“Climate sensitivity” is the amount that the average global surface temperature will rise, given a doubling of the concentration of atmospheric carbon dioxide (CO2) in the atmosphere from its pre-industrial value. This metric is the key to understanding how much global warming will occur as we continue to burn fossil fuels for energy and emit the resultant CO2 into the atmosphere.
The problem is that we don’t know what the value of the climate sensitivity really is.
In its Fourth Assessment Report, released in 2007, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) had this to say about the climate sensitivity:
It is likely to be in the range 2°C to 4.5°C with a best estimate of about 3.0°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded…
In IPCC parlance, the term “likely” means a probability of greater than 66% and “very likely” means a greater than 90% change of occurrence. The IPCC’s 90% range for the climate sensitivity includes values at the low end which, if proven true, would engender very little concern over our use of fossil fuels as a primary energy source, and values at the high end would generate calls for frantic efforts (which would likely fail) to lower carbon dioxide emissions.
While there has been a lot of effort expended to better constrain estimates of sensitivity over the past several decades, little progress has been made in narrowing the range. The IPCC’sFirst Assessment Report, released back in 1990, gave a range of 1.5°C to 4.5°C. It’s not that climate science hasn’t progressed since then, but just that the advanced understanding has not led to substantially better constraints.
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