Climate change feedbacks

Climate change feedbacks are natural processes that influence global temperature changes. When greenhouse gas emissions increase, they act as a forcing agent that pushes the climate system toward warming. Feedbacks determine how much the temperature actually rises in response to that forcing. Scientists categorize these processes into two main types: positive and negative. A positive feedback amplifies the initial warming, making the temperature rise even more. A negative feedback diminishes the warming, helping to reduce the effect.

Physical feedbacks involve the non-living parts of the Earth, such as the atmosphere and ice. One major physical feedback is the water vapor feedback. According to the Clausius–Clapeyron relation, a warmer atmosphere can hold more absolute water vapor. Because water vapor is a powerful greenhouse gas, this extra moisture traps more heat. This creates a loop where warming leads to more water vapor, which leads to even more warming. This process can effectively double the warming caused by CO2 alone.

Another physical process is the surface albedo feedback. Albedo is a measure of how much solar radiation a surface reflects. Bright surfaces like snow and ice have a high albedo and reflect most sunlight. Darker surfaces, like open ocean or soil, have a low albedo and absorb more heat. When warming melts ice, it reveals darker surfaces underneath. These surfaces soak up more energy, which causes more ice to melt. This cycle is a major reason for Arctic amplification. In the Arctic, temperatures have risen nearly four times faster than the global average since 1979.

Negative physical feedbacks help stabilize the Earth's temperature. The most fundamental is the Planck response. As an object like the Earth warms, it emits more thermal radiation. This increase in outgoing radiation back into space helps the planet reach a new equilibrium. The Planck response is so strong that human-caused climate change alone cannot trigger a runaway greenhouse effect. Another negative feedback is the lapse rate feedback. This refers to the rate at which temperature decreases with altitude in the atmosphere. In most parts of the world, this process weakens the greenhouse effect. However, in polar regions, the lapse rate can actually act as a positive feedback.

Biological feedbacks are tied to the carbon cycle. The carbon cycle is the process by which carbon moves between the atmosphere, plants, and oceans. Currently, the ocean and plants absorb more than half of all CO2 emissions every year. This acts as a massive natural sink that slows down warming. However, these biological sinks can become saturated over time. Higher temperatures can also lead to droughts and wildfires. These events can cause plants to release stored carbon back into the atmosphere, changing the feedback from negative to positive.

Scientists use global climate models to estimate these complex relationships. These computer models simulate how different feedbacks will change as the planet warms. While many relationships are well understood, some areas remain uncertain. For example, cloud feedbacks are difficult to model precisely. There is also uncertainty regarding the carbon cycle and permafrost thaw. When permafrost melts, it can release both carbon and methane. These gases are difficult to track and model accurately.

Understanding these feedbacks is vital for predicting the future of our climate. Some feedbacks, like water vapor, respond very quickly to changes. Others, such as the melting of massive ice sheets, are drawn out over several centuries. While the overall sum of feedbacks is currently negative, it is becoming less negative as emissions continue. This means that while negative feedbacks slow the warming, the warming itself will continue to accelerate if emissions are not reduced.