The world must halt its increase in greenhouse gases within the next three years, cut them by 43% in the next seven, and eliminate them entirely by 2050 to avoid catastrophic and irreversible climate change impacts this century, according to a new report released this week by the Intergovernmental Panel on Climate Change (IPCC). But along with dire warnings, the assessment also brings good news: We already have the tools necessary to make these changes, and they're cheaper than ever before.