Climate Sensitivity
Introduction
Climate sensitivity refers to the change in the Earth's global average surface temperature due to changes in its radiative forcing, particularly the increase in carbon dioxide (CO2) concentrations. It is a key factor in predicting potential changes in the Earth's climate in response to increasing greenhouse gases.
Definition and Units
Climate sensitivity is defined in terms of the equilibrium temperature change in response to changes in radiative forcing, which is the difference between the incoming energy from the Sun and the outgoing energy from the Earth. It is usually expressed in degrees Celsius per doubling of CO2 (°C/(2xCO2)).
Types of Climate Sensitivity
There are three main types of climate sensitivity: equilibrium climate sensitivity (ECS), transient climate response (TCR), and Earth system sensitivity (ESS).
Equilibrium Climate Sensitivity (ECS)
ECS refers to the global average surface warming that would occur if the atmospheric concentrations of CO2 were instantly doubled and the climate were given a lot of time to adjust to the new state. It is a measure of the Earth's long-term response to increased CO2.
Transient Climate Response (TCR)
TCR is the temperature change at the time when CO2 has doubled at a rate of 1% per year, which takes about 70 years. It is a measure of the Earth's short-term response to increased CO2.
Earth System Sensitivity (ESS)
ESS takes into account slower feedbacks, such as changes in ice sheets and vegetation, which can alter the Earth's albedo and thus its ability to absorb or reflect incoming solar radiation.
Determining Climate Sensitivity
Climate sensitivity is determined through a combination of theoretical models, laboratory experiments, and historical climate data. These methods include climate models, paleoclimate reconstructions, and observations of recent climate change.
Climate Models
Climate models are mathematical representations of the Earth's climate system. They simulate the interactions between the atmosphere, oceans, land surface, and ice. These models are used to estimate climate sensitivity by simulating the response of the global climate system to changes in CO2 concentrations.
Paleoclimate Reconstructions
Paleoclimate reconstructions involve studying past climates to understand how the Earth's climate system has responded to changes in CO2 concentrations over geological time. This is done by analyzing ice cores, sediments, and other geological records that provide evidence of past climate conditions.
Observations of Recent Climate Change
Observations of recent climate change involve analyzing temperature records and other climate data to estimate the Earth's climate sensitivity. This method relies on the assumption that the recent past is a good indicator of how the climate will respond to future changes in CO2 concentrations.
Uncertainties and Controversies
There is a range of values for climate sensitivity estimated by different methods. The Intergovernmental Panel on Climate Change (IPCC) gives a likely range for ECS of 1.5 to 4.5°C per doubling of CO2, but other studies have suggested a wider range. The uncertainty in climate sensitivity estimates is a major factor in the uncertainty of future climate projections.
Implications for Climate Change
The value of climate sensitivity is crucial for predicting future climate change and for planning mitigation and adaptation strategies. A high climate sensitivity would mean that we can expect a large amount of warming for a given increase in greenhouse gases, while a low climate sensitivity would mean less warming.