Waste heat and its effect on climate

There has recently been some chatter within the Earth science community about the climate impact of heating from human energy use. We are talking here of atmospheric warming due to heat produced as a product of the 15.5 terawatts of power currently used throughout the world. At just three hundredths of a watt per square metre of the planet’s surface, the contribution of waste heat to climate change is relatively small when compared with that of greenhouse gases, but it could have a significant impact on local surface temperature measurements.

In the 16 December 2008 issue of Eos (it can take a few weeks for the wheezing pigeons to land on my windowsill!) is an introduction to the subject by Jos de Laat, an atmospheric physicist based at the Royal Netherlands Meteorological Institute near Utrecht. Access to Eos is by subscription, so for your delectation I shall précis the argument here.

Human energy use is by definition concentrated in population centres, but as anyone who follows the maps presented on TV and Internet weather forecasts will understand, man-made heat in cities can have a regional effect. In the case of London, the sum of human activity leads to a micro-climate that extends at least as far as the M25 orbital motorway. And it’s not just waste heat; weather and climate in urban areas are affected by the presence of industrial aerosols and other pollutants.

Anthropogenic climate change sceptics such as Ross McKitrick & Patrick Michaels have claimed that warming measured over land is exaggerated due to non-climatic effects. However, their study includes a somewhat arbitrary mixing of potentially incompatible datasets, a time series too short for a proper local trend estimation, and a rather simplistic statistical analysis. Oslo-based climate scientist Rasmus Benestad has thoroughly dissected the McKitrick & Michaels paper.

If waste heat can have a significant effect on atmospheric temperature on local and possibly regional scales, could this be affecting measurements that feed into climate models and thus contribute to our understanding of global warming? Possibly, but only to a limited degree. While temperature measurements tend to be made in the vicinity of at least some human activity, there is a growing number of measurement stations in remote locations, the calculation of global average temperature variations is a methodical affair, and there are established techniques to correct for so-called urbanisation effects (e.g., Hansen et al., 2001).

De Laat says that it is unclear what the magnitude and footprint of a waste heat source must be for it to affect local and regional temperatures. That statement may be contested by climate scientists such as Jim Hansen who have considered the issue in depth, but de Laat is right to point out the magnitude of the task required to narrow down the uncertainties associated with the climate impact of waste heat from human activities.

As an interesting historical aside, in the same issue of Eos is a letter from Colby College science historian James Fleming, who challenges pedagogue and former astrophysicist Eric Chaisson for failing to acknowledge in an article published in July of last year an early contribution to the debate concerning waste heat and climate.

Back in 1969, Russian climatologist Mikhail Budyko wrote that heat from human energy use was over and above climate forcing from anthropogenic greenhouse gases, and was likely to result in potentially damaging global warming within 200 years. That prediction was made nearly 40 years ago. Since then greenhouse gas emissions have rocketed, and we have learned a huge amount about atmospheric composition and dynamics.