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Human activity is causing the global climate system to change. The main activity causing climate change is the burning of fossil fuels, which releases greenhouse gases into the atmosphere. Climate changes are already having significant impacts to both environmental systems and human systems, and even larger impacts are expected in the future. Unfortunately, the impacts are predominantly negative. For this reason, people are responding to climate change by adapting to the changes (adaptation) and by reducing greenhouse gas emissions (mitigation). Mitigation is important but difficult given how many human activities emit greenhouse gases. Indeed, greenhouse gas emissions are closely tied to a large portion of human development, in particular industrial activity and much of agriculture. Mitigation is also difficult given that it is a collective action problem at the global scale. Global collective action on climate change is attempted, mainly via treaty negotiations organized by the United Nations. However, these attempts have been largely unsuccessful thus far. Much work remains on mitigation as well as adaptation at all scales from the individual to the global.