Observed trends in Earth System behavior

The behavior of the Earth System over the past two centuries has been dominated by the rapid rise of human activities as a significant geophysical force at the global scale. After the proximate and ultimate human drivers of change in the Earth System are described, the cumulative impact of these drivers on the structure and functioning of the Earth System are explored. Although the imprints of human activities are most profound on the land surface and in the atmosphere, significant effects are also discernible in the coastal seas, in the ocean, and-indirectly-in the cryosphere. The perturbations to the carbon cycle byhumanactivities,most notably the burning of fossil fuels, is the most well-known example of change in global biogeochemical cycling over the past two centuries. However, human modification of the nitrogen cycle is arguably even more pervasive, and other biogeochemical cycles, such as the phosphorus and sulfur cycles, have also been significantly altered by human activities. The changes to the planet's biodiversity over the past two centuries have been profound and continue to accelerate; the Earth is now in the midst of its sixth great extinction event. All of these human-driven changes have implications for the climate system, and in turn are affected by changes in the physical climate. The concept of the Anthropocene-the proposal that the Earth has entered a new geological epoch-is a powerful way to understand the many interacting ways in which over six billion humans have collectively become a geophysical force that rivals the great forces of Nature and are now driving accelerating changes to the behavior of the Earth System.