7.1.1 Terrestrial Ecosystems and Climate

The terrestrial biosphere interacts strongly with the climate, providing both positive and negative feedbacks due to biogeophysical and biogeochemical processes. Some of these feedbacks, at least on a regional basis, can be large. Surface climate is determined by the balance of fluxes, which can be changed by radiative (e.g., albedo) or non-radiative (e.g., water cycle related processes) terms. Both radiative and non-radiative terms are controlled by details of vegetation. High-latitude climate is strongly influenced by snow albedo feedback, which is drastically reduced by the darkening effect of vegetation. In semi-arid tropical systems, such as the Sahel or northeast Brazil, vegetation exerts both radiative and hydrological feedbacks. Surface climate interacts with vegetation cover, biomes, productivity, respiration of vegetation and soil, and fires, all of which are important for the carbon cycle. Various processes in terrestrial ecosystems influence the flux of carbon between land and the atmosphere. Terrestrial ecosystem photosynthetic productivity changes in response to changes in temperature, precipitation, CO₂ and nutrients. If climate becomes more favourable for growth (e.g., increased rainfall in a semi-arid system), productivity increases, and carbon uptake from the atmosphere is enhanced. Organic carbon compounds in soils, originally derived from plant material, are respired (i.e., oxidized by microbial communities) at different rates depending on the nature of the compound and on the microbial communities; the aggregate rate of respiration depends on soil temperature and moisture. Shifts in ecosystem structure in response to a changing climate can alter the partitioning of carbon between the atmosphere and the land surface. Migration of boreal forest northward into tundra would initially lead to an increase in carbon storage in the ecosystem due to the larger biomass of trees than of herbs and shrubs, but over a longer time (e.g., centuries), changes in soil carbon would need to be considered to determine the net effect. A shift from tropical rainforest to savannah, on the other hand, would result in a net flux of carbon from the land surface to the atmosphere.

7.1.2 Ocean Ecosystems and Climate

The functioning of ocean ecosystems depends strongly on climatic conditions including near-surface density stratification, ocean circulation, temperature, salinity, the wind field and sea ice cover. In turn, ocean ecosystems affect the chemical composition of the atmosphere (e.g. CO₂, N₂O, oxygen (O₂), dimethyl sulphide (DMS) and sulphate aerosol). Most of these components are expected to change with a changing climate and high atmospheric CO₂ conditions. Marine biota also influence the near-surface radiation budget through changes in the marine albedo and absorption of solar radiation (bio-optical heating). Feedbacks between marine ecosystems and climate change are complex because most involve the ocean’s physical responses and feedbacks to climate change. Increased surface temperatures and stratification should lead to increased photosynthetic fixation of CO₂, but associated reductions in vertical mixing and overturning circulation may decrease the return of required nutrients to the surface ocean and alter the vertical export of carbon to the deeper ocean. The sign of the cumulative feedback to climate of all these processes is still unclear. Changes in the supply of micronutrients required for photosynthesis, in particular iron, through dust deposition to the ocean surface can modify marine biological production patterns. Ocean acidification due to uptake of anthropogenic CO₂ may lead to shifts in ocean ecosystem structure and dynamics, which may alter the biological production and export from the surface ocean of organic carbon and calcium carbonate (CaCO₃).