Reader Comments

Post a new comment on this article

Double-counting of climate impacts

Posted by RichardBetts on 11 Jun 2015 at 15:27 GMT

One of the other very odd things about this paper is how the authors have decided that it is legitimate to "adjust" the net primary productivity (NPP) projections from the CMIP5 models on the basis of the so-called "unsuitable plant growth days" which have been diagnosed from the CMIP5 climate. Irrespective of Colin Prentice's and Trevor Keenan's concerns (which I share) about the validity of these "unsuitable days", this seems like double-counting of climate change impacts.

The CMIP5 NPP simulations *already* take account of meteorological factors, often on a much finer timescale than Mora et al, and certainly in a way which is process-based as opposed to correlations. The HadGEM2-ES model, for example, uses meteorological and hydrological quantities on time steps of less than one hour to drive the NPP calculations. This means that the model is already not allowing plants to grow when it's too hot, dry etc. This is already factored in to the CMIP5 NPP projections. For these projections to then be adjusted further on the basis of a second (and less sophisticated) interpretation of the meteorology is giving undue weight to the unfavourable conditions.

I also find it strange that the authors pick and choose which components of the CMIP5 models they will use, without realising that all the components interact. They are somewhat dismissive of CO2 effects on plants when it comes to their effects on growth, but overlook the fact that these effects also contribute to the climate change itself through the surface energy and moisture budget. Warming over land is projected to be larger because of CO2 effects on vegetation, and this also affects evaporation and precipitation. By discarding the CO2 effects in one part of the system but not the other, the authors are introducing an inconsistency.

No competing interests declared.

RE: Double-counting of climate impacts

bondlamberty replied to RichardBetts on 11 Jun 2015 at 19:14 GMT

I agree with this comment. Mora et al. dismiss CMIP5 NPP ("likely reflects an overemphasis of CO2 fertilization in modeling NPP while failing to account for the limiting roles of other climatic variables"), as it disagrees with their projections. But the CMIP5 NPP outputs are, at least, fully consistent with the models' internal climatic changes. They're also the products of algorithms that are–however imperfect–more sophisticated than those behind the MODIS NPP product, which has been shown to have significant flaws. Put another way: if you're going to accept the validity of CMIP5 climate outputs, you can't simply dismiss their carbon flux outputs.

It's much more likely, I think, that the authors are underestimating the plasticity and resilience of plants and thus terrestrial NPP. See Trevor Keenan's comment.

No competing interests declared.