What Happens After Global Warming

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This article is an adaptation of the introduction to J. Curt Stager’s Deep Future: The Next 100,000 Years of Life on Earth(Thomas Dunne Books, an imprint of St. Martin’s Press), published here courtesy of the author and publisher.

This excerpt appears in two parts. Part I is printed below. Read Part II here.


            In the past, seemingly intractable debates over environmental issues have sometimes been resolved with the aid of long-term historical perspectives.  When acid rain dominated the environmental media during the 1980s, for example, sediment core records of recent, progressive acidification in lakes from the northeastern United States helped to break rhetorical logjams between activists who correctly insisted that industrial pollution was the cause and polluters who claimed that the lakes had always been naturally acidic.  When global warming later gained wide public attention, ice cores and other such geo-historical records showed that greenhouse gases and climates are indeed closely linked, and that we’re fast approaching conditions that are well beyond the normal range of variability. 

            Although it’s a rather eclectic discipline that straddles the fields of geology and biology, my chosen profession of paleoecology is well known among scientists for its ability to harness truly long-term thinking in order to place current environmental issues into their most meaningful contexts, showing us where we’ve been, how we got here, and where we’re likely to be headed next.

            Today, scientists with such long-term perspectives are once again nudging us beyond currently entrenched arguments over human-driven climate change, but their gaze is aimed forward in time, as well as backward.  A new generation of climate models and the visionaries who wield them are showing that our carbon legacy will last far longer than most of us yet realize, long enough to interfere with future ice ages.  David Archer, an oceanographer and climate modeler at the University of Chicago, sometimes puts it thus; “global warming is essentially forever.”

            Until now, our collective view of future warming has typically ended at 2100 AD, as if it were the edge of a world beyond which we dare not sail.  That’s not surprising, considering that most of us consider “a long time” to be the hours remaining at the office, and even for most scientists the end of this century can seem like an eternity away.  But if the world continues to exist beyond 2100 AD, then what happens after global warming?

            For Archer and experts like him, 2100 AD is just the first step on a long journey that we and our planet are embarking upon together, whether we realize it or not. Sophisticated computer models with names such as CLIMBER, GENIE, and LOVECLIM show that a large fraction of our fossil fuel emissions will remain in the atmosphere for so long that it boggles the mind, not just a few centuries but many tens of thousands of years.  In the most extreme scenarios, which are the ones that we’re heading for at the moment, that persistence could reach half a million years or more. 

            Despite the arcane calculations that support these seemingly outlandish claims, the basic picture of what lies ahead is actually quite simple.  It all boils down to one principle: “what goes up must come down.” 

            Sooner or later, our carbon dioxide emissions will peak and then begin to decline, because sooner or later we’ll either switch to alternative energy sources or simply run out of affordable fossil fuels.  In response, greenhouse gas concentrations will also reach some limit and then begin to fall.  This will trigger a sequential “climate whiplash” period, in which temperatures flip from warming into cooling mode, acidification from CO2 dissolution into the ocean wreaks maximum havoc on marine life before slackening off, and sea levels peak and fall.

            Perhaps most amazingly of all, the intensity and duration of these changes will largely be determined by what we do during this century. 

            If we switch to non-fossil energy soon, then we’ll have emitted roughly a trillion tons of heat-trapping carbon fumes since the Industrial Revolution.  Atmospheric CO2 concentrations will likely peak by 2150-2200 AD and then begin to fall, to be followed by a relatively brief heat spike 2-3°C higher than today, and a later sea level maximum roughly 10 m higher than that of today due to melting in Greenland and western Antarctica.   According to Archer, most of our fossil carbon emissions will dissolve into the oceans during the next few millennia, but the seas can only retain so much.  Roughly one fifth of that carbon will remain stranded in the atmosphere until rock weathering eventually scrubs it away, but those processes work very slowly.  Full recovery in this relatively moderate scenario takes as much as 100,000 years. 

            If, instead, we burn through our coal reserves before utter depletion forces us to find alternative energy sources later on, then much more extreme consequences could follow.  Details vary depending on the models used, but with CO2 concentrations approaching 1900-2000 ppm (five times that of today), temperatures could rise 6-9 °C over a broad peak stretching from 2500 AD to 3500 AD.  Carbon dioxide concentrations and temperatures would not fall close to today’s levels again for hundreds of thousands of years.  All land-based ice would eventually melt, raising sea levels by as much as 70 meters until regenerating ice sheets begin to pull water back out of the oceans.

            Are these just morbid fantasies of an apocalyptic future?  Not at all.  For me, at least, such revelations of our uniquely influential place in deep time are as transformative and energizing as my first look at photos of the Earth floating in deep space.  We are important in the grand sweep of geologic history.  And paleoecological evidence shows that these simulations of possible futures are firmly anchored in scientific fact rather than science fiction, because they’ve happened before.  The Earth has warmed many times in the past, though not because of us, and geo-historical records reveal much about what life on a warmer Earth could be like.

            Take the more moderate scenario outlined here, in which we rein our emissions in as quickly as possible.  The expected outcome is much like what happened the last time world climates came up for air between the last two ice ages, 130,000 years ago.  The so-called “Eemian interglacial” warm period was caused not by greenhouse gases but by natural orbital cycles that warmed Arctic summers enough to destroy the great northern ice sheets. Nonetheless, it lends strong support to the simulations of a best-case future.

            Sediment layers, ice cores, cave stalagmites, and other paleo archives show that the Eemian interglacial was 2-3°C warmer than today and lasted for about 13,000 years. Greenland and Antarctica lost enough ice to hoist sea level by 6-9 meters, but much of their frozen coverings survived the long thaw.  Hickories and black gums that now favor southeastern forests of the Blue Ridge thrived in the Adirondack Mountains of upstate New York, while heat-loving hippos and elephants roamed northern Europe. The environmental changes were enormous but slow enough that few species died out as a result, and most animals and plants simply migrated along with their favored climatic conditions between various latitudes and altitudes.

            The most extreme scenario would more closely resemble a natural super-greenhouse that engulfed the planet 55 million years ago following a catastrophic release of methane and CO2 from undersea deposits, possibly due to volcanism in the North Atlantic.  Global temperatures shot up 11-12°C higher than today over several thousand years – the blink of an eye in geological terms, and on par with a future burn-it-all scenario.  All land-based ice vanished, the Arctic Ocean became a warm, brackish pond rimmed by redwood forests, beech trees covered Antarctica, carbonic acid burned a discolored stripe into the sea floor that still shows up clearly in marine sediment cores, and sea surfaces climbed 70 meters above today’s level.  Recovery in that case took at least 170,000 years. 

            Perhaps surprisingly, even this “Paleoecene-Eocene Thermal Maximum” (PETM) hothouse did little harm to most land-dwelling life, though many marine organisms perished in the oceanic acid bath.  In fact, early mammals thrived in the luxuriant greenery that spread from pole to pole, with newly evolved creatures invading new territories so rapidly that they often seem to have appeared in fossil records everywhere at once. But that’s not necessarily very comforting news.  The PETM predated our earliest hominid ancestors by more than 50 million years, a time at the dawn of mammalian evolution when carnivorous “wolf-sheep” ran about with hooves on the tips of their toes, and proto-whales had legs.  Just because they felt at home during the PETM doesn’t necessarily mean that today’s wolves, sheep, and whales would handle a reprise very well, especially with billions of humans and a global cobweb of roads and cities and farms blocking their migration routes.

But all is not doom and gloom.

Read Part II here, where the author describes the pros and cons of a future world resulting from both the moderate scenario and the extreme scenario.

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