This Is Life in a 400 PPM World

It already ranks as one of the grimmest measurements ever taken. Climate scientists found that for the first time in approximately three million years, the level of carbon dioxide in the atmosphere has reached 400 parts per million. The reason that figure was splashed across the front page of the New York Times—and why top White House advisors find it "truly frightening"—should be well understood by now. Carbon dioxide is a powerful greenhouse gas, and the more that accumulates in the atmosphere, the more sunlight it traps—and the more the globe warms.

We've now added enough CO2 to the atmosphere to change the lives of every human on the planet. This isn't an exaggeration. An increasingly large portion of the CO2 clogging our atmosphere comes from human activity—from our coal-fired power plants, our petroleum burning cars, our factories. Before we had any of those, carbon dioxide accounted for just 280 ppm. That means we've already turned up the dial on the planet's central heating by some 42 percent.

As with most heating units, it will take a little time for the temperatures to catch up with the new setting. But many of those changes are already under way. Life in a world where carbon accounts for 400 ppm is going to be quite different from the old 280 ppm world. The climate is now fundamentally different than it was 40, 30, even 20 years ago.

When I was born, in the mid-1980s, the amount of CO2 that had accumulated in the atmosphere was just enough to account for 350 ppm—the amount climatologists like NASA's Dr. James Hansen have identified as the threshold between a stable climate and an unpredictable, potentially volatile one. Between the 1800s and then, humans—mostly the United States and Europe—had built enough carbon-belching power plants and factories to add 70 ppm to the atmosphere.

Yet in my short life alone, human activity has pumped enough carbon pollution into our skies to raise the bar a full 50 ppm more. That's a huge change—out of the 120 ppm humans have added in total, nearly half of it has occurred in just under 30 years. That's the rest of the world following suit, building fossil fueled power plants and industrializing; the same way the U.S. did.

And that's enough carbon to transform our climate to the point that it better resembles another geologic era entirely: The Pliocene. That era, which took place from 5.8 to 2.6 million years ago, was the last time there was so much CO2 was blanketing the planet. According to the geological record, the CO2 levels of 360-400 ppm that marked the Pliocene made the world a drastically different place than the one that you and I grew up in.

Here are some characteristics of the 400 ppm world then—and those that are likely to be reprised in coming years:

-Sea levels were, on average, between 50 and 82 feet higher.

-Temperatures were 2-3˚C higher, or about 4-6 ˚F, than they are today.

-Arctic temperatures were between 10-20 ˚C hotter.

-Many species of both plants and animals existed several hundred kilometers north of where their nearest relatives exist today.

-Vast swaths of land turned into swamps.

This is our 400 ppm world. Hotter, nastier, even less predictable than the one you got comfortable with. This is the world that your kids are going to be growing up in. And some of the irrevocable damage has already been done.

"We've taken one of the largest physical features on earth--the Arctic--and we've broken it; new data shows 80 percent of the ice that was there 40 years ago is gone. So now we'll find out what disappears between here and 450," Bill McKibben, the environmentalist and author of Eaarth: Life on a Strange New Planet, told me in an email.

What seems like pessimism is actually gloomy pragmatism. McKibben knows that if we keep our factories humming, our cars guzzling, and coal plants firing, we'll hit 450 ppm in less time than we hit 400.

"Sadly, we're shooting right past 400 ppm and likely to commit to at least 450 ppm within a matter of years if we don't begin ramping down our greenhouse gas emissions," the preeminent climatologist Michael E. Mann told me.

And if there's one thing that's worse than a 400 ppm world, it's a 450 ppm world.

"If we cross 450 ppm we likely commit to just under 4˚ F warming of the globe relative to preindustrial," Mann continued. "That's a world where the most extreme summers we've ever seen, like last summer, with its record heat and drought, decimated crops, unprecedented wildfires, and devastating Superstorm Sandy, will be the typical summer. And the extreme summers? There is no analog in our past for what that would look like."

That world is just decades, even years away. I won't recite a full list of dangers a world like this holds—the one that includes displaced climate refugees, tensions over diminishing resources, increased reach of tropical diseases, battered coastal populations—but suffice to say that the 400 ppm world and its successors can be ugly places.

The Arctic is already melted. Sea levels are rapidly rising. We've seen a full 1˚ F of temperature rise since mid-century. Scientists are predicting that climate change is indeed going to devastate plant and animal habitats worldwide, much as it did in the Pliocene. This is the 400 ppm world, and it's upon us. The only question now is if we're going to keep cranking the central heat—are we going to turn this sauna into an inferno? We'd have to embrace a whiplash transition away from fossil fuels and towards clean energy—otherwise we can say hello to planet hotbox.

"Fortunately, there is still time to avoid that future," Mann says. "But not a whole lot of time. Breaching the sobering milestone of 400 ppm simply puts an exclamation mark on that."