The causes of the recent decrease in US greenhouse gas emissions
Since their peak in 2007 GHG emissions in the USA have decreased more in absolute terms than in any other country. The results of this review suggest that approximately 40% of this decrease was caused by the replacement of coal with gas in generating plants, 30% by improvements in the efficiency of internal combustion engines and 30% by growth in low-carbon renewables. Another major contributor was the 2008-9 global recession, although its impact can’t reliably be quantified. Had economic growth continued at historic rates between 2007 and the present US GHG emissions would now be substantially higher than they are.
This review was prompted by an article in Time Magazine written by Michael Bloomberg, one of the world’s richest men and also one of its leading contributors to green causes, entitled “Where Washington Fails to Drive Progress, Cities Will Act”. Bloomberg’s conclusion that the decrease in US emissions was “driven by cities, businesses and citizens”struck me as complete nonsense, and I was going to write more about it but decided it wasn’t worth the effort. But his article did get me thinking about what really did cause US emissions to fall, and in this post I document the results of my efforts to find out.
We begin with emissions. Figure 1 shows total US greenhouse gas emissions by sector between 1990 and 2014 (2015 emissions are not available but would probably be slightly lower than 2014 emissions. The data are from the EPA’s US Greenhouse Gas Inventory Report):
Figure 1: Total US greenhouse gas emissions by sector expressed as CO2 equivalents, 1990-2014. Emissions from land use changes and from US territories are not included.
Note that Figure 1 shows total greenhouse gas emissions, which include CO2 along with methane, nitrous oxide and minor GHGs expressed as CO2 equivalents, from all sectors of the economy, not just the CO2 emissions from electricity generation that we usually get to see.
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