what does it mean
Recently, I read a article, and one sentence i dont understand. "but the main reason is that natural gas is replacing coal everywhere as an energy source ,and gas emits half as much carbon dioxide as coal. ". In the sentence, what does "...and gas emits half as much carbon dioxide as coal" mean？gas emission =0.5 carbon emission=0.5 coal emission?

Question

what does it mean
Recently, I read a article, and one sentence i dont understand. "but the main reason is that natural gas is replacing coal everywhere as an energy source ,and gas emits half as much carbon dioxide as coal. ".  In the sentence, what does "...and gas emits half as much carbon dioxide as coal" mean？gas emission =0.5 carbon emission=0.5 coal emission?

Answer

gas emission =0.25 carbon dioxide 
coal emission = 0.50 carbon dioxide
that is the correct understanding