Union of Concerned Scientists
The Union of Concerned Scientists is the leading science-based nonprofit working for a healthy environment and a safer world. UCS combines independent scientific research and citizen action to develop innovative, practical solutions and to secure responsible changes in government policy, corporate practices, and consumer choices. Their website is full of information about the issues and problems, and has links to actions that can help make a difference. They are extremely well respected in the scientific world, and have been around for several decades making a difference in positive ecological ways.
They have a wonderful section on climate change, with an excellent page on global warming solutions, and their site includes a clear concise list of 10 personal solutions to global warming. They also have an action page with appropriate high level action you can take to make a difference.