Ecosystems' influence on climate

Ecosystems influence climate in at least three ways: altering energy balance, regulating water-vapor dynamics via evapotranspiration, and changing GHG cycling in the atmosphere. The ecosystem processes that influence climate change are also influenced by climate, forming ecosystem-climate feedback systems on local, regional, and global scales.

Background

An ecosystem is a functional system, encompassing all organisms (plants, animals, and microorganisms) and all elements of the nonliving physical environment that interact together in a given area. Organisms extract chemical elements (including water, carbon dioxide, and nutrients) as substrates from the physical environment, using these substrates for their own survival, growth, and reproduction. Physical processes and chemical reactions in the environment are catalyzed by organisms so as to influence and to form biogeochemcial cycles of carbon, water, and other elements within the system. Ecosystems can be bounded on various scales, from a microcosm to the entire planet.

Climate and Geographical Distributions of Ecosystems

Various types of ecosystems exist on Earth, including ocean ecosystems, land ecosystems, and freshwater ecosystems on a broad scale. Within land ecosystems, vegetation displays different patterns, forming different ecosystems at regional scales, such as forests, deserts, grasslands, and croplands. Except for artificial ecosystems, patterns of natural ecosystems are primarily shaped by climate conditions (such as temperature and precipitation). Along a precipitation from wet to dry regions, ecosystem types change from forests, woodlands, and grasslands to deserts. Along a temperature gradient from the equator to the polar region, ecosystems vary from tropical forests, subtropical forests, temperate deciduous forests, temperature mixed forests, and boreal forests to tundra. In polar climate zones with average temperatures below 10° Celsius in all twelve months of the year, ecosystems include tundra and in Antarctica and in inner Greenland. Thus, climate and other physical environmental characteristics determine the distribution of ecosystems on the globe.

Ecosystem Responses to Climate Change

Ecosystems are very sensitive to changes in temperature, atmospheric carbon dioxide (CO₂), and precipitation. Rising atmospheric CO₂ primarily stimulates carbon influx, leading to increases in carbon sequestration and thus potentially mitigating climate change. Rising atmospheric CO₂ concentration has relatively minor impacts on energy balance and water exchange at the surface. Climate warming influences ecosystem feedback related to climate change in several ways, such as exchange of greenhouse gases (GHGs), surface energy balance, and water cycling. It is generally assumed that warming affects carbon release more than carbon uptake, leading to net carbon loss from land ecosystems to the atmosphere. Temperature also affects phenology and length of growing seasons, nutrient availability, and species composition. All these processes influence carbon balances, potentially leading to the net carbon uptake from the atmosphere and negative ecosystem feedback to climate warming.

Increasing temperature also stimulates evapotranspiration, resulting in cooler land surfaces in wet regions and thus negative feedback to climate change. The ecohydrological feedback to climate warming via altering land surface energy balance is weak in dry regions. Altered precipitation regimes (that is, alterations in amount, seasonality, frequency, and intensity) under climate change modify ecosystem carbon cycling, energy balance, and water exchange with the atmosphere. Increased precipitation, for example, usually stimulates plant productivity and ecosystem carbon uptake from the atmosphere. Decreased precipitation generally causes land surfaces to be warmer and generates a higher albedo than does ambient precipitation. Impacts of altered precipitation seasonality, frequency, and intensity are complex and region-specific. In addition, precipitation regimes have long-term impacts on soil development, nutrient availability and vegetation distributions, which can be different from short-term impacts of precipitation on ecosystem processes. Moreover, climate change involves a suite of changes in temperature, precipitation, and GHGs. Those global change factors can interactively influence ecosystem processes and their feedback to climate change.

Ecosystem Regulation of Climate Change Via Energy Balance

Land surface energy balance influences the climate system by causing fluctuations in temperature, winds, ocean currents, and precipitation. The surface energy balance, in turn, is determined by fractions of absorbed, emitted, and reflected incoming solar radiation. One of the key parameters to determine the energy balance at the land surface is albedo, which regulates differences between the amount of absorbed shortwave radiation (input) and the outgoing longwave radiation (output). Different vegetation covers have different values. When land use and land cover changes occur due to either climate change or activities, land surface energy balance is altered. Overgrazing, for example, may increase albedo. As a consequence, decreases with associated decline in energy and moisture transfer to the atmosphere. In general, vegetation absorbs more solar energy, transpires more water, drives more air circulation, and results in more local precipitation in a region with low than high albedo. Thus, ecosystems influence energy balance in the atmosphere and feed back to climate change.

Ecohydrological Regulation of Climate Change

Water vapor exchange at the land surface significantly affects climate dynamics at local, regional, and global scales. Ecosystems receive water input via precipitation and lose water via evapotranspiration. Plant vegetation is the primary regulator of evapotranspiration. Thus, types of ecosystems significantly affect energy and water transfers from ecosystems to the atmosphere.

