Understand Climate Sensitivity in a simpler way

In the context of climate science, sensitivity is the change in atmospheric temperature that results from a change in forcing. For a given level of force, a more sensitive climate will experience a greater change in atmospheric temperature. If you want to know how much temperatures will rise because of climate change, you have to know how sensitive climate is.

The main human impact on the climate has been our carbon dioxide emissions and the resulting increase in the concentration of this gas in the atmosphere. For this reason, when someone simply refers to “sensitivity” they are usually referring to the increase in temperature associated with an increase in CO2 concentrations. When one wants to talk about the sensitivity of the climate with respect to another factor, that other factor is mentioned explicitly.

What is climate sensitivity?

Climate sensitivity is the term used by the Intergovernmental Panel on Climate Change (IPCC) to express the relationship between human-caused emissions that add to the Earth's greenhouse effect (carbon dioxide and a variety of other greenhouse gases ) and temperature changes.

Although scientists don't know exactly what Earth's climate sensitivity is, it's a useful concept for thinking about the range of risks we face from climate change: from slower, more manageable consequences if our climate sensitivity is low, to more rapid changes and more serious if it is high. According to the most recent IPCC report, our climate sensitivity is most likely between 2 and 5 ℃ (3.6 and 9 ℉).

Transient climate response

To simplify the explanation, suppose that the only influence that humans have on the climate is our CO2 emissions. Let's say that our emissions increase the atmospheric concentration of this gas, from about 370 ppm in the year 2000 to 740 ppm in the year 2100. But for that year, thanks to a decline in emissions, the CO2 concentration stabilizes.

The Transient Climate Response, or TCR, is an estimate of how much temperatures will have risen by the time the CO2 concentration has doubled. On page 82 of this IPCC document, you can see a more technical description.

Climatic sensitivity in equilibrium

Think about the example we just mentioned. By the year 2100, the CO2 concentration is double that in the year 2000. As a result, the temperature will have risen, say, 1.5ºC between these two dates. But even if the CO2 level stops rising, temperatures will continue to rise. Equilibrium climate sensitivity (called ECS) refers to the total, long-term warming that results when the amount of CO2 in the atmosphere doubles.

To understand why ECS is almost certainly greater than TCR, think again about the concept of energy imbalance. A positive imbalance means that the Earth is receiving more energy than it is giving off – that is, the planet is warming. Warming only stops when the imbalance disappears or, in other words, when the climate system returns to equilibrium. (Obviously, an imbalance of exactly zero is impossible, and we would not be able to measure it even if it did occur; in this context, "equilibrium" simply means that the imbalance is so small that it does not influence the atmospheric temperature.)

How can we estimate climate sensitivity?

Climate sensitivity cannot be directly measured in the real world. Instead, it must be estimated and there are three main lines of evidence that can be used to do this:

• Historical climate records: Instrumental records of warming since the mid-19th century, combined with estimates of greenhouse gas and aerosol emissions, can be used to assess the global temperature response to CO2 emissions from human activities to date.

• Climate models: We can use climate models, which provide complex simulations of Earth's climate system, to predict future climate sensitivity, since we do not have observations for future climate. These mathematical models are based on our understanding of the physics behind our climate system.

• Paleoclimate records: Ice cores and other records can be used to estimate natural changes in atmospheric temperature and CO2 over thousands of years. These can be used for estimates of the past relationship between the two factors.

So how is the weather sensitivity calculated?

The ideal would be to have a laboratory with the planet Earth in which we double the amount of CO2 and wait to see how much the temperature increases. That's what climate models try to simulate. But if we want to use data from the real world, the only way to calculate (or rather estimate) the sensitivity of the climate is by looking at the evolution of atmospheric temperature, forcing and energy imbalance throughout history.

Suppose that, over a period of time, the forcing has increased by 2 watts per square meter while the temperature has risen 1ºC. This 2 w/m2 would actually be a mixture of two forcings: 3 w/m2 positive for CO2, and 1 w/m2 negative for aerosols.

To calculate TCR, we have to extrapolate from the historical temperature increase to the increase that would occur if a forcing such as that occurs when the amount of CO2 doubles. Since this forcing is 3.8 w/m2, the TCR estimate would be 90% higher than the historical temperature increase (since 3.8 / 2 = 1.9) That is, TCR = 1ºC * 1.9 = 1.9ºC.

But what if the aerosols are 50% more effective than CO2? Then, the real forcing suffered by the climate system would not be 3 – 1 = 2 w/m2, but 3 – 1.5 = 1.5 w/m2. Extrapolation would now give a very different result. Since 3.8 / 1.5 = 2.53, the result would be TCR = 1ºC * 2.53 = 2.53ºC.

What does this mean for global warming?

Climate sensitivity measures allow scientists to easily compare climate models with each other and with observed changes. And because equilibrium climate sensitivity has been in use for more than 30 years, it can help us track how our estimates have changed over time.

But climate sensitivity does not directly tell us what temperature changes we can expect over the next century.

The best guide to future warming is provided by projections from the current generation of climate models. They indicate that warming will largely depend on the level of future emissions, the "scenario."

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