Water Vapour and the Recent Global Temperature Hiatus

  • Date: 29/01/10

Climate scientists, writing in the journal Science, say they may have overlooked a major cause of global warming and cooling. American researchers suggest that the amount of water high in the atmosphere is far more influential on global temperatures than was previously thought.

Lead author Dr Susan Solomon, of the US National Oceanic and Atmospheric Administration, said: “Current climate models do a remarkable job on water vapour near the surface. But this is different — it’s a thin wedge of the upper atmosphere that packs a wallop from one decade to the next in a way we didn’t expect.”

The first thing to be noted in this paper is that it is, once again, predicated on the now well-established fact that the global annual average temperature has remained constant for the past decade. There are still many commentators who deny this. They should read this section from the beginning of this paper in one of the world’s top two peer-reviewed science journals.

“….the trend in global surface temperatures has been nearly flat since the late 1990s despite continuing increases in the forcing due to the sum of the well-mixed greenhouse gases (CO2, CH4, halocarbons, and N2O), raising questions regarding the understanding of forced climate change, its drivers, the parameters that define natural internal variability, and how fully these terms are represented in climate models.”

However, I would have to take issue with the statement made later in the paper, “Over the past century, global average surface temperatures have warmed by about 0.75°C. Much of the warming occurred in the last half century.” That is not borne out by the facts. Most of the warming took place prior to 1940.

The researchers suggest that the recent rise in global temperatures between 1980 and the late 1990’s was exaggerated by the greenhouse effect caused by a temporary increase in water vapour in the atmosphere and that the post 2001 standstill has been caused, to a substantial degree say the researchers, by an observed decline in atmospheric water vapour content.

Increases in stratospheric water vapor levels tend to cool the stratosphere but to warm the troposphere, while the opposite is true for stratospheric water vapor decreases. Previous studies have suggested that variations in stratospheric water vapor might contribute significantly to climate change, but there has been debate about the magnitude of the influence.

Look at their Figure 1A which shows the time series of mid-latitude water vapor in the lower stratosphere based on a variety of measurement sources, terrestrial, balloon-based and satellite. The researchers say that, taken together, these data provide strong evidence for a sharp and persistent drop of about 0.4 parts per million by volume (ppmv) after the year 2000. Prior to this decrease, the data suggest a gradual mid-latitude increase in lower stratospheric water vapor of more than 1 ppmv from about 1980 to 2000.

Click on image to enlarge.


Personally I find the data intriguing but I wouldn’t draw as definite a conclusion about them as the researchers do.

The water vapour data were fed into computer models of radiative transfer in the atmosphere. The simulation was limited in scope, very much a first attempt at the problem, as it focused only on estimating the contributions of stratospheric water vapor changes to recent decadal rates of warming; additional contributions such as from solar variations, aerosols, natural variability, or other processes are not ruled out by this study.

They concluded that decreases in stratospheric water vapor concentrations acted to slow the rate of increase in global surface temperature over 2000-2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases.

More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% compared to estimates neglecting this change.

Overall the paper’s conclusions show that stratospheric water vapor represents an important driver of decadal global surface climate change. However its true level of influence cannot as yet be deduced until other factors that affect decadal temperature variations are taken into account.

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