by Curt Stager
When thermal maximum arrives around 2200 to 2300 AD, global mean temperature may be 3 to 7°F (2 to 4°C) higher than it is today. That peak could come a century or more later than the CO2 peak because of what climatologist Tom Wigley calls “climate change commitment,” a delaying action that mainly reflects the slow response of oceans to heating. Even if we could halt our greenhouse emissions altogether right now, we would still have to face a degree or more of additional warming over the next century. Taking this kind of delay to the extreme, some computer simulations of our moderate scenario by University of Victoria researcher Michael Eby and colleagues push the date of the thermal maximum at least 550 years after the CO2 peak.
As the temperature peak passes, the whiplash effect ushers in a prolonged global cooling-off period. But even though the direction of change has flipped from warming to cooling, the world is still hotter on average than it is today and polar ice continues to melt and flow into the oceans, lifting the sea surface by several feet per century. In addition, the deeper, colder layers of the oceans are still warming and expanding. Thermal expansion in this case could drive global mean sea level an additional 1 to 2 feet (0.5 m) upward, but how fast and far it eventually rises before experiencing its own whiplash reversal will mainly be determined by how much land-based ice melts. In this relatively moderate scenario, one might reasonably expect to lose about half of Greenland’s ice and much of the West Antarctic ice sheet while leaving the huge East Antarctic ice sheet largely intact. As a result, sea-level rise could finally stall at 20 to 23 feet (6 to 7 m) above today’s elevation several centuries or millennia from now.
So far, none of this seems particularly surprising, being standard fare for current descriptions of global warming. But it’s at this point in the climate narrative that Archer and other investigators have begun to grab the scientific world by the collar. Although many who discuss the duration of global warming still limit it to a few hundred years, a growing body of new evidence shows that a century-sized time scale for full climate recovery is far too short to be realistic. “The lifetime of fossil carbon dioxide in the atmosphere is several centuries … plus 25 percent or so that lasts essentially forever,” says Archer.
He reached that conclusion by considering where our massive slug of CO2 may go once we stop pumping additional fumes into the air. By burning fossil fuels, we’re emptying a global graveyard of plants and plankton that took millions of years to fill, and there’ll be no return to those tombs for many thousands of years. Archer’s reference to “forever” is not meant to be taken literally; any carbon imbalance that we impose on the atmosphere will eventually level out again. But it will only happen over time periods that when fully grasped, can take one’s breath away. From the frame of reference of any single civilization, say, ancient Egypt or the Roman Empire or the modern Industrial Age, the span of global carbon recovery time might as well be forever.
This may sound like a wildly extremist claim, but it most certainly is not. If you follow the reasoning behind it step by step, you’ll find that it makes perfect sense. Just keep in mind that the carbon in our CO2 and methane emissions doesn’t leave the planet and it doesn’t break down; carbon atoms are virtually immortal (except for the radioactive isotope carbon-14, which I’ll discuss later). They have to go somewhere when they are released from a burning bit of fossil fuel, and they may wander from place to place after leaving a particular smokestack, flue, or tailpipe.
The computer projections show that, at first, much of our emitted carbon will dissolve in the sea. Gases enter water bodies quite easily from the stirring action of surf and currents as well as the diffusion of air molecules directly into liquid surfaces; that, after all, is how fish manage to breathe dissolved oxygen while swimming in a lake or ocean. The oceans actually contain about fifty times more CO2 than the atmosphere and cover 70 percent of Earth’s surface, and it is this uptake process that will drive the whiplash reversal of CO2 concentrations after our emissions begin to trail off. But there’s a limit to how much gas even an ocean-sized reservoir can hold.
After a millennium or two, the aquatic uptake of our carbon emissions will have slowed to a crawl. When that happens, sometime around 3000 to 4000 AD, between a fifth and a quarter of our CO2 pollution could still be drifting about in the air. In the case of our moderate-emissions scenario, that would represent enough excess greenhouse gas to keep global mean temperatures 2 to 4 degrees (1 to 2°C) higher than they are now, even after a long recovery from the earlier thermal peak.
