Climate Change 2001: The Scientific Basis – C.1 Observed Changes in Globally Well-Mixed Greenhouse Gas Concentrations and Radiative Forcing

Over the millennium before the Industrial Era, the atmospheric concentrations of greenhouse gases remained relatively constant. Since then, however, the concentrations of many greenhouse gases have increased directly or indirectly because of human activities.

Table 1 provides examples of several greenhouse gases and summarises their 1750 and 1998 concentrations, their change during the 1990s, and their atmospheric lifetimes. The contribution of a species to radiative forcing of climate change depends on the molecular radiative properties of the gas, the size of the increase in atmospheric concentration, and the residence time of the species in the atmosphere, once emitted. The latter – the atmospheric residence time of the greenhouse gas – is a highly policy relevant characteristic. Namely, emissions of a greenhouse gas that has a long atmospheric residence time is a quasi-irreversible commitment to sustained radiative forcing over decades, centuries, or millennia, before natural processes can remove the quantities emitted.

Carbon dioxide (CO2)

The atmospheric concentration of CO2 has increased from 280 ppm5 in 1750 to 367 ppm in 1999 (31%, Table 1). Today’s CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The rate of increase over the past century is unprecedented, at least during the past 20,000 years (Figure 10). The CO2 isotopic composition and the observed decrease in Oxygen (O2) demonstrates that the observed increase in CO2 is predominately due to the oxidation of organic carbon by fossil-fuel combustion and deforestation. An expanding set of palaeo-atmospheric data from air trapped in ice over hundreds of millennia provide a context for the increase in CO2 concentrations during the Industrial Era (Figure 10). Compared to the relatively stable CO2 concentrations (280 ± 10 ppm) of the preceding several thousand years, the increase during the Industrial Era is dramatic. The average rate of increase since 1980 is 0.4%/yr. The increase is a consequence of CO2 emissions. Most of the emissions during the past 20 years are due to fossil fuel burning, the rest (10 to 30%) is predominantly due to land-use change, especially deforestation. As shown in Figure 9, CO2 is the dominant human-influenced greenhouse gas, with a current radiative forcing of 1.46 Wm-2, being 60% of the total from the changes in concentrations of all of the long-lived and globally mixed greenhouse gases.

Direct atmospheric measurements of CO2 concentrations made over the past 40 years show that year to year fluctuations in the rate of increase of atmospheric CO2 are large. In the 1990s, the annual rates of CO2 increase in the atmosphere varied from 0.9 to 2.8 ppm/yr, equivalent to 1.9 to 6.0 PgC/yr. Such annual changes can be related statistically to short-term climate variability, which alters the rate at which atmospheric CO2 is taken up and released by the oceans and land. The highest rates of increase in atmospheric CO2 have typically been in strong El Niño years (Box 4). These higher rates of increase can be plausibly explained by reduced terrestrial uptake (or terrestrial outgassing) of CO2 during El Niño years, overwhelming the tendency of the ocean to take up more CO2 than usual.

Partitioning of anthropogenic CO2 between atmospheric increases and land and ocean uptake for the past two decades can now be calculated from atmospheric observations. Table 2 presents a global CO2 budget for the 1980s (which proves to be similar to the one constructed with the help of ocean model results in the SAR) and for the 1990s. Measurements of the decrease in atmospheric oxygen (O2) as well as the increase in CO2 were used in the construction of these new budgets. Results from this approach are consistent with other analyses based on the isotopic composition of atmospheric CO2 and with independent estimates based on measurements of CO2 and 13CO2 in seawater. The 1990s budget is based on newly available measurements and updates the budget for 1989 to 1998 derived using SAR methodology for the IPCC Special Report on Land Use, Land-Use Change and Forestry (2000). The terrestrial biosphere as a whole has gained carbon during the 1980s and 1990s; i.e., the CO2 released by land-use change (mainly tropical deforestation) was more than compensated by other terrestrial sinks, which are likely located in both the northern extra-tropics and in the tropics. There remain large uncertainties associated with estimating the CO2 release due to land-use change (and, therefore, with the magnitude of the residual terrestrial sink).

Process-based modelling (terrestrial and ocean carbon models) has allowed preliminary quantification of mechanisms in the global carbon cycle. Terrestrial model results indicate that enhanced plant growth due to higher CO2 (CO2 fertilisation) and anthropogenic nitrogen deposition contribute significantly to CO2 uptake, i.e., are potentially responsible for the residual terrestrial sink described above, along with other proposed mechanisms, such as changes in land-management practices. The modelled effects of climate change during the 1980s on the terrestrial sink are small and of uncertain sign.