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David Keeling altered our perspectives about global change with his painstaking observations of atmospheric carbon dioxide. The now famous ‘Keeling curve’, measuring both the pulse of Nature and a steadily rising human
impact on atmospheric composition, is invariably hailed as our most rigorous and fundamental measure of global change. Carbon dioxide is joined by other key metrics that help define the causes and consequences of global climate change. The Earth’s history provides our best indication of the levels of change that are likely to have deleterious effects on humans and wildlife, and constitute “dangerous anthropogenic interference” with nature. The Earth’s temperature, with rapid global warming over the past 30 years, is now passing through the peak level of the Holocene, a period of relatively stable climate that has existed for more than 10,000 years. Further warming of more than 1ºC will make the Earth warmer than it has been in a million years. “Business-as-usual” scenarios, with fossil fuel CO2 emissions continuing to increase ~2%/year as in the past decade, yield additional warming of 2 or 3°C this century and imply changes that constitute practically a different planet.