We have a couple of advantages when it comes to dealing with climate change. For one thing, the parameters of the problem are remarkably clear: We can see the Arctic melting, the ocean acidifying, the mercury steadily rising. Droughts and floods reinforce daily our understanding of our predicament.

More, researchers have made it relatively simple to understand what we can and can’t do going forward. If the planet is to hold its temperature increase to two degrees Celsius—and almost every nation agreed to that target in 2009 at the international talks in Copenhagen—then we simply have to leave most of the carbon we know about underground; it can’t be burned. In fact, a powerful article in Nature last January listed all the carbon deposits that would need to go untouched: places like the tar sands of Canada or the oil and gas reservoirs beneath the Arctic. (Scientific American is part of Nature Publishing Group.)

So: easy—or not. Pres. Barack Obama, for instance, traveled to Alaska in early September to draw attention to climate change. He visited native villages, checked out melting glaciers, said all the right things. But the week before he’d given final permission to Shell Oil to drill in the Arctic—exactly the sort of thing that scientists say will make global warming worse.

In so doing Obama exemplified our dilemma. He simply couldn’t bring himself to stand up to the forces that want to dig up every last lump of coal, drill every last drop of oil.

For politicians, this lack of restraint has a simple source: the power of the fossil fuel industry. It is the richest industry on the planet, and it’s historically gotten absolutely everything it wanted. That Obama rejected the Keystone XL Pipeline is a singular display of courage; it was literally the first time a world leader said, “Here’s a project we can’t build because of its effect on the climate.” But usually Big Oil is just too—Big.

Two things might change the equation.

One is the emergence of a real climate movement. It’s getting big, too: Last fall 400,000 demonstrators marched through the streets of New York City, which was the largest demonstration about anything in the U.S. in a very long time. Increasingly this global effort is persuading banks to stop financing the next round of extraction. In midsummer, for instance, the company planning to build the world’s largest coal mine in the Australian desert pulled the plug, their credit lines severed by persistent activism.

But there’s another factor, too. In the past six years the price of solar panels has fallen by 75 percent. That makes renewable energy the cheapest alternative in much of the world. If we committed ourselves, Stanford University professor of environmental engineering Mark Jacobson and his team have shown, every state in the union (and every country on the planet) could supply their needs with clean, reliable electricity by 2030 at an affordable price.

Polls show that most people would like that change. The ones who wouldn’t are the people who own coal mines and oil wells—people who have a good deal of political clout. For instance, the richest man on Earth is the two Koch Brothers taken together. They are oil and gas barons—in fact, they are the largest leaseholders in those Canadian tar sands that are one of the deposits the scientists say we must leave largely untouched. And they have announced that they’ll spend $900 million on next fall’s U.S. elections—that’s more money than the Democratic or Republican parties spent the last time around.

This epic battle is playing out in real time. We can easily measure the carbon dioxide pouring into the atmosphere. This spring, for the first time in human history, CO2 levels passed 400 parts per million. Time is short—if we don’t start showing restraint very soon, then the climate talks in Paris this December won’t much matter, because the changes we’ve kicked off will become self-reinforcing.

Some days, reading the latest science, it's hard not to be pessimistic. But then there are days like the one this fall when Shell canceled its plans for further Arctic drilling, and then the Obama administration took a series of decisions making it harder for others to follow them. Maybe it's a harbinger. Humans are very clever at doing things. The question is, can they not do things?

Bill McKibben is founder of the global climate campaign 350.org and author of the original book for a general audience about climate change, The End of Nature, which was published in 1989.