energy use and development worldwide (as indicated in Figure 6.1). However, recent studies suggest that the rate at which CO2 is removed from the atmosphere by ocean and land sinks may also be declining (Canadell et al., 2007; Khatiwala et al., 2009). The reasons for this decline are not well understood, but, if it continues, atmospheric CO2 concentrations would rise even more sharply, even if global CO2 emissions remain the same. Improving our understanding and estimates of current and projected future fluxes of CO2 to and from the Earth’s surface, both over the oceans and on land, is a key research need (research needs are discussed at the end of the chapter).
To determine how CO2 levels varied prior to direct atmospheric measurements, scientists have studied the composition of air bubbles trapped in ice cores extracted from the Greenland and Antarctic ice sheets. These remarkable data, though not as accurate and precise as the Keeling curve, show that CO2 levels were relatively constant for thousands of years preceding the Industrial Revolution, varying in a narrow band between 265 and 280 ppm, before rising sharply starting in the late 19th century (Figure 6.3). The current CO2 level of 388 ppm is thus almost 40 percent higher Scripps Institution of Oceanography