Executive Summary

Emissions of carbon dioxide, methane, nitrous oxide and of reactive gases such as sulphur dioxide, nitrogen oxides, carbon monoxide and hydrocarbons, which lead to the formation of secondary pollutants including aerosol particles and tropospheric ozone, have increased substantially in response to human activities. As a result, biogeochemical cycles have been perturbed significantly. Nonlinear interactions between the climate and biogeochemical systems could amplify (positive feedbacks) or attenuate (negative feedbacks) the disturbances produced by human activities.

The Land Surface and Climate

• Changes in the land surface (vegetation, soils, water) resulting from human activities can affect regional climate through shifts in radiation, cloudiness and surface temperature.
• Changes in vegetation cover affect surface energy and water balances at the regional scale, from boreal to tropical forests. Models indicate increased boreal forest reduces the effects of snow albedo and causes regional warming. Observations and models of tropical forests also show effects of changing surface energy and water balance.
• The impact of land use change on the energy and water balance may be very significant for climate at regional scales over time periods of decades or longer.

The Carbon Cycle and Climate

• Atmospheric carbon dioxide (CO₂) concentration has continued to increase and is now almost 100 ppm above its pre-industrial level. The annual mean CO₂ growth rate was significantly higher for the period from 2000 to 2005 (4.1 ± 0.1 GtC yr^–1) than it was in the 1990s (3.2 ± 0.1 GtC yr^–1). Annual emissions of CO₂ from fossil fuel burning and cement production increased from a mean of 6.4 ± 0.4 GtC yr^–1 in the 1990s to 7.2 ± 0.3 GtC yr^–1 for 2000 to 2005.^[1]
• Carbon dioxide cycles between the atmosphere, oceans and land biosphere. Its removal from the atmosphere involves a range of processes with different time scales. About 50% of a CO₂ increase will be removed from the atmosphere within 30 years, and a further 30% will be removed within a few centuries. The remaining 20% may stay in the atmosphere for many thousands of years.
• Improved estimates of ocean uptake of CO₂ suggest little change in the ocean carbon sink of 2.2 ± 0.5 GtC yr^–1 between the 1990s and the first five years of the 21st century. Models indicate that the fraction of fossil fuel and cement emissions of CO₂ taken up by the ocean will decline if atmospheric CO₂ continues to increase.
• Interannual and inter-decadal variability in the growth rate of atmospheric CO₂ is dominated by the response of the land biosphere to climate variations. Evidence of decadal changes is observed in the net land carbon sink, with estimates of 0.3 ± 0.9, 1.0 ± 0.6, and 0.9 ± 0.6 GtC yr^–1 for the 1980s, 1990s and 2000 to 2005 time periods, respectively.
• A combination of techniques gives an estimate of the flux of CO₂ to the atmosphere from land use change of 1.6 (0.5 to 2.7) GtC yr^–1 for the 1990s. A revision of the Third Assessment Report (TAR) estimate for the 1980s downwards to 1.4 (0.4 to 2.3) GtC yr^–1 suggests little change between the 1980s and 1990s, and continuing uncertainty in the net CO₂ emissions due to land use change.
• Fires, from natural causes and human activities, release to the atmosphere considerable amounts of radiatively and photochemically active trace gases and aerosols. If fire frequency and extent increase with a changing climate, a net increase in CO₂ emissions is expected during this fire regime shift.
• There is yet no statistically significant trend in the CO₂ growth rate as a fraction of fossil fuel plus cement emissions since routine atmospheric CO₂ measurements began in 1958. This ‘airborne fraction’ has shown little variation over this period.
• Ocean CO₂ uptake has lowered the average ocean pH (increased acidity) by approximately 0.1 since 1750. Consequences for marine ecosystems may include reduced calcification by shell-forming organisms, and in the longer term, the dissolution of carbonate sediments.
• The first-generation coupled climate-carbon cycle models indicate that global warming will increase the fraction of anthropogenic CO₂ that remains in the atmosphere. This positive climate-carbon cycle feedback leads to an additional increase in atmospheric CO₂ concentration of 20 to 224 ppm by 2100, in models run under the IPCC (2000) Special Report on Emission Scenarios (SRES) A2 emissions scenario.

