The extraordinary climate effect of land-use change
Dec 15, 2015
Bishop Hill in Climate: Models, Climate: sensitivity

The latest attempt to conjure up a hypothesis as to why things are going to get much worse on the climate front has appeared in Nature Climate Change, with an author team including Gavin Schmidt and Kate Marvel.

The paper delves deeper into the idea of forcing efficacy, a concept that is explained as follows:

The concept of radiative forcing is used to compare the effects of different physical drivers on the Earth's energy budget. Two forcing agents that produce a similar radiative imbalance might be expected to initiate similar feedbacks and have the same global mean temperature response. However, there can be variations in the size and type of feedbacks engendered by a specific forcing, mainly due to geographical variations in the forcing magnitude. These variations can be characterized by an efficacy that scales for the differences in temperature response. Forcings that project more strongly on the Northern Hemisphere, land or polar regions are systematically more effective at changing temperatures than an equivalent amount of CO2, whose forcing is more uniformly distributed throughout the globe. The converse is true for forcings localized in the Southern Hemisphere or ocean regions.

The authors run a series of climate model simulations, tweaking just one climate forcing at a time, and using the slopes of the resulting graphs of forcing change versus temperature change to estimate the efficacies for each forcing. They then plug the answers into an Otto-type estimate of climate sensitivity from observations. Their conclusions are of course scary, with TCR of 1.9°C and ECS of 3.0°C (compared to 1.3 and 1.9 respectively, for Otto et al).

Whether you find this persuasive or not depends of course on how convinced you are that the GISS climate model the authors used is a fair reflection of the real-world climate. With that in mind it's amusing to consider their estimates of the effect of land-use change on the climate.

If you look at the plot of forcing versus temperature - I show the the TCR one, but its the same story with the ECS one - you can see that all the lines pass through the origin. This makes sense: no change in forcing gives no change in temperature. But in fact there is an exception. Look closely at the the brown dots, the data points for land-use change. Although the authors haven't plotted the line - the points are very close together - you can see that if they did it wouldn't pass through zero. In other words a zero change of forcing is apparently able to produce a non-zero change in temperature.


My point here is not that this will make a difference to their results - the effect of land-use change in their simulations is small anyway. But it does highlight the constant problem with computer simulations, namely that you can kid yourself that they are a reflection of reality, when in fact they are entirely unphysical.

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