Autor: Paulo Saraiva | Publicado: 2012-10-08T22:19:00-07:00

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QUAL A CAUSA DAS
GLACIAÇÕES?





Publicado 26/05/2008 



Os cientistas discutem
constantemente as razões da existência de ciclos climáticos que fazem as
glaciações surgirem e terminarem. A teoria mais famosa, apresentada em 1920
pelo matemático iugoslavo Milutin Milankovitch, defende que o clima da Terra é
determinado pelo volume de energia que ela recebe do sol. Segundo Milankovitch,
essa insolação — incidência de radiação solar — é ditada, por sua vez, por três
fatores astronómicos. 



O primeiro é a natureza irregular
da órbita terrestre em torno do Sol. Em um período de 100 mil anos a órbita do
planeta passa de um círculo quase perfeito para uma forma ligeiramente oval. À
medida que tal excentricidade aumenta, ocorre o mesmo com o afélio — distância
mínima entre a Terra e o Sol —, causando menor insolação e temperaturas mais
baixas. 



O segundo fator é a inclinação do
eixo de rotação da Terra, que varia de 21,8 a 24,4 graus, a cada 40 mil anos. Com o
máximo de inclinação, a insolação e as temperaturas caem. O terceiro fator de
Milankovitch, a precessão, se refere ao modo pelo qual a Terra oscila em seu
eixo, como um pião. Em períodos de 21 mil anos, tal fenômeno afeta a inclinação
do planeta e traz temperaturas mais frias. Quando os três fatores se reforçam mutuamente,
eles mergulham a Terra em uma glaciação.



Embora Milankovitch tenha
demonstrado que as glaciações do passado coincidem com períodos de insolação
mínima, outros cientistas descobriram depois que os fatores astronômicos,
isolados, não fazem baixar drasticamente as temperaturas. 



Tais fatores podem bastar,
contudo, para deflagrar uma cadeia de eventos climáticos que desencadeariam a
próxima glaciação. Na verdade, simulações de computador revelaram que a Terra
poderia passar de seu atual período intergiaclal para uma autêntica Era Glacial
em apenas 60 mil anos; e também que a diminuição das médias de temperatura
necessária para isso seria de apenas 6 graus Celsius.