Not true. While burning natural gas releases about half the CO2 as burning coal, US power plants only account for 32% of greenhouse gas emissions. Even if every power plant in the US was converted to natural gas (it's currently 41% coal, 27% gas) , overall emissions would decrease by about 16%.
If I am reading these charts correctly, then in 2015 Coal and Natural Gas provided about the same amount of energy.
Table ES1.A. Total Electric Power Industry Summary Statistics, 2015 and 2014
Net Generation (thousand megawatthours) July 2015:
• Coal: 139,413
• Natural Gas: 139,997
• All Energy Sources: 399,620
~35%
Coal was down ~7% from last year
Natural Gas up ~22% from last year
http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_es1aIt is interesting to look at the numbers from 2010.
Table ES1.A. Total Electric Power Industry Summary Statistics, 2010 and 2009
Net Generation (thousand megawatthours) July 2010:
• Coal: 132,955 (43%)
• Natural Gas: 76,631(25%)
• All Energy Sources: 307,054
http://www.eia.gov/electricity/monthly/current_year/january2011.pdfPercentages have changed a lot, but the total amount of coal energy has only gone down a bit.
I think that the “20%” claim is based on what would have happened if coal and natural gas were produced in the same proportions as before. (I could be wrong about this reasoning though).
BTW, I think that your 32% number is correct for % for power (megawatthours) generation in the US. There are, however, non-power generation uses of coal around the globe that take that number up to about 40%.