Greenhouse Gases (GHGs)
The increase in greenhouse gases (GHGs) concentration is the main contributor to the changes that affect the atmosphere. The issue of the reduction of greenhouse gases is at the heart of the discussions at COP21 in Paris whose goal is to maintain the global warming underway below 2ºC. These gases tend to increase the absorption of infrared radiation emitted by the Earth surface instead of leaving the Earth and cooling it The corresponding heat is stored in the lower levels of the atmosphere and in the oceans (more than 90%). These GHGs concentration change induce what is called a “radiative forcing “ of the climate system.
A positive value of the radiative forcing contributes to the increase of the global mean surface temperature, whereas a negative one induces a decrease. It is possible to estimate the impact of a particular greenhouse gas to the radiative forcing over a given period of time based on: 1) the variation of its concentration in the atmosphere over this period of time and, 2) the efficiency of this gas in perturbing the radiative equilibrium. Such computation provides what is called the global warming potential. Several main greenhouse gases exist but only a few have a major impact on climate and a significant increase in their concentration is associated to human activities.
Many referenced GHGs
The IPCC has listed more than 40 GHGs, among which we find water vapor (H2O), carbon dioxide (CO2), methane (CH4), ozone (O3), nitrous oxide (N2O) and halocarbons.
Water vapor is essentially of natural origin. It is a powerful GHG that contributes 60 – 90 % of the natural greenhouse effect. Without it our planet would have a temperature of the order of -18ºC and therefore would be frozen. Ozone is a major GHG, continually destroyed and formed by chemical mechanisms. In the lower part of the atmosphere human activity influences ozone concentration through, for instance, the emission of carbon monoxide, hydrocarbons or nitrous oxide. On the contrary, in the upper part of the atmosphere (stratosphere), ozone is destroyed by some gases of anthropogenic origin like CFCs (ozone hole).
What is particularly significant about GHGs is their long lifetime (of the order of 10s of years) compared to other form of pollution. Among the gases that remain several decades in the atmosphere we find the four most active ones:
Carbon dioxide (CO2) – It is found in the highest concentration (400 ppm) in the atmosphere and results from combustion of fossil fuels such as coal, oil and gas, and from biomass burning (soils and forests). In 2004 CO2 represented 77% of all the anthropogenic emissions.
Methane (CH4) – It is very efficient at absorbing infrared radiation (about 70 – 100 times the absorption of CO2) but has a smaller concentration. Like CO2 its concentration has increased dramatically over the last 100 years. Methane results from agricultural activity (rice patties, cattle…), production and distribution of gas and oil. However, a large part is of natural origin. Its concentration has more than doubled between 1990 and 2005.
Nitrous oxide (N2O) – The third most important GHGs has a warming potential 310 times that of CO2. Human activities such as agriculture, fossil fuel combustion, wastewater management, and industrial processes are increasing the amount of N2O in the atmosphere.
Halocarbons – These are the chemical components produced by man that contain carbon and elements from the halogen family (bromide, chlorine and fluorine). These gases have a very long lifetime but are now controlled as part of the Montreal Protocol.
The fifth IPCC report on the evolution of climate published in October 2014 concludes that despite an attempt to take into account climatic impacts and to enact some policies, worldwide GHGs emissions have increased to levels never reached before, so much so that the progression between 2000 and 2010 has been faster that in any decade before.
Proposed scenarios to maintain the increase in global mean temperature below 2ºC require a reduction of worldwide GHGs emissions from 40 to 70% compared to 2010 by the middle of the century and to be fully eliminated by 2100.
Reference article in french by Celine Toubin (theconversation.com)