Resumen de Cambios climáticos: ¿abruptos o graduales? Lecciones del registro paleoclimático
- español
Nuestra civilización, que abarca los últimos 11.000 años, se ha desarrollado en un periodo de gran estabilidad climática, caracterizado únicamente por pequeñas (aunque determinantes en nuestra Historia) variaciones en la temperatura global. El registro paleoclimático, y en especial el análisis del hielo fósil en Groenlandia, nos muestra sin embargo lo excepcional de esa estabilidad reciente. Durante la última glaciación y el interglacial previo (unos 120.000 años), el clima global ha variado repetidamente (a escala de siglos o milenios) y lo ha hecho de forma drástica y rápida (los cambios se han producido en pocos años). Se ha comportado como un sistema complejo biestable, que oscila entre dos estados de equilibrio diferentes. En esos estados y en los cambios que se producen entre uno y otro, resulta determinante la circulación oceánica profunda a escala global. Y ésta es, a su vez, muy sensible a los cambios de salinidad y temperatura que se han producido en la zona septentrional del Atlántico, donde se genera el agua profunda del Atlántico Norte. En la actualidad, esta zona está siendo fuertemente alterada como consecuencia del calentamiento global actual, hecho que puede desencadenar un cambio súbito e inesperado en los patrones climáticos. De ser así, ese calentamiento global que está en boca de todos y del que en parte somos responsables, podría derivar, en pocas décadas o siglos, en un enfriamiento repentino de consecuencias impredecibles en nuestro mundo superpoblado.
- English
Our civilization was born and developed in a period of warm and relative unchanging climate. During the last ten or eleven thousand years, the global climate has experienced only small variations, with maximum changes of one or two degrees Celsius below or above the present day temperature. These changes, which were often capable of modifying the ways of our History, were soft and gradual if compared with the global climate change that our Earth system can now be undergoing. But before extract any conclusions about the near future of climate, we should analyze the evidences yielded by the paleoclimatic record about climatic changes and Earth system behavior during the past tens or hundreds of thousands years.
In the last decade, high-resolution studies of Greenland ice cores and deep sea sediments (as well as data from fossils and lacustrine deposits) have yielded evidence for the exceptionality of that recent climatic stability. During the last glacial time and the preceding interglacial period (about 120,000 years), the climate repetitively experienced very rapid changes, oscillating between relatively warm intervals and much colder and often longer ones. Both types of intervals lasted centuries or millennia, and changes from one to the following interval were always extremely rapid, taking place in decades or even in only few years.
Those oscillations represent major reorganization of Northern Hemisphere circulation patterns, with large changes in precipitation and temperature. They support the notion that climate during the last glaciation was biestable. The system had a chaotic behavior, and alternates between two very different states of equilibrium. Both states and the abrupt changes occurring between them seem to have been strongly controlled by drastic changes in the thermohaline ocean circulation, which is often known as the great conveyor belt.
This conveyor belt, which moves giant water masses throughout the global ocean, has different modes of operation which determine strong differences in the heat transport within the Earth system. At the present time, the main engine of that belt is located in the North Atlantic, where the North Atlantic Deep Water (NADW) forms. This process causes each year an enormous release of heat to the atmosphere that explains the warm temperatures that we enjoy in Western Europe. The mechanism of the NADW formation is however strongly sensitive to changes in salinity or temperature that eventually could occur in the surface of the North Atlantic. Deep marine sediments suggest that, during the last glacial time and simultaneously to the coldest intervals of this period, several catastrophic swarms of icebergs occurred in that area, probably as consequence of autocyclic massive collapses of the Northern Hemisphere ice sheet. These fluxes of icebergs promoted abrupt inputs of freshwater in the North Atlantic, causing salinity to decrease enough to shut down the NADW. This process changed several times the mode of operation of the conveyor belt, and thus the transfer of heat to the Northern Hemisphere, with dramatic consequences in the global climate.
If the major climate changes of glacial time came as the result of mode shifts in the thermohaline circulation, related to the NADW formation, we should suspect that present-day modifications in the area of deep-water generation could drive abrupt climatic changes too. Climate models predict an increase in the global temperature of two or three degrees Celsius for the end of the 21st century. This rapid increase in temperature could modify the density of the surface waters in the North Atlantic by means of (1) rapid melting of the Greenland polar ice, (2) increasing precipitation, and (3) rising the sea - air interface temperature. If sea surface density decreases enough to shut down the NADW formation, the predicted warming will probably ride us into an unpredictable global climate, with extremely cold temperatures, at least in Europe. It is difficult to imagine the dramatic consequences of such climate change in our super-populated world.
