(The Earth Simulator supercomputer helped study the origin of global warming. )
According to the findings of the 2001 IPCC scientists’ report, the most likely cause of this warming in the second half of the twentieth century would be “anthropogenic forcing”, that is, the increase in atmospheric gases. greenhouse effect resulting from human activity. The degree of certainty has increased in the 2007 and 2013 IPCC reports, which describe the fact that global warming is due to human activity as highly probable and then highly likely.
According to current forecasts, global warming will continue during the 21st century, but its amplitude is debated: according to the assumptions used and the models used, the forecasts up to 2100 range from 0.3 to 4.8 °C.
Scientific method: modeling
(Evolution of the average terrestrial temperature modeled according to the different radiative forcings possible during the XX century, compared to the evolutions of observed temperature.)
Their conclusions are drawn from the results of experiments with numerical models. These are computer programs that estimate the relative importance of various natural and human factors through simulations conducted on supercomputers, to identify the factor(s) causing the sudden rise in temperature.
These models take into account two types of mechanisms:
- those that are sufficiently well understood to be translated into equations. It is essentially the circulation of the atmosphere, the phenomena of radiative forcing and the hydrodynamics of the ocean circulation. The accuracy of predictions based on these mechanisms is limited by the spatial and temporal limitation due to the power of the computers and the efficiency of the calculation algorithms used;
- those whose modeling is empirical. This is particularly the effect of clouds. The size of the meshs of the current models makes it possible to treat these only in a statistical aspect. It is the same for the albedo of the vegetation, which is deduced from observation measurements.
Several hypotheses have been tested:
- cyclical fluctuations in solar activity;
- retention of heat by the atmosphere, amplified by greenhouse gases;
- modification of the reflectivity of the terrestrial surface – the albedo – by deforestation, the advance of the deserts, agriculture, the retreat of ice, snow and glaciers, but also by the artificial cirrus created by the contrails of planes and ships, and urban sprawl;
- volcanic emissions.
Some of these causes are of human origin, such as deforestation and the production of carbon dioxide by burning fossil material. Others are natural, such as solar activity or volcanic emissions.
Climate simulations show that the observed warming from 1910 to 1945 can be explained by the only variations in solar radiation. On the other hand, to obtain the observed warming from 1976 to 2006 (see graph), we note that it is necessary to take into account the emissions of greenhouse gases of human origin. Modeling since 2001 estimates that anthropogenic radiative forcing is ten times greater than radiative forcing due to variations in solar activity, although aerosol forcing is negative.
The key point is that net radiative forcing is positive. In particular, the increase in global average temperature since 2001 is in line with the IPCC forecast since 1990 of global warming. Finally, a warming only due to the solar activity does not explain why the troposphere would see its temperature increase and not that of the stratosphere.
(A positive radiative forcing is a reinforcement of the greenhouse effect and a warming, a negative radiative forcing causes a cooling (increase of the albedo). This corresponds to calculations taking into account the concentrations in the atmosphere. )
- In its 2001 report, the IPCC concludes that anthropogenic greenhouse gases “play an important role in global warming”.
- In 2003, the American Geophysical Union states that “natural influences can not explain the rapid rise in global surface temperatures.”
- On June 7, 2005, the G8 academies of science and the three largest oil-consuming developing countries signed a joint declaration in London, arguing that some people’s doubts about climate change do no more justify the inaction and on the contrary, it is necessary to “start immediately” a planetary action plan to counteract this global threat.
- In 2007, the fourth IPCC report states that the probability that global warming is due to human activities is greater than 90%. Many scientists felt that this IPCC report was not clear enough and that an international program should be put in place to drastically reduce the two main sources of greenhouse gases (especially CO2 carbon dioxide): transport and coal plants. Even if the cumulative radiative forcing of methane is less than that of CO2, the reduction of methane emissions must not be forgotten.
- At the end of 2012, climatologists compiled and compared simulations from twenty models and satellites, concluding that changes in temperature of the troposphere and stratosphere are very real and that they are clearly related to human activities.
- The 5th IPCC Report (published in 2014) believes with “high confidence” that in the absence of additional measures taken, “the basic scenarios lead to an increase in global average temperature in 2100 between 3.7 °C and 4.8 °C compared to pre-industrial values (range based on average climate response, range 2.5 °C to 7.8 °C if response uncertainties are included. The role of methane (CH4) (produced by ruminants, paddy fields, natural gas exploitation leaks and permafrost thaw) is reassessed upwards in this fifth report: methane is estimated to have a global warming potential per unit mass at 100 years equals 28 times that of carbon dioxide, but its cumulative contribution given its concentration remains lower.
Other minor causes of global warming
Other anthropogenic causes have been pointed out by the scientific community. The effects of these various factors are often less well known as evidenced by the component graph of radiative forcing.
- Land use has an effect on albedo. For example, cultivated land is generally clearer than forests and therefore reflects more light.
- The hole in the ozone layer could also have a significant effect, but still remains largely unknown. Indeed, stratospheric ozone, by absorbing UV rays, warms the stratosphere; the absence of ozone therefore leads to a cooling of the stratosphere which, according to some analyzes, would lead to an increase in the height of the tropopause, and to a shift of the entire atmospheric circulation (Hadley cell, circulation of humidity, circulation of energy) to the poles. The effect of this change in circulation is also discussed, but it would seem that moving the cloud cover from storm tracks to the poles would reduce the albedo of mid-latitudes and thus participate in global warming.
- Mismanagement of forests can affect the amount of carbon they receive. Several studies show that the efficiency of carbon capture by trees is highly dependent on their health. For example, certain factors (air pollution, parasite proliferation, heat wave) can have a lasting impact on the gross primary productivity of trees, ie the amount of carbon captured. In 2003, because of the heat wave and consequently the stagnant air pollution and the proliferation of tree pests, gross primary productivity decreased by 8% in France and by 15% in northern Italy. This carbon sink deficit could have been reduced through better forest management (mixing tree species, tree spacing).