Can Climate Models Explain the 15-year Slowdown in Warming?

“Can climate models explain the recent stagnation in global warming?” That is the title of a new discussion paper by Hans von Storch, Professor at the Meteorological Institute of the University of Hamburg.

Storch stated the problem his paper explores in a recent (June 20, 2013) interview with Der Spiegel:

SPIEGEL: Just since the turn of the millennium, humanity has emitted another 400 billion metric tons of CO2 into the atmosphere, yet temperatures haven’t risen in nearly 15 years. What can explain this?

STORCH: So far, no one has been able to provide a compelling answer to why climate change seems to be taking a break. We’re facing a puzzle. Recent CO2 emissions have actually risen even more steeply than we feared. As a result, according to most climate models, we should have seen temperatures rise by around 0.25 degrees Celsius (0.45 degrees Fahrenheit) over the past 10 years. That hasn’t happened. In fact, the increase over the last 15 years was just 0.06 degrees Celsius (0.11 degrees Fahrenheit) — a value very close to zero. This is a serious scientific problem that the Intergovernmental Panel on Climate Change (IPCC) will have to confront when it presents its next Assessment Report late next year.

The abstract of Storch’s new paper outlines three possible causes of the divergence between observations and model projections of near-surface global annual mean temperatures:

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.

Whatever the explanation, the divergence has gone on too long for climate scientists to deny, dismiss, or discount. Referring to the large ensemble model projections used to inform the 2007 (AR4) and forthcoming (AR5) UN IPCC climate change assessment reports, Storch writes:

For 10-year trend segments, 6% (CMIP3) or 8% (CMIP5) of the simulated trends are smaller than or equal to the observed trend over the period 1998-2012 — in agreement with a previous positive consistency test for the period 1998-2009. However, for the 15-year trend interval corresponding to the latest observation period 1998-2012, only 2% of the 62 CMIP5 and less than 1% of the 189 CMIP3 trend computations are as low as or lower than the observed trend. Applying the standard 5% statistical critical value, we conclude that the model projections are inconsistent with the recent observed global warming over the period 1998-2012.

Storch puts this finding in layman terms in his Spiegel interview:

SPIEGEL: Do the computer models with which physicists simulate the future climate ever show the sort of long standstill in temperature change that we’re observing right now?

STORCH: Yes, but only extremely rarely. At my institute, we analyzed how often such a 15-year stagnation in global warming occurred in the simulations. The answer was: in under 2 percent of all the times we ran the simulation. In other words, over 98 percent of forecasts show CO2 emissions as high as we have had in recent years leading to more of a temperature increase.

Storch’s paper suggests (although does not explicitly state) that if the warming hiatus persists another five years, the models should be considered invalid:

The inconsistency increases rapidly with increasing trend length. A continuation of the current observed global warming rate for a period of twenty years or longer would lie outside the ensemble of all model-simulated trends.