During the 20^th century, the Earth’s climate and ecosystems underwent overwhelming qualitative and quantitative changes as a direct result of human activity. Our population has grown to over six billion and carries an ever increasing demand for energy derived from natural resources. The average individual energy consumption of modern humans is 200 times that of our primitive ancestors. Presently, 85% of the energy used by our civilization is obtained from the burning of fossil fuels—primarily carbon, petroleum and natural gas. Combustion of these natural resources emits CO2 into the atmosphere, where it is then incorporated into the natural carbon cycle. In the last 150 years the atmospheric concentration of this gas has raised ca. 100 ppm due to human activity.

As a greenhouse gas, CO2 causes a “thermal blanket” effect in the atmosphere. Other greenhouse gasses, including methane, nitrogen oxides and freons, have increased in parallel to CO2 since the Industrial Revolution. There is mounting evidence that our emission of greenhouse gasses causes a warming of the Earth’s atmosphere. It can be affirmed with a probability of 95% that the mean annual temperature increase of the last 100 years, ca. 0.75ºC, is higher than that observed in the last 1,000 years (Mann et al., 1998) and that 2005 has been the year exhibiting higher annual average air temperatures over this period (NOAA, 2006). This year records the highest temperature within a group of the last thirteen years (1995-2007) encompassing the warmest instrumental records of global surface temperature (since 1850) Furthermore, widespread thawing has been observed in mountainous regions as well as in the north and south poles. These effects are concurrent with the thermal expansion of seawater which is causing global sea levels to rise, e.g. an increase of 25-30 cm during the 20^th century in the eastern US coast (Kearney et al., 2002).

The changes occurred so far may not appear to be very significant when considered in the human perspective of daily change and the current variability of meteorological processes. However, when they are examined by reference to natural climate evolution it turns out that strong climate modifications are underway. Thus, increments of 100 ppm of CO2 are equivalent to what occurred naturally every time that our planet underwent transitions from glacial to interglacial periods. Moreover, the present CO2 concentration (388 ppm) is higher than any observed in all of the interglacial (280 ppm) or glacial (190 ppm) ages of the last 650,000 years (Siegenthaler et al., 2005; Petit et al., 1999). Straightforward consideration of these simple facts evidences a need for understanding the planetary processes that may be modified due to this greenhouse gas increase and for anticipating the changes that may occur in the next future.

Evaluation of the present climate change from the perspective of the past climatic history may seem a paradox if the problem is approached from the restricted point of view of a few disciplines. A recurrent question that derives from such limited perspective is whether the presently observed change could be explained by natural fluctuations. However, the information obtained from instrumental records is too short in time as to be able to provide sufficient comprehensive perception. A multidisciplinary point of view that combines past and present studies opens the possibilities for a full understanding of the implications of the present climate change, the human influence on it and the forthcoming evolution. This approach constitutes a scientific challenge since it crosses over different ways and time and space scale differences to address the study of natural processes.

Current evidence indicates that we may be close to the cusp of a much greater transition. The International Panel on Climate Change (IPCC) predicts that by the end of the century, atmospheric CO2 levels may have reached between 400 and 790 ppm depending on the emission. These increments will give rise to best estimate mean temperature increases of 0.6 and 4.0ºC with respect to 1980-1990, and increases in sea levels of between 18 and 59 cm. These predictions are based on extrapolation of present climate data considering accumulative effects. Reorganizations of the climate system either at regional or global scale could involve stronger climate modifications approaching those included within the abrupt effect concept.