I thought I might briefly highlight a recent paper that tries to understand what some call “the pause”, but which isn’t really a “pause” as nothing has actually stopped. The paper is Natural variability, radiative forcing and climate response in the recent hiatus reconciled, by Marcus Huber and Reto Knutti.
What they’ve done is to run an unforced control run of a CMIP5 climate model, from which they extract a segment that matches the observed NINO3.4 index for the time period of interest. They then run a suite of forced climate models, but with updated stratospheric aerosol and solar forcings, and then adjust these using the portion of the unforced model which matches the NINO3.4 index, resulting in the ENSO effect producing a cooling of 0.06oC and the adjustments to the forcings producing a cooling of 0.07oC. The paper then compares the model results with the HadCRUT4 temperature dataset and with the Cowtan & Way (2013) correction to the HadCRUT4 dataset (which includes satellite data for the Arctic to compensate for coverage bias). The basic result is shown in the figure below, and illustrates that with all the corrections, the best fit is to a model with a Transient Climate Response (TCR) of 1.8oC, and an Equilibrium Climate Sensitivity (ECS) of 3oC.
Comparison of model runs and temperature datasets from Huber & Knutti (2014).
So, basically, this result suggests that despite a “pause”/hiatus/slowdown in surface warming we will still likely have warmed by 1.8oC (relative to pre-industrial times) when we’ve doubled CO2 and – if CO2 concentrations remain at that level – we’ll eventually warm by 3oC. Not really all that surprising really.
What can often happen when such papers are published is that some criticise scientists for focusing so much on anthropogenic effects, and ignoring other non-anthropogenic effects. Well, in a sense this paper didn’t. It ran a suite of unforced models to try and extract a segment with NINO3.4 indices similar to the period of interest. Unforced means that there are no external forcings, so the only factors included were internal variability. Did these runs show long-term warming? No, because if there is no change in external forcing, then the equilibrium temperature is essentially fixed. Internal variability can cause temperatures to rise above, and fall below, equilibrium, but it can’t cause long-term warming because temperatures will quickly return to equilibrium. If internal variability could produce a change in forcing, then it could cause long-term warming, but we’re unaware of any internal process that can actually do this.
The paper also considered updates to stratospheric aerosol (volcanoes) and solar forcing, so it did consider other natural effects. Essentially, all of the work that tries to understand the “hiatus” is leading to the same basic conclusion : it is simply a consequence of internal variability that can lead to periods when we warm faster than average, and others when we warm slower, but doesn’t really change what we should expect to happen in the medium- to long-term as a consequence of anthropogenic emissions. Maybe those who keep arguing that there is some kind of natural effect that could both explain this and result in significantly lower climate sensitivities, could actually explain what this natural effect is. Simply proposing something without actually explaining what it is or how it works, is just a little silly. It’s possible that those who seem to think that they might be consensus busting geniuses should really start considering that they’re not.