Incentives for clean energy systems and use of energy efficient technologies have started to bear fruit for the U.S. This week on Friday it was announced by Bloomberg New Energy Finance (BNEF) for the Business Council for Sustainable Energy (BCSE) that U.S. carbon dioxide emissions fell to their lowest since 1994. Carbon dioxide emission level was peaked in 2007 at 6.02 billion tons (Gt). As of today, it is estimated around 5.24 Gt .
There are several factors leading to this result. The biggest one may be the rapid increase in renewable energy technologies across the country for the last couple of years. BNEF report states that cumulative installed solar, wind, geothermal and biomass-based energy sources in the U.S. have doubled since 2008 and become 86 gigawatts  . The use of renewable sources for energy production in the US had been almost at the constant level from 1980 to 2008; however, it peaked in 2012 as a result of steep increase between 2008 and 2012, according to the data of U.S. Energy Information Administration . Secondly, from the same source, it is seen that there has been a trend of switching from coal to natural gas in energy generation since 2008. This has an influence on carbon dioxide emission rates because burning natural gas releases almost half as much carbon dioxide as that of coal as shown in the figure below .
The other factor is the tendency towards energy efficient systems, especially in automotive industry. Hybrid engines and fully-electric-powered cars have significant contribution to lowering down the emissions.
The important thing is that the U.S. achieved that reduction in carbon dioxide emissions while sustaining the incline in GDP, thus, improving the economic standards of the public, according to the president of BCSE .
Taking all that into account, it is seen that the U.S. is paying more attention to environmentally-friendly solutions in energy production. Although U.S. Energy Information Administration (EIA) predicts that carbon dioxide emission level will raise again until 2040 , this prediction is highly dependent on how much popularity the electric vehicles will gain in the near future. Furthermore, the possible change in the attitude of American people towards smaller but highly-efficient engines for fuel economy and environmental purposes plays a role in that prediction. It also affected by improvements in efficiency and installation capacities of renewable energy systems in industry; however, it is reasonable to say that it will be largely determined by trends in transportation.
. Energy Technology and Policy Lecture Notes, ‘Energy Uses in America’ by Dr. Michael E. Webber, 2013