Changes in water vapor and clouds are amplifying global warming

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,

So what we have done here is to look at feedbacks and relationships that relate to climate sensitivity. A number of studies have analyzed the observations, and recent changes in Earth’s temperature, to say something about climate sensitivity. We claim that none of them are really meaningful because there is too much variability in the short record (since 2000 when CERES became active). What we do find is that if one looks at tropospheric average temperature rather than surface temperature, then there is a much stronger relationship with energy flow at the top of the Earth’s atmosphere. We are able to find a water vapor signal that is clearly a positive feedback.

So, there is a lot here in this paper, a good deal of new understanding about our climate and how it will evolve into the future. The group is already exploring how the observed results pan out in climate models. I am anxious to see any follow-on work from this group.