François Gervais: Climate Sensitivity and Carbon Footprints

SCC Volume 1.1

A simple formula is suggested to policy makers to evaluate the impact of Earth’s temperature of fossil fuel emissions or reductions. It is illustrated for main emitters, country by country. Two lists of estimates are compared.

One is based on the last report of the Intergovernmental Panel on Climate Change (IPCC AR5 2013) which retained a range of 1±2.5 °C for the Transient Climate Response (TCR) in case of atmospheric CO2 doubling, a metric that is more relevant than the Equilibrium Climate Sensitivity (ECS) to estimate warming in the next few decades. At the rate of increase of 0.5 % per year since the beginning of this century, a CO2 doubling in the atmosphere will hardly be reached before the end of the century.

The second estimate is based on infrared thermal emission spectra of atmospheric CO2 near the tropopause that constrain the climate sensitivity below 1°C in the absence of feedbacks consistent with 109 studies concluding to low climate sensitivity. An increasing number of their publications is reported during both last decades. They are also confirmed by a plateau observed since 1994 for the temperature of the low stratosphere measured by the Earth System Science Center, University of Alabama in Huntsville (UAH), over a period corresponding to 42 % of the increase of CO2 in the atmosphere since the beginning of the industrial era.

A tendency of “cooling” of climate sensitivity versus year of publication is confirmed for studies based on instrumental records of ocean and surface temperature, whereas CMIP6 climate models are running hotter. The correlation of (i) monthly temperature fluctuations measured by UAH at the Earth’s surface and (ii) CO2 increases in the atmosphere that lag temperature fluctuations instead of driving them, is updated and discussed.

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