Heartland Institute report’s claim that climate models are too sensitive to CO2 does not reflect evidence

CLAIM
"[climate models] systematically over-estimate the sensitivity of climate to carbon dioxide ... and modelers exclude forcings and feedbacks that run counter to their mission"
DETAILS
Unsupported : There is no evidence that climate models systematically overestimate the climate’s sensitivity to CO2. Rather, independent lines of research produces sensitivity estimates consistent with models.
Misrepresents the scientific process : Through research, scientists improve their understanding of the climate system, and this understanding is used to refine climate models as useful scientific tools. Science does not have a “mission” to reach a predetermined conclusion.
KEY TAKE AWAY
Climate models are based on physical processes and our understanding of how the climate system works. Their sensitivity to CO2 is in line with estimates based on modern observations and records of past climate changes.

SCIENTISTS' REVIEW

CLAIM: "GCMs systematically over-estimate the sensitivity of climate to carbon dioxide (CO2), many known forcings and feedbacks are poorly modeled, and modelers exclude forcings and feedbacks that run counter to their mission to find a human influence on climate."

Reto Knutti, Professor, ETH Zürich:
The statement that climate models overestimate the warming in response to CO2 is incorrect; it is based on either too short time periods that are dominated by natural variability, by the comparison of models with datasets that do not have global coverage, by comparing to models that were run many years ago with emissions and forcings that differed from what actually happened, by the use of oversimplified energy balance models1, or a combination of it. Recent studies have shown that once the changes in climate feedbacks over time2, datasets with full coverage are considered3 and all forcings are considered, the agreement between predicted and observed warming is excellent, even over the recent hiatus period4.

It is remarkable that even projections made decades ago with climate models that were much simpler (and were running on computers that were likely slower than a mobile phone today) were quite accurate5,6,7.

Katrin Meissner, Associate Professor, University of New South Wales:
False. Climate Sensitivity has been assessed by the community based on recent observations and proxy data from past climates. Climate models fall within this range of sensitivity. Some recent publications point to an increase in sensitivity with warmer temperatures*.

Patrick Brown, Postdoctoral Research Scientist, Carnegie Institution for Science:
This argument reached a peak in popularity around 2012/2013 when the “hiatus” was still ongoing (i.e. when the divergence between observed and modeled global temperature was at its largest). Even then, however, it was shown that you cannot conclude much about sensitivity to CO2 from such short-term fluctuations1. Similarly, Brown et al. (2015)2 showed that decade-long periods without warming are to be expected and that there was/is a 70% chance of seeing at least one 11-year period with no warming between the years of 1993-2050 under a “middle of the road” emissions scenario.

Since then, observed warming has surged and, as of 2016, observations are warmer than the average prediction from climate models (see figures below).

Figure – Modeled global surface temperature (RCP 4.5 emissions scenario) compared to observed temperature (NASA GISS). Source

Figure – Updated version of IPCC AR5 Figure 11.25a, showing observations and the CMIP5 model projections relative to 1986-2005. The black lines represent observational datasets (HadCRUT4.5, Cowtan & Way, NASA GISTEMP, NOAA GlobalTemp, BEST). Source

 

Published on: 23 May 2017 | Editor:

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