Is temperature the best measure of climate change?

Climate change is most often expressed in numbers as the change in global mean surface temperature. Taking the average temperature of the entire planet is a something of a challenge, relying as it does on consistent measurements combined with statistical analysis of the data. It can be done, however, and the final figures are credible. What they mean is another question.

Is surface temperature the best way of assessing human-induced climate change? Not according to Roger Pielke Sr, who in an article in this month’s Physics Today argues instead for the use of heat accumulation and loss as measured in fundamental energy units (joules). Pielke is a climatologist at the University of Colorado in Boulder.

Pielke’s son Roger Pielke Jr is a political scientist and science policy wonk who for some years has been a thorn in the side of the climate science community, what with his robust criticisms of the Intergovernmental Panel on Climate Change (IPCC) and the way in which the debate is conducted in the public and political spheres, and his sometime association with prominent climate change sceptics and other conservative political forces. Father and son should not be confused.

In his Physics Today article, Pielke the elder questions the value of global average surface temperature on the grounds that it fails to account for the effect of heat sinks such as the world’s oceans. With temperature there is a time lag in its response to radiative forcing in the atmosphere, or the change in the balance between incoming and outgoing energy. But with heat changes there is no time lag involved as the amount of heat present at any time is accounted for.

Does this matter when it comes to the popular presentation of climate change? Maybe, maybe not, but such considerations should not influence the way in which climate scientists go about their work. The issue here is how scientists assimilate and analyse their data in global initiatives such as the IPCC that have a direct impact on policy decisions.

Pielke is calling into question some of the figures put forward by climatologists such as NASA’s James Hansen. If Pielke is right, Hansen’s numbers for upper ocean heat change are too high, and there is either an error in the IPCC values for net radiative forcing, the radiative feedbacks are negative, or both. Feedback mechanisms in the climate system are critical, as for one thing they imply the existence of tipping points beyond which changes are irreversible. It is therefore vital that we get the calculations right.

Another problem, says Pielke, is that global average radiative forcing is a poor measure of climate change brought about by a whole range of mechanisms. In particular it neglects the global effect of regional changes due to natural features such as the El Ñino southern, North Pacific decadal and North Atlantic oscillations, and human-induced changes in atmospheric chemistry.

Pielke acknowledges that there is debate within the climate science community on whether the magnitude of human-induced regional forcing is significant on a global scale. But he points out that global models which incorporate such influences show effects large enough to result in global-scale changes to atmospheric circulation.

One example highlighted by Pielke is aerosols that alter cloud formation and rain patterns, and impact on weather patterns far from where the aerosols entered the atmosphere. Another is nitrogen deposition in soil which alters the growth of vegetation on a regional level.

Humans are altering the climate, says Pielke, but they do so in a variety of ways beyond the radiative effect of atmospheric carbon dioxide. All climate scientists would agree with that statement, so is Pielke justified in his criticism of the IPCC methodology? There is vigourous debate among scientists on these very issues, and progress is being made on the research front. As for average temperature and heat accumulation, Pielke may well have a point.