We asked Dr. Richard Washington, Climatologist and Lecturer at Keble College, Oxford, whether the arguments hold water…
Is the troposphere cooler than it should be?
The two key sources of atmospheric temperature data above the surface in the troposphere are the temperatures derived from balloons (radiosondes) and those from satellite (e.g. using a Microwave Sounding Unit – MSU). These data sets differ in terms of their spatial coverage and also the methods of measurement. It is not surprising then that the temperature trends emerging from these data sets are different.
Satellites measure radiation not temperature. The radiation then has to be converted to temperature. The constants used in these calculations to connect radiation and temperature have been revised many times. Initially the constants were set so that a cooling appeared in the mid troposphere. But subsequent revision to more accurate constants effectively removed this cooling. These revisions were actually undertaken by Spencer and Christy who featured in the Channel 4 documentary. Corrections have also been made to the radiosonde data set so that there is now (in March 2007) much better agreement between these data sets and also with the climate models. The latest consensus (from the IPCC AR4 report) is as follows:
"Previously reported discrepancies between the amount of warming near the surface and higher in the atmosphere have been used to challenge the reliability of climate models and the reality of human-induced global warming. Specifically, surface data showed substantial global-average warming, while early versions of satellite and radiosonde data showed little or no warming above the surface. This significant discrepancy no longer exists because errors in the satellite and radiosonde data have been identified and corrected. New data sets have also been developed that do not show such discrepancies."
All data sets require careful calibration and correction for errors. The atmospheric temperature data sets are no different. What does make the debate different is that the focus is placed on the corrections rather too early in the development stage. Fierce debate surrounding trends in the MSU dataset, for example, were occurring when the data set was still quite short (just over 13 years or so). These debates occur because of the importance of the global warming issue.
Why did global temperature fall during the post-war economic boom?
Global temperatures did indeed fall during the post-war boom when CO2 levels were rising. Temperatures have subsequently risen and this is in line with what the models show in response to CO2 .
But the argument used by the climate change skeptics in the documentary to suggest that this was proof of a hoax was really amongst the weakest arguments they made. This is because the expectation that a simple forcing (e.g. increasing CO2) leads to a simple warming runs up against what we know about the complex climate system. Several of the skeptics in the documentary were not climate scientists and so they could be forgiven for making these errors. They are the kind of errors that I, as a climate scientist would make if I started making claims about ecology.
So the connection between a forcing and a response in the earth system is complicated and non-linear. This is precisely the reason that we need climate models which include the non-linear physics.
So what was going on to cause the cooling in the post-war boom? First, although CO2 levels were increasing during this period, the effect was still quite small. Other relatively more powerful causes of climate variability, such variability of the ocean circulation and patterns of climate like the North Atlantic Oscillation, had a dominant effect at this time. What we see in more recent years is that the dominance of these natural patterns is not enough to cause a downturn in temperatures against the carbon related warming. This relationship strengthens in the model experiments so that beyond 2050 the carbon concentrations are completely in charge.
This article first appeared in the Ecologist March 2007