Doubling the CO₂ in the air, by itself, would warm Earth by only about 1.2°C. Yet scientists expect the real warming from a doubling to be around 3°C. The difference comes from feedbacks: changes set off by warming that then add to it or subtract from it. Understanding feedbacks is the key to the single most important number in climate science.

Forcing and response

Anything that changes Earth's energy balance from outside the climate system is called a forcing. It is measured in watts per square meter. Doubling CO₂ adds a forcing of roughly 4 watts per square meter, a little like placing a small night-light over every square meter of the planet, day and night, forever. Changes in the Sun, volcanic eruptions and pollution particles are forcings too. The climate then responds, and the feedbacks shape how large the response is.

The water vapor feedback

Warm air holds more water vapor, about 7 percent more for each degree Celsius. Since water vapor is itself a powerful greenhouse gas, a little warming from CO₂ loads the air with more vapor, which traps more heat. This is the largest amplifying feedback, and it roughly doubles the warming from CO₂ alone. It is also why water vapor, despite being the most abundant greenhouse gas, is not the control knob: extra vapor rains out within days unless something else keeps the air warm.

Ice and snow

Bright snow and ice reflect most of the sunlight that hits them, while dark ocean and land absorb most of it. As the world warms and ice melts, darker surfaces are exposed, absorb more sunlight and warm further. This ice-albedo feedback is one reason the Arctic is warming roughly three to four times faster than the global average.

Clouds: the biggest uncertainty

Clouds both cool and warm. Low, thick clouds act like parasols, reflecting sunlight. High, thin clouds act more like blankets, trapping infrared. How cloud cover changes with warming is the hardest feedback to pin down, because clouds form at scales much smaller than models can directly represent. The evidence now suggests that, on balance, clouds amplify warming somewhat rather than offset it.

A stabilizing feedback

Not every feedback amplifies. The most basic one works against change: a warmer planet radiates more energy to space, which is what eventually lets it reach a new balance. Changes in how temperature falls with height in the atmosphere also damp the warming a little. Without these, the climate would be unstable.

Carbon cycle feedbacks

Warming can also release more greenhouse gases. Frozen Arctic soils, called permafrost, hold roughly twice as much carbon as the atmosphere does today. As they thaw, microbes break down that material and release CO₂ and methane. Warmer oceans absorb CO₂ less readily, and drought and fire can turn forests from carbon sinks into sources. These feedbacks are slower but could make it harder to stop warming.

The number everyone wants: climate sensitivity

Equilibrium climate sensitivity is how much the planet eventually warms once CO₂ has doubled and the climate has settled. A 1979 US National Academy of Sciences report led by Jule Charney estimated it at 1.5 to 4.5°C, with a best guess of 3°C. Four decades later, the Intergovernmental Panel on Climate Change (IPCC), in its 2021 assessment, narrowed the likely range to 2.5 to 4°C, with a best estimate of 3°C. It did so by combining three independent lines of evidence: the physics of each feedback, the warming observed since 1850, and past climates such as the last ice age.

The ocean slows the response, so the warming we see at any moment lags behind the full eventual warming. Pollution particles called aerosols, from burning coal and ship fuel, also reflect sunlight and have hidden part of the warming. Cleaning up that air pollution saves millions of lives but removes some of the mask.

Tipping points

Some parts of the climate system might change abruptly or irreversibly once pushed past a threshold. Examples scientists study include the collapse of the Greenland and West Antarctic ice sheets, a major slowdown of the Atlantic ocean circulation that warms Europe, and dieback of the Amazon rainforest. The exact thresholds are uncertain, which is itself a reason for caution.

Worked example: how feedbacks multiply

Here is a simple way to see why feedbacks matter so much. Suppose CO₂ alone warms the planet 1.2°C, and the combined feedbacks add 0.6 degrees of extra warming for every degree of warming.

• First round: 1.2°C of warming triggers 0.72°C more.
• Second round: that 0.72°C triggers about 0.43°C more, and so on, each round smaller.
• Add it up: the series settles at 1.2 divided by (1 minus 0.6), which is 3.0°C.
• Change the feedback: at 0.5 the total is 2.4°C; at 0.7 it is 4.0°C.

Notice how a modest change in the feedback strength swings the final answer a lot. That is why scientists work so hard on clouds, and why the uncertainty about sensitivity leans toward higher values rather than lower ones.