In the Nature paper (PDF) that I co-authored with Tom Wigley and Chris Green last spring, we pointed to a disturbing reversal of global trends in energy intensity (energy consumption per unit GDP) and carbon intensity (carbon emissions per unit energy).

The recent UK National Rail Review (PDF) helps to explain why this trend matters in a very specific context in a developed country setting:

Total carbon emissions from fuel consumption increased by 31 ktonnes (less than 1%) in 2006-07. When the data is normalised to take account of increases in traffic volumes – passenger kilometres and net freight tonne kilometres – this shows reductions of 5.8% for passengers and 1.4% for freight.

There were reductions in rail’s total electricity and diesel fuel consumption (by 2.7% and 0.2% respectively). But because of the high price of gas during 2006-07 the electricity industry switched its fuel supply to make greater use of coal, a more carbon-intensive source. As a result the CO2 emissions involved in providing electricity for rail increased. Had the carbon intensity of electricity generation remained constant, CO2 emissions from traction electricity would actually have fallen by 40 ktonnes (2.7%).

So even as rail travel has become less energy intensive (reductions in total electricity and fuel consumption as passenger and freight miles increased), the total carbon emissions from rail nonetheless increased due to the reemergence of coal as a source of electricity.

The UK Rail experience offers a sobering lesson: efficiency gains as a source of carbon dioxide emissions reductions in the context of growing demands for energy are limited when coal is priced cheaper than alternatives, and decision makers and the public care about having cheap energy. In this context, making coal more expensive seems pretty unlikely, since a much less carbon-intensive fuel (natural gas) is already available at a higher cost but disfavored as being too expensive, even by a government with ambitious emissions reductions goals and a general public very concerned about climate change. Does anyone really think that policy makers anywhere will meaningfully increase the costs of energy?

And if you think the increasing carbon intensity in the UK is a temporary blip, think again. The Center for Global Development projects continuing increases in carbon intensity in both the United States and Europe.