During the Alaskan Gold Rush thousands of mostly nameless<span> men sought their fortune in a harsh environment and lost</span>
Human activities contribute to climate change by causing changes in Earth's atmosphere in the amounts of greenhouse gases, aerosols (small particles), and cloudiness. The largest known contribution comes from the burning of fossil fuels, which releases carbon dioxide gas to the atmosphere.
In the introduction but it depends where you think your thesis would make your essay more understandable.