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Q & A: global warming

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Most recent answer: 07/04/2010
I've been thought that global warming is mainly caused by humans because of the green house effect. But some people say global warming is just a natural phenomenon and humans have very little to do with it. They say climate changes all the time, and there's nothing we can currently do about it. And they also say that the way the average atmospheric temperature was measured is not reliable, and according to satelite observations, the earth is actually cooling! One of their arguments regarding CO2 and the green house effect is that the rise in atmospheric temperature will form more clouds and they will balance things out by reflecting the infrared rays. They have tons of theories like this. So, what they are saying is that we can produce as much carbon dioxide as we want and there will be no detectable problems directly caused by it. I understand that things like this cannot be proved or disproved in a way that everyone will accept since it's all about theories, yet. But, as a physicist and an educator, what is your opinion? I'm very confused.
- Eric (age 10)
Grand Rapids, MI, USA
That's a nice set of questions. I'll try to work through them.

1. On satellite measurements, those sources are plain wrong. The satellites also show the lower atmosphere warming. Even more importantly, they show the stratosphere (upper atmosphere) cooling. That's just what you expect if the cause of the lower warming is that it's getting harder for heat to escape to higher regions. That's what's called the greenhouse effect.

2. Warming has a variety of effects on water vapor (itself a powerful greenhouse gas), clouds, ice cover. etc. Some of those effects tend to amplify the warming, some tend to make it smaller. The main difficulty in making predictions is to figure out what the net amplification factor is. Models are used, but it's even more important to use the history of the earth's temperature over the last 100 years. Since the main things driving change (changes in the sun's input, changes in greenhouse gases, changes in reflecting particles) are approximately known, one can approximately calculate how much the changes have been amplified.  When scientists talk about uncertainties in exactly how much warming will be caused by a given amount of CO2 and methane, this is where the uncertainties come from. The uncertainties in the standard international reports include these effects.

Mike W

(published on 07/04/2010)

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