Changing feedbacks in the climate–biosphere system

© The Ecological Society of America www.frontiersinecology.org T factors that determine properties of both ecosystems and the climate system have changed more rapidly in the past 50 years than during the previous 10 000 years (Steffen et al. 2004; Figure 1). Our children will probably see even more profound changes during their lifetimes. Some of these, such as changes in climate and atmospheric composition, alter the dynamic interactions between land, ocean, and atmosphere and, therefore, future transformations in climate and the ecosystems on which society depends (IPCC 2007a). Development of policies that reduce rates of climate change, while sustaining the services provided by ecosystems, requires a clear understanding of these dynamics. Changes in ecosystems influence the climate system through several processes (Figure 2), including (1) emission of greenhouse gases, which causes an imbalance in the Earth’s energy budget at the top of the atmosphere; (2) altered albedo (the proportion of solar radiation that the Earth’s surface reflects back to space), which influences the amount of heat transferred from ecosystems to the atmosphere; (3) altered evapotranspiration (evaporation from the Earth’s surface plus that from leaves), which cools the surface and provides moisture to form clouds and fuel atmospheric mixing; (4) altered long-wave radiation, which depends on surface temperature and cloudiness; (5) changes in production of aerosols (small particles that scatter and absorb light); and (6) changes in surface roughness, which determines the strength of coupling between the atmosphere and the surface and, therefore, the efficiency of water and energy exchange. For trace gases and aerosols, the impact of an individual constituent on climate depends on the magnitude of the instantaneous forcing and the turnover time of each constituent in the atmosphere (ie the total quantity divided by the average rate of input and loss; Table 1). In general, energy, water, and highly reactive compounds from fossil-fuel emissions (eg nitric oxide, sulfur dioxide) have such short atmospheric lifespans that they have strong local or regional effects, as well as global consequences. In contrast, the effects of greenhouse gases, such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), are globally dispersed, so their impacts are averaged over the entire planet. Discussions about, and efforts to reduce human impacts on, the climate system have generally focused on greenRECENT ADVANCES SERIES RECENT ADVANCES SERIES