New paper finds climate models cannot explain the global warming stagnation over past 15 years

A new paper by prominent German climatologists Dr. Hans von Storch and Dr. Eduardo Zorita, et al, finds "that the continued [global] warming stagnation over fifteen years, from 1998 -2012, is no longer consistent with model projections even at the 2% confidence level." In other words, there is a greater than 98% probability that climate models are unable to explain the stagnation in warming over the past 15+ years. The authors suggest 3 possible explanations for this:

1. the models underestimate natural climate variability

2. the climate models fail to include important forcings such as ocean oscillations and solar amplification

3. the models assume exaggerated climate sensitivity to man-made CO2

The authors point out that even if climate sensitivity to CO2 was greatly reduced future models, it is still "hardly feasible" that the models would reproduce the 15 year stagnation of temperature, stating, "a recalibration [with lower CO2 sensitivity] reproducing the reduced warming of the last 15 years appears hardly feasible." All of which suggests that CO2 is not the control knob of climate and natural variability is. 

Can climate models explain the recent stagnation in global warming?

Hans von Storch (1) , Armineh Barkhordarian (1) , Klaus Hasselmann (2) and Eduardo Zorita (1)

(1) Institute for Coastal Research, Helmholtz-Zentrum Geesthacht, Geesthacht, Germany

(2) Max-Planck-Institute for Meteorology, Hamburg, Germany

In recent years, the increase in near-surface global annual mean temperatures has emerged as considerably smaller than many had expected. We investigate whether this can be explained by contemporary climate change scenarios. In contrast to earlier analyses for a ten-year period that indicated consistency between models and observations at the 5% confidence level, we find that the continued warming stagnation over fifteen years, from 1998 -2012, is no longer consistent with model projections even at the 2% confidence level. Of the possible causes of the inconsistency, the underestimation of internal natural climate variability on decadal time scales is a plausible candidate, but the influence of unaccounted external forcing factors or an overestimation of the model sensitivity to elevated greenhouse gas concentrations cannot be ruled out. The first cause would have little impact of the expectations of longer term anthropogenic climate change, but the second and particularly the third would.

Estimates of the observed global warming for the recent 15-year period 1998-2012 vary between 0.00370C/year NCDC)(1), 0.00410C/year (HadCRUT4)(2) and 0.0080C/year (GISS)(3). These values are significantly lower than the average warming of 0.020C/year observed in the previous thirty years 1970-2000(4). Can models explain the global warming stagnation?

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What do these inconsistencies imply for the utility of climate projections of anthropogenic climate
change? Three possible explanations of the inconsistencies can be suggested: 1) the models underestimate the internal natural climate variability; 2) the climate models fail to include important external forcing processes in addition to anthropogenic forcing, or 3) the climate model sensitivities to external anthropogenic forcing is too high,.

The first explanation is simple and plausible. Natural climate variability is an inevitable consequence of a slow system (climate) interacting with a fast system (weather) (10) . The forcing of the slow system by the (white noise) low-frequency components of the fast system produces a “Brownian motion” of the slow system, represented by a red variance spectrum - in qualitative agreement with observations.However, the details of the response depend strongly on the internal dynamics of the slow system in the time scale range of interest - in the present case, on decadal time scales. It is long known, from successive reports of the Intergovernmental Panel on Climate Change (4), that contemporary global climate models have only limited success in simulating many such processes, ranging from the variability of the ocean circulation, ENSO events, various coupled ocean-atmosphere oscillation regimes, to changes in sea ice, land surface, atmospheric chemistry and the biosphere. The inability to simulate the statistical internal climate variability may have been artificially compensated in the past by tuning the models to prescribed external forcings, such as volcanic eruptions and tropospheric aerosols.This would explain why simulations with historical forcing by different GCMs tend to be very similar and follow closely the observed record. This artificial "inflation”  (11) of forced variability at the expense of unpredictable natural variability works, however, only in the period of tuning, and no longer in the post-tuning phase since about 2000. The net effect of such a procedure is an underestimation of natural 3 variability and an overestimation of the response to forced variability. .  

Nevertheless, the second explanation cannot be ruled out: in the spirit of traditional model tuning, the recent stagnation in global warming could be assigned to an external forcing that is not included, or not included satisfactorily, in contemporary models. Volcanic eruptions and variations in solar insolation are frequently proposed candidates. However, while both explanations have supporters, a significant increase in recent volcanic activity has not been recorded, while variations in solar insolation or activity still require rather speculative amplification mechanisms that could contribute to the observed recent decrease in global warming (12,13)

Finally, the model overestimation of the global warming in the period 1998-2012 could be partially corrected by a reduction in the assumed model sensitivity to radiative forcing. In principle, climate model sensitivities are calibrated by fitting the climate response to the known seasonal and latitudinal variations in solar forcing, as well as by the observed climate change to increased anthropogenic forcing over a longer period, mostly during the 20th century. It would be difficult to modify the model calibration significantly to reproduce the recent global warming slow down while still satisfying these other major constraints. While adjusting the effect of aerosols may help to reconcile differences between observed and simulated long term trends 14 , and a recent study (15) argues that the true sensitivity may indeed lie at the lower range of the of the contemporary climate models, a recalibration reproducing the reduced warming of the last 15 years appears hardly feasible. Whether or not a later calibration of the CMIP5-models was undertaken is not known, but the CMIP3 models were run before the recent stagnation emerged.

We do not wish to suggest which of the three possible explanations is the most probable, leaving this for others to decide. Quite possibly, all three factors contribute to some extent. But we hope that the need to understand the origin of the recent stagnation in global warming will accelerate efforts to achieve a more reliable simulation of climate variability on decadal time scales, and the ability to disentangle the relative contributions of forced (deterministic) and internal [natural] (stochastic) variability.