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1.5.2. Anthropogenic greenhouse gas emissions
Since the beginning of the industrial revolution, the quantity of greenhouse gases (GHGs) injected into the atmosphere by human activities (CO2, CH4, etc.) has been considerable (see Table 1.1).
These concentrations represent a major disruption in the evolution of the climate compared to pre-industrial natural evolution. These anthropogenic gases reinforce the greenhouse effect and prevent infrared radiation from leaving the lower layers of the atmosphere, which leads to the warming of the air, surface ocean layers and soils. The increase in GHGs is mainly linked to the use of fossil fuels (coal, gas and oil). However, agriculture also contributes to this increase, in particular through deforestation, which is partly responsible for the emission of carbon dioxide (CO2), rice growing and the breeding of ruminants that release methane (CH4), and pig farming and the spreading of manure as fertilizer, which is responsible for the increase in nitrous oxides (N2O). These changes are intimately linked to changes in consumption patterns and to the evolution of the world population explosion since the 19th Century, unprecedented in the history of mankind.
Table 1.1. Global annual average surface area abundances and trends of the main greenhouse gases of the Global Atmosphere Watch (GAW) of the World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) global greenhouse gas (GHG) monitoring network
| CO2 | ch4 | N2O | |
| Average overall abundance in 2018 | 407.8 ± 0.1 ppm | 1,869 ± 2 ppb | 331.1 ± 0.1 ppb |
| Average overall abundance in 1750 | 278 ppm | 722 ppb | 270 ppb |
| Relative abundance in 2018 compared to 1750 | + 147% | + 259% | + 123% |
| Absolute increase between 2017 and 2018 | 2.3 ppm | 10 ppb | 1.2 ppb |
| Relative increase between 2017 and 2018 | + 0.57% | + 0.54% | + 0.36% |
| Annual average of absolute growth over the last 10 years | + 2.26 ppm per year | + 7.1 ppb per year | + 0.95 ppb per year |
COMMENT ON TABLE 1.1.- Units are molecular fractions of dry air and uncertainties are 68% confidence limits. A number of stations are used for the analyses: 129 for CO2, 127 for CH4 and 96 for N2O (source: WMO 2019).
The most optimistic scenarios of global warming are based on a decrease or stabilization of GHG emissions. However, if we look lucidly at the consumption trajectories of contemporary societies, the most pessimistic scenarios remain the most likely. In order to achieve a neutralization of Co2 emissions, the necessary changes in energy consumption, transportation, industrial production, agricultural production linked to changes in food consumption, societal choices and therefore political choices are radical and therefore unlikely in the short term, even though many solutions exist.
1.5.3. Volcanism
Major volcanic eruptions can inject huge amounts of gases into the stratosphere, including carbon dioxide Co2, sulfur dioxide So2 or hydrogen sulfides H2S. Sulfur gases (So2 and H2S) lead to the formation of liquid sulfate aerosols in the stratosphere with a lifetime of a few years. These aerosols scatter incoming solar radiation and absorb infrared radiation. They then lead to a clear reduction in the radiation reaching the earth's surface and thus induce cooling.
Major periods of volcanic activity in the Earth's geological past, such as the one that allowed the Deccan traps in India to form, have resulted in climatic changes that have caused major environmental crises, such as the most famous mass extinction of the late Cretaceous period, 65 million years ago, which contributed to the extinction of the dinosaurs. The multi-millennia history of societies also includes a number of volcanic eruptions that have modified the climate, with repercussions at different scalar levels: climatic, agricultural, sanitary, demographic and political (Eldgja in 939-940, Samalas in 1257, Laki in 1783, Tambora in 1815, Krakatoa in 1883). Two major volcanic eruptions in 536 and 540, the names of the active volcanoes involved are still not known with certainty, are believed to have caused a 2°C drop in temperature in the northern hemisphere during the decade 536-545. A positive feedback loop of spatial extension of the Arctic ice pack would thus have amplified volcanic-induced atmospheric cooling by the combined increase in albedo and reduced ocean-atmosphere interactions (Toohey et al. 2016).
Reconstructions of summer temperatures for the Northern Hemisphere can be made for the last 1500 years based on tree ring widths and maximum wood density. For the Samalas eruptions of 1257, the summer cooling is estimated to have been -1.3°C for the extra-tropical regions of the Northern Hemisphere and -0.8°C for the Tambora eruption in 1815. These coolings continued 4-5 years after the Samalas eruption and 2-3 years after the Tambora eruption (Stoffel et al. 2015). Analyses based on glacial records from Greenland and Antarctica since 500 BC show that the 20th Century, even though it saw major eruptions such as the Bezymianny in Kamchatka in 1955, the Agung in Bali in 1963 or Mount St. Helens in the USA in 1980, did not experience as much volcanic forcing as in previous centuries (Toohey et al. 2017).
Thus, the thermal variations over the last millennium (medieval optimum and Little Ice Age) could be explained by a combination of natural climate forcings, linked in particular to variations in solar activity and volcanic activity (Khodri et al. 2015). on the other hand, simulations of future climate change for the 21st Century generally do not take into account likely major volcanic eruptions. one study, however, attempts to model how 60 eruptions could influence climate change by 2100 and concludes that, beyond annual or decadal variability, they are unlikely to be able to mitigate contemporary climate warming on this secular scale (Bethke et al. 2017).
1.5.4. Albedo and the radiation balance
Figure 1.12. Elements of the global energy balance and albedo of different surfaces in the Arctic
(source: design D. Mercier, drawing by F. Bonnaud, Faculty of Arts, Sorbonne University, 2020). For a color version of this figure, see www.iste.co.uk/mercier/climate.zip
Although albedo occurs everywhere on the planet's surface, its role in today's global warming is best illustrated in the Arctic basin. The ocean (sea ice) and land (glaciers, snow) surfaces of the Arctic cryosphere have a high albedo potential (see Figure 1.12). Increases in air and ocean temperatures are contributing to a
