With my rudimentary knowledge of chemistry (and physics), I fail to
see how CO2 contributes to global warming. There is not much of it, it
is heavy, and without it we would starve.
I've also seen a few 'authoritive' dismissals.

The greenhouse effect results because certain gases in the atmosphere
(water vapor, carbon dioxide, nitrous oxide, and methane, for
example)permit incoming solar radiation to reach the surface of the
Earth unhindered but restricts the outward flow of infrared radiation.
 They absorb and reradiate this outgoing radiation, effectively
storing some of the heat in the atmosphere, thus producing a net
warming of the surface.

According to National Geographic
(http://www.nationalgeographic.com/xpeditions/lessons/07/g912/co2.html)
:-

** Causes of the greenhouse effect ** 
Carbon dioxide, methane, nitrous oxide, and other gases are
transparent to incoming sunlight, allowing the heat from the sun to
enter Earth's atmosphere. These gases trap the heat close to the
Earth's surface, warming the atmosphere. Fuel combustion is the
largest human-made source of carbon dioxide. Deforestation is the
second largest human-made source.

** Possible outcomes related to the greenhouse effect **
From 1860 to 1994 carbon dioxide in the atmosphere rose from 280 to
more than 350 parts per million. A network of scientists organized by
the United Nations predicts that by 2100 continued emissions of carbon
dioxide at current rates might raise global temperatures and sea
levels. Islands and shorelines could be inundated, climate zones could
shift, and weather could grow more turbulent.

** Uncertainties ** 
Due to the climate system's complexity, computer simulations of
warming and its impacts are, by nature, imprecise. Skeptics say that
conclusions drawn from such simulations form a weak basis for
international action. Major uncertainties include the role of clouds
in warming or cooling the atmosphere and the role of oceans in
absorbing atmospheric heat and carbon dioxide.

For further information & myths associated with the Green house effect
you can checkout these links : -
http://www.science.gmu.edu/~zli/ghe.html
http://www.epa.gov/globalwarming/kids/greenhouse.html

Also it's not factually correct that "There is not much of it (CO2)" .
In nature, carbon is plentiful and dynamic. Its natural cycle is
thought to have been in a relatively steady state for thousands of
years until the industrial revolution. Then, humans perturbed the
carbon cycle by clearing forests and burning fossil fuels (coal, oil,
and natural gas), which increased the CO2 content of the atmosphere.
To understand the Carbon Cycle & its impact on our atmosphere please
refer to this detailed paper available at the following link :-
http://www.llnl.gov/str/Duffy.html

I hope this helps !!

Have a wonderful Day,
regards

Actually a long time ago I read a letter in the Times from a Professor
of Forestry (Aberdeen), in which he said that rain forests are pretty
much 'jungles in a jar'.

I rather suspect that there is a lot more malign water vapour (H2O =
18 per molecule rather than N2 = 28) lurking around than, say methane.

Yet, I have heard somewhere (and this sounds ridiculous) that the
largest source of UK methane is from cows.

CO2 is pretty heavy, and given a choice it sits at ground level, by
the time solar radiation has got through (? thousand) miles, why
should it get interested in a few inches of 'foliage'.

BTW: Atomic weights are from a seriously raddled memory.

?authoritative? dismissals of the contribution of CO2 to the
greenhouse effect are usually biased by agenda.

Though CO2 is indeed quite heavy (for a simple gas), diffusion ensures
pretty even distribution within the atmosphere.

The point about CO2 is that it is very effective in trapping IR so not
a lot is needed to make a sizeable impact. In extremes, you end up
with Venus.

Best

I read recently, from what seemed like an authoritative source, whose
name I inconveniently cannot remember, that the real problem is water
vapour
- and anyway what we take as global warming is just cyclical.
- we are only 13,000 years out of the last Ice Age
- and reliable statistics go back say 400 years

I'm also not sure about the dispersement of C02, I have memories of
school experiments pouring it from jar to jar.

Dear frde-ga, mostly right, but.....

Water vapour is implicated in global warming but that?s largely chicken and egg.

Rapid global warming / cooling is cyclic but the present rate appears
to exceed ?natural? rates.

Reliable (reasonably) statistics of global temperature go back tens of
millions of years.

CO2 can indeed be poured, but leave the jar open for a few hours and
it will diffuse out.

Best