Re: If there were no Earth

Re: If there were no Earth

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Posted by James Risbey on September 12, 1997 at 12:30:26:
In Reply to: If there were no Earth posted by Urs on September 12, 1997 at 12:04:57:
: If there were no Earth, would there be a green house effect?
: I suppose that the job of the Earth is to transfer short-wave radiation (from the sun) into long-wave radiation (heat) which can't escape the greenhouse. So, if there were no Earth, but just greenhouse gases, would there be a greenhouse effect?
Urs,

Having an earth ensures that all the shortwave radiation that is not
reflected or absorbed in the atmosphere will be absorbed (and reflected) at the
surface, since the surface (land/ocean) won't transmit any shortwave
energy out the other end -- if the shortwave can't pass through the
earth it will have to be absorbed within it.

If there is no earth, that means that the half the incoming shortwave
that is normally received at the surface will just pass right out of
this hypothetical no-earth system into space. Of the half that
doesn't reach the surface, about half that is reflected to space and
about half is absorbed in the atmosphere. The fraction that is
absorbed in the atmosphere will heat up the atmospheric constituents
that absorb it. These constituents (clouds, particles, and gases)
will then emit longwave radiation. Some of this emitted longwave
radiation will pass out to space and some will be absorbed by
atmospheric constituents that absorb longwave radiation and then
re-emitted. If this re-emission takes place on average at colder
temperatures (higher up in the atmosphere), then this will create a
greenhouse effect. When there is a warm surface present in the system
doing most of the initial absorbing, then the reabsorption/reemission
in the atmosphere is guaranteed to take place at lower temperatures
(since the temperature decreases away from the surface), creating a
greenhouse effect. In the no-earth case, the reaborption and
reemission will mostly be above the clouds that do most of the
shortwave absorption and so will be at a slighly lower temperature,
creating a much weaker greenhouse effect than in the case where a
surface is present.

So, the short answer to your question is 'yes', but a much weaker one.

James

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: : If there were no Earth, would there be a green house effect? : : I suppose that the job of the Earth is to transfer short-wave radiation (from the sun) into long-wave radiation (heat) which can't escape the greenhouse. So, if there were no Earth, but just greenhouse gases, would there be a greenhouse effect? : Urs, : : Having an earth ensures that all the shortwave radiation that is not : reflected or absorbed in the atmosphere will be absorbed (and reflected) at the : surface, since the surface (land/ocean) won't transmit any shortwave : energy out the other end -- if the shortwave can't pass through the : earth it will have to be absorbed within it. : : If there is no earth, that means that the half the incoming shortwave : that is normally received at the surface will just pass right out of : this hypothetical no-earth system into space. Of the half that : doesn't reach the surface, about half that is reflected to space and : about half is absorbed in the atmosphere. The fraction that is : absorbed in the atmosphere will heat up the atmospheric constituents : that absorb it. These constituents (clouds, particles, and gases) : will then emit longwave radiation. Some of this emitted longwave : radiation will pass out to space and some will be absorbed by : atmospheric constituents that absorb longwave radiation and then : re-emitted. If this re-emission takes place on average at colder : temperatures (higher up in the atmosphere), then this will create a : greenhouse effect. When there is a warm surface present in the system : doing most of the initial absorbing, then the reabsorption/reemission : in the atmosphere is guaranteed to take place at lower temperatures : (since the temperature decreases away from the surface), creating a : greenhouse effect. In the no-earth case, the reaborption and : reemission will mostly be above the clouds that do most of the : shortwave absorption and so will be at a slighly lower temperature, : creating a much weaker greenhouse effect than in the case where a : surface is present. : : So, the short answer to your question is 'yes', but a much weaker one. : : James
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