ABSTRACT
The global-mean temperature at Earth’s surface has increased by about 0.6°C (1.1°F) over the past century. Most of this warming is due to the excess greenhouse gases emitted to the atmosphere by human activities such as fossil fuel use, agriculture practices, and land-use change. Many changes in natural systems have already resulted from this warming, including melting of glaciers and ice caps, increasing sea level, extended growing seasons, changes in precipitation regimes, and changes in the geographical distributions of plant and animal species. Current projections, based on reasonable assumptions of future energy-use practices, indicate that Earth will continue to warm during the 21st century and beyond, in part because parts of the Earth system respond slowly to changes in greenhouse gas levels.
