Positive retroaction and tipping points

The so-called 'positive retroaction' mechanisms are likely to make climate change even worse. Positive retroaction corresponds to higher CO2 emissions, with anthropogenic emissions releasing greenhouse gases trapped in the soil, the ocean, the earth and the biosphere.

Large quantities of methane trapped in the permafrost2 could therefore be released due to a temperature increase. The methane is trapped in crystals formed in the presence of water, the hydrates. Once the temperature is high enough to cause decomposition of the hydrates, methane is quickly released. Since methane has a relatively high GWP, there is a considerable risk for the planet. The Siberian permafrost has started to thaw, suggesting that this change could occur in the relatively short term.

Maritime streams such as the Gulf Stream could be modified, with consequences for the climate of the coastal regions. Climate change could have also an impact upon interaction phenomena between ocean water layers and winds, such as those which produce El Niño. El Niño is a strong maritime stream, which occurs periodically in the Pacific Ocean. It is provoked by dominant winds, which induce an oscillating mechanism in the atmosphere coupled with a displacement of surface ocean layers. This oscillating mechanism results in modifications of climatic conditions and of rain occurrence.

Disastrous effects have been observed as a consequence of El Niño, such as droughts and famines, forest fires and floods3, which illustrate the possible consequences of such a climatic disorder.

2 Permanently frozen ground of the Arctic regions.

3 The disastrous episode of massive forest fires which struck Indonesia in 1997 can be related to El

Nino.

Other positive retroaction phenomena are likely to increase the imbalance: thus, disappearance of the reflective effect of pack ice with respect to solar radiation, the so-called 'albedo' effect, will contribute to increasing the quantity of heat absorbed by the ground and therefore to heating the surface.

The temperature increase could also affect the tropical forests, which currently capture some of the CO2 in the atmosphere. If these forests should disappear, they would release the carbon they have stored until now. These risks concern in particular the subtropical forests, whose disappearance could accelerate if the temperature reaches a certain threshold. A reduction in their area, by modifying the rainfall, would reduce the quantity of water received by the remaining forest, causing a chain reaction. Repeated heat waves also promote outbreaks of forest fires. The Amazonian forest could therefore disappear in a relatively short time.

Lastly, a major risk identified by the IPCC in its last report concerns a reduction, caused by a rise in temperature, of the ability of the oceans to act as a carbon sink, thereby increasing the atmospheric CO2 content.

All these examples demonstrate that the effects of climate change could increase suddenly above a certain point which could be reached in the relatively short term. Survival of all ecosystems on the surface of the Earth is concerned with such 'tipping points'.

The dramatic consequences of a temperature elevation are increasing rapidly. For a 1 °C elevation, it is estimated that acidification of the oceans would cause serious damage to the coral reefs and to the ecosystems of the Arctic region.

For a 1.5 °C elevation, we would observe irreversible thawing of the polar ice cap in Greenland. Between 2 ° C and 3 °C, numerous ecosystems, especially the Amazonian ecosystem, would be threatened. At 3 °C, we would be faced with conditions leading to massive extinction of numerous plant and animal species, under conditions similar to those which led to the major extinctions which occurred during the geological ages [21].

It therefore seems vital not to exceed a temperature elevation of 2 °C. According to the models produced, to achieve this it is essential to limit the atmospheric CO2 content which will be reached in 2050 to a value of between 400 ppm and 450 ppm.

The vulnerability of each country varies according to its geographic position and its ability to adapt. The northern industrialised countries are less vulnerable than the developing countries in the southern hemisphere, in view of their latitudes and also their ability to invest to adapt to climate change. Emerging countries like India and China are highly exposed to the risk of climate change, which could hinder their development. A very large proportion of the world population is therefore seriously concerned [22].

Should we consider that there will be winners and losers, and that the countries in the north (or the extreme south) will benefit from climate change? Such an attitude would be hazardous and risky. Certain countries could partially benefit from the situation, provided that the climate change remains moderate. If the change was to intensify, they would themselves suffer from the dramatic consequences, and the tensions resulting from the distress of the worst hit countries would escalate out of control. Famine, water shortage, floods and the increased risks of epidemics would inevitably lead to massive migrations and conflicts which would be all the more fierce for being quite literally a fight for survival. Despite the remaining uncertainties it is therefore urgent to combat global warming, which would threaten our entire planet, with every possible means.

According to the Stern report, if no appropriate measures are taken immediately, the financial impact of climate change would amount to over 5500 billion Euros [13]. It reveals that inaction may turn out to be far more expensive than the measures which must be taken immediately to prevent any worsening of the phenomena related to climate change.

A vigorous action plan is required. The Kyoto Protocol raised public awareness regarding the problem of climate change, but much more radical measures must be set up in the future.

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