We all happen to be alive at an extremely interesting moment—the exact moment when human beings have grown so large that they’ve begun to affect everything around them.

Human civilization—the period we have records for, when people lived lives somewhat like our own—stretches back about 10,000 years. Scientists call that period the “Holocene.” The earth’s climate stayed stable for those 100 centuries—there were storms and droughts and floods, but if your great-grandmother had grown corn in a particular field, you could count on growing it there too. People were pretty small.

But about 250 years ago, people learned how to burn coal, and then gas and oil. These are called “fossil fuels,” because they come from the bodies of plants and animals that have piled up over hundreds of millions of years. Each time we burn some of that fossil fuel, we put a little more carbon into the atmosphere. And since the molecular structure of carbon traps heat, the planet keeps getting a little bit warmer. We finally noticed this in the last few decades—it’s what scientists call climate change, or global warming.

By this point, it’s pretty bad. The world is warm enough now that the polar ice caps have begun to melt. During summer in the Arctic, boats can now sail through open water, where before there had been feet of ice. The world’s great glaciers are sliding into the sea and raising the level of the ocean. Because warm air is moister than cold air, we see more evaporation in dry areas like California (where a terrible drought is underway) and more downpours and flooding in wet areas (like in Pakistan, where the worst flood of the modern era drove 20 million people from their homes in 2010).

One way of understanding this is that human beings have gotten much bigger. For most of our history, we were just one species. We changed the world by planting fields or cutting down trees, but our impact ended at the edge of the field or the border of the forest. When we learned to burn fossil fuels, we suddenly got very large. Now our impact is changing the temperature of the planet. There’s still almost no one living in Antarctica, but the ways we live are melting that remote continent. We’ve done away with the basic physical stability that marked the Holocene era, so scientists have come up with a new name for our time: the “Anthropocene.” The era made by man.

This new world that we’re creating by raising the temperature is already causing a lot of problems, especially for the poorest and most vulnerable people on earth. On low-lying islands in the Pacific and Indian oceans, for instance, people have begun to have to evacuate their countries—the rising tides make it impossible to live where their ancestors have lived for generations. In other places, the rapid spread of mosquitoes (who like the warm, wet world we are creating) brings with it new disease. Drought and flood make life hard for peasant farmers. In some big cities like Sao Paulo, it’s gotten so dry that people turn on their taps and nothing comes out.

But worse problems lie ahead. So far human beings have raised the planet’s temperature about one degree Celsius, which doesn’t sound like much, but it’s been enough to melt the ice caps. The scientists who study our carbon emissions say that on our current trend we will probably raise the temperature four or five degrees before the end of the century. That would make our planet hotter than at any time since primates began evolving. It would be very hard to grow enough food or find enough water on a planet like that.

The good news is, we don’t have to keep burning coal and oil and gas if we don’t want to. In the last few years—the same decades we’ve learned about climate change—we’ve also learned how to use the sun and the wind to generate much of the power we need. Engineers have worked hard to make solar panels cheaper and more efficient; now, in many places in the world, it’s cheaper to get your electricity from the sun than from a coal-fired power plant. Inventors have even made practical and reliable electric cars—so if, like me, you have solar panels on your roof, you can essentially drive on the energy of the sun.

A few countries have tried very hard to put all this technology to good use, and they’ve had spectacular success. In Germany, there are days when 80% of the electricity is supplied from solar panels. In Denmark, 40% of the country’s power comes from windmills. Every other country in the world could do the same thing—in fact, in many places it would be much easier. Germany is far to the north, and gets about the same amount of sunshine as Alaska. Think how much solar power you could produce in Arizona or Florida, in India or Africa!

In too many places, however, the development of renewable energy has been slowed or stopped because the fossil fuel industry is fighting against it. For most of us, sun and wind are a great blessing—once you have a solar panel or wind turbine, you get electricity for free. But for the coal, gas, and oil industries, they’re a threat. If everyone had cheap, clean energy, they wouldn’t need to buy the old stuff any more. Electric cars don’t have gas tanks, after all!

So those fossil fuel companies have given lots of money to politicians to pass laws making it hard to switch to renewable energy. Luckily, lots of other people are fighting back. Around the world, there’s a powerful movement, led mostly by young people, to stand up to the coal and gas and oil companies. People have gone to jail to block new mines or pipelines. They’ve elected new kinds of politicians who don’t take money from fossil fuel companies.

They’ve even made giant works of art to wake people up to our crisis. I remember one great piece of artwork made by several thousand people in Santa Fe, New Mexico. Because of a deep drought, their river had dried up. Then, on one particular hour, on one particular day, thousands went down into the river bed and stood next to each other. They each were carrying a blue blanket, and right at the moment they knew a satellite was passing overhead, they held the blankets up high in the sky. On the photograph snapped by the satellite, it looked like blue water—it looked like the river was coming back to life.

To me, that’s the key. I’ve spent most of my life working on this crisis, because I think the world we were born into is so beautiful. I’ve lived most of my life deep in the woods of the eastern United States and know firsthand all of the seasons—the spring, when doe hide their tiny, day-old fawns in the woods from predators; the summer, when the hemlocks shade the streams, keeping them cool enough for trout; the fall, when the leaves turn neon for a few weeks; the calm quiet of cold, unbroken winter. My friends in other places know different kinds of beauty—the tranquility of Pacific Islands, the stark grandeur of high Tibetan mountain villages, shady oases in the blazing Australian desert. Wherever we are, that sense of place runs so deep; it is a great human inheritance.

Of course, we are the creatures that can pay witness to that glory. It’s one of our jobs, even as the planet is in trouble. We need to take time to get outdoors, to be part of the big and beautiful. And then we need to come together and fight for its future.

Our planet is in trouble, but it’s not doomed yet. We still have time, if we stand together, to move toward a different kind of future. I hope you’ll get involved. By acting carelessly, human beings have changed the world for the worse. Now, if we’re careful and committed, we can make some changes for the better.