Climate change

The term “climate change” refers to an abnormal, quick and important modification of the system of climates observed on earth. This modification concerns the average climatic conditions and the probability that extreme meteorological events occur.

The international scientific community first became interested and started to study climate change in the early 1970’s. Decades of research and thousands of peer-reviewed scientific studies later, a broad consensus began to emerge on three basic conclusions: first, climate change is occurring. Second, a large share of the observed climate change is linked to human induced emissions of greenhouse gases (“GHGs ”). Third, significant and urgent reductions of these emissions are required to limit the worst risks of climate change.

The expected effects of climate change are many. According to the IPCC , the average worldwide temperature would be likely to increase from 1.1°C to 6.4°C by the end of the century. This increase in average temperatures, which would be larger in some regions than others, will induce water cycle perturbation. While uncertainties about the exact future impacts of climate change still exist, the best science today suggests that we can expect: large-scale ice-melt, an increase in global average sea level, more severe droughts in those regions already drought affected, and an increase in the frequency of scorching heats, floods, storms and other extreme meteorological events.

Further information:

1. What are greenhouse gases?

The greenhouse effect is a natural phenomenon. Water vapour, which can account for between 0.4 to 4 % of the make-up of the atmosphere, plays the largest role in it. Other GHGs , either naturally occurring or induced by humans, make up just under 0.1 % of the atmosphere.

There are six human-induced GHGs :

Carbon dioxide (CO₂) mainly comes from burning of fossil fuels (petroleum, natural gas, and coal) and tropical deforestation;
Methane (CH₄) is mainly emitted by the decomposition process of organic matter in landfills and agriculture activities, such as ruminant cattle digestion;
Nitrous Oxide (N₂O) mainly arises from agricultural production (in particular from the use of fertilisers, animal waste, soil and land-management) plus industrial processes.
Synthetic gases (HFC, PFC and SF6), used in the preparation of sprays, for refrigeration, or aluminium smelting.

The power of each GHG to warm the Earth by re-radiating some of the outbound energy from the planet back towards the surface is measured by its Global Warming Potential (GWP), which is the ratio between the energy reflected towards the surface over 100 years per 1 kg of the gas and that which would be reflected by 1 kg of CO₂ over the same period. GWP can be used to calculate the total quantity of greenhouse gases emitted in the same unit, the ton of CO₂ equivalent (tCO₂e). In 2004, human-induced GHG emissions amounted to 49 billion tonnes of CO₂ equivalent according to IPCC .

Greenhouse gas emissions worldwide per economic sector in 2005

Source : IPCC , 2007.

2. The increase in atmospheric concentration of greenhouse gases

Historically, global temperatures and atmospheric CO₂ concentrations have shown a strong tendency to move together. While the link between the two variables involves complex feedback effects – the precise nature which remain the subject of scientific debate –, the two values are connected. The current concentration of CO₂ is 30% higher than the maximum observed over the 450,000 years of weather records, due to GHG emissions by human activities.

Temperature and concentration of CO₂ in the atmosphere over the last 400 000 years

Source: World Data Center for Paleoclimatology, Boulder & NOAA Paleoclimatology Program.

3. Consequences of climate change

Temperature Increases

The global average temperature has increased by approximately + 1°C over the last century. This increase is particularly apparent over the last 25 years. The expected increase in temperature varies according to latitude. Thus, warming is less in the tropics than at the poles. Equally, the increase in temperature in coastal regions is less than it has been further inland.

Estimated global mean temperature since 1850

Source: IPCC , 4th report of the 1st working group, 2007.

Sea level rise

Data readings indicate a continued increase in levels since the 1870s. The two principal growth factors for global sea levels are thermal expansion and the melting of terrestrial ice deposits (i.e. glaciers, polar ice caps, snow cover and permafrost). Increases in sea level are expected to lead to population migration. Population living in low-lying coastal regions and areas prone to flood will be most strongly hit by even relatively small growth of sea levels.

Global average sea level since 1850

Source: IPCC , 4th report of the 1st working group, 2007.

Extreme weather events

These refer to a number of extreme climatic events such as tornadoes, hurricanes, heat- and cold-waves and abnormally heavy rain or snowfall. Because of the complexity of extreme weather events, it is difficult to attribute any individual event to climate change. But their increase in frequency and changes in their average intensity are understood to be related to a warming global climate.