Alternative facts and carbon cycle. By Colin Prentice

In today’s issue of Science, Bastin et al. hit back at critics (no less than four technical comments and three letters published today draw attention to errors and/or omissions in their original paper!).

They have, at least, withdrawn their misleading claim that tree planting is the “best solution” to climate change.

They say that they recognize the difference between the amount of carbon that has been emitted and the amount of carbon that has remained in the atmosphere. However, they do not address the corollary, that the removal of carbon from the atmosphere­ as a result of tree planting will only be about half of the amount of carbon sequestered. Moreover, they continue to defend their claim regarding the amount of carbon that could be sequestered. In fact they describe it, astonishingly, as a “conservative” estimate.

Their defence is a masterpiece of obfuscation.

They dismiss the simple argument, made in the technical comment by Lewis et al., that it’s impossible that complete reforestation could put back many times more carbon than has been released by deforestation. To do so, they simply rubbish the standard estimate of how much carbon that was. They ignore the fact that if a very large amount of carbon had been released by deforestation before 1750, we would know about it, because it would show up in the Holocene CO2 and δ13C records. It doesn’t.

They dismiss the key argument (presented in several technical comments) that they double-counted the pre-existing carbon in their supposed potential restoration areas. Today, they assert that they did subtract the pre-existing carbon; but there is no evidence of this in the “more detailed description” of their methods, which they have now provided.

They do now provide a Table showing the numbers they used to assess tree cover and carbon storage per biome. This Table contains only four distinct numbers for carbon density (shared out among 14 biomes). We learn – for example – that “tundra” has a median tree cover of 80%.

Thus, Bastin et al. stick by their inflated estimate of the amount of carbon that could be sequestered by “forest restoration” while confirming that yes, their estimate does include boosting tree cover to 100% in savannas and other ecosystems  where tree cover is naturally less than their estimate of  potential tree cover. (They explain the low tree cover in some non-forest ecosystems by recourse to “desertification”.)

A comprehensive debunking may have to wait for the next IPCC report… In the meantime, this group has taught us that the technique of “rebutting” criticisms by repeating indefensible statements is no longer confined to politics – and is, apparently, condoned by one of the world’s most prestigious scientific journals.

 

Colin Prentice

18 October 2019

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