A decline in U.S. carbon emissions in recent years is unlikely to continue over the long term unless there is a significant shift in how the nation produces and uses its energy, according to a new analysis. While several factors have triggered a 9 percent decline in annual carbon emissions in the U.S. since 2005 — including a decrease in the use of coal-fired electricity as a result of the natural gas boom — the most significant factor has been the economic recession, according to the group Climate Central. As the economy recovers, any reductions in greenhouse gas emissions will likely be offset by increased incomes that drive an greater demand for vehicles, electrical appliances, and other consumer products. The report calculates that U.S. carbon emissions can be reduced 38 percent below 2005 levels by 2035 if several hypothetical changes occur. These include the number of miles driven remaining at today’s level and average vehicles achieving fuel efficiency of 55 miles-per-gallon; significant gains being made in the efficiency of energy-consuming equipment; and natural gas continuing to reduce the share of coal-burning technologies.