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Climate change

Serge Janicot
Catherine Aubertin
Martial Bernoux
et al.

Part 1. Observing and understanding climate change

Observing and understanding climate change

Texte intégral

A GREATICE team on a glacier of the Antisana volcano (5,700 m).

A GREATICE team on a glacier of the Antisana volcano (5,700 m).

© IRD/B. Francou

1Emissions of greenhouse gases resulting from human activities have increased since the beginning of the industrial era to the extent of having a significant impact on the increase in mean global temperature and, more generally, on the evolution of the climate. The influence of these emissions on the climate has been identified increasingly clearly in the successive reports of the Intergovernmental Panel on Climate Change (IPCC), set up in 1988 to evaluate scientific information on climate change. Climate projections for the coming decades confirm the present heating pattern whose scale will depend on future greenhouse gas emissions. The scientific community considers that we are thus entering a new epoch, the Anthropocene, in which the influence of man on the global system is predominant. Although its chronology has not been determined definitively, the Anthropocene is taking the global climate system out of the Holocene, the geological period that encompasses the last 10,000 years.

2Human activities are thus adding an anthropic marker to the main geological eras. At this time scale, past evolution of the climate features alternate glacial periods and hot and humid or more temperate periods (interglacial periods). These glacial transitions are caused by the redistribution of the solar energy received by the earth as a result of the slow changes in the parameters of the earth’s orbit. Its eccentricity varies in two periods: 400,000 and 100,000 years. Its obliquity varies about every 40,000 years and, finally, the precession of the equinoxes varies in a cycle lasting some 22,000 years. On a more short-term basis, volcanic activity also has an impact on climate by dust emission that reduces solar radiation for several years. These natural forcings determine most of the variability of the climate. Their effects are in turn amplified or reduced by internal retroactions in the atmosphere-ocean-continental surface system. The natural greenhouse effect of the atmosphere is an example of retroaction (in contrast with the additional greenhouse effect resulting from anthropic emissions) caused mainly by atmospheric water vapour. It is estimated that without an atmosphere, the average temperature at the surface of the earth would be -18°C and not +15°C as is observed.

3Finally, the atmosphere-ocean-continental surface system also has its own natural variability. This ‘internal’ climatic variability is seen at different time scales ranging from seasonal, such as monsoons, to pluriannual, such as for example the El Niño and La Niña phenomena. These climatic variability modes have a strong impact in the tropical zone. Furthermore, certain physical processes between the surface and the atmosphere or within the atmosphere have strong amplification or reduction effects on the internal variability of the climate system. Internal variability can thus modulate or even replace the effects of anthropic forcing.

4 In this context, one of the difficulties is that of knowing how to ‘attribute’ the cause of climate change observed at a regional or local scale—whether to natural forcings, the natural internal variability of the climate or anthropic impact. Climate scientists use the notion of ‘time of emergence’, that is to say the time required, depending on the region and the climatic variable considered (temperature, precipitation, etc.), for the signal of the forcing of the anthropic greenhouse effect to exceed the ‘noise’ of internal climate variability. In terms of temperature for example, this emergence is fastest in the tropics as internal temperature variation is smaller than elsewhere. This question of attribution is all the more complex as anthropic activity is expressed by other environmental changes such as land use (deforestation, agriculture) that can cause local climate modifications.

5The first part of this book illustrates research aimed at understanding present climate change. Paleoclimatology sets this change in a much longer climate context in order to evaluate amplitude and rapidity. Monitoring the ongoing evolution of climate and interpreting the changes observed and determining the probable cause/s require the use of long-term multidisciplinary observation networks and also modelling facilities that represent the complexity of the processes and physical mechanisms involved. It is thus possible to identify the various spatial and time scales and better understand the functioning of the climate and environmental system of the world. Observation networks and models are also essential for evaluating the confidence and uncertainty level of climate projections.

6At IRD, this research is strongly focused on the tropical regions; these have their own dynamics of response to natural and anthropic climate forcings. The tropics also affect the various global climatic variations. The classic example is the El Niño phenomenon that starts in the tropical Pacific and is the main mode of climate variability at the global level. Stress is laid in the first part of the book on the importance of the research conducted in the tropics for understanding climate change. The most recent advances are described, accompanied by coverage of the limits of the models and information about the main issues of climate research sciences it possible to interpret present regional climate variations and also climate change at the global level.

Table des illustrations

Titre A GREATICE team on a glacier of the Antisana volcano (5,700 m).
Crédits © IRD/B. Francou
Fichier image/jpeg, 177k

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