Post-human Earth: How the planet will recover from us
* 30 September 2009 by Bob Holmes Magazine issue 2728 New Scientist.
WHEN Nobel prize-winning atmospheric chemist Paul Crutzen coined the word Anthropocene around 10 years ago, he gave birth to a powerful idea: that human activity is now affecting the Earth so profoundly that we are entering a new geological epoch.
The Anthropocene has yet to be accepted as a geological time period, but if it is, it may turn out to be the shortest - and the last. It is not hard to imagine the epoch ending just a few hundred years after it started, in an orgy of global warming and overconsumption.
Let's suppose that happens. Humanity's ever-expanding footprint on the natural world leads, in two or three hundred years, to ecological collapse and a mass extinction. Without fossil fuels to support agriculture, humanity would be in trouble. "A lot of things have to die, and a lot of those things are going to be people," says Tony Barnosky, a palaeontologist at the University of California, Berkeley. In this most pessimistic of scenarios, society would collapse, leaving just a few hundred thousand eking out a meagre existence in a new Stone Age.
Whether our species would survive is hard to predict, but what of the fate of the Earth itself? It is often said that when we talk about "saving the planet" we are really talking about saving ourselves: the planet will be just fine without us. But would it? Or would an end-Anthropocene cataclysm damage it so badly that it becomes a sterile wasteland?
The only way to know is to look back into our planet's past. Neither abrupt global warming nor mass extinction are unique to the present day. The Earth has been here before. So what can we expect this time?
Take greenhouse warming. Climatologists' biggest worry is the possibility that global warming could push the Earth past two tipping points that would make things dramatically worse. The first would be the thawing of carbon-rich peat locked in permafrost. As the Arctic warms, the peat could decompose and release trillions of tonnes of carbon into the atmosphere - perhaps exceeding the 3 trillion tonnes that humans could conceivably emit from fossil fuels. The second is the release of methane stored as hydrate in cold, deep ocean sediments. As the oceans warm and the methane - itself a potent greenhouse gas - enters the atmosphere, it contributes to still more warming and thus accelerates the breakdown of hydrates in a vicious circle.
"If we were to blow all the fossil fuels into the atmosphere, temperatures would go up to the point where both of these reservoirs of carbon would be released," says oceanographer David Archer of the University of Chicago. No one knows how catastrophic the resulting warming might be.
That's why climatologists are looking with increasing interest at a time 55 million years ago called the Palaeocene-Eocene thermal maximum, when temperatures rose by up to 9 °C in a few thousand years - roughly equivalent to the direst forecasts for present-day warming. "It's the most recent time when there was a really rapid warming," says Peter Wilf, a palaeobotanist at Pennsylvania State University in University Park. "And because it was fairly recent, there are a lot of rocks still around that record the event."
By measuring ocean sediments deposited during the thermal maximum, geochemist James Zachos of the University of California, Santa Cruz, has found that the warming coincided with a huge spike in atmospheric CO2. Between 5 and 9 trillion tonnes of carbon entered the atmosphere in no more than 20,000 years (Nature, vol 432, p 495). Where could such a huge amount have come from?
Volcanic activity cannot account for the carbon spike, Zachos says. Instead, he blames peat decomposition, which would have happened not from melting permafrost - it was too warm for permafrost - but through climatic drying. The fossil record of plants from this time testifies to just such a drying episode.
Carbon spike
If Zachos and colleagues are right, then 55 million years ago Earth passed through a carbon crisis very much like the one feared today: a sudden spike in CO2, followed by a runaway release of yet more greenhouse gases. What happened next may give us a glimpse of what to expect if our current crisis hits full force.
Geochemists have long known that when a pulse of CO2 enters the air, much of it quickly dissolves in the upper layer of the ocean before gradually dispersing through deeper waters. Within a few centuries, an equilibrium is reached, with about 85 per cent of the CO2 dissolved in the oceans and 15 per cent in the atmosphere. This CO2 persists for tens or hundreds of thousands of years - what Archer believes will be the "long tail" of the Anthropocene. Until recently, though, climate modellers were a bit fuzzy on what this tail would look like.
