Greenhouse Effect
What Is the Greenhouse Effect?
By Adam Mann
The greenhouse
effect is what keeps our planet warm enough to support life.
Earth is said to be
in a perfect "Goldilocks zone" away from the sun (not too cold, and
not too hot), which enables life to thrive on the planet's surface.
But Earth's balmy
temperatures would not be possible without the greenhouse effect, which traps
solar energy on Earth's surface and keeps the planet warm.
The greenhouse
effect arises from Earth's atmosphere.
Visible light from
the sun, as well as invisible ultraviolet and infrared wavelengths, can
penetrate the gaseous layer that blankets our world.
Roughly 70% of
these energetic rays are absorbed by Earth's oceans, land and atmosphere, while
the remaining 30% are immediately reflected back into space, according to NASA
Earth Observatory.
Diagram depicting
how the greenhouse effect works on Earth.
As the planet's
surface heats up, it releases some of the infrared energy that it had absorbed,
but that energy doesn't make it back out of Earth's gaseous atmosphere.
Instead of shooting
back out into space, the infrared energy closely hugs our planet and,
therefore, raises Earth's overall temperature.
This is similar to
how a human-built glass greenhouse works, trapping heat from the sun to keep
plants warm in the winter.
Without an
atmosphere, our world would be as cold as the lifeless moon, which has an
average temperature of minus 243 degrees Fahrenheit (minus 153 degrees Celsius)
on its far side.
Because of the
greenhouse effect, Earth maintains an overall average temperature of around 59
F (15 C).
Greenhouse gases
and climate change
Greenhouse gases
include several naturally occurring molecules — like water vapor, carbon
dioxide, methane, nitrous oxide and ozone — as well as several manufactured
ones, like chlorofluorocarbons, according to the Australian Department of the
Environment and Energy.
Over the past
century or so, human activities — such as the burning of fossil fuels,
intensive agriculture, livestock raising and land clearing — have dramatically
increased the concentrations of greenhouse gases in Earth's atmosphere, to the
point where it's changing our planet's climate.
Factory smoke
stacks releasing clouds of greenhouse gases.
Greenhouse gases
produced by human activity are the most significant driver of climate
change.
Since the middle of
the 20th century, greenhouse gases produced by humans have become the most
significant driver of climate change, according to the U.S. Environmental
Protection Agency.
Carbon dioxide levels in the atmosphere have increased by more than 40% since the start of the Industrial Revolution, from roughly 280 parts per million (ppm) to more than 400 ppm today.
Fossils show that
forests grew in the Canadian Arctic during the Pliocene, and savannas and
woodlands spread over what's now the Sahara desert.
While some people
still doubt the reality of human-induced climate change, the evidence for it is
overwhelming.
Since the 1850s,
the average global surface-air temperature has risen by around 1.4 F (0.8 C),
and ocean temperatures are now at the highest levels ever recorded.
Increases in
greenhouse gases in the coming decades are expected to harm human health,
increase droughts, contribute to sea level rise, and decrease national security
and economic well-being throughout the world.
The greenhouse
effect on other planets
Because the
greenhouse effect is a natural process, it affects other bodies in the solar
system, too.
And, in some cases,
that provides a warning about how things can go awry.
A perfect example of this is Venus, which is roughly the same size as Earth and not that much closer to the sun.
Billions of years
ago, when the sun was cooler and dimmer, Venus may have had a temperate climate
that could have allowed for liquid water oceans on its surface.
Simulations suggest
that the planet's average temperatures ranged from a low of 68 F (20 C) to a
high of 122 F (50 C) for about 3 billion years, potentially even allowing Venus
to support life.
But as the sun aged
and grew brighter, excess water vapor would have entered Venus' atmosphere.
This potent
greenhouse gas trapped heat and raised the planet's surface temperature,
leading to a vicious feedback cycle in which hotter temperatures led to more
water vapor in the atmosphere, further heating the world — a process known as
the runaway greenhouse effect.
When Venus' oceans
vaporized, its planetary plate tectonics would have ground to a halt, as there
was no water left to help lubricate the shifting of geological plates.
The increasingly
thick atmosphere might have created a drag on Venus' rotation period, leading
to its bizarrely slow spin, in which a year goes by with only two days passing.
The dense cloud
cover also led to hellish surface temperatures on present-day Venus, with an average
of 700 F (370 C) — hot enough to melt lead.
"I think Venus is an important warning: Greenhouse
atmospheres are not theoretical," Ellen Stofan, director of the
Smithsonian's National Air and Space Museum and former chief scientist at NASA,
previously told Space.com.
On Mars, greenhouse
gases such as water and carbon dioxide might have been released during ancient
impact events.
Some scientists
speculate that such wallops could have raised Mars' overall temperature enough
for the planet to have liquid water on its surface for significant lengths of
time.
However, because
Mars is smaller than Earth, it's gravitational pull is weaker. Therefore, these
gases drifted away, and eventually, the Red Planet reverted back to the cold
and dry world it is today.
Saturn's distant
moon Titan, which has a thick nitrogen atmosphere with about a thousand times
greater concentration of methane as Earth, is also subject to the greenhouse
effect.
With data from the
European Space Agency's Huygens probe, which landed on Titan in 2005,
researchers are getting a better understanding of how methane absorbs
short-wavelength infrared radiation and are using that information to develop
climate change models of our planet.
The greenhouse
effect is also expected to warm the worlds of other star systems.
Many astronomers
speak of a narrow habitable zone around a star — the area where a planet would
be at the perfect distance to maintain liquid water on its surface, between
0.95 and 1.4 times the Earth-sun distance.
However, others have
argued that such models need to be broadened.
A thick atmosphere of molecular hydrogen, which is a potent greenhouse gas, could potentially give a world clement temperatures even if it were 15 times farther from the sun than Earth is.
Adam Mann is a journalist specializing in
astronomy and physics stories. His work has appeared in the Wall Street
Journal, Wired, Nature, Science, New Scientist, and many other places. He
lives in Oakland, California, where he enjoys riding his bike. Follow him on
Twitter @adamspacemann.
https://www.space.com/greenhouse-effect.html
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