Greener, Not Browner

A recent Science paper by J-F. Busteri and 30 named coauthors assisted by 239 volunteers found, looking at global drylands (about 40% of land areas fall into this category), that we had undercounted global forest cover by a whopping “at least 9%.” 239 people were required to examine over 210,000 0.5 hectare (1.2 acre) sample plots in GoogleEarth, and classify the cover as open or forested. Here’s the resultant cool map:

This has been the subject of a flood of recent stories, blog posts, tweets, and whatever concerning Bastin et al. But here at the Center for the Study of Science, we’re value added, so here’s some added value.

Last year, Zaichin Zhu and 31 coauthors published a remarkable analysis of global vegetation change since satellite sensors became operational in the late 1970s. The vast majority of the globe’s vegetated area shows greening, with 25-50% of that area showing a statistically significant change, while only 4% of the vegetated area is significantly browning. Here’s the mind-boggling map:

Trends in Leaf Area Index, 1978-2009. Positive tones are greening, negative are browning, and the dots delineate where the changes are statistically significant. There is approximately 9 times more area significantly greening up than browning down. 

Hope you're sitting down for the money quote:

We show a persistent and widespread increase of growing season integrated LAI (greening) over 25% to 50% of the global vegetated area, whereas less than 4% of the globe shows decreasing LAI (browning). Factorial simulations with multiple global ecosystem models show that CO2 fertilization effects explain 70% of the observed greening trend…

And the other greening driver that stood out from the statistical noise was—you guessed it—climate change.

Now, just for fun, toggle back and forth between the two maps. As you can see, virtually every place where there’s newly detected forest is greening, and a large number of these are doing it in a statistically significant fashion. This may lead to a remarkable hypothesis—that one of the reasons the forested regions were undercounted in previous surveys (among other reasons) is that there wasn’t enough vegetation present to meet Bastin’s criterion for “forest,” which is greater than 10% tree cover, and carbon dioxide and global warming changed that.

References:

Bastin, F-L., et al., 2017. The extent of forest in dryland biomes. Science 356, 635-638.

Zhu, Z., et al., 2016. Greening of the earth and its drivers. Nature Climate Change, DOI: 10.1038/

NCLIMATE30004.