CO2 Now

To find the definition for a particular global warming or climate change term, RealClimate.org may be the best place to start your search:

RealClimate.org Glosssary

The site organizers recognize the room to make a good thing even better.  Although no comprehensive climate science glossary exists, RealClimate.org suggests the following climate-related glossaries:

1. National Oceanic and Atmospheric Administration (NOAA, US) plus a separate page for paleo-scientific terms 
2. Bureau of Meteorology (Australia)
3. Environment Protection Authority (Austrlia) 
4. Environmental Protection Agency (EPA, US),
5. Western Regional Climate Center (WRCC, US).
6. Wikipedia glossary for climatological terms.
7. European Union for more bureaucratic terms
8. UN Framework Convention on Climate Change (UNFCCC) for more bureaucratic terms
9. The Encyclopedia of Earth for other related resources

CO2Now.org extends the list by adding the Glossary appended to the 2007 assessment report [pdf document] of the International Panel on Climate Change (IPCC).  

For more information about climate glossaries, see the RealClimate.org blog of August 9, 2008.