Going beyond carbon essential to appraise Climate Benefits of Nature-based Solutions
The contribution of reforestation to climate change mitigation is typically appraised and reported in terms of how much carbon dioxide (CO2) can be taken up. Two recent papers, in Science and Nature Communications, highlight how focussing exclusively on this measure can overlook other important details that impact climate.
When trees are planted or naturally regrow on a previously deforested area, they remove atmospheric carbon dioxide through photosynthesis, fixing it in an accumulating stock of carbon in biomass and soil. As long as these carbon stocks are maintained, this carbon sequestration can provide a valuable contribution to climate change mitigation.
At the same time, increased tree cover typically darkens the land surface, meaning that less sunlight is reflected and more energy is absorbed, causing warming. The reflectiveness of a surface is technically described as its ‘albedo’, with a higher albedo meaning greater reflection. Hasler et al. report that neglecting to factor in reductions in albedo could exaggerate the climate benefits of tree restoration by up to 81% in some locations.
Furthermore, forests release a variety of biogenic volatile organic compounds (BVOCs) that can give rise to methane and ozone, which are also greenhouse gases, and so may further offset the climate benefit from any carbon removed. Deploying Earth System Models to simulate the climate effects of largescale forestation, Weber et al. found that 1/3 of the cooling provided by carbon removals was offset by decreased albedo and these non-CO2 greenhouse gas effects.
The potential for ecological restoration to capture carbon but increase methane emissions is well recognised for peatlands – and is something we are exploring as part of our project on uncertainties in habitat greenhouse gas emissions – but is only rarely noted in relation to other habitats, warranting further attention.
It is not all bad news for Nature-based Solutions contributing to climate change mitigation, however. Another effect of the BVOCs released by trees can be to increase organic aerosols – particles suspended in the air – which can scatter incoming sunlight and provide a cooling effect in the same way that reflective clouds do. Between 50 and 100% of the warming caused by albedo reductions was overcome by this effect, depending on the modelling setup.
Both studies also highlight that despite these additional effects reducing the degree of cooling provided, tree restoration still universally contributes to climate change mitigation. As the carbon stored and degree of albedo offset expected following tree restoration both vary significantly across the world, appropriately targeting the most effective areas can maximise the climatic value of reforestation. Indeed, many models do already take these factors into consideration, such as earlier NbSI work quantifying contribution of nature-based solutions to reducing peak warming, which excluded restoration of areas likely to have major albedo offsets.
The conclusion is not, therefore, that we should stop encouraging Nature-based Solutions for climate change mitigation, but that we do need to recognise and build on the latest science to provide accurate and comprehensive estimates of their value, and factor in emerging details in targeted deployment. The need for a more holistic approach goes beyond just climate mitigation assessment; the true value of Nature-based Solutions is in their ability to contribute to multiple goals, from climate change adaptation, to livelihoods for local communities, and of course, benefiting biodiversity.
Authors: John Lynch and Xiao Zhang
