terça-feira, junho 14, 2005

A radiação dos gases com efeito de estufa de origem natural

A atmosfera é quase “transparente” à radiação solar (só retém 70 W/m2) mas absorve grande parte da radiação infravermelha terrestre (390 W/m2 – 40 W/m2 = 350 W/m2). Esta absorção é realizada, fundamentalmente, por alguns gases concentrados no ar e chamados gases com efeito de estufa (GEE) – vapor de água, dióxido de carbono, metano, ozono, etc. – e pelas nuvens.

O azoto e o oxigénio não participam neste processo de absorção. Além de absorver o calor emitido tanto pelo Sol como pela Terra, os GEE transformam-no e emitem calor. Este é emitido em radiações de ondas longas, infravermelhas, em todas as direcções, para o espaço extraterrestre e para a superfície.

Para o topo da atmosfera vai um fluxo de 200 W/m2 de radiação infravermelha emitida pelo ar e pelas nuvens, que se perde no espaço extraterrestre, enquanto para a superfície planetária vai um fluxo de 324 W/m2. Desta forma, a superfície terrestre recupera grande parte da energia que ela própria emitiu para cima.

Este calor recebido na superfície (324 W/m2) proveniente da atmosfera representa quase o dobro do proveniente do Sol (170 W/m2). Pela soma dos dois, a superfície recebe um fluxo médio de 494 W/m2 (o mesmo que emite, 80 W/m2 + 24 W/m2 + 390 W/m2). Donde, também existe um balanço nulo à superfície terrestre tal como existe no topo da atmosfera.

No topo da atmosfera perde-se continuamente por radiação infravermelha e por reflexão do albedo o mesmo calor que se recebe do Sol. É a situação de equilíbrio já referida. As medições das radiações infravermelhas feitas pelos satélites indicam que sai do topo da atmosfera um fluxo médio de 240 W/m2 que se escapa para o espaço, valor esse que é igual ao da radiação em ondas curtas que o Sol acaba por enviar em direcção ao planeta depois de se ter reflectido uma parte pelo albedo (342 W/m2 – 102 W/m2).

A energia que atravessa o topo da atmosfera por radiações infravermelhas provém dos GEE (165 W/m2), das nuvens (35 W/m2) e da radiação infravermelha procedente directamente da superfície terrestre (40 W/m2) que não é absorvida pelos GEE e pelas nuvens.

Note-se que a radiação infravermelha emitida a partir da superfície terrestre, 390 W/m2, é muito maior do que a radiação infravermelha que sai pelo topo da atmosfera, 240 W/m2. Não existiria esta diferença se não houvesse GEE.

Então, da superfície terrestre emitir-se-ia uma radiação semelhante à que sai pelo topo da atmosfera, isto é, 240 W/m2 (uns 150 W/m2 menos do que se emite realmente). A temperatura média da superfície terrestre, que hoje é de +15 ºC, seria a correspondente a esta emissão inferior, ou seja de -18 ºC, isto é, o ganho térmico devido à radiação infravermelha é de 33 ºC.

A temperatura mais elevada à superfície da Terra deve-se à existência da sua atmosfera.


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