How does the greenhouse effect work?

by Deb on September 9, 2009

The latest buzzwords seem to be Anthropogenic Global Warming, but if you are about the same vintage I am you probably grew up hearing about the Greenhouse effect.

In order to understand the Greenhouse effect you need to know a little bit about light, or more specifically the electromagnetic spectrum.  Very briefly, there are a lot of things that we don’t traditionally think of as ‘light’ that all go together, like X-rays, microwaves, radio waves, visible light and heat.  They are the same particles travelling back and forth as waves, but with different wave lengths.  This is the distance it takes to go back and forth.  The important thing is that because it is the same particle just travelling slightly differently, one type of radiation can be changed into other types fairly easily, basically by running into other particles.

electromagnetic-spectrum

 When visible light from the sun reaches earth some is reflected back to space by the atmosphere, but some is absorbed by the earth and heats it.  This is because light is a type of energy and putting energy into things generally makes them hot.  Some of this energy is changed then radiated back out by the earth as infrared energy or heat.  This is what warms the atmosphere, not the sun.

Most of the infrared energy from the earth goes back out into space, but there are some gases in the atmosphere that reflect or scatter it.  Water is particularly good at this, so are methane and carbon dioxide.  So instead of going out to space, this scattered infrared energy comes back to the earth and heats it more.

greenhouse-effect

So in effect there are two sources of heat for the earth – the sun and the atmosphere. 

This is perfectly normal on earth, without this greenhouse effect it would be far too cold for us.  In fact there was possibly a time in the earth’s past when it was a mostly frozen ‘snowball,’ so we seem to owe our existence to it.  And we are actually in quite a cold phase of the earth’s history right now – it’s unusual geologically to have polar ice caps, dinosaurs lived quite happily at the north pole.  So as far as the planet is concerned it bounces between heat and cold, and the greenhouse effect is fine.  What’s unusual about now is that we are releasing carbon dioxide and methane faster than they would normally build up.  And unfortunately there is a positive feedback loop, where warming the oceans releases some of the methane stored deep down as types of ice.  There are already methane plumes forming, but none seem to have reached the surface yet.  When the methane ices start to melt they will introduce huge amounts of methane into the atmosphere, which reacts to form carbon dioxide.  To absorb this the oceans turn acidic, which has pretty obvious effects on the things living in it.

So climate change is not just about getting a bit hotter, it is about a whole cascade of effects.  This is why you will often hear climate scientists talk about ‘tipping points.’  The oceans will turn acidic and dissolve the shells of corals and shellfish.  Changes in temperature will change the ocean currents, which will change the nutrient distribution, which will affect where food is for the ocean ecosystems.  Changes in ocean currents and temperatures also have enormous weather effects – el nino and monsoons – which affect what can be grown where.

The planet has done it before quite happily.  The speed of change is going to be really tough on many species, which is terrible for them as individuals but again won’t really affect the planet.  Extinction is a normal geological event, even mass extinctions happen periodically.  But then you have a question of ethics – is there a difference between an impact or vulcanism causing an extinction and us?  The biggest impact is going to be on us.  We have a liking for coastal plains and river systems.  We need vast areas to grow crops to feed all of us.  Then we need working transport systems to get it to people – look at Africa to see what happens when you live far away from where the food is grown.  So reducing the human impact on the greenhouse effect and global warming is not some optional thing that’s nice for the cute furry little animals.  It’s something that will decide whether your grandchildren eat.

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