Transcript The Greenhouse Effect

Theme 9 – The Greenhouse Effect
ASTR 101
Prof. Dave Hanes
The Greenhouse Effect
A First Important Point
A large fraction of the sun’s energy reaches us as
visible light. The Earth’s atmosphere is transparent
to visible light. (We can see the stars!)
On Reflection…
Q: If visible light passes
through the atmosphere
on the way in, what will it
do if it reflects from a
snow-covered surface?
(Here, England and Ireland.)
Answer: it will pass back through the atmosphere
and go out into space! (That’s how astronauts
can see the continents, oceans, and so on.) The
atmosphere is transparent in both directions.
In other words, reflected light will not heat the surface.
But Not All the Sunlight is Reflected!
A dark roadway looks
black because it absorbs
the light – and thus the
energy, which heats it.
Similarly other surfaces and
objects.
Trapping the Energy
The warmed surface glows in
the infrared… but the
infrared light cannot flow
through!
It is absorbed by the socalled greenhouse gases in
the atmosphere, heating it
from below.
The Usual Suspects
The famous greenhouse
gases are H2O, CO2, CH4
and O3 (water vapour,
carbon dioxide, methane
and ozone.)
Because of their abundance,
water vapour (distributed in
the air, not just in clouds)
and carbon dioxide are the
most important.
A New Balance
The trapping of energy does not go on without cease!
An eventual equilibrium is reached, with an
atmosphere that is warmer than expected. The
upper part of the troposphere emits infrared
radiation out into space, and all is in balance.
WITHOUT greenhouse gases, our planet would be
colder by 30-50 degrees at the surface… so some
greenhouse effect is welcome and indeed essential!
The Earth, as Seen by ‘Infrared Eyes’
we see only the warm blanket of air, glowing in the IR
But Why Greenhouse Effect?
A greenhouse works in an
analogous way: the glass
prevents the infrared radiation
from escaping.
So: the visible light warms all
the plants, earth and pots. But
the re-emitted IR cannot get
out through the glass! So the
greenhouse stays warm.
(Note that in this case the atmospheric gases are not responsible.)
Glass is Opaque to Infrared
Although visible light is streaming out through these
windows on a cold winter night, very little heat is lost.
Danger! – The Effect Can be Deadly
Conversely: The Desert at Night
– Why is it So Cold?
The soil heats up in the day, but there is no water vapour
in the air (deserts are dry!) to prevent the heat radiating
away during the night.
Although some greenhouse effect is welcome,
We Want to Avoid a Runaway!