"Until we had some case studies from the past, there was always some degree of uncertainty in the models," says Zachos. His studies are beginning to clear up these doubts. Carbonate rocks laid down on the sea floor during the carbon spike, for example, reveal that the oceans quickly became very acidic (Science, vol 308, p 1611). But this extreme acidification lasted just 10,000 or 20,000 years, barely a blink of an eye by geological standards, after which the oceans returned to near-normal conditions for the next 150,000 years. Even the stores of peat and methane hydrates must have regenerated within 2 million years, Zachos says, because at that time the planet underwent another, smaller carbon crisis, which must also have involved peat or methane hydrates. That suggests that the long tail of the Anthropocene is unlikely to last longer than 2 million years - still not long at all by geological standards.
However, today's carbon spike differs from that of the late Palaeocene in one important way: our planet is much cooler than it was back then, so warming is likely to have a more profound effect. During the late Palaeocene, the world was warm and largely ice-free. Now we have bright, shiny ice caps which reflect sunlight back into space. These will melt, giving way to dark, energy-absorbing rock and soil. And with all that meltwater, sea levels will rise and permafrost will thaw more rapidly, boosting warming still further.
This extra nudge could conceivably tip the Earth out of its present cycle of glacials and interglacials and return it to an older, warmer state. "The Earth was ice-free for many millions of years. The current ice ages started only about 35 million years ago, so we might kick ourselves out of that," says Pieter Tans, an atmospheric scientist at the US National Oceanic and Atmospheric Administration in Boulder, Colorado. Even so, the newly ice-free world would merely be reverting to a familiar state. On this reading of the evidence, even the most drastic climate catastrophe would have little chance of pushing the Earth's physical systems into uncharted territory.
Not so, says James Hansen, director of NASA's Goddard Institute for Space Studies. He argues that past episodes are a poor guide to what will happen in the future, for the simple reason that the sun is brighter now than it was then. Add that to the mix and the release of methane hydrates could lead to catastrophic, unstoppable global warming - a so-called "Venus syndrome" (PDF) that causes the oceans to boil away and dooms the Earth to the fate of its broiling neighbour.
So much for the Earth itself - what of life? If Hansen is right, Earth is heading for sterility. But if the lesser scenario plays out instead, it's a very different story.
Conservation biologists say we may already be in the midst of an extinction event that could potentially turn into one of the greatest mass extinctions ever - one that would alter the trajectory of evolution.
Oddly enough, the climatic turmoil of the thermal maximum led to very little loss of biodiversity. "Nobody has ever picked the Palaeocene-Eocene boundary as a major extinction interval. It's not even in the second tier," says Scott Wing, a palaeobotanist at the Smithsonian Institution in Washington DC. Instead, the fossil record shows that species simply migrated, following their preferred climate across the globe.
Today, of course, that is often not possible because roads, cities and fields have fragmented so many natural habitats. Polar and alpine species may find their habitat vanishes entirely, and this is not to mention all the other ways people imperil species.
"We're a perfect storm as far as biodiversity is concerned," says David Jablonski, a palaeontologist at the University of Chicago. "We're not just overhunting and overfishing. We're not just changing the chemistry of the atmosphere and acidifying the oceans. We're not just taking the large-bodied animals. We're doing all this stuff simultaneously." Even so, Jablonski thinks humans are unlikely to be capable of causing an extinction comparable to the one at the end of the Permian, 251 million years ago, when an estimated 96 per cent of all marine species and 70 per cent of terrestrial ones bit the dust.
Whether the Anthropocene mass extinction eventually ranks with the Permian or with lesser ones, it would still reshuffle the evolutionary deck. Once again, the past gives us some idea of what we could expect.
The fossil record tells us that every mass extinction plays out differently, because each has its own unique causes. However, there is one common factor: the species at greatest risk are those with the narrowest geographic ranges. Jablonski's studies of fossil marine snails show that species with planktonic larvae - which disperse widely - fare better than species with a more restricted distribution (Science, vol 279, p 1327).
