2. Projected Changes in Future Climate

Temperature in the United States has warmed over the past 100 years, with high rates of warming in Alaska (∼4.5 °C) and the West (∼1.5 °C), whereas precipitation has increased in the East and South and decreased in the Southwest. Global climate models project a steady increase in future temperature through the end of the twenty-first century. Compared to 1971 through 2000, average annual air temperature will likely increase from 0.8 to 1.9 °C by 2050, from 1.4 to 3.1 °C by 2070, and from 2.5 to 5.3 °C by 2099, where the range is bounded by the B2 (low) and A2 (high) greenhouse gas emission scenarios. Temperature increases will be higher in northern and interior areas of the United States, especially during the winter, and extreme droughts are expected to increase. Changes in precipitation are expected to be small (higher in some regions, lower in others), although potential changes in timing and spatial distribution of extreme precipitation events may occur. Sea level may rise by as much as 2 m, affecting coastal forests and human communities. Most climate models project similar climatic trends until around 2050, but diverge considerably after that. Users of climate information often represent future climate with a range of output from different climate models and emission scenarios. Given that greenhouse gas emissions will likely increase unabated for at least the next few decades, using a high emission scenario will provide a more accurate future climate for forest management and planning.