The UN Intergovernmental Panel on Climate Change has released the most comprehensive synthesis of what climate scientists know about how we’re currently affecting the Earth’s climate. Start with the summary overview.
The International Geosphere-Biosphere Programme runs an excellent site providing a wealth of information on ocean acidification.
Skeptical Science is a helpful site offering an accessible breakdown of a variety of climate-science questions — and answers to climate skeptics.
The International Energy Agency’s World Energy Outlook 2013 has an excellent overview of energy trends in the world — and what would need to happen for the world to curtail its greenhouse gas emissions and avoid significant global warming.