It's a mistake to think the climatic effects of our carbon emissions will be over within a few decades or centuries. Our intergenerational responsibilities run much deeper into the future.
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In this new Anthropocene epoch, the “Age of Humans,” we have become so numerous, our technology so powerful, and our lives so interconnected that we are now a force of nature on a geological scale. By running our civilization on fossil fuels, we are both creating and destroying climates that our descendants will live in tens, if not hundreds, of thousands of years from now.
Carbon atoms do not disappear when we burn them into carbon dioxide. Isotopic tracer studies show that they work their way into the very fabric of life on Earth. Some of them travel up the food chain from the atmosphere to plants to animals to our dinner plates. Roughly one-eighth of the carbon in your flesh, hair and bones recently emerged from smokestacks and tailpipes. We are not only a source of air pollution — we are air pollution, and our waste fumes will henceforth be woven into the bodies of our descendants, too.
This inert fossil fuel carbon inside us has no direct effect on our health, although mercury and other pollutants that often accompany it amid industrial and automotive emissions may harm us. Most of the airborne carbon will eventually dissolve into the oceans, leaving a sizable fraction of it aloft until it, too, is removed by chemical reactions with carbonate and silicate minerals in rocks and sediments.
That’s the good news.
The bad news is that the natural mopping up of our mess will be extremely slow. Research by the University of Chicago oceanographer and climate scientist David Archer and others shows that the cleanup will take tens of thousands of years even if we switch quickly to renewable energy sources. When the Earth’s slow cyclic tilting and wobbling along its eccentric orbital path once again leads to a major cooling period some 50,000 years from now, enough of our heat-trapping carbon emissions will still remain in the atmosphere to warm the planet just enough to weaken that chill. In other words, our impacts on global climate are so profound that we will have canceled the next ice age.
It is now too late to stop human-driven warming altogether. Even if we wean ourselves from fossil fuels within the next few decades, our descendants will still face temperatures significantly higher than they are now — for millenniums to come. But that is no reason to delay or despair. If we don’t make the switch soon, our descendants will later be forced to do so under duress because of the depletion of finite reserves, and the artificially hotter Earth will be even poorer in species, habitats and lifestyles for thousands more generations.
What will it be like to live in a warmer world? Geological history contains numerous examples of previous natural hot spells that offer clues. First, consider the milder scenario.
If we switch quickly from fossil fuels, climates might come to resemble those of the interglacial warm periods that punctuated ice ages of the last two million years. During the last interglacial, which began 130,000 years ago and lasted about 13,000 to 15,000 years, global average temperatures were 2 to 5 degrees Fahrenheit higher than today. Enough of the ice in Greenland and Antarctica melted to lift sea levels by about 20 feet, but most polar ice survived. Stronger monsoons turned the Sahara lush and watery, and intense droughts parched the American Southwest.
Many species and ecosystems adapted to the changes that didn’t suit them by simply migrating toward the poles. Polar bears survived, presumably because they found enough icy refuges in the high Arctic to keep them going. The warmth coaxed southern Appalachian oak-hickory-black gum forests north to upstate New York and sent hippos, elephants and other typically African animals north through Europe.
Unfortunately, our Anthropocene cities, roads, farms and fences now block future migration routes, and as our excess carbon dioxide soaks into the ocean, there will be no place for shell-bearing marine creatures to migrate to as the seawater grows increasingly acidic. Furthermore, the heat-trapping gases that we release in the most moderate scenario will warm the Earth for much longer than a typical interglacial, on the order of 100,000 years.
This best-case scenario is troubling, but Earth history shows us that the alternative is unacceptable. If we burn all remaining coal, oil and gas reserves within the next century or two, we could introduce a more extreme, longer-lasting hothouse much like one that occurred about 56 million years ago: the Paleocene-Eocene Thermal Maximum, or PETM.
Unlike the relatively mild interglacials driven by the tilt, wobble and orbit of the Earth, the PETM fundamentally transformed the planet. Experts speculate that it was set off by volcanism in the Atlantic Ocean, thawing of permafrost, melting of methane hydrates, or a combination of such factors. Whatever caused the PETM, it spewed trillions of tons of carbon dioxide into the air and oceans. Global average temperatures climbed 10 degrees or more, erasing cold-loving species and habitats from the planet. With atmospheric carbon dioxide concentrations several times higher than today, a combination of warming and carbonic acid buildups in the oceans exterminated many deep-sea creatures and dissolved limy minerals and shells from the ocean floor.
The spectacular rise of the PETM lasted several thousand years. The Arctic Ocean became a tepid, brackish cove surrounded by deciduous redwood forests. Antarctica was covered in beech forests and so rain-soaked that silty runoff clouded the surrounding ocean. If we were to de-ice the planet in similar fashion again, global mean sea level would rise well over 200 vertical feet.
Many descriptions of the PETM focus on its onset and peak to illustrate today’s warming, but that was only the first chapter in a much longer story. What goes up must also come down, and the PETM’s temperature profile resembled a child’s playground slide with a steep ladder to the top and a long ride down as carbon dioxide was absorbed by seawater, rocks and sediments in a recovery that lasted more than 100,000 years. At the top of that slide, temperatures flipped from warming to cooling in a dramatic period of “climate whiplash.”
From the perspective of future generations, the whiplash and subsequent cooling that follows our own thermal peak could be as challenging as the warming. Species and cultures that will have adapted to centuries of rising temperatures, retreating ice, and advancing sea levels will then have to face strange new kinds of environmental change in reverse. For example, when global temperatures eventually begin to fall, the oceans will continue to swell because climates will still be warm enough to continue melting what remains of the polar ice sheets for thousands of years. Those who live through that long, strange period will face sea level rise and global cooling at the same time.
WE are not only warming the planet but also constructing and demolishing artificial worlds of the deep future. The thermal peak of a PETM reprise could last many thousands of years, long enough for future cultures and habitats to grow older than Babylon, and long enough for a greenhouse Earth to seem normal for hundreds of generations. But climate whiplash will eventually pull the rug out from under our later descendants. In that far future, there will be no more fossil fuels left to burn in order to sustain the artificial hothouse, and only a reduced, heat-tolerant fraction of today’s cultural and biological diversity will remain to face an age of global cooling that could last as long as half a million years, far more than the entire history of anatomically modern humans up until now.
A switch from finite fossil energy to cleaner, renewable energy sources is inevitable: We are only deciding how and when to do it. That is what world leaders and policy makers will be grappling with at the United Nations Conference on Climate Change that begins Monday in Paris. Much of the environmental harm that we have already done was unintentional, but now that science has exposed our role in it a new moral dimension has been added to our actions. Pope Francis’ recent encyclical on the environment makes it clear that to continue taking a profligate carbon path is to sin against future generations and our own human dignity.
As pioneers of the Anthropocene, we are an immensely powerful force of nature and can accomplish great things if we not only learn what is scientifically true, but also do what is morally right. Pope Francis tells us that “there is nobility in the duty to care for creation.” As a climate scientist who welcomes international action to address climate change, I offer a heartfelt “Amen” to that.
Curt Stager is a professor of natural sciences at Paul Smith’s College and the author of Deep Future: The Next 100,000 Years of Life on Earth.