Currently, the atmospheric carbon dioxide concentration is >400 ppm, an increase of more than 50% over typical values of 260 ppm for an interglacial climate like today when glaciers and ice sheets are relatively small. This represents the highest atmospheric CO2 concentrations seen in at least 800,000 years and probably longer. These levels are unprecedented for a time when the geologic and biologic sinks (removal processes) for carbon dioxide are relatively strong and favor an atmosphere with low CO2.

Barring an immediate and dramatic change in the types of energy we use and how we use it, an atmospheric carbon dioxide concentration of 600 ppm will likely be reached by the end of this century. A higher concentration may even be attained if the ocean and/or biosphere become net sources of carbon dioxide as they respond to climate change; currently both are net sinks of carbon dioxide and have taken up about 1/3 of the carbon released by fossil fuel use.

From a purely scientific point of view, this is the grandest experiment ever undertaken by human hands: change the composition of Earthâs atmosphere and see how the climate, oceans, and biosphere react to that change. We are only just beginning a large change in the way the Earth system operates in response to enhanced atmospheric carbon dioxide; these changes will continue for several generations and some will be irreversible. At what point should we end this experiment? How high will we allow atmospheric carbon dioxide to get (and how much change in climate, ocean, and biosphere will we tolerate) before we choose to address this issue in a meaningful way? Or, should the "experiment" be allowed to continue? Why?