7.6 Concluding Remarks
Biogeochemical cycles interact closely with the climate system over a variety of temporal and spatial scales. On geological time scales, this interaction is illustrated by the Vostok ice core record, which provides dramatic evidence of the coupling between the carbon cycle and the climate system. The dynamics of the Earth system inferred from this record result from a combination of external forcing (in this case long-term periodic changes in the orbital parameters of the Earth and hence solar forcing) and an array of feedback mechanisms within the Earth environment (see Chapter 6). On shorter time scales, a range of forcings originating from human activities (conversion and fragmentation of natural ecosystems, emissions of greenhouse gases, nitrogen fixation, degradation of air quality, stratospheric ozone depletion) is expected to produce planet-wide effects and perturb numerous feedback mechanisms that characterise the dynamics of the Earth system.
A number of feedbacks that amplify or attenuate the climate response to radiative forcing have been identified. In addition to the well-known positive water vapour and ice-albedo feedbacks, a feedback between the carbon cycle and the climate system could produce substantial effects on climate. The reduction in surface carbon uptake expected in future climate should produce an additional increase in the atmospheric CO2 concentration and therefore enhance climate forcing. Large differences between models, however, make the quantitative estimate of this feedback uncertain. Other feedbacks (involving, for example, atmospheric chemical and aerosol processes) are even less well understood. Their magnitude and even their sign remain uncertain. Potentially important aerosol-cloud interactions such as changes in cloud lifetime and aerosol effects on ice clouds can influence the hydrologic cycle and the radiative budget; however, the scientific understanding of these processes is low. The response of the climate system to anthropogenic forcing is expected to be more complex than simple cause and effect relationships would suggest; rather, it could exhibit chaotic behaviour with cascades of effects across the different scales and with the potential for abrupt and perhaps irreversible transitions.
This chapter has assessed how processes related to vegetation dynamics, carbon exchanges, gas-phase chemistry and aerosol microphysics could affect the climate system. These processes, however, cannot be considered in isolation because of the potential interactions that exist between them. Air quality and climate change, for example, are intimately coupled (Dentener et al., 2006). Brasseur and Roeckner (2005) estimate that the hypothetical removal from the atmosphere of the entire burden of anthropogenic sulphate aerosol particles (in an effort to improve air quality) would produce a rather immediate increase of about 0.8°C in the globally averaged temperature, with geographical patterns that bear a resemblance to the temperature changes found in greenhouse gas scenario experiments (Figure 7.24). Thus, environmental strategies aimed at maintaining ‘global warming’ below a prescribed threshold must therefore account not only for CO2 emissions but also for measures implemented to improve air quality. To cope with the complexity of Earth system processes and their interactions, and particularly to evaluate sophisticated models of the Earth system, observations and long-term monitoring of climate and biogeochemical quantities will be essential. Climate models will have to reproduce accurately the important processes and feedback mechanisms discussed in this chapter.
Figure 7.24. Effect of removing the entire burden of sulphate aerosols in the year 2000 on (a) the annual mean clear sky TOA shortwave radiation (W m–2) calculated by Brasseur and Roeckner (2005) for the time period 2071 to 2100 and (b) on the annual mean surface air temperature (°C) calculated for the same time period. (c) temporal evolution of global and annual mean surface air temperature anomalies (°C) with respect to the mean 1961 to 1990 values. The evolution prior to the year 2000 is driven by observed atmospheric concentrations of greenhouse gases and aerosols as adopted by IPCC (see Chapter 10). After 2000, the concentration of greenhouse gases remains constant while the aerosol burden is unchanged (blue line) or set to zero (red line). The black curve shows observations (A. Jones et al., 2001; Jones et al., 2006).