Journalists delving into climate change are likely to be bombarded by news releases, reports and offers of interviews with selected "experts" from a bevy of environmental groups. They can be useful in highlighting the latest developments in scientific research and in surveying the responses by government or business. However, it's important to remember that these are not disinterested groups, and the completeness and even accuracy of information conveyed varies.
Just as skeptics tend to inflate normal scientific back-and-forth into a great debate or even a hoax, environmentalists often portray climate change as a looming apocalypse for the entire planet which only immediate, drastic action can avert. Some groups do a better job than others in acknowledging there are still uncertainties about some of the science, but many — in the interests of prompting action — tend to stress only the most extreme outcomes among the range of possible impacts.
The best way to gauge the state of scientific knowledge about climate change — or any scientific issue, for that matter — is to keep up with the latest findings published in peer-reviewed journals. While technical reports in journals may not be easy for many journalists to parse, there are more accessible reports assessing the state of scientific knowledge, such as the IPCC's mentioned above, which summarize peer-reviewed research in less technical language and provide context for individual findings. And, unlike some journals, the IPCC report is free.
Sierra Club 
Sierra is perhaps the largest and most politically active of U.S. membership-based environmental groups. Their long-established climate change program is focused on grass-roots organizing and policy advocacy. Wikipedia . Press Contact .
Center for Climate and Energy Solutions (C2ES) 
Formerly known as the Pew Center on Global Climate Change, C2ES is still helmed by longtime climate maven Eileen Claussen. It is a centrist, non-profit advocacy organization that "provides independent analysis and innovative solutions to the twin challenges of energy and climate change." C2ES works intensely with Fortune 500 companies. Wikipedia . Press Contact .
Climate Institute 
The Climate Institute, founded in 1986, is possibly the oldest single-issue group focused on climate change. Its style is more informational than advocacy-driven, and large conferences and symposia are characteristic activities. 
Climate Action Network 
CAN is actually a coalition of over 365 NGOs worldwide pushing for action on climate change. They follow international climate treaty negotiations closely. Press Contact .
Greenpeace is a media-savvy advocacy group historically focused on environmental, anti-nuclear, and energy issues. It has both a U.S.-based and a worldwide organization. They specialize in direct-action and investigative research. Wikipedia . Newsroom . Press Contact .