That fall, I took a leave from college, determined to do something about the insanity of global warming. I would never return, choosing instead to co-found the Energy Action Coalition and grow it into the world's largest youth advocacy organization working on the climate crisis. As we won thousands of small victories, getting cities, college campuses, and companies to begin reducing their carbon footprints, I and those around me felt empowered -- confident that we would prevail in the greatest challenge of our generation.

Then came 2008. After the election, we saw an opportunity to win both federal climate legislation and to secure an international climate deal in Copenhagen. When both went down in flames, many climate activists (myself included) fell into a kind of depression.

Fast forward to 2012. We're living through the warmest year in American history. Wildfires and droughts are plaguing the West, prompting experts to warn of a looming food crisis, and Bill McKibben's tour-de-force Rolling Stone piece "Global Warming's Terrifying New Math" has been viewed 1.2 million times in two weeks. The listervs I'm on are filling up with huge threads with subject lines like, "I'm scared."

What happened? What do we do now? I and many other members of the millennial generation have spent the past few years developing answers to these questions. The good news is that we now know a great deal about what works, and we know what we need to do.

First, it's important to recognize that this not only a dangerous time, but also a time of immense opportunity. We are living in a world of dueling exponential curves. On one hand, there are the hockey stick slopes, the terrifying and skyrocketing lines of environmental degradation and carbon. But not far behind is another wave of fast-growing curves representing a solution set that could sustainably feed, shelter, and power the planet.

In his Rolling Stone piece, McKibben mentions that Germany recently met nearly half its noonday power demand with solar energy. What he didn't mention was that as recently as 2000, solar power comprised only 0.01 percent of Germany's power supply. A similar story of renewable energy growth has played out around the world. Late last year, the International Energy Agency came out with a stunning revision of its forecast for the future energy mix of the planet, saying solar could produce most of the world's power in less than 50 years. In the U.S., the rate of uptake of wind and solar technologies has blown expert predictions out of the water.

Taking note of new realities, the National Renewable Energy Laboratory recently issued a statement that would have been unimaginable five years ago: "Renewable electricity generation from technologies that are commercially available today is more than adequate to supply 80% of total U.S. electricity generation in 2050 while meeting electricity demand on an hourly basis in every region of the country."