Emissions of carbon dioxide from the burning of fossil fuels have ush-ered in a new epoch where human activities will largely determine the evolution of Earths climate. Because carbon dioxide in the atmosphere is long lived, it can effectively lock Earth and future generations into a range of impacts, some of which could become very severe. Therefore, emissions reductions choices made today matter in determining impacts experienced not just over the next few decades, but in the coming centuries and millen-nia. Policy choices can be informed by recent advances in climate science that quantify the relationships between increases in carbon dioxide and global warming, related climate changes, and resulting impacts, such as changes in streamflow, wildfires, crop productivity, extreme hot summers, and sea level rise.Since the beginning of the industrial revolution, concentrations of green-house gases from human activities have risen substantially. Evidence now shows that the increases in these gases very likely (90 percent chance) account for most of Earths warming over the past 50 years. Carbon dioxide is the greenhouse gas produced in the largest quantities, accounting for more than half of the current impact on Earths climate. Its atmospheric concentration has risen about 35 percent since 1750 and is now at about 390 ppmv, the highest level in at least 800,000 years. Depending on emissions rates, carbon dioxide concentrations could double or nearly triple from todays level by the end of the century, greatly amplifying future human impacts on climate. Society is beginning to make important choices regarding future green-house gas emissions. One way to inform these choices is to consider the projected climate changes and impacts that would occur if greenhouse gases in the atmosphere were stabilized at a particular concentration level. The information needed to understand such targets is multifaceted: how do emissions affect global atmospheric concentrations and in turn global warming and its impacts.