It is well known that the past decade or so has seen less global warming than might have been expected – but what is the cause? This is more of a discussion post, rather than any new analysis.
The most recent decade has seen observed global temperatures at the lower limit of the model projections. There seem to be 3 possibilities for this relative slowdown in the rate of warming:
1) Internal climate variability
2) The assumed radiative forcings are wrong
3) The climate simulators used are too sensitive to greenhouse gases
I think there is evidence that all 3 possibilities are playing some role. Firstly, climate simulators show a range of internal variability behaviours, and a decade with no global warming (or even a cooling) is not implausible – various analyses indicate that around 5% of decades should exhibit a cooling trend globally, perhaps because the warming is in the deeper ocean. In fact, we might have expected a cooling decade sometime in the next few decades anyway. If internal variability is the sole cause, then we might expect a more rapid warming over the next decade.
However, another option is that the radiative forcings used in the climate simulations are somehow incorrect. The most obvious culprit would be the emission of aerosol precursors, which help cool the planet. The scenarios used by the IPCC optimistically tend to show a rapid reduction in aerosol emissions from 2005 onwards, which may not have happened. So, perhaps some of the relative lack of warming is due to the fact that we are not using the observed forcings, but projected forcings after 2005? In addition, even if the projections have the correct forcings, the models may be too sensitive to aerosol reductions, and so the projections produce a more rapid warming than that observed. Other candidates for producing incorrect forcings are stratospheric effects or volcanic aerosols.
The final possibility is that the higher climate sensitivity models are too sensitive to greenhouse gases. Recent analyses in the ‘Detection & Attribution’ framework have suggested this, along with initial work on examining hindcasts from decadal predictions.
What would help answer this? More time to see what happens, of course. A better understanding of recent aerosol trends, and whether the models are responding correctly would be very beneficial. Also, more regular updates to observed emissions and radiative forcings to allow more concrete detection and attribution of trends.
My suspicion is that all 3 possibilties are playing some role. The next few years will be very interesting indeed!