What is a first-order climate forcing?

The figure here gives one estimate for how many of those forcings have changed over the industrial period (1750-2000). This assessment is from my own lab and so I may be a little biased, but although there are significant uncertainties (particular for the aerosol indirect effects), it probably gives a reasonable idea of the current thinking. The forcings illustrated here are from the well mixed greenhouse gases (GHGs) (CO2, CH4, N2O, CFCs), tropospheric and stratospheric O3, direct aerosol effects (from sulphates, nitrates, organic and black carbon), land use change, solar irradiance, volcanic aerosols, and various indirect effects (on clouds, stratospheric water vapour, snow albedo etc.). Reasonable estimates of these 16 different effects (and counting…) were included in the GISS simulations for the upcoming IPCC assessment.

The land use change used in the figure is related to the deforestation dataset of Ramankutty and Foley (1999) and includes the effects of albedo and vegetation type change, but not the impacts of increased irrigation or the ‘greening’ of the high latitudes (due to climate changes and possible CO2 fertilisation effects) . These latter two effects are expected to lead to slight warming, but the overall impact of land use changes is expected to be negative (i.e. a cooling) (Myhre and Myhre, 2003), although the uncertainty is still significant (maybe 0.5 W/m2 either way).

To my mind, the ‘first-order’ forcings would be the ones without which you can’t really do without in assessing global climate change. I would therefore argue that for the global mean the well-mixed GHGs and the counterbalancing reflecitve aerosol effects are ‘first-order’ – without GHGs there is no appreciable warming signal, and without the aerosols, the warming from GHGs is excessive and important changes in the diurnal cycle and cloudiness are not captured. Everything else (apart from volcanos, which are a special case) is in the noise. If we were to break it down even further, I would argue that CO2, CH4 and sulphates (the main non-soot aerosol) were the only ‘first order’ forcings. It is curious to note that this is the combination of forcings that were predominantly used in the simulations discussed in IPCC (1995) where the conclusion was made that the ‘balance of evidence’ supported the notion of ongoing human-caused climate change.

Before the emails come streaming in, let me make it clear that this isn’t to say that ‘second order’ forcings are unimportant. On the contrary, many of these effects have very specific signatures in the climate system (in the stratosphere, in the Arctic and in the tropics) that need to be understood much better – however they are unlikely to have a big impact on the global mean temperature. Thus when it comes to global warming, neither land use change/vegetation type, nor for instance, the biogeochemical effect of increased CO2 are ‘first order’. The first example is clearly important locally (impacts of deforestation, urban development etc.) while the second effect is as yet inadequately unquantified but there doesn’t appear to be any a priori reason to think it is globally important. It doesn’t therefore make much sense to claim that some of the smaller forcings are ‘first-order’ despite their importance, and conceivably dominance, at smaller scales.

To be sure, some of these effects (such as the impact of irrigation on surface water vapour, or land use changes on evapotranspiration) are not easily dealt with in terms of the tropospheric radiative forcing – a point that was well made in the National Academies report on radiative forcing (on which Dr. Pielke was an author). However, the dominance of well-mixed greenhouse gases on the anthropogenic forcing over the last few decades is robust to almost any estimate of the uncertainty in the other forcings. This is clearly a different opinion to that held by Dr. Pielke. However, this is probably due to our different perspectives in what we feel are important questions (local vs. global), rather than a disagreement over fundamentals.


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