Reactive Gases and Climate

• Observed increases in atmospheric methane concentration, compared with pre-industrial estimates, are directly linked to human activity, including agriculture, energy production, waste management and biomass burning. Constraints from methyl chloroform observations show that there have been no significant trends in hydroxyl radical (OH) concentrations, and hence in methane removal rates, over the past few decades (see Chapter 2). The recent slowdown in the growth rate of atmospheric methane since about 1993 is thus likely due to the atmosphere approaching an equilibrium during a period of near-constant total emissions. However, future methane emissions from wetlands are likely to increase in a warmer and wetter climate, and to decrease in a warmer and drier climate.
• No long-term trends in the tropospheric concentration of OH are expected over the next few decades due to offsetting effects from changes in nitric oxides (NO_x), carbon monoxide, organic emissions and climate change. Interannual variability of OH may continue to affect the variability of methane.
• New model estimates of the global tropospheric ozone budget indicate that input of ozone from the stratosphere (approximately 500 Tg yr^–1) is smaller than estimated in the TAR (770 Tg yr^–1), while the photochemical production and destruction rates (approximately 5,000 and 4,500 Tg yr^–1 respectively) are higher than estimated in the TAR (3,400 and 3,500 Tg yr^–1). This implies greater sensitivity of ozone to changes in tropospheric chemistry and emissions.
• Observed increases in NO_x and nitric oxide emissions, compared with pre-industrial estimates, are very likely directly linked to ‘acceleration’ of the nitrogen cycle driven by human activity, including increased fertilizer use, intensification of agriculture and fossil fuel combustion.
• Future climate change may cause either an increase or a decrease in background tropospheric ozone, due to the competing effects of higher water vapour and higher stratospheric input; increases in regional ozone pollution are expected due to higher temperatures and weaker circulation.
• Future climate change may cause significant air quality degradation by changing the dispersion rate of pollutants, the chemical environment for ozone and aerosol generation and the strength of emissions from the biosphere, fires and dust. The sign and magnitude of these effects are highly uncertain and will vary regionally.
• The future evolution of stratospheric ozone, and therefore its recovery following its destruction by industrially manufactured halocarbons, will be influenced by stratospheric cooling and changes in the atmospheric circulation resulting from enhanced CO₂ concentrations. With a possible exception in the polar lower stratosphere where colder temperatures favour ozone destruction by chlorine activated on polar stratospheric cloud particles, the expected cooling of the stratosphere should reduce ozone depletion and therefore enhance the ozone column amounts.

Aerosol Particles and Climate

• Sulphate aerosol particles are responsible for globally averaged temperatures being lower than expected from greenhouse gas concentrations alone.
• Aerosols affect radiative fluxes by scattering and absorbing solar radiation (direct effect, see Chapter 2). They also interact with clouds and the hydrological cycle by acting as cloud condensation nuclei (CCN) and ice nuclei. For a given cloud liquid water content, a larger number of CCN increases cloud albedo (indirect cloud albedo effect) and reduces the precipitation efficiency (indirect cloud lifetime effect), both of which are likely to result in a reduction of the global, annual mean net radiation at the top of the atmosphere. However, these effects may be partly offset by evaporation of cloud droplets due to absorbing aerosols (semi-direct effect) and/or by more ice nuclei (glaciation effect).
• The estimated total aerosol effect is lower than in TAR mainly due to improvements in cloud parametrizations, but large uncertainties remain.
• The radiative forcing resulting from the indirect cloud albedo effect was estimated in Chapter 2 as –0.7 W m^–2 with a 90% confidence range of –0.3 to –1.8 W m^–2. Feedbacks due to the cloud lifetime effect, semi-direct effect or aerosol-ice cloud effects can either enhance or reduce the cloud albedo effect. Climate models estimate the sum of all aerosol effects (total indirect plus direct) to be –1.2 W m^–2 with a range from –0.2 to –2.3 W m^–2 in the change in top-of-the-atmosphere net radiation since pre-industrial times, whereas inverse estimates constrain the indirect aerosol effect to be between –0.1 and –1.7 W m^–2 (see Chapter 9).
• The magnitude of the total aerosol effect on precipitation is more uncertain, with model results ranging from almost no change to a decrease of 0.13 mm day^–1. Decreases in precipitation are larger when the atmospheric General Circulation Models are coupled to mixed-layer ocean models where the sea surface temperature and, hence, the evaporation is allowed to vary.
• Deposition of dust particles containing limiting nutrients can enhance photosynthetic carbon fixation on land and in the oceans. Climate change is likely to affect dust sources.
• Since the TAR, advances have been made to link the marine and terrestrial biospheres with the climate system via the aerosol cycle. Emissions of aerosol precursors from vegetation and from the marine biosphere are expected to respond to climate change.