Robert Howarth has been criticized by ANGA (America’s Natural Gas Alliance) for using an unconventional time horizon in his study “Methane and Greenhouse-Gas Footprint of Natural Gas from Shale Formations.” While Howarth’s Paper is seriously flawed (the existing data on natural gas leakage rates is extremely limited), a few pieces are worth salvaging from the flames.
The Intergovernmental Panel on Climate Change uses a baseline 100 year time horizon to measure the radiative forcing caused by gases in the atmosphere, while the Howarth paper uses 20. MIT Energy Initiative ED Melanie Kenderdine went as far as to say “What he has done in his analysis is deviated from what are accepted standards, accepted by EPA, DOE, the IPCC, European Trading Scheme, California Air Resources Board, where essentially the denominator that they use to calculate the impacts of various greenhouse gases is an agreed upon hundred years; Professor Howarth uses 20 years.” Kenderdine suggests that Howarth is setting up his GHG comparison outside the realm of scientifically accepted standards—similar to claiming statistical significance at a .20 alpha.
Kenderdine’s characterization is misleading. While the IPCC does use the 100 year time frame, they also use 20 year, and 500 year periods. They note that “the choice of the time horizon depends in part on whether the user wishes to emphasize shorter-term processes…or longer term processes that are linked to sustained alterations of the thermal budget. In addition, if the speed of potential climate change is of greatest interest (rather than the eventual magnitude), then a focus on shorter time horizons can be useful.”
What time frame is most relevant here? The Howarth paper was clearly aimed at the natural gas industry, in the height of the shale boom. The golden age of gas is upon us. If global use of natural gas expands at projected rates, emissions from gas production, transportation, processing, storage, and consumption will have a greater impact due to increased volumes. The EPA’s recent CO2 rule will likely result in the majority of new power plants built in the US to run on natural gas. Natural gas leaked into the atmosphere has a Global Warming Potential of 21 over the 100 year time horizon. Under the 100 year time horizon, it appears very likely that a switch to natural gas could result in a reduction of net radiative forcing over the century—however, the “net” may not be the optimal way to look at the situation. In 100 years, the damage may well be done. Consider the following:
1.) Switching a short time frame GHG for a long time-frame GHG is essentially trading a steeper short term rise for an earlier slope downward. Natural gas instead of coal, for example, would make short term temperature increases more severe, but long-term temperatures more stable. This would be OK if ecosystems and economies were robust enough to take the rapid climb—however many impacts are related to seasonal temperature, not long-term averages.
2.) Feedback loops in the artic make short term changes to temperature more important. Short-term warming of the permafrost releases additional GHGs. Sea ice formation is a central hub of the artic ecosystem, and depends to multiple factors, all of which have the potential to ‘snowball’ with near-term warming. Previously sequestered methane in the artic will be increasingly released into the atmosphere, further increasing the rate of warming as well as extending the period of rapid increase in temperatures. The most potentially catastrophic example of this has to do with the release of the EIA’s favorite resource for tomorrow: Methane Hydrates. Previous releases in the Paleocene resulted in 1-8 degree (C) rises in temperature, dependent on latitude. Methane Hydrates are currently stabilized along continental margins, and the exact amount of forcing that would trigger a release is unknown.
Because climate stabilization must occur before the ‘tipping point,’ I would argue that short-time frames are essential in consideration of emissions. How fortunate that policymakers operate in 1-4 year time horizons.
 US Inventory of Greenhouse Gas Emissions and Sinks 1990/2007 (EPA, 2009)