The overwhelming majority of climate scientists (98%) agree that human activity is a significant factor in global climate change. The global warming observed by lead scientists over the past 50 years is due primarily to human-induced emissions of heat-trapping gases--mainly from the burning of fossil fuels (coal, oil, and gas) but also from the clearing of forests. Consensus as strong as the one that has developed around global warming is rare in science. Government must play a role in addressing global climate change, although regulated, market-based solutions are likely the most effective approach. The challenge is how to effectively convince policymakers and polluting nations to act upon what is scientific consensus. Fierce political opposition from those who are unconvinced by the science of climate change has made it difficult to address the problem. We must move to reduce greenhouse gas emissions soon before the planet's climate system enters into runaway global warming. And we must look to government and non-government solutions to this potentially devastating problem.