Because water transpired through leaves comes from the roots, rooting systems play a critical role in ecohydrological regulation of climate. Woody encroachment to grasslands, for example, can accelerate the ecosystem and then influence climate dynamics because trees usually have deep taproots to take up water from deep soil layers. Conifer forests can transpire water from the soil to the atmosphere in early spring and late fall and have longer seasons of transpiration than deciduous forests. Conversion of grasslands to winter wheat croplands accelerates evapotranspiration in winter and early spring when wheat actively grows and grasses are dormant. However, evapotranspiration is lower in fallow fields after wheat harvest than in grasslands in summer and fall. In addition, rooting systems are highly adaptive to climate change. When climate warming increases soil temperature and water stress, plants grow more roots to take up water. The adaptive rooting systems can significantly regulate climate change.

Carbon-Climate Feedback

Ecosystems can regulate climate change via changes in uptake and releases of GHGs. The GHGs involved in ecosystem feedbacks to climate change include CO₂, methane (CH₄), nitrous oxide (N₂O), and ozone (O₃). Their uptakes and releases are modified by changes in temperature, precipitation, atmospheric CO₂ concentration, land use and land cover changes, and nitrogen deposition. For example, ecosystems absorb CO₂ from the atmosphere by photosynthesis and release it back to the atmosphere via respiration. Photosynthetically fixed carbon from the air is converted to organic carbon compounds. Some of the carbon compounds are used to grow plant tissues while others are used for plant respiration. Plant tissues die, adding litter to soil. Litter is partly decomposed by microorganisms to release CO₂ back to the atmosphere and partly incorporated to soil organic matter. The latter can store carbon in soil for hundreds and thousands of years.

Many factors and processes can alter the carbon cycles and then influence carbon-climate feedback. For example, usually results in net release of carbon from ecosystems to the atmosphere, enhancing climate change. Rising atmospheric CO₂ usually stimulates plant growth and ecosystem carbon sequestration, mitigating climate change. Climate warming can stimulate both and respiration. Most models assume that respiration is more sensitive than photosynthesis to climate warming and predict a positive feedback between terrestrial carbon cycles and climate warming. Field experiments, however, suggest much richer mechanisms driving ecosystem responses to climate warming, including extended growing seasons, enhanced nutrient availability, shifted species composition, and altered ecosystem-water dynamics. The diverse mechanisms likely define more possibilities of carbon-climate feedbacks than projected by the current models.

Context

Ecosystems are basic units of the biosphere. The latter is the global ecological system integrating all living organisms and their interaction with the lithosphere, hydrosphere, and atmosphere. Biosphere-atmosphere interactions occur via exchanges of energy, water, and GHGs in ecosystems. Specifically, ecosystems interact with the atmosphere via emission and absorption of GHGs so as to influence energy balance in the atmosphere; variations in albedo to influence the amount of heat transferred from ecosystems to the atmosphere; and changes in evapotranspiration to cool the land surface, to influence water vapor dynamics, and to drive atmospheric mixing. In addition, ecosystems can influence climate dynamics by changes in production of aerosols and surface roughness and coupling with the atmosphere. Thus, understanding ecosystem processes that regulate energy balances, water cycling, and carbon and nitrogen dynamics is critical to Earth-system science.

Key Concepts

• albedo: the percentage of the solar radiation of all wavelengths reflected by a body or surface
• biome: a geographically defined area of similar plant community structure shaped by climatic conditions
• canopy: the upper layers of vegetation or uppermost levels of a forest, where energy, water, and greenhouse gases are actively exchanged between ecosystems and the atmosphere
• evapotranspiration: processes through which water on surfaces or in plants is lost to the atmosphere
• greenhouse gases (GHGs): atmospheric gases that trap heat, preventing it from escaping into space
• latent heat flux: the flux of thermal energy from land surface to the atmosphere that is associated with evaporation and of water from ecosystems
• photosynthesis: a metabolic pathway that absorbs inorganic carbon dioxide from the atmosphere and converts it to organic carbon compounds using sunlight as an energy source
• respiration: metabolic reactions and processes to convert organic compounds to energy that release CO₂ as a by-product
• sensible heat flux: the flux of thermal energy that is associated with a rise in temperature
• stoma: a pore in the leaf and stem epidermis that is used for gas exchange

Bibliography

Chapin, F. Stuart, III, et al. “Changing Feedbacks in the Climate-Biosphere System.” Frontiers in Ecology and the Environment 6 (2008): 313-320.

Chapin, F. Stuart, III, Harold A. Mooney, and Pamela Matson. Principles of Terrestrial Ecosystem Ecology. New York: Springer, 2002.

"Ecosystems." US Climate Resilience Toolkit, 17 Apr. 2024, toolkit.climate.gov/topics/ecosystems. Accessed 19 Dec. 2024.

Field, C. B., D. B. Lobell, and H. A. Peters. “Feedbacks of Terrestrial Ecosystems to Climate Change.” Annual Review of Environmental Resources 32 (2007): 1-29.

Luo, Y. Q. “Terrestrial Carbon-Cycle Feedback to Climate Warming.” Annual Review of Ecology Evolution and Systematics 38 (2007): 683-712.