Question

why can helium be compressed?

Answer 1

Yes. All gases can be compressed.

Answer 2

which is a greenhouse gas.

Answer 3

First, Jaqcueline, if you have a question, it is much better to put it in as a separate post in <Post a New Question> rather than attaching it to a previous question, where it is more likely to be overlooked.

Since this is not my area of expertise, I searched Google under the key words "<I>greenhouse gas</I>" to get these possible sources:

http://en.wikipedia.org/wiki/Greenhouse_gas
http://www.eia.doe.gov/bookshelf/brochures/greenhouse/Chapter1.htm
http://www.epa.gov/climatechange/emissions/index.html
(Broken Link Removed)

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.

I hope this helps. Thanks for asking.

Answer 4

A greenhouse gas is a gas which, when added to the atmosphere, prevents the Earth's infrared radiation from being reradiated back to space, and therefore acts as an insulating "blanket" to make the Earth warmer. Water vapor is not considered a greenhouse gase because there is so much of it, that radiation cannot escape at its broad range of absorbing wavelengths anyway. The most important greenhouse gases causing global warming are CO2 and CH4.