Global warming: Hypothesis of an additional greenhouse effect

(Temperature variation, solar activity and CO₂ concentration. )

The greenhouse effect is a natural phenomenon: part of the infrared radiation emitted by the Earth to the Earth’s atmosphere remains trapped by the so-called “greenhouse gases”, thus increasing the temperature of the lower atmosphere (troposphere). These gases are essentially water vapor and carbon dioxide. About a third of the latter was produced by man. Without this effect, the surface temperature of the Earth would be on average less than 33 °C, that is, -19 °C.

The observed increase in the amount of greenhouse gases, such as CO₂, is contributing to a stronger greenhouse effect. Since 1750, it has been estimated that 2,000 gigatons of carbon dioxide were emitted into the atmosphere as a result of human activities, and that 800 gigatons of this total have remained accumulated there. The current levels of CO₂ in the atmosphere far exceed the rates of the last 650,000 years. They increased from 280 ppm (parts per million) in 1750 to 379 ppm in 2005. The level of 400 ppm has been exceeded locally since 2013. However, according to the WMO Bulletin, the annual average for 2014 was found at 397.7 ppm. The Mauna Loa reference observatory in Hawaii reported a concentration of 399.06 ppm during the week of November 1, 2015. Methane concentrations rose from 715 ppb (parts per billion) in 1750 to 1,774 ppb  (parts per billion) in 2013, and 1,833 ppb in 2014, or 254% of its pre-industrial level.

In addition, the rate of growth of CO₂ in the atmosphere also increases, from +1.5 ppm per year from 1970 to 2000, to +2.1 ppm per year between 2000 and 2007. It has been proved by the isotopic study of carbon in the air that this increase in the quantities of greenhouse gases is due for more than half to the combustion of fossil carbon material, the other part being due essentially to massive deforestation.
According to the fourth IPCC report, in 2004 49 billion tons of CO₂ equivalent are emitted annually by human activities, broken down as follows:

• the share due to the energy sector is 25.9%;
• followed by industry at 19.4%;
• the forestry sector at 17.4%;
• agriculture at 13.5%;
• transport at 13.1%;
• dwellings at 7.9%;
• waste and wastewater at 2.8%.

The hypothesis of a link between the average global temperature and the rate of carbon dioxide in the atmosphere was formulated for the first time in 1895 by the Nobel Prize in Chemistry Svante Arrhenius. Arrhenius has shown that increasing the concentration of CO₂ in the atmosphere could significantly increase the temperature of the planet. He calculated that a doubling of the CO₂ content could cause a warming of 4 to 6 °C, values ​​consistent with the modelings of the twenty-first century.

In 1938, the British engineer Guy Callendar, then in 1956 the American physicist Gilbert Plass, established and theorized the relationship between the increase of industrial CO₂ emissions and the first observations of global warming. In this context, in 1957, the Americans set up measurements of the CO₂ concentration of the atmosphere in Hawaii. This allowed the American climatologist Charles Keeling to produce in 1961 a first curve confirming a steady increase in the concentration of CO₂. In the early 1960s, the arrival of large computers allowed the implementation of the first climate models and a first simulation was published by Syukuro Manabe. In the 1970s, Bert Bolin established that a quarter of anthropogenic carbon dioxide emissions were due to tropical deforestation. “Global warming” was described in August 1975 by the US geochemist Wallace Smith Broecker (Columbia University), who for the first time uses this term, with a fairly accurate forecast of the increase in CO₂ concentrations he predicted a rate of 400 ppm around 2010. It was in 1979, at the first world climate conference in Geneva, that the possibility of an impact of human activity was publicly advanced for the first time on the international scene. on the climate. Jule Gregory Charney estimated this impact to a warming of 3 °C (at plus or minus 1.5 °C) in case of doubling of concentration of carbon dioxide. Until the fifth report of the Intergovernmental Panel on Climate Change173 in 2014, this estimate was not modified by subsequent work that relied on more accurate climate models and the study of climate change old. In 1988, Veerabhadran Ramanathan showed that the radiative forcing was also due to methane and nitrous oxide.

The increase in the greenhouse effect induced by all greenhouse gases is estimated at 2.3 W/m², resulting in an increase in temperature. The variations of energy radiated by the Sun during its activity cycles are ten times lower. The hypothesis of a possible influence on the formation of solar-modulated galactic cosmic ray clouds, proposed in the 2010s, was invalidated in 2016 following studies conducted at CERN on the formation of aerosols of the atmosphere (Cloud experience)