Polar bears in the Arctic use the sea ice that covers the North Pole as a platform for hunting seals, the mainstay of their diet. Recent data suggest that Arctic sea ice may
have thinned as much as 40% over the past 50 years. If the polar ice continues to
melt, it is likely that the polar bear will become extinct. Many scientists believe that
the melting of the Arctic sea ice is due primarily to human activities. Which of the
following human activities would have the most direct effect on the thickness of the
polar ice cap?