Do California's Deserts Help Control Climate Change?

Carbon sink | Photo: Jon Sullivan/Flickr/Creative Commons License

Living ecosystems in the Western United States sequester a substantial amount of carbon, meaning that preserving those intact landscapes may prove crucial in mitigating the effects of greenhouse gas-driven climate change. That's according to a report released today by the U.S. Geological Survey. Included in those carbon-storing ecosystems are the warm deserts of the southwest, currently being eyed for massive development for utility-scale solar power plants.

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The report, imposingly titled "Baseline and Projected Future Carbon Storage and
Greenhouse-Gas Fluxes in Ecosystems of the Western United States" and edited by Zhiliang Zhu and Bradley C. Reed, is the second in a series of USGS publications looking at the role of North American landscapes in sequestering carbon. The first, released in 2011, examined the Great Plains area.

Ecosystems sequester carbon in a number of ways: in living biomass, in dead biomass such as leaf litter, and as organic soil carbon that results from the breakdown of biomass. Though warm deserts such as the Mojave often hold less carbon per square meter than, say, the old-growth forests of the Pacific Northwest, both warm and cold desert shrublands turn out to sequester a surprising amount of carbon. According to the report, basing its statements on data from the US Forest Service, warm deserts such as the Mojave sequester an average of 6.7 kilograms of carbon per square meter, and cold deserts such as the sagebrush steppes of the Mono Basin up that to 7 kilograms.

Those figures are likely to be revised as more research is done into desert ecosystems and carbon sinks. Researcher Michael Allen, Professor of Biology at UC Riverside, cites annual carbon uptake figures -- the amount of carbon added to that sequestered -- a tenth of a kilogram per year.

This becomes relevant to renewable energy policy in that many thousands of acres of desert are slated for renewable energy development on a massive scale. Industrial development of living landscapes displaces the biological communities that sequester carbon, and may in fact cause the release of carbon already sequestered.

Even worse, disruption of desert landscapes may cause the release of inorganic carbon, the well-known "caliche" layer of calcium carbonate that underlies most of the sites suitable for development in solar facilities. Caliche generally consistes of carbon taken out of the atmosphere during the Pleistocene or earlier; disturbance of caliche layers on a massive scale can conceivably cause the carbon it contains to be released to the atmosphere about like we're doing with fossil fuels. The USGS report doesn't discuss carbon contained in caliche, but Allen did, in 2011, and he said that loss of caliche

[M]ay be especially critical following removal of the vegetation for thermal solar power units.... This net loss of caliche could continue or even increase as temperatures warm for centuries or more, given the incredibly large amount stored in our California desert valleys and vegetation recovery following disturbance for developing desert lands can also take a century or more.

The USGS report concurs that warm deserts may actually start to contribute carbon to the atmosphere if we let the planet warm.

All the more reason to put as many PV panels on rooftops and carports as we can.

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