If Earth were made entirely of water, the gas satiation of the oceans would mark the end of the recovery process and a greatly reduced airborne remnant of extra CO2 would be marooned above the waves eternally. But much of the planetary surface consists of dry land, and geological features underlie the watery regions, as well. These features, the rocks and sediments of Earth’s crust, will clean up whatever remains of our pollution legacy. Unfortunately, they work slowly. Very, very slowly indeed.
The fastest repair mechanisms in the geological tool kit, or rather the least sluggish ones, involve chemical reactions with lime-rich carbonate materials such as limestone, chalk, and the shells of marine organisms. These are the kinds of alkaline substances that fizz when you drop acid on them, like the baking soda that you may have dribbled vinegar on in science class so you could watch it erupt into foam. Carbonate rocks and sediments that lie exposed to the elements can be attacked by natural acids in precipitation, soils, and water, most importantly by CO2-derived carbonic acid that now increasingly contaminates raindrops and water bodies worldwide.
Some of the first of such cleanup mechanisms to kick into gear will be those that lie in submerged marine environments. As our CO2 pollution dissolves into the oceans during the early stages of the Anthropocene future, it will acidify them enough to dissolve some of the alkaline muds, corals, and shells that litter the bottoms of the ocean basins. As you might imagine, this will not be a welcome change for many sea creatures—but there is a positive side to it as well. As carbonate molecules move from those solid forms into solution, they will help to neutralize the acidification over the course of millennia, and by readjusting the chemical balance of the oceans in this manner they will also help them to absorb even more CO2 from the air. In terms of the long-range global cleanup, it will be like feeding antacid pills to the ocean so it can continue to feast a bit more on the carbon pollution banquet.
Meanwhile, the carbonate rocks and soils on land will also be pitching in, but they will be dealing with rainwater, not seawater. You may be familiar with industrial acid rain, the strong stuff that kills forests in Europe and sterilizes lakes in my home region of the Adirondacks. Natural carbonic acid rain is different: in mild doses, carbonic acid is just another normal component of rainwater, and much weaker than the sulfuric and nitric acids that coal-fired power plants and automobile engines produce. It comes from the diffusion of CO2 from the air into the water droplets in clouds, and it makes even the purest rain slightly acidic. When carbonic acid rain falls on, say, a pale lump of limestone, you won’t see the rock foam up in response. But if you could look much closer, down on the microscopic scale, you’d see the wetted spots on the stony surface begin to crumble ever so slightly.
Imagine that we’re watching this take place on some beautiful geological formation in a region where limestone is abundant, as in the jagged, spectacularly etched landscape near Guilin and Yangzhou in southern China, that is commonly depicted in traditional watercolors. Perhaps you’ve seen such paintings in which the steep-sided hills resemble conical towers or canine teeth, seemingly too tall, narrow, and sharply pointed to be real. Those dramatic shapes were sculpted over the ages by the erosive effects of naturally acidic water on soft, soluble rock through processes that also shape other limestone-rich landscapes around the world.
Let’s say that we’re standing on top of one such spire overlooking a bend in the Li River on a gray, misty day, and it’s starting to drizzle. Small puddles pool around our feet, and t
hey funnel their slightly acidic contents into flowing cracks and crevices where carbonic acid molecules tear at the crystal lattices of the limestone. Those reactive acids then morph into new, less corrosive molecules, called bicarbonate, each of which still carries its original atmospheric carbon atom with it, and tumble downhill with other substances that have fallen away from the slowly decaying stone and dissolved into the watery runoff.
The bicarbonate-enriched rivulets pour their contents into the river, which eventually delivers its load to the South China Sea. When the bicarbonates join the immensity of the ocean, they help to neutralize the marine acid buildup just as the seafloor carbonates did, and their additional dose of natural antacid helps the seawater to take another few bites of CO2 from the air. Over time, bicarbonates and carbonates may also drag their carbon atoms down with them for more permanent burial on the ocean floor because marine organisms use such substances for photosynthesis and the building of shells and skeletons, turning them into heavy ballast when the host organisms die.
