The U.S. Environmental Protection Agency released a draft report this week that indicates the nation's greenhouse gas emissions declined 2.2 percent in 2015, continuing a generally downward trend since U.S. emissions peaked in 2007.
Overall, net emissions in 2015 were 11.6 percent below 2007 levels, according to the report. Except for 2012, when emissions were slightly lower, they have not been this low since 1993.
The decline in recent years has been mostly attributable to reductions in the electric power industry as a result of mild winters and power plants switching their fuel from coal to natural gas. Consumption of electricity decreased slightly in 2015, but power plants still accounted for the largest single source of greenhouse gas emissions (29%), followed by transportation (27%), and industries (22%).
"Emissions from industry have in general declined over the past decade, due to a number of factors, including structural changes in the U.S. economy (i.e., shifts from a manufacturing-based to a service-based economy), fuel switching, and energy efficiency improvements," according to the report.
As the EPA acknowledges, when manufacturing shifts from the U.S. to another country, like China or Mexico, the carbon emissions follow, which negates some of the gains seen in the U.S.
"The remaining 21 percent of U.S. greenhouse gas emissions were contributed by, in order of magnitude, the agriculture, commercial, and residential sectors, plus emissions from U.S. Territories," according to the report.
But, not all greenhouse gases have an equal effect on global warming, the report notes. While carbon dioxide (CO2) accounted for 82.2 percent of the nation's emissions in 2015, other gases like methane, nitrous oxide and industrial gases known as HFCs, PFCs, SF6 and NF3 can trap heat significantly more and stay in the atmosphere much longer than CO2.
Between 1990 and 2015, total emissions of CO2 increased by 5.6 percent while emissions of methane and nitrous oxide have decreased 16.7 and 6.8 percent, respectively. During the same period, HFCs, PFCs, SF6 and NF3 gas emissions have increased 86.2 percent.
"Despite being emitted in smaller quantities relative to the other principal greenhouse gases, emissions of HFCs, PFCs, SF6 and NF3 are significant because many of these gases have extremely high global warming potentials and, in the cases of PFCs and SF6, long atmospheric lifetimes," the report states.
The EPA uses a "common and consistent mechanism" that enables parties to the United Nations Framework Convention on Climate Change (UNFCCC) to compare the relative contribution of different emission sources and greenhouse gases to climate change. To ensure that the U.S. emissions inventory is comparable to those of other countries, the estimates were calculated using methodologies recommended by the Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories.
The draft report is required to be produced annually under the U.N. agreement. It is currently open for public comment and scheduled to be finalized in April, according to the EPA.
Former President Barack Obama announced in August the EPA’s final Clean Power Plan, which was intended to cut U.S. carbon pollution from the power sector by 870 million tons, or 32 percent below 2005 levels, by 2030. Similar emission reductions were announced in the transportation sector as new standards for heavy-duty trucks were established in 2016.
It remains to be seen whether or how much the Donald Trump administration will roll back these new regulations. Trump has famously suggested global warming is a "hoax" and has threatened to curtail federal efforts to reduce greenhouse gas emissions.