What is the greatest threat this planet faces?
There is no threat to this planet. To even ask the question misses an important point. To best illustrate the point that is missed, I'll quote a chapter from the novel
Jurassic Park…
Hammond seemed to revive, and began bustling around, straightening up. “Well,” he said, “at least that disaster is averted.”
“What disaster is that?” Malcolm said, sighing.
“Well,” Hammond said, “They [dinosaurs] didn't get free and overrun the world.”
Malcolm sat up on one elbow. “You were worried about that?”
“Surely that's what was at stake,” Hammond said. “These animals, lacking predators, might get out and destroy the planet.”
“You egomaniacal idiot,” Malcolm said in a fury. “Do you have any idea what you are talking about? You think you can destroy the planet? My, what intoxicating power you must have.” Malcolm sank back on the bed. “You can't destroy this planet. You can't even come close.”
“Most people believe,” Hammond said stiffly, “that the planet is in jeopardy.”
“Well it is not,” Malcolm said.
“All the experts agree that our planet is in trouble.”
Malcolm sighed. “Let me tell you about this planet” he said. “This planet is four and a half billion years old. There has been life on this planet for nearly that long. Three point eight billion years. The first bacteria. And, later, the first multicellular animals, then the first complex creatures, in the sea, on the land. Then the great sweeping ages of animals—the amphibians, the dinosaurs, the mammals, each lasting millions upon millions of years. Great dynasties of creatures arising, flourishing, dying away. All this happening against a background of continuous and violent upheaval, mountain ranges thrust up and eroded away, cometary impacts, volcanic eruptions, oceans rising and falling, whole continents moving… Endless constant and violent change… Even today, the greatest geographical feature on the planet comes from two great continents colliding, buckling to make the Himalayan mountain range over millions of years. The planet has survived everything in its time. It will certainly survive us.”
Hammond frowned. “Just because it lasted a long time,” he said, “doesn't mean it is permanent. If there was a radiation accident…”
“Suppose there was,” Malcolm said. “Let's say we had a bad one, and all the plants and animals died, and the earth was clicking hot for a hundred thousand years. Life would survive somewhere—under the soil, or perhaps frozen in the Arctic ice. And after all those years, when the planet was no longer inhospitable, life would again spread over the planet. The evolutionary process would begin again. It might take a few billion years for life to regain its present variety. And of course, it would be very different from what it is now. But the earth would survive our folly. Life would survive our folly. Only we,” Malcolm said, “think it wouldn't.”
Hammond said, “Well, if the ozone layer gets thinner—”
“There will be more ultraviolet radiation reaching the surface. So what?”
“Well. It'll cause skin cancer.”
Malcolm shook his head. “Ultraviolet radiation is good for life. It promotes mutation, change. Many forms of life will thrive with more UV radiation.”
“And many others will die out,” Hammond said.
Malcolm sighed. “You think this is the first time such a thing has happened? Don't you know about oxygen?”
“I know it's necessary for life.”
“It is now,” Malcolm said. “But oxygen is actually a metabolic poison. It's a corrosive gas, like fluorine, which is used to etch glass. And when oxygen was first produced as a waste product by certain plant cells—say, around three billion years ago—it created a crisis for all other life on our planet. Those plant cells were polluting the environment with a deadly poison. On earth, the concentration of oxygen was going up rapidly—five, ten, eventually twenty one percent! Earth had an atmosphere of pure poison! Incompatible with life!”
Hammond looked irritated. “So what's your point? That modern pollutants will be incorporated, too?”
“No,” Malcolm said. “My point is that life on earth can take care of itself. In the thinking of a human being, a hundred years is a long time. A hundred years ago, we didn't have cars and airplanes and computers and vaccines… It was a whole different world. But to the earth, a hundred years is nothing. A million years is nothing. This planet lives and breathes on a much vaster scale. We can't imagine its slow and powerful rhythms, and haven't got the humility to try. We have been residents here for the blink of an eye. If we are gone tomorrow, the earth will not miss us.”
“And we might very well be gone,” Hammond said, huffing.
“Yes,” Malcolm said, “we might.”
“So what are you saying? We should not care about the environment?”
“No, of course not.”
“Then what?”
Malcolm coughed, and stared into the distance. “Let's be clear. The planet is not in jeopardy. We are in jeopardy. We haven't got the power to destroy the planet—or to save it. But we might have the power to save ourselves.”