Carbon dioxide emissions from the American energy sector are at their lowest level since 1991, federal officials said Wednesday.

The United States energy sector produced 2.5 billion metric tons of carbon dioxide during the first six months of 2016, the federal Energy Information Administration (EIA) reports. That is the lowest level of emissions produced between January and June since 1991.

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The EIA predicted annual energy sector emissions would total nearly 5.2 billion metric tons for all of 2016, the lowest annual level since 1992.

The agency attributed the lower carbon emissions to a handful of factors.

First, it cited mild weather. The first six months of 2016 saw the lowest demand for heating since at least 1949, with warmer winter weather lowering the demand for heating fuels. Total energy consumption fell 2 percent between 2015 and 2016, the agency said.

The energy mix itself has also changed, the agency reported. Consumption of some fossil fuels fell from their 2015 levels, with coal tumbling 18 percent and natural gas falling 1 percent. Petroleum use, however, increased by 1 percent due to low gasoline prices.

Renewable energy use surged 9 percent from 2015 levels. Wind energy accounted for half of that increase, with hydroelectric power, at 35 percent, and solar, at 13 percent, following. The EIA predicts solar power will have the largest capacity increase of any fuel this year.