Greenhouse Gases: Explaining the Impact of CO2 & CH4 on Global Temperature

[ftary]

How would I answer the question: explain why even small amounts of CO2 and CH4 can have a large effect on global temperature?

 

[D H]

For various reasons, we don't discuss anthropogenic global warming at this site. I changed the title of the thread from "Global Warming" to "Greenhouse Gases" for this reason.

If you want to ask how greenhouse gases moderate atmospheric temperatures, that's fine. If you want to discuss whether humans are responsible for global warming, that's not fine. It's against our rules. We don't have the expertise onboard to distinguish sense from nonsense, and the arguments get very acrimonious.

 

[D H]

It's important to keep in mind that the Earth would be a cold, cold place if there weren't any greenhouse gases in the atmosphere. It is those greenhouse gases that prevent the atmosphere from achieving a thermodynamic equilibrium, with a more or less constant temperature profile from the near the surface to the top of the atmosphere.

The atmosphere instead exhibits a temperature that decreases with altitude, a drop of about 6.4°C/km on average. This is the atmospheric lapse rate (google that term). While this lapse rate is driven by adiabatic cooling, this nonetheless goes against the grain of the second law of thermodynamics. Something has to be responsible driving the atmosphere to this non-equilibrium condition. That something is a combination of the heating by the Sun and the presence of greenhouse gases in the atmosphere.

The predominant components of the atmosphere are diatomic oxygen and nitrogen. Being diatomic gases, these are more or less transparent to infrared. The atmosphere would need to be much thicker to get a greenhouse-type effect from oxygen and nitrogen. On the other hand, multiatomic gases such as water vapor, carbon dioxide, and methane are rather opaque to infrared, even in dilute quantities. It is this opacity to infrared that drives the atmosphere toward having something close to an adiabatic lapse rate as opposed to a constant temperature from near the bottom to near the top. Increasing the quantity of these multiatomic gases changes the temperature profile of the atmosphere.

 

[TumblingDice]

Welcome to PF, ftary!

How would I answer the question: explain why even small amounts of CO2 and CH4 can have a large effect on global temperature?

That's actually a statement soliciting support for itself. Putting a question mark at the end doesn't make it a question.

Also, 'small' is a relative term. What do you consider large or small? For example, methane represents 0.000179 per cent of the atmosphere according to Wikipedia. That's really small to begin with compared to the other 99.998 % of the gas content:
http://en.wikipedia.org/wiki/Earth's_atmosphere]Earth's[/PLAIN] Atmosphere

I'm confident that members will be eager to help you with clearer, better defined questions. Good luck getting started!

 

[sophiecentaur]

Would anyone actually have a problem with understanding what the OP was asking? (Despite breaking the rules a bit.)
I think the clue is in the words"large effect". In fact, the effect, seen on another planet, or in some simple experiment would not be regarded as "large". The effect is not large but it's highly Significant in the context of Human civilisation. We are in a very narrow Goldilocks zone of conditions and a 0.5C change in mean temperature has a much greater effect than 0.5/300 (the fractional difference in absolute temperature) would seem to imply.

There was a radio program on BBC R4 ('Frontiers') last night in which they were discussing the possibility of thermoregulating the Earth by squirting water droplets to an appropriate height in the stratosphere to alter the reflectivity and reduce the surface temperature. Their calculations seemed to suggest that fewer than 100 modest sized ships could steam around the Earth and inject enough water in a matter of a few years to undo any effects that may have been introduced since the Industrial Revolution. Now, that's not a lot of water 'in global terms'; it's just a matter of introducing it into the right places. I was only listening with half an ear but I think I got is more or less right. I know that BBC Science programmes are not peer reviewed Journals but they are worth listening to and even quoting on occasions
UK listeners can play this programme on BBC iPlayer but it may not be available abroad.

 

[russ_watters]

Given the confusing wording of the OP and D_H's point about our rules, the unfortunately named Global Warming Potential (GWP) factor (should have called it GHP to be less politically charged) provides a lot of information about the issue raised. It is a measure of how effective a gas is at being a greenhouse gas versus an equal mass of carbon dioxide.
http://en.m.wikipedia.org/wiki/Global-warming_potential

 

[klimatos]

I think DH's third paragraph sums up the situation rather nicely. It also gives water vapor its rightful pride of place in the Earth's longwave heat budget, since that component (in all three of its phases) accounts for a full 70% of all atmospheric longwave absorption. Carbon dioxide (20%) and methane and nitrous oxide (4%) are relatively minor players.