Global Warming - Feedback

Feedback

Feedback is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback increases the change in the first quantity while negative feedback reduces it. Feedback is important in the study of global warming because it may amplify or diminish the effect of a particular process.

The main positive feedback in the climate system is the water vapor feedback. The main negative feedback is radiative cooling through the Stefan–Boltzmann law, which increases as the fourth power of temperature. Positive and negative feedbacks are not imposed as assumptions in the models, but are instead emergent properties that result from the interactions of basic dynamical and thermodynamic processes.

A wide range of potential feedback processes exist, such as Arctic methane release and ice-albedo feedback. Consequentially, potential tipping points may exist, which may have the potential to cause abrupt climate change.

For example, the "emission scenarios" used by IPCC in its 2007 report primarily examined greenhouse gas emissions from human sources. In 2011, a joint study by the US National Snow and Ice Data Center and National Oceanic and Atmospheric Administration calculated the additional greenhouse gas emissions that would emanate from melted and decomposing permafrost, even if policymakers attempt to reduce human emissions from the A1FI scenario to the A1B scenario. The team found that even at the much lower level of human emissions, permafrost thawing and decomposition would still result in 190 Gt C of permafrost carbon being added to the atmosphere on top of the human sources. Importantly, the team made three extremely conservative assumptions: (1) that policymakers will embrace the A1B scenario instead of the A1FI scenario, (2) that all of the carbon would be released as carbon dioxide instead of methane, which is more likely and over a 20 year lifetime has 72x the greenhouse warming power of CO₂, and (3) their model did not project additional temperature rise caused by the release of these additional gases. These very conservative permafrost carbon dioxide emissions are equivalent to about 1/2 of all carbon released from fossil fuel burning since the dawn of the Industrial Age, and is enough to raise atmospheric concentrations by an additional 87±29 ppm, beyond human emissions. Once initiated, permafrost carbon forcing (PCF) is irreversible, is strong compared to other global sources and sinks of atmospheric CO₂, and due to thermal inertia will continue for many years even if atmospheric warming stops. A great deal of this permafrost carbon is actually being released as methane instead of carbon dioxide. IPCC 2007's temperature projections did not take any of the permafrost carbon emissions into account and therefore underestimate the degree of expected climate change.

Other research published in 2011 found that increased emissions of methane could instigate significant feedbacks that amplify the warming attributable to the methane alone. The researchers found that a 2.5-fold increase in methane emissions would cause indirect effects that increase the warming 250% above that of the methane alone. For a 5.2-fold increase, the indirect effects would be 400% of the warming from the methane alone.