Droughts increased emissions of carbon dioxide and other harmful air pollutants in some Western states when grid operators turned to natural gas- and coal-fired generation to make up for hampered hydropower production, climate scientists from Stanford University found.
Drought-induced emissions tied to shifts in electricity supplies added 100 megatons of carbon dioxide, 44.8 kilotons of sulfur dioxide and 56.9 kilotons of nitrogen oxides to the atmosphere in 11 western states between 2001 and 2015, according to the report titled "Response of electricity sector air pollution emissions to drought conditions in the western United States" by scientists at Stanford's School of Earth, Energy & Environmental Sciences. The increase in carbon emissions is equivalent to adding 1.5 million vehicles per year to roads in those states, said the report, which was published Dec. 21 in Environmental Research Letters.
Scientists have tied human-induced carbon emissions to global warming, and emissions of sulfur dioxide and nitrogen oxides can irritate lungs and contribute to acid rain and smog. The electric sector is the second-biggest contributor of carbon emissions in the U.S.
The Stanford scientists found that California, Idaho, Oregon and Washington experienced the greatest emissions uptick tied to drought and reduced hydropower availability. Three of those states have set ambitious goals of reducing emissions from 1990 levels. By 2050, California aims to curb greenhouse gas emissions by 80% while Oregon and Washington have set reductions goals of 75% and 70%, respectively.
Western states in recent years have experienced the kind of intense droughts that scientists forecast will become more common due to climate change and global warming. The new research suggests that failure to prepare for the emissions impact of such droughts could make achieving climate and air quality goals more difficult, particularly for states that rely heavily on hydropower supplies. And the research has broader global implications, given that hydropower supplies about 16% of the world's electricity.
"These drought-induced emissions are large enough to pose challenges to western states' progress towards their carbon dioxide emissions targets," the scientists wrote.
The research focused on reduced water runoff and stream flows to hydropower plants. Runoff levels, among other things, can be tied to the extent to which droughts prevented significant amounts of snow from accumulating in the mountains. That snowpack melts in spring and summer months and provides much of the water hydropower plants use to produce electricity.
In addition to reducing runoff, droughts also can drive up temperatures and the use of air conditioning. As a result, customers consume even more electricity on those days than in milder weather, which in turn increases the likelihood that fossil fuel generation will be used.
For the electricity part of the study, the researchers used monthly state-level electricity generation and sales data from the U.S. Energy Information Agency and subtracted total net in-state electricity generation from total electricity retail sales. Emissions data was pulled from the U.S. Environmental Protection Agency, and the authors aggregated power plant-level emissions to the state level.
The authors of the report also took steps to account for changes in market dynamics and state policies that occurred over the 15-year-period to help eliminate the potential that those factors could mask the actual impacts of droughts on emissions.