The phrase "largest source" refers to something that is considered as the biggest or most significant contributor among other things. It indicates a primary, major, or chief element in a particular context.
Sentences with «largest source»
They are the single largest source of greenhouse gases, accounting for 26 per cent of our emissions. (newscientist.com)
And the oil and gas industry is the single largest source of methane pollution in the country. (edf.org)
For example, soil is second only to oceans as the planet's largest carbon sink, while agriculture and land use changes represent the second largest source of greenhouse gas emissions. (sciencedaily.com)