According to the US Department of State -
"Energy use for heating and cooling is directly responsive to weather variation. The AEO forecast of CO 2 emissions assumes 30-year average values for population-weighted heating and cooling degree-days. Unlike other sources of uncertainty, for which deviations between assumed and actual trends may follow a persistent course over time, the effect of weather on energy use and emissions in any particular year is largely independent from year to year." I get it, the weather will either be the same or different and that is certainly not an uncertainty.