It’s known as “single study syndrome”. When a new scientific paper is published suggesting that the climate is relatively insensitive to the increased greenhouse effect, potentially modestly downgrading the associated climate change threats, that sort of paper will generally receive disproportionate media attention. Because of that media attention, people will tend to remember the results of that single paper, and neglect the many recent studies that have arrived at very different conclusions.

Clouds Point to a Sensitive Climate



For example, there have been several recent studies finding that the global climate models that most accurately simulate observed changes in clouds and humidity over the past 10–15 years also happen to be the ones that are the most sensitive to the increased greenhouse effect. For example, a 2012 paper by Kevin Trenberth and John Fasullo concluded,

These results suggest a systematic deficiency in the drying effect of either subsident circulations or spurious mixing of moister air into the region in low-sensitivity models that directly relate to their projected changes in cloud amount and albedo … the results strongly suggest that the more sensitive models perform better, and indeed the less sensitive models are not adequate in replicating vital aspects of today’s climate.

A 2014 paper led by Steven Sherwood took a similar approach with similar results. The paper concluded,

The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.

Figure (derived from Sherwood et al. 2014, Fig. 5c) showing the relationship between the models’ estimate of Lower Tropospheric Mixing (LTMI) and sensitivity, along with estimates of the same metric from radiosondes and the MERRA and ERA-Interim reanalyses. Source: RealClimate.

Another 2014 paper published by scientists from CalTech and UCLA arrived at a similar conclusion, as lead author Hui Su explains,

This study used an index that represents how models capture the observed spatial structure of the Hadley Circulation and associated humidity and cloud distributions. We showed that the inter-model spread in climate sensitivity and cloud feedback is closely related to models’ Hadley Circulation change and present-day circulation strength varies systematically with models’ climate sensitivity (Figure 9). The stronger Hadley Circulation in the models, the higher climate sensitivity the models have (Figure 10). The observed circulation strength is on the high end of the modeled ones.

Clouds Hold the Key



Clouds are a key to determining the Earth’s climate sensitivity. We know that by itself, a doubling of the amount of carbon dioxide in the atmosphere will cause about 1.2°C global warming. A warmer atmosphere will hold more water vapor, and as another greenhouse gas, we know that increase in water vapor will roughly double that carbon-caused warming (a “positive feedback”). We also know of some other significant positive feedbacks, like melting ice decreasing the reflectivity of the Earth’s surface, causing it to absorb more energy from the sun.

Those who argue that the Earth’s climate is relatively insensitive to the increased greenhouse effect need a big negative feedback to offset those factors we know amplify global warming. Clouds represent the only such plausible mechanism, because we don’t have a very good grasp on how different types of clouds will change in a hotter world.

For example, climate scientist contrarian Richard Lindzen came up with what’s known as the “iris hypothesis” in 2001, suggesting that in a warmer world, high cirrus clouds will contract like the iris on an eye to allow more heat to escape. That hypothesis has not withstood the test of time, however, with four studies published within a year of Lindzen’s paper effectively refuting the hypothesis. One recent paper found that even if the iris effect is real, it would reduce the Earth’s climate sensitivity by no more than 20%, still well within the range of possible values outlined by the IPCC.

Not only have the aforementioned studies found that changes in humidity and clouds are consistent with simulations from more sensitive climate models, but previous research led by Andrew Dessler and more recently by Kevin Trenberth and colleagues has shown that observed changes in water vapor amplifying global warming as expected, and that clouds are thus far acting to weakly amplify global warming. These observations are inconsistent with the strong cloud dampening effect contrarians need to justify arguments for low climate sensitivity.

Low Sensitivity Single Study Syndrome



There have been a few recent studies using what’s called an “energy balance model” approach, combining simple climate models with recent observational data, concluding that climate sensitivity is on the low end of IPCC estimates. However, subsequent research has identified some potentially serious flaws in this approach.

These types of studies have nevertheless been the focus of disproportionate attention. For example, in recent testimony before the US House of Representatives Committee on Science, Space and Technology, contrarian climate scientist Judith Curry said,

Recent data and research supports the importance of natural climate variability and calls into question the conclusion that humans are the dominant cause of recent climate change: … Reduced estimates of the sensitivity of climate to carbon dioxide

Curry referenced just one paper (using the energy balance model approach) to support that argument – the very definition of single study syndrome – plus an interpretation of a second paper whose author objected, saying,



others have used my findings to suggest that Earth’s surface temperatures are rather insensitive to the concentration of atmospheric CO2. I do not believe that my work supports these suggestions, or inferences. As fond as I am of my own ideas, one should resist concluding too much, too soon, from a single study.

Real Skeptics Consider all the Evidence

True skepticism requires considering all available evidence. While some studies suggest that climate sensitivity is on the low end of the estimated range, other studies suggest it’s on the high end. As Andrew Dessler told me,

There certainly is some evidence that climate sensitivity may be below 2°C. But if you look at all of the evidence, it’s hard to reconcile with such a low climate sensitivity. I think our best estimate is still around 3°C for doubled CO2.

Andrew Dessler discusses climate sensitivity.

Ultimately when we consider all the available scientific evidence and risk management principles, there’s no case to be made for delaying action to curb global warming.

Update: yet another paper has just been published finding that the models that most accurately simulate the observed changes in the Earth’s atmosphere are the ones that are most sensitive to the increasing greenhouse effect.

