Annual US CO2 emissions and reductions by source (in million metric tonnes), 1990-2017, from energy in black, with estimated reductions by factor shown by colored wedges. Top chart shows zoomed-in reductions with a truncated y-axis, while bottom chart shows the same chart with a y-axis starting at zero. Graphic: Carbon Brief

By Zeke Hausfather
15 August 2017

(CarbonBrief) – Before 2005, U.S. carbon emissions were marching upwards year after year, with little sign of slowing down. After this point, they fell quickly, declining 14% from their peak by the end of 2016.

Researchers have given a number of different reasons for this marked turnaround. Some have argued that it was mainly due to natural gas and, to a lesser extent, wind both replacing coal for generating electricity. Others have suggested that the declines were driven by the financial crisis and its lasting effects on the economy.

Here Carbon Brief presents an analysis of the causes of the decline in US CO2 since 2005. There is no single cause of reductions. Rather, they were driven by a number of factors, including a large-scale transition from coal to gas, a large increase in wind power, a reduction in industrial energy use and changes in transport patterns.

Declines in US CO2 have persisted despite an economic recovery from the financial crisis. While the pace of reductions may slow, many of these factors will continue to push down emissions, notwithstanding the inclinations of the current administration.

Carbon Brief’s analysis shows that in 2016:

  • Overall, CO2 emissions were around 18% lower than they would have been, if underlying factors had not changed, and 14% lower than their 2005 peak.
  • Coal-to-gas switching in the power sector is the largest driver, accounting for 33% of the emissions reduction in 2016.
  • Wind generation was responsible for 19% of the emissions reduction.
  • Solar power was responsible for 3%.
  • Reduced electricity use – mostly in the industrial sector – was responsible for 18%.
  • Without these changes, electricity sector CO2 emissions would have been 46% higher than they are today.
  • Reduced fuel consumption in homes and industry was responsible for an additional 12% of the overall emissions reductions.
  • Changes in transport emissions from fewer miles per-capita, more efficient vehicles, and less air travel emissions per-capita account for the final 15%.

The “big picture”

US emissions peaked in 2005 at just below 6,000 million tonnes of CO2 (MtCO2), and have declined to below 5,200 Mt CO2 in 2016. The figure [above] shows actual US emissions in black, as well as the reductions in emissions due to each different cause as a colored “wedge”.

If underlying factors driving emissions had not changed, a growing population would have led to emissions increasing rather than declining over the past decade. [more]

Analysis: Why US carbon emissions have fallen 14% since 2005

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