I think that the issue of global warming has become so politicized on both sides that one of the fundamental concerns is going to be how one can assess the validity of different sources. The truly "great" source might be one that is replete with coherent analysis and is more dispassionate about its presentation of the effects of global warming on the ecosystem. In this light, I tend to think that the National Geographic's detailing of results from the warming of the ecosystem can help to provide some very compelling evidence about its overall effects.
Another such great source I found was through the National Aeronautics and Space Administration report about the Vital Signs of the Planet with an emphasis on how global warming will impacts the development of the planet. In both of these sources, a very scientific and methodical approach is taken to detail how global warming's effects will transform the planet. Both sources talk about the effects it will have will bring about more negative attributes to the planet and make its sustenance a critical issue in the future. These sources are two of many out there that take a balanced and rational approach to the problem and seek to minimize the politicizing that has become such a part of the issue.