The upshot of this seaward migration of carbon atoms, which happens day and night, year after year, is that carbonate rocks and sediments on land will help to remove excess carbon from the air and store it in the oceans in much the same manner as the marine deposits did. After about five millennia of combined cleanup efforts, by which time the worldwide supply of carbonate materials has done about as much as it can, only 10 to 20 percent of our carbon excess remains in the atmosphere. But even so, that’s still enough to hold global average temperatures 2 to 4°F (1 to 2°C) higher than now. Far out in 7000 AD, the lingering gases still hold enough greenhouse warmth to keep leftover polar ice melting and sea levels rising.
But even this is not the end of the story. Amazingly, 7000 AD is only the entry ramp to what Archer calls “the long tail of the CO2 curve.” Our atmospheric carbon pollution will continue to decrease, but the remaining recovery will proceed so slowly that another 5,000 years will barely dent the carbon residue. In 12,000 AD—ten millennia after our fossil fuel emissions have ended—global average temperatures could still be at least 2°F (1.1°C) warmer than today.
The waning tail of the carbon curve is so long because, once the oceans and carbonates do what they can, the last major cleanup mechanism left will be the hard crystalline frameworks of more resistant silicate rocks such as pink or white granite and dense black basalt. Their response to weathering by acidified water is somewhat similar to that of carbonates, but it is achingly slow. Just how long will it take to return to our present CO2 concentration of 387 ppm by this route? Most computer models show that similar concentrations won’t be seen again for at least 50,000 years, and complete recovery could take several times longer still.
Although the world will be cooling slightly throughout the long tail-off, those falling temperatures will still be higher than they would have been without our influence. That will allow plenty of time for polar ice sheets to lurch and dribble into the sea, potentially raising ocean levels by tens of feet after many millennia of mild but sustained heating.
What Archer and researchers like him are telling us here is that even the emissions scenarios that many investigators call moderate are locking us into immensely long chains of environmental consequences. But this example of a relatively mild carbon pollution scenario pales in comparison to the more extreme side of the emissions spectrum. What if we throw caution and everything else flammable to the winds and spew a whopping total of 5,000 Gtons of carbon into the air? This would fall at the extreme upper end of the IPCC list of possible futures, something like the fossil fuel–intensive A2 emissions scenario carried well beyond 2100 ad. It is often said that we’re undertaking a huge ecological experiment by running our civilization on carbon-based fuels, but if the preceding 1,000-Gton scenario is just an experiment, then an extreme 5,000-Gton scenario is more like a crash test. If we go down that route, most likely by continuing on our current path until we burn through what remains of our coal reserves, then we can rightly be called crash-test dummies.
Atmospheric carbon dioxide concentrations for the next 100,000 years under two emissions scenarios. Recovery from the more extreme scenario takes much longer than 100,000 years. After Archer, 2005
Scenario 2: A Super-Greenhouse
Because it takes a long time for us to consume all of our easily accessible coal, CO2 emissions reach their peak later than in the previous scenario, say, between 2100 AD and 2150 AD, and they continue at decreasing rates for another couple of centuries.
Atmospheric CO2 concentrations in most computer simulations peak near 1900 to 2000 ppm, five times higher than today, around 2300 AD before reversing into a long-term decline. The carbonic acid spawned by all that CO2 dissolving into the oceans becomes a corrosive solvent to shell-bearing sea creatures from pole to pole.
Temperatures peak as early as 2500 AD or as late as 3500 AD, depending on the model and the environmental parameters used; most simulations stretch the broad thermal maximum out over several centuries. The massive peak is so flattened and long-lived that it can be difficult to call it a point of climatic whiplash; it’s more of an all-consuming high tide than a steeply cresting wave. Global mean temperatures during that thermal maximum could be at least 9 to 16°F (5 to 9°C) higher than today, or more than twice as high as the peak in the more moderate scenario. And this range merely represents the worldwide average; northern high latitudes might warm twice as much. Under those conditions, Europe, Scandinavia, and most of the United States are largely snow-free in winter.
