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Interesting paper from @CrowtherLab, but I share concerns on communication expressed by @pepcanadell. It should be recognised that getting to a net-zero CO2 budget requires us foremost to reduce CO2 emissions close to 0. This can unfortunately not be achieved with planting trees.
A few additional points not addressed in @pepcanadell's thread, which are important to consider when planning reforestation:
First, reforestation may lead to a net warming in some regions, due to albedo effects (e.g. snow shading), as shown in e.g. papers of @richardabetts.
First, reforestation may lead to a net warming in some regions, due to albedo effects (e.g. snow shading), as shown in e.g. papers of @richardabetts.
Also evapotranspiration plays a role. Hence, one should be careful not to equate 1m2 of forest in Siberia with 1m2 of Amazon rainforest. See eg work of @mikewin_climsci (also with @TWCrowther), recently recognized by @theAGU [paper in preparation...]:
Second, and just as important: Forest sinks are not guaranteed. They may be threatened by pest outbreaks, fires and climate extremes, possibly also as a result of on-going global warming.
In fact, our research has shown a strong interannual correlation between land water availability & global atmospheric CO2 growth rate. Likely due in part to drought effects on ecosystems, incl. forest fires. Climate models underestimate this relationship
To conclude: Afforestation can contribute to reduce climate change, but it is no silver bullet & cannot make up for the large continuing CO2 emissions. Particularly important is to avoid deforestation in ecosystems with large CO2 uptake & cooling effect on climate, e.g. Amazonia.
