I will not even try do full justice here to the basic theory
of AGW theory. I highly encourage you to check out RealClimate.org. This is probably
the premier site of strong AGW believers and I really would hate to see AGW
skeptics become like 9/11 conspiracists, spending their time only on
like-minded sites in some weird echo chamber.
If you are reading this, you probably know that CO2 is what
is called a greenhouse gas. This means that it can temporarily absorb
radiation from the Earth, slowing its return to space and thereby heating the
troposphere (the lower 10KM of the atmosphere) which in turn can heat up the
Earth's surface. You probably also know that CO2 is not the only
greenhouse gas, and that water vapor, for example, is actually a much stronger
and more prevalent greenhouse gas.
It is important to understand that the greenhouse gas effect
is well-understood in the laboratory. No one really disagrees that, all
other effects held constant in a laboratory, CO2 will absorb certain
wavelengths of reflected sunlight. What may or may not be
well-understood, depending on your point of view, is how this translates to the
actual conditions in our chaotic climate. Does this effect dominate all
other climate effects, or is it trivial compared to other forces at work?
Does this greenhouse effect lead to runaway, accelerating change, or are there
opposing forces that tend to bring the climate back in balance? These are
hugely complex questions, and scientists are a long way from answering them
But wait, that can't be right -- scientists seem so
sure! Well, some scientists, particularly those close to microphones,
seem sure. Their proof usually follows one or both of these paths:
- Some scientists argue that they believe they have
accounted for all the potential natural causes, or "forcings," in the
climate that might cause the warming we have observed over the last century,
and they believe these natural forcings are not enough to explain recent
temperature increases, so therefore the changes must be due to man.
This seems logical, until I restate their logic this way: "the
warming must be due to man because we can't think of anything else it
- Scientists have created complicated models to predict
future climate behavior. They argue that their models show man-made
CO2 causing most 20th century warming. Again this sounds good,
until one understands that when these models were first run, they were
terrible at explaining history. Since these first runs, scientists
have tweaked the models until they match historical data better. So,
in effect, they are saying that manmade CO2 is the cause of historical
warming because the models they tweaked to match history"¦ are very good at
matching history; and because the models they programmed with CO2 as the
major driver of climate show that"¦CO2 is the major driver of
climate. We will see a lot of such circular analysis in later
The best evidence we could expect to find (lacking a second
identical Earth we can use as a control in an experiment) is to find a historic
correlation between temperature and CO2 that is stronger than the correlation
between temperature and anything else (and of course, even this would not imply
causation). There is a lot of argument whether we have that or not, a
topic I will cover in the next chapter. Of course, the lack of unequivocal
evidence at this point does not make the AGW theory wrong, just still"¦
Before we get to the historical evidence, though, there may
be a few other facts about CO2 and warming that you don't know:
- CO2 is a really, really small part of the atmosphere.
Currently CO2 makes up about 0.0378% of the atmosphere, up from an
estimated 0.0280% before the industrial revolution. (Just to give an
idea of scale, if you were flying from Los Angeles to New York City,
traveling 0.0378% of the distance would not even get you off the runway at
LAX. AGW advocates are arguing that a CO2 concentration increase of
0.009% has heated the world over a half a degree C.
- The maximum warming should, by greenhouse gas theories,
occur in the troposphere (the first 10km or so of atmosphere).
Global warming theory strongly predicts that the warming in the
troposphere should be higher than warming at the ground. We will see
later that the opposite is actually occurring.
- The radiated energy returning to space consists of a wide
spectrum of wavelengths. Only a few of these wavelengths are
absorbed by CO2. Once these few wavelengths are fully absorbed,
additional CO2 in the atmosphere has no effect whatsoever. Also,
these absorbed frequencies overlap with the absorption of other gasses,
like water, which further lessens the incremental effect of extra CO2.
What does this mean? In
effect, the warming effect of CO2 is a diminishing return relationship.
The first increase of, say, 100 parts per million (ppm) in the atmosphere has a
greater effect than the next 100 ppm, and so on until increased CO2 has
essentially no effect at all.
I once bought a house that had
fuchsia walls in the kitchen and family room (really). I spent all night
painting the rooms with a coat of white paint, and when I was done, I found
that some of the fuchsia still showed through the white paint, making it
kind of light pink. A second coat of white made the wall nearly perfectly
white. The effects of CO2 in the atmosphere are similar, with the first
"coat" making for the most warming and later "coats" having much less effect
but still adding a bit. At some point, the wall is white and more coats
have no effect.
This relationship of CO2 to warming
is usually called sensitivity, and is often expressed as the number of degrees
of global warming that would result from a doubling in global temperature.
There are lots of values floating
around out there for sensitivity, but a preponderance (I won't say consensus)
seem to center on an increase of one degree C for a doubling of CO2 levels from
the pre-industrial figure of about 280ppm. Note that you will see numbers
much higher than this, but these generally include feedback loops, which we
will get to later. Without feedbacks, 0.5 to maybe 1.5 degrees seems like
a fairly well accepted number for sensitivity, though there are people on both
side of this range.
Motl provides a handy approximation of the diminishing return effect from
CO2 concentration on temperature. I have taken his approximation and
graphed it below.
This is a very crude approximation,
but the shape of the curve is generally correct (if you exclude feedbacks,
which we will discuss in MUCH more depth later). Other more
sophisticated approximations generally show the initial curve less steep, and
the asymptote less pronounced. Never-the-less, it is generally accepted
by most all climate scientists that, in the absence of feedbacks, future
increases in atmospheric CO2 will have less effect on world temperature than
past increases, and that there is a cap (in this chart around 1.5 degrees C) on
the total potential warming.
Note that this is much smaller than
you will see in print. The key is in "feedbacks" or secondary effects
that accelerate or slow warming. We will discuss these in more depth
later, but typically AGW supporters believe these will triple the sensitivity
numbers, so a non-feedback sensitivity of one degree would be tripled to three
degrees. Remember, though, these three points:
· Warming from CO2 is a diminishing return, such that future CO2 increases
has less effect than past CO2 increases
· In the absence of feedback, a doubling of CO2 might increase
temperatures one degree C
· In the absence of feedback, the total temperature increase from future
CO2 increases is capped, maybe as low as 1-1.5 degrees C.