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The contemporary phenomenon of global change usually refers to the suite of global-scale socio-economic and environmental changes that have been occurring over the past several decades or perhaps a century. Global change is readily observable in the biophysical world. The Earth’s cover of snow and ice is now changing rapidly, from the loss of remnant glaciers on tropical mountains to the retreat of Arctic sea ice in the northern hemisphere summer and the growing evidence of instability in the Greenland and perhaps West Antarctic ice sheets. The Earth’s atmosphere is also changing rapidly, with the global accumulation of greenhouse gases and the regional and seasonal accumulation of aerosol particles. The oceans are also changing, particularly marine ecosystems, with the intensive pressure applied by mechanised fisheries. The Earth’s land surface, however, has been transformed the most profoundly of all aspects of the Earth, with about 50% of ecosystems extensively modified by direct human action. The human imprint can also be seen in the operation of biogeochemical cycles. Human perturbation of the carbon cycle is well known, but human activities have more extensively modified the nitrogen cycle.