Cambio global

CAMBIO CLIMÁTICO GLOBAL

¿QUÉ SE ENTIENDE POR PALEOCLIMA? ¿QUÉ EVIDENCIA EXISTE DEL CAMBIO CLIMÁTICO EN ÉPOCAS ANTERIORES? ¿QUÉ CAUSA QUE EL CLIMA CAMBIE? --------------------------------------¿QUÉ PODEMOS DECIR ACERCA DE LOS POSIBLES CAMBIOS CLIMÁTICOS FUTUROS? ¿CÓMO FUNCIONAN LOS MODELOS DE CAMBIOS CLIMÁTICOS Y CUÁLES SON SUS PREDICCIONES PARA EL FUTURO?

This, roughly speaking, isthe vegetation cover that existed 500 years ago as Europeans began to arrive in the Americas. In the eastern USA, forest was predominant with warmth-adapted temperate forest in the south-east. To the west, decreased rainfall and the resulting fires prevented trees from growing, and a wide grassland belt (prairie) existed. In the mountains and plains of the far west, sparse rainfall gave semidesert,open scrub and open conifer woodlands. To the north, various conifer forest belts extended across Canada, opening out into the meadow-like tundra where climates were too cold for trees to grow.

This vegetation map showing the eastern USA during the period 28,000-25,000 14C y.a. has been compiled by Paul & Hazel Delcourt. An ice sheet already covered most of Canada and extended south of theGreat Lakes. Boreal conifer woodlands and forests predominated in what is now the cool temperate forest zone, and the cool and warm temperate forest belts were compressed southwards.

At the most extreme stage of the last glaciation, most of Canada and much of the northern USA were covered by an ice sheet thousands of metres in thickness. Colder and often drier than present conditionspredominated across most of the USA. The eastern deciduous and conifer forests were replaced by more open conifer woodlands with cooler-climate species of pines and a large component of spruce. The open spruce woodland and parkland extended somewhat further west than present, into what is now the prairie zone. As a result of aridity and lowering of sea level (which lowered inland water tables), much ofFlorida was covered by drifting sand dunes. Notably moister than present conditions occurred across much of the south-west, with open conifer woodlands and scrub common in areas that are now semi-desert.

At around 13,000 14C y.a., retreat of the the western and eastern North American ice sheets exposed an 'ice free' corridor linking Alaska to the land to the south. The Bering Straits at this timealso remained dry land. Cold climate conifer forest began to colonize areas exposed by the ice sheet in the eastern USA, and cool temperate forest vegetation started to spread northwards.

Just before the beginning of the Younger Dryas cold phase, continuing retreat of the ice sheets and recolonization of vegetation allowed conifer forests to return across the Great Lakes region. In the 'icefree corridor' of western Canada, spruce parkland (and, locally, spruce forest) had colonized. Alaska remained as tundra, and areas most recently exposed by the retreating ice were also still tundra. In the eastern USA, the cool temperate forest belt was returning, but it was generally somewhat further south than its present northern limit. Cool temperate forest had in fact reached as far as Maine inthe north-eastern USA, but in this area it was eliminated and replaced by boreal forest during the subsequent Younger Dryas cold period.

Although climates were as warm or slightly warmer than at present around 8,000 14C y.a., large ice masses remained in eastern Canada because there had not yet been enough time for the ice sheet to melt away completely. Retreating rapidly and cooling theclimate at their immediate periphery, the ice masses exposed a rim of new land that took time for the forest to colonize fully. In Alaska too, the present forest cover had not yet returned fully, with ongoing ecological succession. Most other areas, however, had somewhat similar vegetation cover to the present. Drier conditions in the prairie zone caused greater eastward and northward extent of the...