Seeing the wood for the trees

A new study in the journal Science ($) claims to have detected… “increases current estimates of global forest cover by at least 9%”, by using new “high resolution satellite images covering more than 200,000 half-hectare-sized plots” in dryland areas.

“The extent of forest area in dryland habitats, which occupy more than 40% of Earth’s land surface, is uncertain compared with that in other biomes. Bastin et al. provide a global estimate of forest extent in drylands, calculated from high-resolution satellite images covering more than 200,000 plots. Forests in drylands are much more extensive than previously reported and cover a total area similar to that of tropical rainforests or boreal forests. This increases estimates of global forest cover by at least 9%”

Sounds like very good news. But I suppose there are three niggling questions here, which immediately spring to mind.

1) Is that 9% figure within the margin of variability for a sample of 200,000 one-acre plots, from an area as massive as whole of the Earth’s dryland tree-growing areas? I mean, if you made a basic Earth model in 3D and pointed a virtual camera at 200,000 random plots, what would be the variability of the results arising simply from chance? Could I re-run it with different samples, multiple times, and then pick one result from a range of -10% to +20%?

2) What proportion of this tree cover has always been there, hiding in plain sight? And to what extent is this 9% figure measuring the carbon fertilisation effect, the ‘global greening’?

3) What part of the 9% is measuring the ‘leafing out’ of the massive tree-planting efforts in the developing world? A chart from the paper, published on a short Science blog post, sort-of-helps with that, but not much…

My hunch would be that a large chunk of of the 9% is natural re-greening in Africa and Russia, plus the huge levels of tree planting in Asia which is now ‘leafing out’ in a manner which makes the saplings visible on high-res satellite images.

Sadly the paper itself is behind a paywall, so it’s difficult to ask the above questions of it. But, effectively, it appears humanity has discovered the equivalent of ‘another Amazon’ sucking down a whole lot of carbon — something which hasn’t yet been fed into the greenhouse warming models.