The following summary contains some material more fully discussed in Chapter 1 (which was extracted from the 1994 IPCC Report): Bullets containing significant new information are marked “***”; those containing updated information are marked “**”; and those which contain information which is essentially unchanged are marked “*”.
Climate change can be driven by changes in the atmospheric concentrations of a number of radiatively active gases and aerosols. We have clear evidence that human activities have affected concentrations, distributions, and life cycles of these gases. These matters, discussed in this chapter, were assessed at greater length in IPCC WGI report “Radiative Forcing of Climate Change” (IPCC 1994).
* Carbon dioxide concentrations have increased by almost 30% from about 280 ppmv in the late 18th century to 358 ppmv in 1994. This increase is primarily due to combustion of fossil fuel and cement production, and to land-use change. During the last millennium, a period of relatively stable climate, concentrations varied by about ±10 ppmv around the pre-industrial value of 280 ppmv. On the century time-scale these fluctuations were far less rapid than the change observed over the 20th century.
*** The growth rate of atmospheric CO2 concentrations over the last few years is comparable to, or slightly above, the average of the 1980s (∼1.5 ppmv/yr). On shorter (interannual) time-scales, after a period of slow growth (0.6 ppmv/yr) spanning 1991 to 1992, the growth rate in 1994 was higher (∼2 ppmv/yr).