist at the UK’s Natural Environment Research Council. This meant most DOC [Dissolved organic carbon] stayed put, and didn’t run off into surrounding rivers and lakes. But as soil sulphur concentrations dropped, DOC became unstuck.
In the mid 1990s, Evans and two colleagues were among the first to notice rising DOC levels. A decade ago, their research revealed that concentrations in 22 rivers in the UK had increased by an average of 91 per cent over the preceding 15 years. Two years later, Evans collaborated with a larger group to reveal that rising DOC wasn’t restricted to the UK. Their results, published in Nature in 2007, showed that 522 remote lakes and streams in North America and northern Europe had seen nearly a doubling of DOC concentrations between the 1990s and 2004. They also firmly tied the trend to decreased sulphur deposition, which had halved during the same period.
The result?
The link between browning and climate change is yet to be confirmed, but one thing is clear: more browning is bad news. One survey of 168 lakes in Norway found that while initial increases in DOC were linked to increases in brown trout numbers, continued rises caused the population to steadily drop. The initial benefits were probably due to DOC’s ability to block UV rays and the fact that when it drains into watercourses it often brings phosphorus and nitrogen too, key nutrients that fuel the growth of organisms at the bottom of the food chain. However, DOC levels reach a tipping point when the water turns a deep brown, says Anders Finstad at the Norwegian University of Science and Technology in Trondheim, who led the study. This prevents sunlight from reaching bottom-dwelling algae and, if the water is dark enough, free-floating plankton. No sunlight means no photosynthesis, and no food at the base of the food web.
From pothole to pothole, it seems. Sometimes I wonder if we’ll find our way to a relatively stable position. That would actually be rather unnatural, though.
