In particular, delegations from 195 nations, including the United States, adopted the Paris Agreement in 2015, creating an international structure for nations to pledge to abate their GHG emissions, adapt to climate change, and cooperate to achieve these ends, including financial and other support. Pursuant to the Paris Agreement, the United States pledged (in 2015) to reduce GHG emissions by 26%-28% by 2025 compared to 2005 levels. In 2017, President Trump announced his intention to withdraw from the Paris Agreement. Under the provisions of the Paris Agreement, this cannot be completed before November 4, 2020. GHG Emission Sources

GHG emissions are generated throughout the United States from millions of discrete sources: power plants, industrial facilities, vehicles, households, commercial buildings, and agricultural activities (e.g., soils and livestock). As the figure indicates, GHG emissions in the electric power sector have historically accounted for the largest percentage of total U.S. GHG emissions. CO2 Emissions from Fossil Fuel Combustion

 Figure 4 illustrates the 2017 U.S. CO 2 emission contributions by sector from the combustion of fossil fuels. CO2 Emission Projections in the Electricity Sector

As the electricity sector contributes a large percentage (35%) of CO 2 emissions from fossil fuel combustion, policymakers and stakeholders are paying attention to both recent trends and future projections of CO 2 emissions in the electricity generation sector. Multiple factors will likely impact CO 2 emission levels from the electricity sector. In recent years, several groups, including EPA and EIA, have prepared projections of CO 2 emission levels in the electricity sector. When comparing a reference case to a CPP scenario in the same model, the results generally indicate that the 2015 CPP final rule would have an impact on 2030 CO 2 emission levels from electricity generation. Concluding Observations

International negotiations and domestic policy developments continue to generate congressional interest in current and projected U.S. GHG emission levels. Recent changes in the U.S. electricity generation portfolio between 2005 and 2017 played a key role:
Coal's contribution to total electricity generation decreased from 50% to 30%; Natural gas's contribution to total electricity generation increased from 19% to 32%; and Non-hydro renewable energy (wind and solar) generation increased from 2% to 10%. Accurately forecasting future emission levels is a complex and challenging endeavor. In general, actual emissions have remained well below projections.
International negotiations and domestic policy developments continue to generate congressional interest in current and projected U.S. greenhouse gas (GHG) emission levels. In December 2015, delegations from 195 nations, including the United States, adopted an agreement in Paris that creates an international structure for nations to pledge to abate their GHG emissions, adapt to climate change, and cooperate to achieve these ends, including financial and other support. Pursuant to that agreement, the United States pledged (in 2015) to reduce GHG emissions by 26-28% by 2025 compared to 2005 levels. In 2017, President Trump announced his intention to withdraw from the Paris Agreement, but under the provisions of the agreement, this cannot be completed before November 4, 2020.
GHG emissions are generated throughout the United States from millions of discrete sources: vehicles, power plants, industrial facilities, households, commercial buildings, and agricultural activities (e.g., soils and livestock). Of the GHG source categories, carbon dioxide (CO2) emissions from fossil fuel combustion account for the largest percentage (76%) of total U.S. GHG emissions. Among the sectors, transportation contributes the largest percentage (36%) of CO2 emissions from fossil fuel combustion, with electric power second at 35%.
Recent changes in the U.S. electricity generation portfolio played a key role in the CO2 emission decrease. The electricity portfolio affects CO2 emission levels, because different sources of electricity generation produce different rates of CO2 emissions per unit of electricity (zero in the case of some renewables). The figure below illustrates the changes in electricity portfolio between 2005 and 2017. Highlights include:
Coal's contribution to total electricity generation decreased from 50% to 30%; Natural gas's contribution to total electricity generation increased from 19% to 32%; and Non-hydro renewable energy (wind and solar) generation increased from 2% to 10%.
In recent years, several groups have prepared projections of CO2 emission levels in the electricity sector. The results generally indicate that the 2015 Clean Power Plan would have an impact on CO2 emission levels from electricity generation. In addition, reference case scenarios in more recent studies (2018) project lower emissions by 2030 when compared to reference cases from earlier studies.
Multiple factors will likely impact electricity sector CO2 emissions levels, including the electricity generation portfolio, the relative prices of fossil fuels, federal and/or state policy developments, economic impacts, and improvements in demand-side energy efficiency. Accurately forecasting future CO2 emission levels is a complex and challenging endeavor. A comparison of actual CO2 emissions between 1990 and 2017 with selected emission projections illustrates this difficulty. In general, actual emissions have remained well below projections.