 The Washington Post
(All of the following Q&As originally ran on November
12, 1997.)
Q: Is Earth really warming_
A: Yes, but not a lot-so
far. Nearly all experts agree that its average surface temperatures have
risen between 0.5°F and 1.1°F (0.3 to 0.6°C) since about 1860,
when dependable record-keeping began.
 Most climate scientists attribute at least some of that increase to
greater human pollution of the air with "greenhouse" gases, such
as carbon dioxide, that trap heat in the lower atmosphere before it can
radiate away.
The warming has not been uniform. For example, most has occurred at
night, so days are about as warm as ever, while evenings are warmer. In
general, as an international group of scientists writes in the May 1997
Scientific American, "the minimum [land] temperature has increased
at a rate more than 50 percent greater than the maximum temperature."
By one estimate, this has given the northeastern United States approximately
11 more frost-free evenings a year than it experienced 40 years ago. Another
study shows that the growing season has lengthened by as much as one week
a year during this century.
Moreover, much of the warming has occurred in cold months, so summers
are not much hotter on average, but winters are notably less cool. Higher
latitudes of the northern hemisphere have warmed more than equatorial regions.
Of course, that's where the thermal action is. Water warms less rapidly
than land, and about two-thirds of the world's land is in that hemisphere.
The century's 10 warmest years have occurred since 1980, and some observations
indicate that the '90s have been hotter yet. But the trend since 1900 has
been oddly uneven: Global average temperatures increased from 1910 to the
end of World War II, then decreased slightly until the mid-1970s (despite
the enormous expansion of many postwar economies), then started upward again.
One possible explanation is the surge of fossil-fuel burning during
the postwar development years, which may have placed such a quantity of
planet-shading sulfate compounds into the atmosphere that it temporarily
more than offset global warming.
Q: What's the 'greenhouse effect_'
What's bad about it_
A: The term refers to the
way certain gases trap heat in the atmosphere, much as the glass in a greenhouse
prevents rising warm air from escaping.
Enhancement of this effect by human activity-not the effect itself,
which is entirely natural-has experts worried. In fact, Earth, 93 million
miles from its energy source, the sun, would be about 60°F colder if
certain atmospheric gases did not trap heat. Instead of an annual average
temperature of about 60°F, it would be about 0°F. We probably owe
existence of life on this planet to the greenhouse effect.
It works like this: Half of sunlight that strikes the planet's atmosphere
is in the form of electromagnetic radiation that we call visible light,
with a peak wavelength of about 0.5 microns. A micron is one-millionth of
a meter.
Much of the rest is infrared radiation (IR), the invisible but warmth-inducing
long waves you feel while holding your hands next to a radiator. Incoming
sunshine contains relatively little ultraviolet light and few X-rays or
gamma rays, and most of them are filtered by absorption in the upper atmosphere.
Whether radiation is absorbed depends on the size and type of objects
through which it travels. Because molecules of air, chiefly oxygen and nitrogen
in two-atom combinations, are comparatively small, they catch most short
waves as sunlight passes down through the atmosphere. One familiar example
is the way three-atom molecules of oxygen in the stratospheric "ozone
layer" absorb ultraviolet radiation at about 0.3 microns.
About half of sunlight that hits the outer atmosphere reaches the surface,
where it transfers energy to land and water. The surface then sheds that
energy as heat, largely infrared radiation with a wavelength of about 3
to 30 microns. If the air contained nothing but its main components (21
percent oxygen, 78 percent nitrogen), almost all of the energy emitted at
the surface would radiate uninterrupted into space.
But in fact, nearly 90 percent of that long-wavelength radiation is
caught by clouds and gases, which send much of it down again. The astonishing
result is that Earth's surface is hit daily with twice as much energy from
re-radiated IR out of the atmosphere as from incoming sunlight.
That is because some kinds of molecules, known collectively as "greenhouse"
gases, are just the right size and configuration to trap long-wavelength
IR and re-radiate it. Most elude natural chemical cleaning processes in
the atmosphere, and many stay aloft from decades to centuries.
 Q: Even if it
does warm up a couple of degrees in a century, is that really such a big
deal_
A: In a way, the term "global
warming" is deceptive because it suggests that the paramount issue
is average temperature.
In fact, in many ways that's the least significant htmlect of the entire
panoply of effects that worldwide warming would entail. One of the most
important is a change in the way water moves around.
As Earth's surface warms, most of the energy goes into evaporating water.
The heat involved in changing something from one phase, such as a liquid,
to another such as a gas is called latent heat. When rising water vapor
condenses again, it sheds that heat into the air. Even a modest amount of
global warming would cause a substantial increase in air moisture because
for every 1°C that its temperature rises, the atmosphere can hold 6
percent more water vapor.
What would that mean_ There is a loose consensus that substantial warming
would produce: (1) more clouds and rain on average but varying drastically
from one area to the next; (2) more violent precipitation events locally
and regionally with aggravated risk of flooding; and (3) severe drying of
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