It is only during the past few decades, practically since the end of 1980s, that scientists are beginning to understand the climate system that results from the energy of the sun, and is characterized by complex interaction of numerous factors of influence. These include sun activity, the wobbling of the earth's axis of revolution, oceans, the atmosphere, the lower and upper troposphere, the relative position and surface of continental land masses seas and oceans relative to the poles, the polar snow and ice masses, plant life - forests, croplands, rangelands and wetlands -, and all animal life including modern man with its population of +6 billion (2003), and the volcanic and tectonic activity of the earth. All of these factors are parts of this complex climate system and contribute to its equilibrium at any time. Changes - even small - of any part of the system, may result in a change of climate, due to the necessity of reaching a new equilibrium of all the factors of influence. The ways by which the earth reacts to changing factors are still largely unknown.
Observations on climate change are concerned with the present state of the Earth's climate system because we perceive some evolution of the system may be occurring and we fear consequences for the future of our species, even its survival as we now know species decrease or increase in their numbers, or even become extinct, depending on the evolution of environmental conditions. The genetic and cultural evolution of modern man is linked with the natural change of climate that occurred at the end of the pleistocene, the last glacial maximum, and the advent of the warmer holocene period that permitted the development of agriculture. This was followed by a number of leaps forward that culminate in today's social, scientific, technical and economic advance including world population of more than 6 billion.
The main evidence claimed in favour of climate change is the observed worldwide retreat of terrestrial glaciers, arctic polar ice caps covering land masses, and sea ice with increased numbers of icebergs breaking off, notably in the northern hemisphere pole. It is also claimed that there is a rise of average global temperature as measured by worldwide meteorological stations and of upper atmosphere temperatures as measured by meteorological satellites, and also rise of sea level (10-20cm during the 20th century).
After a period of confusion and non consensus within the scientific community (see history of climate change), today there is a broad consensus that global warming is occurring fast and which may produce dramatic consequences. See IPCC links in the left frame. Although the global warming trend is now acknowledged by the majority of the scientific community, there is no consensus on what are the causes of this trend. What are the effects of natural causes, or human activities, including the production of greenhouse gases, deforestation and change of land use, on climate change that we observe?.
But with new data available, there are more and more objections to the view that the Earth is warming, and specially that it is due to human activity. See dossier on these objections.
Our species Homo.sapiens evolved and developed during the past 200 000 years because of the warming of the earth's climate after a severe glaciation. A logical step is therefore to try to understand how the climate changed over the geologic age of the Earth i.e. 4.5 billion years. Unfortunately, we have little evidence of how the Earth's climate changed over such a long period of time. The evidence we have in the distant past are the forms of life that existed: for example during the carboniferous period which lasted 64 million years from -354 to 290 million years BP, most of the earth's reserves of coal were formed from the decay of plants and trees. See another site on the carboniferous period.
But we have evidence of climate change in the most recent past i.e. first from historical records of the past millenium, and thanks to the progress of science, during the pleistocene which extends over 1.8 million years before present. And still more recently by the analysis of ice cores in central Greenland ice cap, in Siberia Vostok ice core project, and the Antarctica ice core project , which cover about 200 000 years. All data collected suggest that climate changes in the geologic near past appear to have ranged from slow and gradual to surprisingly fast and dramatic. See our state of knowledge on paleoclimates.
This much we have learned about the climate system; but beyond this, we are less knowledgeable. How do all the parts of the system interact? How will specific changes in one part affect the others and, ultimately, the climate? What processes occur to produce the changes we observe such as the cycle of advances and retreats of glaciers and polar ice caps? What effects on climate change can result from our own activities i.e. the increased production of so-called greenhouse gases or deforestation of large areas in the amazon or indonesian archipelago?
Humanity's production of waste gases, water vapour, CO2, nitrous oxide, sulfuric and nitric acids, CFC's and other "greenhouse" gases, as well as our direct impact on large ecosystems like forests and changes of land uses, makes our understanding of how the climate system works, imperative. From what we know now, climate change could raise the average temperature a few to several degrees centigrade over the next decades in addition to altering weather patterns. If so, the potential exists for severe, or catastrophic, disruption of the Earth's environmental and climate systems with dramatic consequences on populations and their economies.
Ice cores, cylinders of ice drilled out of glaciers and polar ice sheets, have played an important role in revealing what we know about the recent history of climate and its evolution in the recent geological past. Scientists completed an ice coring project in Greenland with a view to answering questions about how the climate system functions, based on knowledge of the past. Drilling for "Greenland Ice Sheet Project N°2 (GISP2)" began in 1989 and was completed in 1993. When they reached the bottom of the ice sheet, 3000+ meters thick, in 1993 they had recovered the longest, most detailed, continuous record of climate available from the northern hemisphere stretching back 200 000 years or more through two glacial/interglacial (cold/warm) cycles. While long ice core records exist from Antarctica, fewer comparable records exist from the northern hemisphere. In parallel, within the framework of the joint European Greenland Ice Core Project (GRIP) a similar program of ice coring was performed.
In addition to this, the newest theories on the transitions between glacial (cold) and interglacial (warm) periods involve changing ocean current circulation patterns in the North Atlantic Ocean. The modern atlantic ocean circulation pattern includes the well known Gulf Stream. This flow of water brings significant amounts of heat, in the form of warm water, from the tropics to the North Atlantic. When it reaches the North Atlantic and cools, by giving its heat off to the atmosphere, it becomes dense enough to sink to the bottom thus creating a pump effect. This water flows slowly through the depths of the Atlantic to the Indian and Pacific oceans and eventually returns, approximately 500-1000 years later, to the North Atlantic. It is surmised that during glacial periods, this circulation was shut off, changing temperatures in the polar regions and in the north atlantic. Because prevailing winds in the northern hemisphere are west to east oriented, this caused low temperatures in western europe, and prevented populating of the area by Homo.sapiens until the neolithic.
Also, because CO2 is soluble in seawater, the cessation of this circulation also has important implications for CO2 in the atmosphere. Since the North Atlantic is where this circulation is driven - the cooling, sinking water sets the circulation in motion - it has become a region of significant interest and means that GISP2 will provide a unique record for evaluating these theories.
Ultimately though, a new equilibrium will necessarily be reached. The CO2 that we are ejecting to the atmosphere as we burn the carbon sequestered in fossil fuels over the past geological periods of hundreds of millions of years, will be sequestered again by the different processes (see diagram of such processes).
Also read an overview of carbon sequestration.
and "Woods Hole Research Institute" website on the carbon cycle, which refers to the "missing carbon sink".
The question is time. How long will this new equilibrium need to take place.
Voir aussi essai: Sur le dilemme "croissance économique indéfiniment" et développement durable; est-ce possible?updated on 17/10/2006 by Pierre Ratcliffe Contact: (pratclif@gmail.com)
