- François Gervais
- Faculty of Sciences and Techniques, University of Tours, France
- Dedicated to the memory of Professor Nils-Axel Mörner
A simple formula is suggested to policy makers to evaluate the impact on 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.
In 2020, the concentration of carbon dioxide in the atmosphere measured by NOAA at the observatory of Mauna Loa, detrended from seasonal oscillations, reached 414 parts per million (ppm). 1 ppm corresponds to 7.8 Gigatons of CO2 (GtCO2). The atmosphere, therefore, was composed in 2020 of 3.2 1012 tons of CO2. The transient climate response (TCR) is defined as the increase of average Earth’s temperature when the atmospheric CO2 concentration would double. At the average rate of increase of 2.2 ppm per year observed since two decades as is detailed in Figure 6 of Section 4, viz. 2.2/414 = 0.5 %/year, doubling will hardly be achieved during this century.
Section 1 complements the Summary for policymakers of IPCC AR5 (2013) by evaluating a key point that is missing, viz. the impact of the emission (or of reduction of emission) of one ton of CO2 on the Earth’s temperature, a metric that is more relevant than the carbon footprint in terms of climate. Results for largest emitter countries will illustrate their own climatic impact at their rate of emissions during 2019.
Section 2 is a review of published values of climate sensitivity lower than 1 °C that have not been considered in IPCC AR5 (2013) which retained for the TCR the interval from 1°C to 2.5 °C only.
In Section 3, the infrared thermal emission spectrum of atmospheric CO2 near the tropopause – not shown in IPCC AR5 (2013) – is scrutinized. A TCR lower than 1 °C is deduced, confirming data of Section 2. Results of Section 1 are complemented with this value for comparison.
Section 4 updates the correlation of Earth’s temperature measured by satellites and the yearly increase of CO2, discuss them and focus on specific points.