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Is CO2 fertilisation our get-out-of-jail-free card?

Response to Matt Ridley by Richard Betts

In Monday's Times newspaper, Matt Ridley wrote an article entitled: "Now here’s the good news on global warming: Activists may want to shut down debate, but evidence is growing that high CO2 levels boost crops and nourish the oceans". He cited my tweet from last week that CO2 fertilisation is not news, and was already covered in IPCC reports. I was referring to a report by Indur Gokl...any published by the Global Warming Policy Foundation. However, Ridley neglected to cite my criticisms of that report - I pointed out that Goklany was inconsistent in favourably citing a climate modelling study whilst later criticising climate models, effectively having his cake and eating it.

Beyond the specific criticisms of Goklany's report, I have wider disagreements with Ridley's article. He claims that "the assessments used by policy makers have underestimated the direct benefits of carbon dioxide", despite the fact that he had earlier agreed with me that this was covered in the IPCC reports which are used by policymakers.

Moreover, when it comes to the future impacts of climate change, Ridley seems strangely complacent. He says "surely the climate harms will one day outweigh the growth benefits? Not necessarily" and also "The IPCC’s forecast warming range includes the possibility that we will still be enjoying net benefits by the end of the century". These are very weak statements to make in support of his case that this is all "good news". He's implicitly acknowledging that there are a wide range of possible outcomes for future climate change, some of which are extremely severe and some less so, which is why IPCC reports emphasise risk as opposed to attempting to make firm predictions. However, Ridley is focussing on the "possibility" that the outcomes will be at the very low end, and "not necessarily" harmful. This seems like somewhat shaky grounds for the conclusion that this is "good news".

I have published extensively on the effects of CO2 on vegetation and been at the forefront of including this important process in climate models. I am not alone in this - many of my colleagues have also worked on this topic. To pretend that it is somehow overlooked in mainstream climate science is plain wrong. Assessments of the impacts of climate change on ecosystems and crops do include CO2 effects as well as physical climate effects, and while there are uncertainties in both, current understanding suggests that CO2 effects on photosynthesis will tail off while the impacts of climate change itself continue. The harms may well outweigh the benefits, especially when we remember that sea level rise is an inevitable consequence of a warming world. There are no good reasons to assume that that the effect of CO2 on plants is some sort of "get out of jail free" card.