We are not lacking hypothesis about the recent hiatus - or stagnation- in the global mean temperature: the ocean is taking up more heat, stratospheric water vapour has decreased, the sun has recently weakened and volcanic activity has also gathered a quicker clip. A preliminary look at the structure of the stagnation may, or may not, offer some clues about how likely which of these hypothesis, or which combination, may end up being the correct one.
The global mean temperatures observed in the last 15 years are indeed at the very lower edge of the ensemble of trends simulated by climate models. This is the main message of our manuscript and the paper by Fyfe et al.. It has been argued that the starting year in the 15-year period used to compute the trends is misleading because 1998 experienced y very strong El Niño, but by now almost everyone recognizes that, whereas in reality the causes of the hiatus may still very well compatible with the effect of greenhouse gas forcing, current climate models have serious difficulties in simulating such low temperature trends as observed in this period when they are forced by the present external forcings and reasonable extrapolations of this forcing into the future. One important caveat here is that climate projections into the future assume constant solar activity and no volcanic activity, which arguably is not totally realistic.
One intriguing aspect of the stagnation is that it has not been equally distribution across all 12 months of the annual cycle. It has mainly occurred during the months of December, January and February, with the trends in June, July and August over the last 15 years being more similar to the corresponding trends in the period 1980 to 1997.
This means that temperature trends have decelerated much more during the boreal winter months. Actually the trend over the last 15 years in these months is remarkably negative, as can be seen in the following diagrams:
Form the CMIP5 ensemble of climate models that is being used in the 5th IPCC Assessment Report we can derive an ensemble of 15-year periods chosen from the simulations driven by the RCP4.5 scenario from 2005 until 2060 (from a total of 109 simulations, 6104 overlapping 15-year segments) Only 2 per thousand of these 6104 15-year trends are lower than the trends observed during December-to-February in 1998-2012 (HadCRUT4). This occurs in 5% of the trends of the boreal summer season (June-to-August), and in 2 % of the annual means.
What is the spatial 'fingerprint' of the stagnation, or in other words, in which regions are the recent temperature trends more strongly subdued - or even become negative- and in which regions the trends continue unabated ? The following figure tries to give a visual impression of this spatial structure for the December-to-February case. For each grid cell in the HadCRUT4 data set we have computed the linear trend in two periods: 1998-2012 and 1980-1997, and then taken the difference. Negative values indicate the grid-cells where temperature trends have recently slowed ; positive values, where they have recently increased relative to the 'base period' 1980-1997.
This has been also found by Cohen et al. applying slightly different methods. It is obvious that, although trends have recently become smaller in general, the region and the season bearing the brunt of the temperature stagnation is Eurasia in wintertime. We will later call this pattern the DJF stagnation fingerprint. At first sight this does not seem to dovetail with an increased heat uptake by the ocean nor with a driving role of the Tropical Pacific, as suggested by the recently published paper by Kosaka et al. . It looks to us rather like the effect of the North Atlantic Oscillation on surface temperatures with some additional global contribution. The NAO has shown up in its negative phase in the recent Northern Hemisphere winters, favouring a more meridional circulation and causing polar air intrusions in Eurasia. It is, however, not totally clear how an atmospheric circulation mode, which in principle just shuffles air masses around, can influence global mean temperatures. Wallace et al suggested long ago that the NAO would modify the heat fluxes to the ocean by advecting cold/warm air masses from the continents to the ocean surface. The NAO might also modulate cloud cover that, in turn, could modify the global radiation balance, but this is not widely accepted and, to our knowledge, these lines of research has not be followed up.
Even if the temperature stagnation is indeed related to the NAO, the main question mark in this puzzle still remains : is the stagnation due to internal stochastic variations or to the external forcings ? The NAO is known to be an internal model of climate variability, but it is clearly influenced by the external forcing as well. Future climate projections participating in the CMIP3 project showed that the NAO would tend to shift to a more positive model under greenhouse has warming. Shindell et al. found in climate simulations conducted with the GISS model a few years ago that the NAO could also be affected by solar forcing - and this is the interesting link I would like to put forward here - provided that the climate model includes a well-resolved stratosphere with the corresponding ozone-related chemistry. They found that low solar irradiance would nudge the NAO toward a negative state, and this could explain the extreme European cold temperatures during the Late Maunder Minimum. Is this happening now again, albeit in a lesser magnitude ? If it is, then a weaker sun would explain a global tendency to weaker temperature trends, with a stronger slow down over Eurasia. The following figure shows two time series: one describes the strength with which DJF stagnation fingerprint appears in each boreal winter, denoted here stagnation index (in the winters in which the index is strongly positive the spatial fingerprint of the stagnation is particularly strongly represented in the data of that boreal winter); the second time series displays the Total Solar Irradiance. The correlation between both is just suggestive of a weak link, but it is clearly not a definitive proof. Clearly, this fingerprint is expressing itself very strongly in the last few years, but without a corresponding drop in solar activity. Something else is happening that is not captured by this quite preliminary explanation.
Finally, another question mark. Over the past millennium, solar activity and volcanic activity appear statistically anti-correlated. In fact, this is one of the difficulties to disentangle the effect of each of these forcings on surface temperatures in the last centuries. And, intriguingly enough, we see the same phenomenon again in the last 15 years: a weaker sun and more volcanic aerosols. Does anyone dare an explanation ?
UPDATE to comment #7
UPDATE to comment #7