A climate feedback is a process in the climate system that responds to an initial change and then influences the magnitude or direction of that original change. A positive climate feedback amplifies the initial perturbation, whereas a negative climate feedback counteracts or dampens it. The terms positive and negative refer to the effect on the original change rather than whether the feedback is beneficial or harmful.
Climate feedbacks are essential because the climate system contains interacting components including the atmosphere, oceans, land surface, cryosphere and biosphere. A change in one component can alter another, which can then feed back into the first. For example, warming can melt snow and ice, reducing surface reflectivity and allowing more solar energy to be absorbed, thereby amplifying warming. Feedbacks are different from external forcings, which are factors that perturb the Earth's energy balance from outside the internal response being considered.
The ice albedo feedback is a classic positive feedback. Snow and ice have relatively high reflectivity, or albedo, so they send a substantial fraction of incoming sunlight back to space. When warming reduces snow or sea ice, darker land or ocean surfaces become exposed and absorb more solar radiation. The additional absorbed energy can cause further warming and further loss of reflective surfaces.
The feedback is especially important in polar and high latitude environments, where snow and sea ice respond strongly to temperature. The strength of the feedback varies with season, surface type, cloud conditions, solar angle and the extent and duration of snow and ice cover. Sea ice loss also changes interactions between the ocean and atmosphere because open water exchanges heat and moisture with the atmosphere differently from an ice covered surface.
Water vapour provides another important positive feedback. Water vapour is itself a greenhouse gas, but its atmospheric concentration is strongly controlled by temperature and the hydrological cycle rather than being an independent long lived forcing in the same way as carbon dioxide. Warmer air can hold more water vapour, approximately following the Clausius Clapeyron relationship under equilibrium conditions. If warming increases atmospheric water vapour, the additional water vapour enhances infrared absorption and contributes further warming.
This creates a positive feedback that amplifies the initial warming. Clouds complicate the picture because changes in humidity, cloud amount, altitude, droplet and ice properties and cloud radiative effects can either warm or cool depending on the circumstances. Cloud feedback is therefore more complex and uncertain than the basic water vapour feedback.
The carbon cycle contains several feedbacks that can influence climate. Warming can alter vegetation productivity, soil respiration, decomposition, wildfire activity and ocean carbon uptake, potentially changing how much carbon dioxide remains in the atmosphere. For example, warmer conditions can increase decomposition of some soil organic matter and release additional carbon dioxide, producing a positive feedback.
Conversely, increased plant growth under certain conditions can remove additional carbon dioxide from the atmosphere, creating a potentially negative feedback, although nutrient limitations, water stress, heat extremes and ecosystem changes constrain this response. Ocean warming can reduce the solubility of carbon dioxide in seawater, potentially weakening the ocean's capacity to absorb additional atmospheric carbon dioxide. These processes occur simultaneously and vary by region, making the net carbon cycle feedback a complex result of multiple interacting mechanisms.
Clouds are among the most important and difficult climate feedbacks to quantify. Clouds reflect incoming sunlight, producing a cooling effect, but they also absorb and emit infrared radiation, producing a warming effect. The balance depends strongly on cloud altitude, thickness, particle properties, coverage, latitude, surface conditions and atmospheric temperature.
High thin clouds can produce substantial greenhouse warming because they are cold and emit infrared radiation less efficiently to space, while low thick clouds can strongly reflect sunlight and therefore have a cooling influence. Climate warming can alter atmospheric circulation and humidity in ways that change cloud distributions and properties. Because these changes operate across many spatial and temporal scales, representing clouds accurately in climate models requires detailed physical parameterisations or increasingly high resolution simulations. Cloud feedback remains an important source of uncertainty in estimates of equilibrium climate sensitivity.
Some climate feedbacks operate through oceans and atmospheric circulation. Warming increases evaporation, which transfers additional water vapour into the atmosphere and affects cloud formation, precipitation and latent heat transport. Ocean warming and changes in freshwater input can modify seawater density and circulation, potentially affecting the distribution of heat and carbon. Vegetation can change surface albedo, evapotranspiration and roughness, influencing regional climate. Forest loss may increase surface reflectivity in some environments while reducing evapotranspiration and carbon storage, producing competing climatic effects. Changes in permafrost can release carbon dioxide and methane when previously frozen organic material becomes biologically active, potentially adding greenhouse gases to the atmosphere. These examples demonstrate that feedbacks can operate through physical, chemical and biological pathways simultaneously.
Feedbacks are commonly studied using perturbation experiments and climate models. Scientists can impose a small change in a climate variable, such as surface temperature, and calculate how different components respond. A feedback parameter quantifies how much additional radiative forcing results from a given temperature change under a particular framework.
Climate sensitivity is closely related to the combined effect of feedbacks because the initial radiative forcing from greenhouse gases produces a temperature response that alters water vapour, clouds, surface albedo, lapse rates and other processes. Feedbacks can also be nonlinear, meaning that their strength may change as the climate state changes. A feedback that is weak in one temperature regime may become stronger or weaker under substantially different conditions. Consequently, simple linear descriptions are useful for understanding mechanisms but do not capture every aspect of a changing climate system.
Climate feedback should not be confused with climate forcing, correlation or ordinary cause and effect. A forcing initiates or alters an energy imbalance, while a feedback modifies the subsequent response to that perturbation. The distinction depends on the chosen system boundary and timescale, so the same process can be described differently in different analytical frameworks. Feedbacks can be local or global, rapid or slow, and strongly positive, weakly positive, negative or approximately neutral.
They are crucial to understanding why Earth's temperature does not respond to greenhouse gas changes through a simple one step relationship. Some feedbacks amplify warming, others oppose it, and several interact with one another. The resulting climate response is therefore an emergent property of the coupled atmosphere, ocean, cryosphere, land and biosphere rather than the direct effect of greenhouse gases in isolation.
