Figure 1: GLOBAL CARBON CYCLE: Arrows show the fluxes (in petagrams of carbon per year) between the atmosphere and its two primary sinks, the land and ocean, averaged over the 1980s. Anthropogenic fluxes are in red; natural fluxes in black.The net flux between reservoirs is balanced for natural processes, but not for anthropogenic fluxes. Within the boxes, black number give the preindustrial sizes of the reservoirs and red numbers denote the changes resulting from human activities since preindustrial times. For the land sink, the first red number is an inferred terrestrial land sink whose origin is speculative; the second one is a decrease due to deforestation. Numbers are slight modifications of those published by the Intergovernmental Panel on Climate change. NPP is net primary production.

Figure 2: ANTHROPOGENIC CARBON DIOXIDE LEVELS have increased by more than 30% since the industrial revolution started in the mid 18th century. Before then, atmospheric CO2 had been at 280ppm for several millenia. Data before 1958 stem from measurements of air bubbles trapped in ice cores recovered from several sites in Antarctica. The CO2 data displayed in the inset were measured on air samples taken by Charles D. Keeling and collaborators at Mauna Loa, Hawaii. The seasonal variations evident there reflect the seasonal breathing of the terrestrial biosphere in the Northern hemisphere: CO2 is removed by strong growth in spring and summer and returned by respiration and remineralization in the fall and winter.

Figure 3: GROWTH RATE OF CARBON RESERVOIRS: Since 1958, the yearly accumulation rate of atmospheric carbon dioxide has grown, on average, from about 1PgC/yr to about 3.0 PgC/yr (light blue area). Over the same period, fossil fuel emissions (red line) have grown from about 2.5 PgC/yr to about 6.5 PgC/yr. Net uptake by the ocean or terrestrial biosphere (green region) must account for the difference. Note the large interannual variation in the annual atmospheric CO2 growth rate. Higher growth rates generally appear to be associated with El Nino episodes (arrows), the exception being the period following the eruption of Mt. Pinatubo in the early 1990s.

Figure 4: ANTHROPOGENIC CARBON CONCENTRATIONS IN THE OCEAN along the track shown as a red line in the inset.The black area represents the depth of the sea floor. The anthropogenic carbon dioxide concentrations are separated from the natural background by using a recently developed analysis applied to high-precision measurements of dissolved inorganic carbon. Uncertainties in this separation technique are so large south of 60S that they are shown as contour lines only. Anthropogenic carbon has penetrated significantly below about 2000 meters of the water column only in the North Atlantic, where surface waters sink directly into the abyss.

Figure 5: CHANGES IN LAND USE IN THE US and their impact on carbon stocks. As the eastern US was settled in the 18th and 19th centuries, primary vegetation (green) was converted to crops (yellow) and pastures (blue), as seen in the left panels. The impact of old growth logging is symbolized by a direct conversion from primary to secondary vegetation (orange). As the primary areas for cropland and pasture migrated west, some eastern and southern lands became abandoned and started to regrow secondary vegetation, a process that was nearly complete by the late 20th century. This sequence of land use changes had decimated the high carbon stocks contained in the eastern and southern forests by 1920, but 1990, the stocks were slowly bouncing back (right panels). Carbon stocks have also increased in the southwestern US, thanks to fire suppression and other land management practices.

Figure 6: FUTURE DISTRIBUTION OF CARBON afrom fossil-fuel emissions, as estimated by (a) the Hadley Centre model and (b) the Institut Pierre Simon Laplace model. The two models differ strongly in their sensitivity to global warming. The primary difference is the land sink, which is far weaker in the Hadley model and actually becomes a source of carbon (yellow area) by the century's end.