By 4000 AD, many models drop atmospheric CO2 concentrations only as low as 1,000 to 1,300 ppm or so, still about three times higher than those of today. However, a model described by oceanographer Andreas Schmittner and colleagues in Global Biogeochemical Cycles keeps those concentrations even higher, more like 1,700 ppm, a level close to that obtained by Michael Eby’s team, as well. In that situation, sea-surface temperatures are also much warmer than they are now, perhaps 11 to 13°F (6 to 7°C) warmer in the tropics and 18°F (10°C) at higher latitudes. Such a global hothouse would sharply reduce the climatic differences between latitudes and leave the world more climatically homogeneous.
In 7000 AD, sluggish silicates continue to pick at the carbon leftovers in the atmosphere. One paper published by Archer and German climatologist Victor Brovkin suggests a cooling recovery of less than 2°F (ca. 1°C) during those first five millennia, after which CO2 concentrations still hang close to 1,000 to 1,100 ppm. Even 10,000 years down the time line, between a tenth and a quarter of our greenhouse pollution still hangs in the air, keeping global average temperatures 5 to 11°F (3 to 6°C) warmer than those of today. Carbon dioxide concentrations don’t resemble those of today until 100,000 AD or so, and full recovery in this extreme scenario takes at least 400,000 to 500,000 years.
Detail of near-term changes expected for an extreme 5,000 Gton emissions scenario. After Schmittner et al., 2008
With such prolonged and intense heating, Greenland’s ice sheet eventually shrinks to bedrock and drains enough meltwater into the North Atlantic to lift sea levels by 23 feet (7 m). The loss of even more ice from Antarctica eventually pushes those levels ten times higher, gradually turning low coastal plains into submarine extensions of the continental shelves.
The differences between the moderate 1,000-Gton path and the devil-may-care 5,000-Gton route are profound and disturbing. In both cases, most of the environmental changes that they cause will occur slowly in comparison to our lifespans, but it’s their magnitude and duration, more than their speed, that boggle the mind, particularly in the extreme case.
In the moderate scenario, most of the largest environmental disturbances are over and done with during the first millennium or two, though a small fraction of our carbon pollution lingers for tens of thousands of years. But an extreme 5,000-Gton emissions release lifts the long tail of the CO2 curve much higher and stretches it out much further into the future, not just because our emissions were greater in that case but also
because the warming itself launches feedback mechanisms that release natural stores of carbon and boost temperatures even further. The thermal maximum in this scenario is at least twice as high as in the more moderate case and it ends several centuries later.
Believe it or not, this sketch of an extreme hothouse future is actually conservative. One simulation published by oceanographer James Zachos and colleagues holds CO2 concentrations nearly flat within the 500 to 600 ppm range to the end of the 100,000-year model run. And another study puts the end of the silicate cleanup a full million years in the future, even for a moderate 1,000-Gton scenario.
As we contemplate the expansive time scales that these changes will play out on, it can be difficult to believe that they are real. Looking so deeply into the future can be like sailing off the edge of a flat world in our imaginations, and for many of us nothing that is concrete and familiar lies beyond the mysterious horizon of our own lifetimes. Will people even exist in 100,000 AD?
When I’ve presented this question to my students and colleagues, most of them have said that they expect the human race to be extinct long before that date. Some expect a natural or human-made disaster to kill us off, and others believe in various religion-based dooms for humanity. I find this fatalism disturbing, because our survival as a species is central to all discussions of fossil fuel consumption, climate, and life on Earth. If we’re not going to be here for much longer, then who cares how long our CO2 will hang around in the air? And if there’s not much time left for our descendants to live in, then why shouldn’t we simply use up our natural resources today and enjoy life selfishly while we can?
Claiming that humans will not exist in the deep future can also be a cop-out. Eliminating a long-term future makes it easier to ignore the immense longevity of our carbon pollution and pretend that nobody will have to deal with the environmental changes that we’re setting in motion today. But there’s no way out from under this yoke of responsibility, because humankind is really not going anywhere. The lifestyle decisions that we make today will affect many generations to come, and the least we can do on their behalf is to acknowledge that fact.