MODERN TIMES
Art Hobson
ahobson@uark.edu
NWA Times 28 Feb 2009
The plausibility
of global warming
A
2008 Pew Poll found that less than half of all Americans agree with the
scientific consensus that global warming is real and human-caused. This contrasts with the scientific
consensus, as expressed in the 2007 Intergovernmental Panel on Climate Change
report by over 2000 climate scientists who assessed tens of thousands of
peer-reviewed scientific papers published during 2000-2005. That report stated that "warming
of the climate system is now unequivocal" and that there is "very
high confidence" that this is at least partly due to humans.
I
witnessed this consensus last week at a national meeting of the American
Association for the Advancement of Science, probably the world's most prestigious large scientific
organization. Al Gore was invited
to speak. The large lecture hall
was overflowing. Gore received a
standing ovation both at the beginning and end of his talk. The enthusiasm for this man who has so
raised public awareness of global warming was palpable.
Public
skepticism about global warming stems partly from a gut feeling that human
actions are too puny to warm our huge planet.
Allow me to explain why this gut feeling is wrong. For this, we must understand the
greenhouse effect of Earth's atmosphere.
It's
true that the human energy input is relatively small: 99.98 percent of Earth's energy comes from the sun and only
0.0075 percent of the total comes from fossil and nuclear fuels. The energy from the sun must, on the
long-term average, be radiated by Earth back into space. If, for example, Earth radiated less
energy than the energy that comes in from the sun, our planet would warm and
the increased temperature would cause Earth to radiate more strongly until,
eventually, the radiated energy balanced the incoming energy from the sun.
The amount of
energy radiated by Earth is determined by Earth's overall temperature, just as
a hot plate's radiative properties (white hot, red hot, dim-red, no visible
glow) are determined by its temperature.
When this is put into the form of a couple of simple equations, the
overall Earth temperature required to balance the incoming energy from the sun
can be calculated to be minus two degrees Fahrenheit.
But
Earth's surface isn't this cold.
Minus two degrees would freeze the oceans, for one thing. The answer to this dilemma is that
minus two degrees is the temperature of the top of the atmosphere; this extreme
outer surface must be at minus two degrees for the planet to "glow"
sufficiently to balance the sun's energy.
Like a warm but not visibly glowing hot plate, this low-temperature glow
is invisible "infrared" radiation, not visible light.
Earth's
surface is much warmer than minus two degrees, because the surface lies under a
thick atmospheric blanket. Like
the blanket on your bed on a cold night, the bottom of this blanket is much
warmer than the top. Just like a
blanket, the atmosphere warms us by trapping some of the infrared radiation
that the surface emits toward space.
This trapping is called the "greenhouse effect" but
"blanket effect" is more accurate.
The
warming "insulation" in Earth's greenhouse blanket involves only a
few trace chemicals known as "greenhouse gases," not the nitrogen,
oxygen, and argon that form far more than 99 percent of the atmosphere. This insulation is almost entirely
water vapor and carbon dioxide (CO2), forming far less than one percent of our
atmosphere. These few water and
CO2 particles are such efficient absorbers of infrared radiation that they
cause the bottom of the greenhouse blanket to be nearly 60 degrees warmer than
the top, resulting in a pleasant 58-degree average surface temperature.
Three-quarters
of the greenhouse effect is caused by water vapor, one-fifth by CO2, and the
remainder by other trace chemicals.
Humans and ongoing natural processes can't alter the amount of water
vapor by much, because the atmosphere can hold only so much water before it
condenses into droplets and "rains out." But humans can and have altered the amount of CO2. Before the industrial age, for at least
the past 800,000 years there were never more than about 280 CO2 particles in
the atmosphere for every million air particles. Now there are 386.
We know that most of the excess CO2 comes from fossil fuels (the rest
comes from de-forestation) because this long-buried fossil carbon is physically
different (it contains no radioactive carbon-14) from natural atmospheric
carbon and can thus be detected in the atmosphere.
So
humans have increased the greenhouse blanket's CO2 insulation by nearly 40
percent. In view of the 60-degree
natural greenhouse effect, it's not surprising that this has raised Earth's
temperature by 1.5 degrees, with 6 degrees expected by 2100 if present
emissions continue. What we've
done is analogous to replacing a cotton blanket with a wool blanket. On a cold night, the top of both
blankets are at the same temperature but the bottom of the wool blanket is much
warmer because wool retains more of your body's heat.
So
global warming is unsurprising. A
thin trace of water vapor and CO2 causes nearly all 60 degrees of natural
greenhouse warming, and we've increased CO2 by 40 percent. Since CO2 causes 20 percent of the
greenhouse effect, we should expect that we've increased the greenhouse effect
by several percent, or a few degrees.
It's not the total energy input by humans that's causing global warming,
it's the greenhouse gas input.
What is surprising is humankind's lackadaisical and skeptical response. We'd better get smart fast.