by Judith Curry
”The dramatic warming predicted after 2008 has yet to arrive.”
Jeff Tollefson has published a news feature in Nature, entitled Climate change: the forecast for 2018 is cloudy with record heat. Excerpts:
In August 2007, Doug Smith took the biggest gamble of his career. After more than ten years of work with fellow modellers at the Met Office’s Hadley Centre in Exeter, UK, Smith published a detailed prediction of how the climate would change over the better part of a decade. His team forecasted that global warming would stall briefly and then pick up speed, sending the planet into record-breaking territory within a few years.
The Hadley prediction has not fared particularly well. Six years on, global temperatures have yet to shoot up as it projected. Despite this underwhelming result, such near-term forecasts have caught on among many climate modellers, who are now trying to predict how global conditions will evolve over the next several years and beyond. Eventually, they hope to offer forecasts that will enable humanity to prepare for the decade ahead just as meteorologists help people to choose their clothes each morning.
These near-term forecasts stand in sharp contrast to the generic projections that climate modellers typically produce, which look many decades ahead and don’t represent the actual climate at any given time. “This is very new to climate science,” says Francisco Doblas-Reyes, a modeller at the Catalan Institute of Climate Sciences in Barcelona, Spain, and a lead author of a chapter that covers climate prediction for a forthcoming report by the Intergovernmental Panel on Climate Change (IPCC). “We’re developing an additional tool that can tell us a lot more about the near-term future.”
In preparation for the IPCC report, the first part of which is due out in September, some 16 teams ran an intensive series of decadal forecasting experiments with climate models. Over the past two years, a number of papers based on these exercises have been published, and they generally predict less warming than standard models over the near term. For these researchers, decadal forecasting has come of age. But many prominent scientists question both the results and the utility of what is, by all accounts, an expensive and time-consuming exercise.
To make its climate prediction, Smith’s team used its standard climate model, but broke the mould by borrowing ideas from the way meteorologists forecast the weekly weather. Typical climate projections start some way back in the past, often well before the industrial era, in a bid to capture the average climate well enough to forecast broad patterns over the long term. Weekly weather forecasts, however, begin with the present.
“It’s fair to say that the real world warmed even less than our forecast suggested,” Smith says. “We don’t really understand at the moment why that is.”
The answer may lie in the oceans. Although the atmosphere largely controls day-to-day weather, the slow-moving oceans hold so much more energy and heat that they dominate how the climate changes from year to year. Researchers suspect that much of this variability is tied to widespread cycles, such as the El Niño warming and La Niña cooling system in the eastern tropical Pacific. In theory, the fact that salt water circulates more slowly than air should also make the oceans a little easier to model.
Despite their faults, such efforts helped spark a wave of research among modellers who are hungry for ways to test and improve their calculations. The global climate-modelling groups that took part in the IPCC’s experiments invested a substantial portion of their modelling time to produce the first systematic predictions of how the global climate will evolve in the coming years. These models predict cooler temperatures: on average 15% less warming over the next few decades compared with standard climate projections.
To determine whether these projections are likely to hold, the groups ran the usual test of seeing how well their models performed when hindcasting, or predicting the past. The teams plugged in all of the observational data and ran decadal climate predictions at least every five years beginning in 1960, comparing the resulting hindcasts to the actual climate as well as standard climate models. In one such analysis4, Doblas-Reyes and his colleagues say that their model anticipated the slowdown in global warming up to five years in advance. Their paper also bolstered the theory that the deep oceans, notably the Atlantic and tropical Pacific, had stalled atmospheric warming by absorbing much of the heat being trapped by rising concentrations of greenhouse-gas concentrations in the air.
It is one of the biggest mysteries in climate science: humans are pumping more greenhouse gases into the atmosphere today than ever before, yet global temperatures have not risen much in more than a decade. That trend does not undermine the idea that greenhouse gases will eventually push global temperatures into uncharted territory, but it does have scientists puzzled.
One partial explanation is natural variation: temperatures are expected to plateau occasionally even during a warming climate. And the world remains a very warm place. The ten hottest years on record have all occurred since 1998.
Yet with the stalled warming now approaching its 15th year, researchers are seeking some deeper explanation. “The heat must be going somewhere,” says Ed Hawkins, a climate scientist at the University of Reading, UK. “The question is where.”
But scientists cannot yet fully explain the recent trends, and the larger question is whether the lack of warming today portends less warming in the future.
Michael Ring and his colleagues at the University of Illinois at Urbana-Champaign argue that Earth might in fact be less sensitive to greenhouse gases than previously believed. Whereas the Intergovernmental Panel on Climate Change estimates that doubling atmospheric carbon dioxide levels would ultimately increase global temperatures by 2–4.5 °C, with a best estimate of 3 °C, the Illinois group says that the rise is more likely to be between 1.5 °C and 2 °C.
Other researchers argue the opposite, and the issue remains unsettled. Besides, the continuing climb in global emissions means that a lower climate sensitivity would cause only a slight delay in global warming, says Alexander Otto, a climate policy researcher at the University of Oxford, UK. “The impacts we were expecting in 2050 would happen a decade later,” he says. “There is certainly no reason for complacency.”
JC comments: You may recall a previous Climate Etc. post CMIP5 decadal hindcasts, which describes this modeling effort and provided the first multi-model verification analysis. The initialized decadal simulations starkly point out the deficiencies that climate models have in simulating natural internal variability.
While I think this focus on decadal simulations is very important for illuminating the deficiencies of climate models and developing insights for their improvement, I suspect that it will be a long time before climate models can do a better job on decadal timescales than statistical models, based either on persistence or climate dynamics.
Finally, I would like to take issue with this statement by Alexander Otto:
Besides, the continuing climb in global emissions means that a lower climate sensitivity would cause only a slight delay in global warming, says Alexander Otto, a climate policy researcher at the University of Oxford, UK. “The impacts we were expecting in 2050 would happen a decade later,” he says. “There is certainly no reason for complacency.”
So, cutting the climate sensitivity in half means the impacts we are expecting in 2050 would happen a decade later? Huh?