Contributions of weather and fuel mix to recent declines in US energy and carbon intensity

A recent (1996–2000) acceleration of declines in energy and carbon intensity in the US remains largely unexplained. This study uses Divisia decomposition and regression to test two candidate explanations—fuel mix and weather. The Divisia method demonstrates that fuel mix does not explain the declines in carbon intensity. The fuel mix, both overall and for electricity generation, became slightly more carbon intensive over the study period (though the slight trend reversed before the end of the period). A regression-based correction to the Divisia indices, accounting for variation in heating- and cooling-degree-days, indicates that warmer weather accounts for approximately 30% of the total declines. This leaves declines of more than 2% per year (and an acceleration of more than 1% over the previous decade) remaining to be explained.