The world's greatest lake, holding 20 percent of the planet's surface fresh water, has long remained one of the most pristine places on earth. Now, as Russia's economy booms and its climate warms, the Siberian lake faces new threats.

When it comes to setting climate change policy, science can only tell us so much. Ultimately, a lead report author for the Intergovernmental Panel on Climate Change writes, it comes down to making judgments about what is fair, equitable, and just.