Current Greenhouse Emissions
Currently the US produces 25% of the world’s carbon emissions, totaling around 1.5 billion tons C. The graph below shows how human created emissions have influenced atmospheric carbon levels:
CO2 and other emitted gases such as methane, SO2, and CFCs are known as greenhouse gases for their heat trapping potential. Without going into too much detail, the atmosphere of earth is a beautifully balanced system of negative feedback loops that have maintained conditions viable for life for millions of years. Currently a majority of scientists agree that anthropogenic sources of greenhouse gases have the potential to throw these systems out of balance. Most models show the effects to be in the form of global warming, although the extent is debatable (anywhere from 1 to 8 degrees Celsius).
Greenhouse gas emissions, therefore, are one major reason why we care about the energy sector, its future, and possible energy sources with reduced emissions.
Here is a chart breaking down the national energy sector:
As you can see, fossil fuels account for around 85% of energy consumption in the US. The top figure represents all energy use, including vehicles etc., while the bottom figure is only for electricity production. Mainly because of hydro and nuclear power, non-fossil energy production is higher at 30%.