What is climate sensitivity?

A central question posed by climate research is how much warmer it will become on Earth if the concentration of greenhouse gases increases by a certain amount. One measuring rod for this is called climate sensitivity. In the following, we explain what this term means.

Greenhouse gases like CO₂ and methane affect the energy balance of our planet: 
Energy from the sun enters the atmosphere as short-wave radiation, which people perceive in part as visible light. The Earth’s surface absorbs some of the radiation, converts the energy it contains into heat and emits this back into the atmosphere. Greenhouse gases take up a portion of the long-wave thermal radiation and radiate it back into outer space and also in the direction of the Earth’s surface, warming it up more in the process. In other words, the higher the concentration of greenhouse gases in the atmosphere, the more heat they retain. When the Earth and its atmosphere absorb more energy than they give back off, the temperature near the Earth’s surface rises. This is referred to as a positive radiation budget. How much energy the Earth absorbs and emits is measured in watts. 

Climate sensitivity states how many degrees the temperature of the Earth’s surface will change when the energy balance rises by one watt per square meter – averaged globally and over a year. Put in simple terms, it is often described as the increase in temperature at double the CO₂ concentration in the atmosphere, specifically in correlation between today's rising CO₂ and temperature levels compared to the pre-industrial age. Yet, climate sensitivity is not a constant parameter. It depends on the observed state of the Earth’s surface and atmosphere. 

With solar radiation as a constant, there are multiple parameters impacting the energy balance: 

Every gas in the atmosphere and material on the Earth’s surface can alter the incident solar radiation or the heat radiation emitted. Light-colored materials like snow absorb less energy from solar radiation than dark areas such as bodies of water or forests. Gases or water vapor reflect or absorb radiation and thereby give it back off into outer space or retain it in the atmosphere. For many individual materials like snow, water or CO₂, this effect can be measured in the laboratory. Under real climate circumstances, an interplay of many factors takes place: The effects of various materials interact additively or compensate each other. Feedback phenomena trigger further changes. For example, the warmer the atmosphere is, the more water vapor it absorbs. This, in turn, drives further warming. Changes in vegetation, the size of ice-covered surfaces or the oceans’ temperatures all have an effect on the energy balance of the overall system. 

Given that so many different dynamic factors and feedback effects need to be considered, it is no easy task to calculate the energy balance of the Earth and state a value for climate sensitivity. In order to understand the complex synergy between all these factors better, climate scientists are studying the development of climate sensitivity over the course of the Earth's history. For that purpose, they take ice cores from the Antarctic that are several hundred thousand years old and use them to reconstruct temperatures and atmospheric CO₂ concentrations, for example. 

To predict the future development of the currently advancing climate change, it is crucial to determine the current climate sensitivity of the Earth's system as accurately as possible. To calculate a value for this, researchers are developing high-performance computer models to help them illustrate and map as many relevant factors as possible as precisely as possible.