But I do belong to Mad Science, although I have not been active for a while, given my day job and all... oh and all this blogging takes time, too.
Mad Science takes questions from the public and sends them off to practicing scientists for answers. The scientists' answers are posted on the website, and (if well-enough written) hopefully contribute to broader understanding of issues. It's a dedicated group, and I wish I had more time to actively participate again.
This is a question sent to me by a grade 10-12 student, waaay back in 2001:
What is the significance of the increase and decrease of atmospheric CO2?
The level of CO2 in the atmosphere rises and falls naturally in response to seasonal changes each year. This has probably occurred for several tens of millions of years. What is more serious is an overall, very rapid increase in the "background" level of CO2 that we can trace back to the beginnings of the industrial revolution.
The amount of carbon dioxide in the atmosphere is a balance between the things producing it (the "sources:" breathing, burning, decomposition) and the things removing it from the air (the "sinks:" plant growth, mineral formation, dissolving in water).
Why does this concentration change seasonally? Because when spring arrives, all the plants begin growing again, and this process uses up CO2 -- so during fall & winter carbon dioxide builds up, and during spring and summer this supply is drawn down.
But if it is spring in the Northern Hemisphere, then it is fall in the Southern Hemisphere -- doesn't this all just balance out? No, for two reasons. First, if you look at a map of the whole Earth, you will notice that there is a lot more land in the Northern Hemisphere than in the Southern Hemisphere. This means there are a lot more plants, so the variation in the Northern Hemisphere is much greater. Second, the air in the Northern Hemisphere doesn't mix very quickly with the air in the Southern Hemisphere -- it takes longer than a year to mix thoroughly, so differences between the hemispheres aren't "lost" with all the stirring.
The following figure shows the concentration of CO2 (the vertical axis) as a function of time (left to right) and as a function of latitude (in and out of the page). Notice how much bigger the waves in CO2 concentration are in the Northern Hemisphere compared to the Southern Hemisphere. Note also that when it is high in the North, it is low in the South. The final thing to notice is that the whole pattern is climbing as we go to the right--this is the "background" level of CO2 that is rising, on top of which the seasonal ripples go up and down.
What about the oceans? What role do they play? Carbon dioxide dissolves in water, so if we increase the concentration of CO2 in the air, it will slowly seep into the oceans. However, the oceans are so huge -- and mix so slowly--that this increase in CO2 in the atmosphere has affected only the top few hundred meters of the ocean.
Why has it probably been this way for millions of years? The fact that there is more land in the Northern Hemisphere than the Southern is due to plate tectonics--the relative movement of the continents on the surface of the Earth. Now that's a really slow process--the fastest plates move about as fast as you fingernails grow, so the pattern of land and water does not change very fast at all.
So why is an increase in CO2 serious? An increase in carbon dioxide has both good and bad effects, but we are not sure which are more serious. It is good in that certain kinds of plants (some of which we use for food) grow better with more CO2 so we might be able to feed more people. It is also bad because CO2 is a "green house gas" that traps heat from the sun, so that the average temperature on the surface would get hotter, making for more intense weather patterns. Basically, we think more CO2 will mean it will get hotter where it is already hot, colder where it is already cold, drier where it is dry, and wetter where it is wet. Bigger deserts, bigger floods, more strong hurricanes. But we will also have more usable cropland in Alaska, Canada and Siberia. We might be able to grow more crops, but they might also get wiped out more often!
Other sources of info:
EPA Global Warming Web Site: Climate System, Emissions, Impacts and Actions
Common Questions about Climate Change: A good overview from the World Meteorological Organization
GCMD Learning Center - Q&A: Climate Change, Global Warming, Greenhouse Gases, Sea Level Rise, El Ni?o
ARM Education Site: Global Warming
Climate Change: State of Knowledge
U.S. National Assessment - The Potential Consequences of Climate Variability and Change
I then went on to give a much longer, more technical answer, but Blogger seems to choke on that amount of text, and I'm getting tired of re-entering it, so here's the direct link.