Arguments
Changes in water vapor and clouds are amplifying global warming
Posted on 23 April 2015 by John Abraham
A very new paper currently in press shines light on climate feedbacks and the balance of energy flows to and from the Earth. The paper was published by Kevin Trenberth, Yongxin Zhang, John Fasullo, and Shoichi Taguchi. In this study, the authors ask and answer a number of challenging questions. Their findings move us a big step forward in understanding what is happening to the planet now, and how the climate will evolve into the future.
So, what did the scientists do? First, they used measurements at the top of the Earth atmosphere to count the energy coming into the Earth system and the energy leaving the planet. The measurements were made by satellites as part of the Clouds and Earth’s Radiant Energy System project (CERES for short). By subtracting one energy flow from the other, they found what is called the Earth’s energy imbalance. Most studies show that the energy imbalance is in the range of 0.5 to 1 Watt per square meter of surface area, which is causing ongoing global warming.
What the authors then asked is, how does this imbalance change? It turns out, the imbalance changes a lot over time. On a monthly basis the balance might change 1 Watt per square meter of surface area. The changes are caused principally by changes to clouds and water vapor, and other short-term weather patterns. Clouds have the ability to reflect sunlight back to space; however, clouds also have the ability to trap more heat within the Earth’s atmosphere. So, short-term fluctuations in clouds have large impacts on the net rate of heat gain by the Earth.
The authors also correlated the observed temperatures, the amount of water vapor in the atmosphere, and the flow of radiant energy to explore how they affect each other. They found a strong relationship between the outgoing long wavelength radiation (infrared energy) and temperature; however, this relationship varies substantially across the planet. In fact, the relationship switches sign in some regions, such as the tropics. Measurements of the absorbed incoming radiation from the sun provided direct indications of the effects of clouds on that quantity.
By looking at the relationships among measured variables, such as temperature, radiation heat transfer, water vapor, and others, the authors were able to extract how changes to cloud cover influence global temperature. We know that clouds have a net cooling effect on the planet. But what we really want to know is whether this cooling effect will get larger or smaller in the future. If the cooling effect gets smaller, it means the Earth will warm more than expected. If the cooling effect of clouds gets bigger, it means the Earth will warm less than expected.
What the present paper shows is that future changes to clouds will cause slightly more warming. Scientists describe clouds as a “positive feedback” on global warming. This finding is consistent with the work of Dr. Andrew Dessler. He had published work here and here showing changes in clouds are making the Earth warm more than otherwise expected.
The results of this study harken back to prior work by one well-known skeptic Richard Lindzen who published work on climate feedbacks in 2009, and by another well-known skeptic Roy Spencer who wrote an article in 2011. Those works, among others, reportedly show that the Earth is less sensitive to increases in greenhouse gases. This new work confirms the opposite; it turns out Dr. Dessler was correct after all.
I asked for a comment from Dr. Trenberth. He wrote,
The links to the works of Dr. Dressler don't work here (they are active in the Guardian article). Some way to fix it?
[JH] Link inserted.
Interesting development on a thorny issue.
"This new work confirms the opposite; it turns out Dr. Dessler was correct after all."
Confirms? Wouldn't "corroborates" be more appropriate? This paper is one more step towards understanding. How does John Abraham know it is definitive?
Here's what I wonder:
If the TLT measurements turn out to be underestimating warming (which seems likely), how does that impact the conclusions of the study?
Some statements in this article seem contradictory. It states "We know that clouds have a net cooling effect on the planet", but then says scientists describe clouds as "positive feedback". How can clouds have a net cooling effect yet be considered positive feedback? It seems like this study is saying clouds actually have a net heating effect.
rkrolph
Actually they aren't contradictory. There is a difference bewteen the net effect clouds have when the climate is in one state and the impact a change in their conribution has.
I am going on memory here but I think from the last IPCC report the contribution from clouds as 50 watts/m2 of cooling and 30 watts/m2 of warming for a net effect of 20 watts/M2 of cooling.
Now hypothetically, if in a warmer world the cooling aspect increased to 55 watts/M2 and the warming aspect to 37 watts/M2, the net cooling is now 18 watts/M2. A change of +2 watts/M2. So they have acted as a warming feedback because they now contribute less cooling.
Recommended supplemental reading:
The return of the iris effect? by Andrew Dressler, Real Climate, Apr 24, 2015
Glenn Tamblyn
Thanks for putting some numbers to this. Since clouds do have a cooling effect (although less in a warmer world), would it be possible that if cloud cover was underestimated for the future, then you could still get a net cooling effect. For example, using your numbers, if equivalent cloud cover was 20% greater, then you would have 66 watts/M2 cooling, and 44.4 watts heating, giving a delta of 21.6 watts/M2 net cooling, which would be greater than the current IPCC value of 20 watts/M2, and therefore a cooling feedback. It's not that I think this may be a likely scenario, but am just wondering if I am understanding this correctly.
I guess a simpler way of putting this would be asking if in the future we were to somehow artificially induce more clouds, would that help cool the planet, with everything else being equal?
Kevin Trenberth has the dubious honour of belonging to a select cadre of scientists, i.e. those whose words have been most widely and wildly misconstrued by fake sceptics.
Those unfamiliar with the "travesty" story may care to look at an SkS article from December 2009 which provides a background.