Increases in CO2 Are Making Earth's Arid Regions More Leafy

Since the 1980s foliage cover in several arid regions of the world has increased, correlated with the marked increase in carbon dioxide in the atmosphere, a new study in Geophysical Research Letters has found.

Scientists from Australia's Commonwealth Scientific and Industrial Research Organization examined via satellite data the changes in greenery in the US Southwest, the Australian Outback, the Middle East, and parts of Africa, determining that from 1982 to 2010—after accounting for other changes such as in precipitation and land-use changes—there has been an 11 percent increase in foliage due to the "fertilization effect" of there being more CO2 in the air.

At the start of the time period there was roughly 340 parts per million of CO2 in the atmosphere. In 2010 there was about 390 ppm. In May of this year the Mauna Loa observatory in Hawaii recorded 399.89 ppm, the highest amount ever recorded instrumentally and the highest amount in millions of years. Initial reports showed that concentrations higher than the ominously symbolic 400 ppm mark had been recorded, but this was revised downward very slightly afterwards. Last week we hit 399.92 ppm—so technically we're still not past 400 ppm by these readings, but for all intents and purposes we're there.

Though additional CO2 in the atmosphere does little to help plants in humid tropical areas to grow much more foliage, the authors say, in warm and dry places, "if elevated CO2 causes the water use of individual leaves to drop, plants will respond by increasing their total numbers of leaves." Furthermore, the researchers observed that the type of vegetation in these arid regions has changed, with trees invading grasslands.

It's an interesting observation for sure, confirming something that has long been predicted could happen as CO2 rises and our climate changes. What I think is probably one big variable in this, as CO2 continues to rise, is how long might it be before precipitation changes in the areas examined start overwhelming the situation.

According to the graphic above, by the end of this century the very areas examined in this research are some of the worst-affected by declines in precipitation. Look at the map: The American southwest is solid in the brown, with a decline of 10-20 inches of liquid water; the Sahara in Africa sees little change, but some of the arid-but-not-desert regions seeing declines in the 20-30 inches range; ditto for the Middle East and slightly less for all of Australia.

We'll obviously have to see how it plays out, but just because there's been an increase in vegetation in these arid regions over the past two decades, corresponding to the roughly 14 percent rise in CO2 since then, doesn't mean such an increase will continue in the future.

I say this because it might be tempting to look at this research and make a leap to say something along the lines of "see, I told you global warming would be good for plants." It's an old climate denier talking point to make this claim—well refuted by the folks at Skeptical Science—and that's probably the wrong conclusion to draw from this research, for the future.