Cockroach world
Add to that massive habitat disturbances, says Jablonski, and a picture emerges of life after the Anthropocene extinction. Small body sizes, fast reproductive rates and an ability to exploit disturbed habitats will all prove advantageous. "It's a rats, weeds and cockroaches kind of world," says Jablonski.
The wave of extinctions is likely to sweep through species in a fairly predictable way. "First we would probably lose the species that are already endangered, then it would work its way down," says Barnosky. "Eventually it would hit some of the species that we don't consider at risk today - for example, many of the African herbivores that today seem to have healthy populations."
However, predictions about the fate of any particular species are almost impossible, as luck will also play a role. The survivors will probably be a more-or-less random selection of weedy plants and opportunistic animals, notes Doug Erwin, a palaeobiologist at the Smithsonian Institution.
If the Anthropocene does end with a mass extinction, the fossil record tells us a lot about what the recovery might look like. Whether the news is good or bad depends on your perspective. "Recoveries from mass extinctions are geologically rapid, but from a human point of view grindingly long. We're talking millions of years," says Jablonski.
Recoveries from mass extinctions are geologically rapid, but from a human point of view grindingly long. We're talking about millions of years
Immediately after a mass extinction, the fossil evidence suggests that ecosystems go into a state of shock for several million years. For many millions of years after the Permian extinction, for example, marine environments the world over were dominated by the same 25 or 30 species. "It's pretty boring," says Erwin.
Something similar happened on land after the Cretaceous extinction. Pre-extinction plant fossils from western North America testify to flourishing ecosystems, with a variety of insects feeding on a wide assortment of plants. After the extinction, though, both plant and insect diversity drops dramatically, with some insect feeding methods vanishing almost completely.
After that, confusion reigns for 10 million years. There are fossil assemblages with only a few insects and plants, ones with many insects but few plants, others with many plants but few insects - just about everything except what ecologists would call "normal" (Science, vol 313, p 1112). "At no time did we have what I would call a healthy ecosystem, with diverse insects feeding on diverse plants," says Wilf. All the while biodiversity remains low, with few new species evolving. "You're just trying to hang on," says Erwin.
A study of marine fossil diversity bears this out. Nearly a decade ago, James Kirchner of the University of California, Berkeley, and Anne Weil of Duke University in Durham, North Carolina, took a database of all known marine fossils and used it to work out how closely peaks of speciation follow peaks of extinction (Nature, vol 404, p 177). "We went into this thinking, like everybody else, that when you have an extinction, you begin repopulating almost immediately," says Kirchner, now at the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf. Instead, they found that speciation peaks lagged about 10 million years behind extinction peaks. "We pretty much fell out of our chairs," he says.
In fact, for the first few million years after an extinction the speciation rate actually falls. "That suggests to us a sort of wounded biosphere. Extinction events don't just remove organisms from an ecosystem, leaving lots of opportunity for new species to diversify. Instead, what we think happens is that the niches themselves collapse, so you won't have new organisms emerging to occupy them. The niches themselves don't exist any more," says Kirchner.
Eventually, though, evolution wins the day, and after a few tens of millions of years biodiversity rebounds. Sometimes, as after the Ordovician mass extinction 440 million years ago, the new regime looks a lot like the old one. But more often a new world emerges. "You're not re-establishing the old chessboard, you're designing a whole new game," says Erwin.
In the Permian, the oceans were dominated by filter-feeding animals such as brachiopods and sea lilies, which lived their whole lives attached to the bottom. Predators were rare. All that changed after the extinction, leaving a more dynamic and richer ecosystem. "From my point of view, the end-Permian mass extinction was the best thing that ever happened to life," says Erwin.
In a perverse way, then, the bottom line is an encouraging one. Even if we manage to overpopulate and overconsume ourselves back to the Stone Age, the Earth will probably survive. Life will go on. By the time the long tail of the Anthropocene is over, what little was left of humanity will probably be gone. A new geological age will dawn. Shame there won't be anybody around to give it a name.
Bob Holmes is a consultant for New Scientist based in Edmonton, Canada
Source: New Scientist
