Planetary energy imbalance?

For their part, the model simulations that have been run for the IPCC AR4 have tried to simulate the climate of the last hundred or so years using only known and quantifiable forcings. Some of these forcings are well known and understood (such as the well-mixed greenhouse gases, or recent volcanic effects), while others have an uncertain magnitude (solar), and/or uncertain distributions in space and time (aerosols, tropospheric ozone etc.), or uncertain physics (land use change, aerosol indirect effects etc.). Given these uncertainties, modellers nevertheless make their best estimates consistent with observations of the actual forcing agents. The test for the modellers is whether they reproduce many of the elements of climate change over that period. Some tests are relatively easy to pass – for instance, we have discussed the model skill in response to the Mt. Pinatubo eruption in a number of threads.

The overall global surface temperature is also well modelled in this and other studies. While impressive, this may be due to an error in the forcings combined with compensating errors in the climate sensitivity (2.7 C for a doubling of CO2 in this model) or the mixing of heat into the deep ocean. Looking at the surface temperature and the ocean heat content changes together though allows us to pin down the total unrealised forcing (the net radiation imbalance) and demonstrate that the models are consistent with both the surface and ocean changes. It is still however conceivable that a different combination of the aerosol forcing (in particular (no pun intended!)) and climate sensitivity may give the same result, underlining the continuing need to improve the independent estimates of the forcings.

So how well does the model do? The figure shows the increase in heat content for 5 different simulations in the ensemble (same climate forcings, but with different weather) matched up against the observations. All lines show approximately the same trend, and the variability between the ensemble runs being greater than the difference with the observations (i.e. this is as good a match as could be expected). The interannual variability, predominantly related to ENSO processes, is different but that too is to be expected given the mainly chaotic nature of tropical Pacific variability, the short time period and the models’ known inadequacy in ENSO modelling. The slope of these lines is then related to the net heat imbalance of around 0.60+/-0.10W/m2 over 1993-2003, and which the models now suggest has grown to around 0.85+/-0.15 W/m2. The distribution of heat in the ocean in the different runs is quite large (figure 3 in the article) but clearly spans the variations in the observations, which is of course just one realisition of the actual climate.

What does this imply? Firstly, as surface temperatures and the ocean heat content are rising together, it almost certainly rules out intrinsic variability of the climate system as a major cause for the recent warming (since internal climate changes (ENSO, thermohaline variability, etc.) are related to transfers of heat around the system, atmospheric warming would only occur with energy from somewhere else (i.e. the ocean) which would then need to be cooling).

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