Climate sensitivity

Climate sensitivity is a fundamental metric in climate science. It describes how much the Earth's surface temperature will rise in response to a doubling of atmospheric carbon dioxide (CO2) concentrations.

This concept is vital because it helps scientists predict the magnitude of climate change. Understanding sensitivity allows researchers to model how the entire planetary system reacts to changes in energy. This information is essential for creating global policies and economic plans to manage future warming.

To understand this mechanism, we must look at how energy moves through our atmosphere. The Earth maintains a balance between incoming solar radiation and outgoing heat radiation. When greenhouse gases like CO2, methane, or nitrous oxide increase, they cause radiative forcing. Radiative forcing is an imbalance where more energy enters the system than leaves it.

This imbalance is often measured in Watts per square meter (W/m2). As the planet absorbs this extra energy, the temperature rises until a new balance is reached at a higher temperature.

This warming process is often amplified by climate feedbacks. Feedbacks are secondary effects that occur as the temperature changes. Some are self-reinforcing, meaning they increase the warming. For example, higher temperatures cause ice to melt. This reduces the planet's albedo, which is the measure of how much sunlight a surface reflects.

As reflective ice disappears, darker surfaces absorb more heat. Another reinforcing feedback involves evapotranspiration. This process increases atmospheric water vapour, which is itself a greenhouse gas that traps even more heat.

Scientists distinguish between two main types of climate sensitivity. The first is the transient climate response, or TCR. The TCR measures the initial rise in global temperature during the period when CO2 levels are doubling.

The second is the equilibrium climate sensitivity, or ECS. The ECS represents the larger, long-term temperature increase after the entire climate system has fully adjusted. The ECS is typically higher because some processes, like the deep ocean, take many centuries to reach a new steady state.

Researchers use several methods to estimate these values. They analyze temperature and gas concentrations since the Industrial Revolution began around 1750. They also use indirect measurements from Earth's distant geological past.

Additionally, they use complex computer simulations to model future scenarios. These methods help narrow down the uncertainty regarding how strong various feedbacks might be.

Historical data provides specific numbers for these changes. In the 18th century, CO2 levels were approximately 280 parts per million (ppm). By 2020, these levels had risen to over 415 ppm.

Between the start of the Industrial Revolution and 2020, the Earth's temperature rose by a little over 1°C. Scientists have found that a doubling of CO2 causes a radiative forcing of about 3.7 W/m2. Without any feedbacks, this would cause roughly 1 K of warming, but feedbacks make the actual result much higher.

Climate sensitivity has massive implications for global society. The Paris Agreement aims to limit global warming to well below 2°C. However, if the ECS is higher than certain estimates, meeting this goal may be impossible.

High sensitivity means we must achieve carbon neutrality much faster. Some studies suggest that reducing uncertainty in TCR estimates could save trillions of dollars in economic costs. The more sensitive the climate, the more likely we are to experience extreme temperature swings.