The most serious impact of modern human activities is the release of large quantities of carbon dioxide and methane into the atmosphere primarily as a result of our use of fossil fuels.
(The most serious impact of modern human activities; is; the release of large quantities of carbon dioxide and methane into the atmosphere primarily as a result of our use of fossil fuels)
The most serious impact of modern human activities
is
the release of large quantities of carbon dioxide and methane into the atmosphere primarily as a result of our use of fossil fuels