So, how do scientists figure out how "significant" a greenhouse gas is? Well, they study how the various gases behave in the atmosphere based on the laws of physics, considering all of the factors we've discussed in this thread, and then they predict what the laws of physics say will happen they change something in the system. They divide things that change the temperature in the atmosphere into "forces" and "feedbacks". Roughly speaking, a "force" is a change that comes from outside the atmospheric system, while a "feedback" is a change that occurs in response to a forcing. For example, changes in the solar radiation that enters the atmosphere is a "forcing." If solar irradiance increases, atmospheric temperature increases. That increased temperature may result in melting of sea ice. Because sea ice is a more efficient reflector of short wave solar radiation than the surface of the ocean, less of that radiation is reflected back into space and more is re-radiated as longer wave radiation (heat). This is known as the albedo feedback. The increased solar heat (forcing) reduces the albedo, which then causes additional heating (feedback).
Human burning of fossil fuels has increased the amount of CO2 in the atmosphere. That's a forcing.
https://en.wikipedia.org/wiki/Carbon_di ... atmosphere The graph is a little out of date. We're at about 400 ppm now.
Water vapor in the troposphere, because of its extremely short residence time, is treated as a feedback. If we inject water vapor into the troposphere, it quickly precipitates out, returning to roughly the same overall relative humidity. For that reason, tropospheric water vapor is treated as a feedback.
http://www.realclimate.org/index.php?p=142 (Small amounts of water vapor are added to the stratosphere through volcanoes, oxidation of methane, and jet exhaust. This additional water vapor is treated as a forcing.)
This post, which you may have read upthread, describes the first calculation of the effect of CO2 that analyzed the effect layer by layer, taking into account the changing composition of the atmosphere.
http://www.realclimate.org/index.php/ar ... -argument/ Although the calculation was crude by modern standards, it showed that doubling the amount of CO2 in the atmosphere, without considering any feedbacks, would result in an increase in average temperature of about 1C. Although the calculations have become more complex to reflect additional knowledge about how the atmosphere works, the generally accepted number for the effect of doubling CO2 (without consideration of any feedbacks) is about 1C.
https://en.wikipedia.org/wiki/Climate_sensitivity.
As the atmosphere warms, it can and does take up more water vapor. This is the water vapor feedback described in the above link. The total water vapor feedback the results from a 1C increase in atmospheric temperature is also around 1C.
https://en.wikipedia.org/wiki/Climate_sensitivitySo, here's what the science tells us about the significance of water vapor v. CO2 under the current conditions we are experiencing (significantly increasing CO2): Doubling CO2 will increase the atmospheric temperature 2C -- 1C from the direct radiative effect of CO2 and 1C from the water vapor feedback. In once sense, we could say they are equally significant because they are contributing equal temperature increases. But in a very important sense, the CO2 is much more significant because it controls the amount of water vapor.
And that is why these statements from Tobin are just flat out wrong and/or misleading.
Tobin wrote:The idea that human beings (at our current level of progress) can meaningfully influence how the climate develops is patently absurd.
Tobin wrote:However, even when it is tripled the CO2 concentrations will be less than 1/50th of 1% of the atmosphere.
Tobin wrote:CO2 is such a tiny part of our atmosphere (less than 1/100th of 1%), it can't do much on its own. The major greenhouse gas in the Earth's atmosphere is water. After the Sun, it is the primary cause of global warming. That is why I get a chuckle out of you global warming nuts. If people found out that you wanted to rein in the SUN and WATER, you'd be laughed out of the room.
Tobin wrote:Most of the problem is CO2 is really a nonfactor in our atmosphere. While increases in CO2 corresponds to higher temperatures, water is really the primary greenhouse gas that causes global warming since it is a major portion of our atmosphere
Tobin wrote:Which number is bigger? .4/100th of 1% or 4%? Which number do you think represents CO2? Which number is H2O? Which one is significant? Which one is insignificant?
Tobin wrote:I'll just note in response to your question that CO2 is measured in parts per MILLION and CH4 is measured in parts per BILLION. CO2 and CH4 are not a million (or a billion) times more powerful than water vapor as greenhouse gases.
Tobin wrote:When you are talking about gases that represent mere parts per million and billion, it is very fair to state they have negligible effects on global warming. Water vapor represents often 4% of the atmosphere per volume. If there is a greenhouse gas of significance, that is the one. And as I've also pointed out, good luck getting people to rally around the cause of removing water from the atmosphere. It's a ridiculous notion.
Tobin wrote:But that is the thing. We AREN'T talking about percentages!!! We are talking about green house gases that are measured in microscopic amounts in parts per MILLION and parts per BILLION. Even if you double or triple the amount of these gases (which would take CENTURIES) in the atmosphere, it isn't a meaningful amount in comparison to a green house gas like water vapor.
Tobin wrote:It isn't a percentage. It's a fraction of a percent. Obviously it doesn't compute for you, but the 4 major gases N2, O2, H2O, and Ar make up virtually all of the atmosphere. The remaining trace gases don't even add up together to 1% of the atmosphere.
Tobin wrote:400 PPM is an insignificant number in comparison to 4%.
Tobin wrote:It's a narrow band absorber which overlaps with another greenhouse gas H2O. And since there is vastly more H2O in the atmosphere that already absorbs the same spectrum as CH4, there really isn't any contribution made by CH4 to global warming.
Tobin wrote:it should be clear that at the levels of CO2 currently in the atmosphere (or the amount that could ever be produced in hundreds of years of continual use of carbon emitting sources) that doing so won't have any significant impact on global warming at all.
“The ideal subject of totalitarian rule is not the convinced Nazi or the dedicated communist, but people for whom the distinction between fact and fiction, true and false, no longer exists.”
― Hannah Arendt, The Origins of Totalitarianism, 1951
