Climate change feedbacks

Climate change feedbacks are processes in the climate system which amplify or diminish the effect of forces that initially cause the warming. Positive feedbacks enhance global warming while negative feedbacks weaken it.^[4]: 2233 Feedbacks are important in the understanding of climate change because they play an important part in determining the sensitivity of the climate to warming forces. Climate forcings and feedbacks together determine how much and how fast the climate changes.^[5]: 11

Feedbacks can be divided into purely physical and partially biological. Physical feedbacks include increased thermal radiation as the planet warms, increased water vapor from evaporation, altered cloud distribution, decreased surface reflectivity as snow and ice cover diminishes, and an amplification of the rate at which atmospheric temperature falls with rising altitude. Biological feedbacks are mostly associated with changes to the rate at which plant matter accumulates CO₂ as part of the carbon cycle.^[6]: 967 Some feedbacks rapidly impact climate sensitivity, while the feedback response from ice sheets is drawn out over several centuries.^[6]: 967 Feedback strengths and relationships are estimated through global climate models, with their estimates calibrated against observational data whenever possible.^[6]: 967

While the overall sum of climate change feedbacks is negative, it is becoming less negative as greenhouse gas emissions continue. This means that warming is slower than it would be in the absence of feedbacks, but that warming is accelerating in response to greenhouse gas emissions.^{[2]: 95–96} Feedbacks staying negative also means that a runaway greenhouse effect effectively cannot occur due to anthropogenic climate change.^[7][8] Feedbacks will stay negative largely because of increased thermal radiation as the planet warms, which is an effect that is several times larger than any other singular feedback.^[2]: 96 Additionally, the carbon cycle absorbs more than half of CO₂ emissions every year into plants and into the ocean.^[9]: 676 Over the long term the percentage will be reduced as carbon sinks become saturated and higher temperatures lead to effects like drought and wildfires.^{[9]: 698 [2]: 96 [5]: 20}

Climate change scenarios consider both how the Earth will be warmed by greenhouse gas emissions, and how this warming will be modified by feedbacks.^[10] While the basic relationships are well understood, there is still uncertainty regarding feedbacks. Cloud patterns are varied and often occur in remote areas over the ocean, which makes cloud feedback difficult to estimate accurately.^[6]: 975 Models where cloud feedback is very large project faster and stronger warming than the rest.^[11][12] Carbon cycle uncertainty is driven by the large rates at which CO₂ is both absorbed into plants and released when biomass burns or decays. For instance, permafrost thaw produces both CO₂ and methane emissions, in ways that are difficult to model.^[9]: 677 While their long-term importance is limited,^{[6]: 975–976} permafrost emissions substantially reduce the available carbon budgets for 1.5 °C (2.7 °F) and 2 °C (3.6 °F) warming in the near term.^{[13][9]: 678}