Climate Change

We can’t wait any longer

Who could still doubt the reality of global warming? The final diagnosis issued in February 2007 by the scientists of the IPCC (1) highlighted the need to face up to the facts and take immediate action.

The melting of polar ice caps is proof of global warming. The ice cap in the Arctic and Greenland in 1979 and 2005. The melting of polar ice caps is proof of global warming. The ice cap in the Arctic and Greenland in 1979 and 2005.
© WWF/photos NASA
In the possible catastrophic scenarios, the entire Mediterranean Basin would suffer a serious drought. In the possible catastrophic scenarios, the entire Mediterranean Basin would suffer a serious drought. Below, the impact of human actions on the wetlands of the Camargue studied by the CEFE (Centre for Functional and Evolutionary Ecology) in Montpellier. This work focuses on micro-organisms in sediments, particularly with regard to greenhouse gas emissions.
© CNRS/Gilles Pinay

Global warming is, without doubt, a reality.’ Standing in front of a sea of cameras gathered from all corners of the globe, Susan Solomon, co-chair of one of the IPCC working groups and researcher at the Earth System Research Laboratory in Boulder (USA), addressed the audience in a neutral tone, as befitting a strictly scientific message. It is 2 February 2007 and we are in Paris, where the IPCC is publishing its fourth evaluation report, the product of six years of work undertaken by the world climate community.

Her voice may have been devoid of emotion but the information being issued set off alarm bells. ‘Essentially, we consider that the increase in average global temperatures observed since the mid-20th century, is most probably due to the increase observed in anthropogenic greenhouse gases,’ continued the scientist. ‘The phrase most probably, which replaces the word probably used in the previous report, should be understood as signifying a significant change, considering that it comes from a group of scientists rather than conservationists,’ insists David Wratt, director of the National Climate Centre in Wellington, New Zealand.

Wratt knows what he is talking about, as he is one of the forty lead authors who put the document together. He stresses that the term ‘most probably’ indicates a probability greater than 90 %. The message from Achim Steiner, Executive Director of the United Nations Environment Programme (UNEP), is the same: ‘This day will probably go down in history as being the day when any remaining doubt concerning man’s effect on the climate was effectively eradicated.’

Reliable models, alarming scenarios

This is not a revolution, but a confirmation and a consolidation of what the IPCC has been saying for a number of years. Our planet is warming up; during the 20th century, the earth’s temperature rose by approximately 0.7 °C. To make matters worse, the increase in temperature is getting faster: it is currently rising at a rate of 0.2 °C per decade. And what about tomorrow? ‘The results of global models, which hold the answer to this question, are becoming more reliable,’ explains Susan Solomon. Reliable because they are improving and are therefore providing a better representation of physical phenomena. Reliable because there are more of them: currently around 25, compared with barely 10 or so at the time of the previous report. And finally, because it has been possible to run them more – seven or eight simulations inevitably provide more information than just one.

The key messages contained in the report warn of the certainty of an alarming future. In the absence of any particular strategy to combat global warming, the increase in global temperatures during the 21st century is likely to lie somewhere between 1.8 °C and 4 °C, according to the possible scenarios. Even so, we need to bear in mind that predicting the evolution of human societies is at least every bit as complicated as predicting the climate. Thus, for these various scenarios, specialists have designed consistent systems of hypotheses regarding the global population, technological options (for example, a lot of coal or a lot of nuclear/renewable energy), and type of growth (service and information-orientated or material consumption, for example). Climatologists have then endeavoured to predict the consequences of these various future situations.

The figures range from 1.8 °C for the best-case scenario (called B1) to 4.0°C for the worst-case scenario (called A1FI to denote ‘fossil intensive’). The figures are quoted with their possible margins of error. Thus, according to the IPCC ranges, A1FI could give rise to an increase ranging from +2.4 °C to +6.4 °C. Why this difference? One of the features of global warming is that its effects are intensified as we move further away from the tropics and its intensity will be greatest on continents and at high latitudes. We could make a rough estimate that we would need to add 50 % to the global figure to obtain the figures for warming in western Europe and anything up to 300 % for polar regions and the northernmost parts of the continent. A1FI thus truly depicts a climate cataclysm.

The catastrophe programme

However, the increases noted represent averages for the planet and do not represent a continuous and even rate of warming. The average temperature of the globe is purely a mathematical concept, used to provide a measure of human interference. The precise forms taken by this interference are much more difficult to predict. At a regional or local level, various phenomena on a small or medium scale may aggravate or mitigate overall trends. However, recent models enable us to imagine several probable developments.

Heat waves and peaks in temperature will become more frequent and more intense and will last for longer. This will lead to a reduction in the amount of snow cover around the world and a marked increase in the melting of the ice layer in permafrost (2)regions. Sea ice is likely to significantly regress in the Arctic and Antarctic with some simulations pointing to its total disappearance during the summer in the northern hemisphere from 2050 onwards.

As regards rainfall, one of the most important aspects of the climate for human activity, increases are expected at higher latitudes, while reductions of up to 20% are expected in the majority of subtropical land regions. The entire Mediterranean Basin is likely to become drier, an effect which will be felt across southern Europe. The models also point towards an intensification of hurricanes and typhoons in the tropics, while storms in medium latitudes will see their courses redirected towards the poles. Finally, the rise in sea levels appears to have gathered momentum over the last 20 years, with increases reaching approximately 3.1 mm/year. All projections confirm that this phenomenon is set to continue throughout the 21st century.

Three enigmas

Despite the extent to which knowledge has progressed, key uncertainties about the climate remain. While there is no doubt regarding the general trend, scientists are struggling to come up with a satisfactory model for a certain number of phenomena. Three major enigmas continue to pose a challenge. The first concerns what are known as the positive feedback of the carbon cycle. In other words, to what extent could the effects of global warming on vegetation aggravate the situation? It has been shown, for example, that the heatwave in Europe in 2005 resulted in significant damage to vegetation, which not only stopped absorbing carbon from the atmosphere but also released its own carbon as it decayed. However, this type of retrospective effect is very tricky to model.

Another uncertainty is the atmospheric behaviour of aerosols, tiny particles of natural (erosion particles) or human origin (particularly sulphates generated by industry). Depending on their size and colour, these reflect or absorb light, and so have different effects on the atmosphere and cloud cover. Aerosols contribute, in particular, towards the formation of clouds - which also has an impact on radiation, although this impact is very difficult to predict. Depending on whether the cloud is light or thick, low or high, it can contribute towards the cooling down or heating up of the atmosphere. Finally, scientists are puzzled by the future behaviour of continental ice caps in Antarctica and Greenland. It is possible that global warming will lead to the formation of a layer of meltwater which could act as a kind of lubricant at the base of these enormous masses of ice. This would accelerate their natural progression towards the sea and could cause them to melt at a more constant rate, with an impact on sea levels which, for the moment, no one will dare to predict.

  1. Intergovernmental Panel on Climate Change
  2. Permafrost is the term used to describe permanently frozen subsoil.