4. What is climate sensitivity?
Climate sensitivity is a measure used for climate modeling, which describes the amount of warming in the atmosphere associated with increases in atmospheric carbon dioxide (CO2) as a result of the anthropogenic greenhouse effect. Even though only a tiny amount of the gases in Earth’s atmosphere are greenhouse gases, they have a huge effect on climate. Climate sensitivity indicates how much the Earth’s surface temperature would increase if pre-industrial CO2 concentrations were doubled.
Svante Augustus Arrhenius was the first to calculate global climate sensitivity to changes in atmospheric CO2 concentration. Arrhenius calculations showed temperature increases with 4°C, which are quite close to modern estimates. According to modern reports by the Intergovernmental Panel on Climate Change (IPCC), the possible range of climate sensitivity to doubling of atmospheric CO2 is from 1.5 to 4.5°C and the probable date of CO2 doubling will be reached between the years 2050 and 2100.
Estimating climate sensitivity
Climate sensitivity is typically estimated in three ways; by using observations taken during the industrial age, by using temperature and other data from the Earth’s past, and by modeling the climate system. There are two types of climate sensitivity: Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR).
The ECS is the amount of warming achieved when the entire climate system reaches equilibrium to a doubling of CO2. The ECS is likely to be 1.5°C to 4.5°C and extremely unlikely to be less than 1°C and not greater than 6°C. These estimations for temperatures are repeated in assessment reports every 6 years. These reports have so far shown a good consistency in the temperature values with only minor variations.
The TCR considers the changes that would occur if CO2 levels increase by 1% per year until they double. If atmospheric CO2 concentrations were held at double pre-industrial concentrations, the planet would still continue to warm. This is because the world’s oceans take a long time to heat up in response to the enhanced greenhouse effect. The TCR is likely to be 1°C to 2.5°C and extremely unlikely to be greater than 3°C.
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