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Climate Hustle

Overlooked evidence - global warming may proceed faster than expected

Posted on 30 April 2015 by dana1981

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 mostsensitive 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.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, butprevious 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 whose author objected, saying, 

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Comments

Comments 1 to 3:

  1. This is great, evidence is clearer each day, and denyers one less rock to hide under.

    But I have a question: Suppose there is a way to precipitate out an Amazon's flow from the air continuously. This means that the latent heat would concetrate higher in the atmosphere where it radiates into space faster with the 4th power of temperture K. 

    Would this not reduce the problem? We still have the CO2 in the atmosphere, but we systematically reduce vapor content. Does this reduce the feedback from clouds? Or does it not help because then we have less short wave radiation reflected by less cloud cover, but still have CO2 reflection of low wave radiation? 

    I was under the impression that low level clouds reflect more short wave solar energy, but higher level clouds refelct more long wave heat back to earth. That issue seems to be ingnored in this paper.

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  2. It is good to raise this issue of single study syndrome because far too many grand conclusions are drawn from just one new piece of evidence or from yet one more model.

    It also highlights the point that there is no clear conclusion of the individual, let alone cumulative, effects of both negative and positive feedback mechanisms and how they offset each other.

    It's also good to see a recognition of how the impact of clouds is so poorly undertood and yet potentially significant

    It can't be said too often that new pieces of work that authors and commentators may  exaggerate the significance of for obvious reasons, are frequently challenged by later pieces of work but that challenge is not always examined and recorded.

    Keep on being sceptical might be a good mantra

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  3. Carbon in soils is another thing that seems as poorly understood as cloud feedback.

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