Recently The Economist caused a stir by featuring
the recent slowdown in global temperatures, illustrated above and described as follows (see also The Telegraph
Temperatures fluctuate over short periods, but this lack of new warming is a surprise. Ed Hawkins, of the University of Reading, in Britain, points out that surface temperatures since 2005 are already at the low end of the range of projections derived from 20 climate models (see chart 1). If they remain flat, they will fall outside the models’ range within a few years.
The mismatch between rising greenhouse-gas emissions and not-rising temperatures is among the biggest puzzles in climate science just now. It does not mean global warming is a delusion. Flat though they are, temperatures in the first decade of the 21st century remain almost 1°C above their level in the first decade of the 20th. But the puzzle does need explaining.
The mismatch might mean that—for some unexplained reason—there has been a temporary lag between more carbon dioxide and higher temperatures in 2000-10. Or it might be that the 1990s, when temperatures were rising fast, was the anomalous period. Or, as an increasing body of research is suggesting, it may be that the climate is responding to higher concentrations of carbon dioxide in ways that had not been properly understood before. This possibility, if true, could have profound significance both for climate science and for environmental and social policy.
The fact that the atmosphere is not warming in line with climate model predictions let The Economist
to state rather provocatively:
If climate scientists were credit-rating agencies, climate sensitivity
would be on negative watch. But it would not yet be downgraded.
The responses have been predictably tribal
. Below I reproduce a blog post that my father and I wrote back in 2006
warning advocates of action not to put too many bets on the short-term evolution of the climate system based on climate model predictions. Like experience with credit-rating agencies, surprises could have been avoided.
Big Time Gambling With Multi-Decadal Global Climate Model Predictions
Roger A. Pielke Sr. and Roger A. Pielke Jr.
8 August 2006
Many advocates for action on climate change, including the IPCC
assessments and recent documentaries have promoted a view that global
warming will continue through the 21st century, with global warming
defined as a steady increase in global average temperatures. This
prediction of warming is based on the output of multi-decadal general
circulation models and is primarily due to the radiative forcing effect
of anthropogenic emissions of CO2. In such models only relatively minor
year-to-year variations in global average temperatures are forecast in
the upward trend, except when major volcanic eruptions cause short-term
(up to a few years) of global cooling. For example, see these projections
of the most recent IPCC — none of the models has an obvious multi-year
(i.e., >2) decrease in global average temperatures over the next
Such predictions represent a huge gamble with public and policymaker
opinion. If more-or-less steady global warming does not occur as
forecast by these models, not only will professional reputations be at
risk, but the need to reduce threats to the wide spectrum of serious and
legitimate environmental concerns (including the human release of
greenhouse gases) will be questioned by some as having been oversold.
For better or worse, a failure to accurately predict the changes in the
global average surface temperature, global average tropospheric
temperature, ocean average heat content change, or Arctic sea ice
coverage would raise questions on the reliance of global climate models
for accurate prediction on multi-decadal time scales. Surprises or
experience that evolve outside the bounds of model output would likely
raise questions even among some of those who have so far accepted the
IPCC reports as a balanced presentation of climate science. (for a
perspective different than the IPCC on applications of climate models
The National Research Council published a report in 2002 entitled “Abrupt Climate Change: Inevitable Surprises”
(of which RP2 was a committee member). The report raised the issues of
surprises in the climate system. One of the surprises (to many) may be
that the global climate models are simply unable to accurately predict
the variability and trends in the climate metrics that have been adopted
to communicate human-caused climate change to policymakers. Among the
climate metrics with the most public visibility are the long term trends
in global average surface temperature, global average tropospheric
temperature, summer arctic sea ice areal coverage, and ocean heat
There is some emerging empirical evidence to suggest, however, that
the concerns expressed here are worth consideration. The recent dramatic
cooling of the average heat content of the upper oceans, and thus a
significant negative radiative imbalance of the climate system for at
least a two year period, that was mentioned in the Climate Science weblog posting of July 27, 2006,
should be a wake-up call to the climate community that the focus on
predictive modeling as the framework to communicate to policymakers on
climate policy has serious issues as to its ability to accurately
predict the behavior of the climate system. No climate model that we are
aware of has anticipated such a significant cooling, nor is able to
reproduce such a significant negative radiative imbalance. Meaningless
distinctions between “projections” and “predictions” will be unlikely to
convince consumers of climate models to overlook experience that does
not jibe with modeled output.
There is no greater danger to support for action on important issues
of human impacts on the environment than an overselling of what climate
science can provide. If the climate behaves in ways that are unexpected
or surprising it will be more than just credibility that is lost.
Advocates for action should think carefully when gambling with the
unknown predictive abilities of climate models. The human influence on
the climate system is real, but the climate may not always cooperate.