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Satellite stories - what do new findings on tropospheric warming mean for climate change?

Data from satellites have fuelled debate over climate science. NASA via Wikimedia Commons

If you’ve followed climate contrarian talking points over the years, you’ll be familiar with the argument over satellite observations of global warming.

In 1990 it was reported that the satellite record of temperatures in the troposphere (the part of the atmosphere that should warm more or less with the surface) did not show any warming since 1979, the year satellites began measuring temperature.

Since the satellites were not calibrated accurately enough to measure climate change without delicate efforts to correct for biases, mainstream scientists were sceptical. Indeed the contrarians retreated in 2005, when revisions of their data began to show atmospheric warming consistent with that measured by surface thermometers.

Focus on the tropical upper troposphere

Most considered the debate over at that point.

However, a few sceptics established a fallback position: that the particularly rapid warming expected at high altitudes in the tropics (called either the “hot spot” or the tropical upper troposphere or TUT) is not happening. They still point to a set of warming estimates derived from weather balloons by the UK Hadley Centre, shown in the 2001 IPCC report.

But tropical observations are sparse, and these data have since been found by their own developers to be too inaccurate in the TUT region to say whether warming occurred there or not. Several other estimates of the warming have appeared, at least one clearly contradicting sceptics’ favoured satellite record outside the tropics.

But no record is definitive, and answers for the TUT region range from no warming to even more than predicted. A few sceptics have stubbornly maintained that the only observational reconstructions they trust are those which happen to disagree with models.

New studies find models are missing something

Two new studies suggest the sceptics have not been completely wrong in this. A research team from the University of Washington, well known for its previous work on satellite climate records, reports (in an article to appear in Geophysical Research Letters¹) a careful combination of information from different channels on the satellites to distinguish temperature in the upper and lower parts of the troposphere.

What they find is that the TUT has warmed since 1979, but at a rate hardly exceeding that at lower altitudes. Basic theory predicts a significant excess, and according to detailed models the odds of so little excess are about the same as the odds of rolling 12 on a pair of dice. Not impossible, but unlikely enough to suggest that the models are missing something.

Before speculating on what this means, there are two important caveats to consider. First, this new finding still suffers from the same calibration issues as other attempts to get warming trends out of satellites. Second, the period since 1979 captures only a fraction of the era of man-made global warming.

That brings us to a second study, one based on weather balloons by a team of authors led out of the UK Hadley Centre, including the developers of the 2001 data, appearing in this month’s Journal of Geophysical Research².

This one also finds that TUT warming appears weak since 1979 (though again cautioning that the data are not quite accurate enough to be sure about this). Significantly, however, the same data show much stronger warming prior to 1979.

If you look at change over the longest period available (since 1958), everything matches up pretty well, a finding on which several studies now agree. This means we must be careful not to over-generalise observations during the shorter, satellite era.

So is global warming exaggerated?

Is the “missing,” or more accurately, “inconsistent hot spot” aloft an indicator that models exaggerate global warming? This conclusion, leapt to by many climate deniers, makes no sense.

First, we care about warming near the surface, and there is no reason why less warming aloft would mean a more or less sensitive climate (an argument you may hear relating to the water-vapour feedback rests on a misunderstanding of climate physics).

Second, the different trends before and after 1979 suggest either a natural oscillation, or some influence on climate that has not been put into the models correctly, superimposed on global warming.

Strong candidates for missing influences are particulate air pollutants or changes in ozone amounts, neither of which were adequately represented in the last round of climate model simulations. This should improve somewhat in the simulations going on now for the next IPCC report in 2013, although air pollutants in particular are very tough to handle.

If upper tropospheric temperatures are indeed flopping around unexpectedly, this would challenge our current understanding about how energy is transported within the atmosphere.

That would have no direct repercussions for global warming, which is instead fundamentally about energy exchanges between the planet and space. But it would be very interesting to those of us who study how the atmosphere shifts heat around and, when sorted out, might indeed revise our estimates of future warming up or downward.

The ongoing debate is a reminder that Earth’s atmosphere may still have some secrets up its sleeve.

  1. Fu, Q., S. Manabe, and C. Johanson (2011), On the warming in the tropical upper troposphere: Models versus observations, Geophys. Res. Lett., doi:10.1029/2011GL048101, in press. (accepted 24 June 2011)
  2. Thorne, P. W., et al. (2011), A quantification of uncertainties in historical tropical tropospheric temperature trends from radiosondes, J. Geophys. Res., 116, D12116, doi:10.1029/2010JD015487.

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