A third theory says that because ice ages are dusty, more dust blows into the ocean when glaciers are at maximum. This dust is rich in nutrients, particularly iron, which fertilizes the growth of plankton. That removes carbon dioxide from the water, then sequesters it in the depths when the plankton die and sink to the bottom. Deliberately dusting the ocean with iron (a key nutrient) has even been proposed as a method of fighting global warming, but in a pilot-scale test, called the Southern Ocean Fertilization Experiment (SOFeX), it looked to be impractical.[7]
[Footnote 7: There were several articles on this topic in the April 16, 2004 issue of Science.]
All three mechanisms would produce CO2 levels that fluctuate cyclically with the glaciers: exactly what the ice cores show. Thus, carbon dioxide, like methane, should have reached a peak 11,000 years ago and dropped ever since. But it, too, dropped for only the first part of that cycle, then started to rebound—so much so that at the start of the industrial age, the level was already 15 percent too high.
Other scientists have posited a variety of natural theories for this reversal. One is that changes in ocean chemistry are causing the seas to disgorge large quantities of previously absorbed carbon dioxide. Another is that it is due to a natural decline in forests, which remove CO2 from the air to form branches, leaves, bark, and roots. But Ruddiman again suggests that humans might be the cause.
Studies of pollen particles trapped in lakebed sediments allow scientists to trace the spread of wheat, peas, lentils, flax, and barley across regions that were naturally forest. They reveal that as far back as 10,000 years ago, people were beginning to cut down forests to make room for farming. These bogs, Ruddiman says, also reveal increasing levels of sun-loving weeds from cleared land, plus soot from slash-and-burn agriculture.
These facts may have been overlooked by climate modelers, but Ruddiman discovered that they are well known to historical geographers. In 1989, Ian G. Simmons of the University of Durham, England, wrote that by 2,000 years ago, large segments of Southeast Asia, China, Southwestern Asia, and the Mediterranean region were “greatly” deforested. And in a 2003 book, Deforesting the Earth, Oxford geography professor Michael Williams reported that humans were already cutting down European forests 6,000 years ago.
Even North America was affected. As far back as 7,000 years ago, Williams wrote, Native Americans were clearing forests in the Mississippi River Valley to plant squash, sunflowers, maize, and beans.
By the time of the Roman and Chinese empires, the effect had become quite pronounced. “Most of Eurasia was deforested by the time of Christ,” Ruddiman said in his 2003 lecture.
In an effort to quantify the amount of preindustrial deforestation, Ruddiman turned to the Domesday Book, a census of Britain conducted by William the Conqueror in 1086 AD. In addition to counting people, William's census-takers tallied the extent of forests, fields, and pastures. According to figures in the Domesday Book, the 1.5 million people then living in England had already cut down 85 percent of their nation's trees.
Extrapolating these per-capita land-clearing figures to the 57 million people living in China a thousand years earlier, plus the 140 million more in India, Southeast Asia, and the Roman Empire, Ruddiman calculates that 2,000 years ago, tree-cutting had released 700 to 900 billion tons of carbon dioxide to the air—enough to offset the natural decline and start driving levels of the gas back up again, thousands of years before anyone was using significant quantities of oil.[8]
[Footnote 8: Our ancestors did use peat. And by 3,000 years ago, the Chinese had discovered coal. In his book, Ruddiman estimates that emissions from these could have put another 120 billion tons of carbon dioxide into the air. Note, by the way, that I have stated these figures as tons of carbon dioxide. In academic books and papers, the same figures are often given as tons of carbon. A ton of carbon dioxide contains only]
Ruddiman's bottom line: All those years ago “humans were doing things at a scale that can explain why the natural trends went haywire."
He backs up his tree-cutting theory by pointing to several dips in the atmospheric level of carbon dioxide that occurred over the past 2,000 years. None was large—only a few parts per million—but they appear to be too much to be explained by natural factors such fluctuations in the rate of volcanic emissions.
One dip occurred during the late years of the Roman Empire. Another was in the 1400s, and a third was between 1500 AD and 1750 AD. All three, Ruddiman says, link to periods when plagues killed off sizeable fractions of the world's population.
The first occurred at a time when bubonic plague killed 20 million people in China and the Roman Empire: roughly one-tenth of the world's then-population. The second correlates to the Black Death, which killed one-third of the people of Europe in its first year alone. The third was during an era when 90 percent of the 50 million to 120 million people living in Central and South America died of smallpox, measles, and other European diseases: the single largest mass mortality in history.
When that many people die, farms are abandoned, and trees grow back quickly enough to take significant amounts of carbon dioxide back out of the air. Historical accounts of the Black Death, Ruddiman says, are full of stories about millions of abandoned farms. “These accounts don't give numbers of farms or acreage,” he said, “but it's immense."
Another intriguing aspect of these plagues is that the last one more or less coincides with an era called the Little Ice Age. During the heart of that period, from about 1550 to 1850, northern climates saw a temperature drop of about one or two degrees. That may not sound like a lot, but it allowed glaciers to surge in Alaska and froze the canals of Holland memorably enough that the Dutch are still speedskating fanatics. Could the Little Ice Age have represented the Earth's attempt to return to its normal cooling trend, thanks to the reduction in human-caused CO2? If so, a lot of low technology alternate-history books and fantasy novels need to be rewritten to include more ice and snow.
* * * *
Bye-bye Ice Age
The timing of these wiggles adds yet another line of support to Ruddiman's claim that land clearing was the driving force behind the preindustrial increase in carbon dioxide. Over the course of 8,000 years, he says, enough of the Earth was deforested to raise CO2 levels by 40 ppm over what they “should” be.[9]
[Footnote 9: That 40 ppm is comprised of a 20 ppm actual rise, plus 20 ppm of normal drop that]
Combined with the increase in methane, Ruddiman argues, that's enough to warm the Earth by about 1.4 degrees F—roughly the same amount that industrial-era emissions are believed to have warmed it to date (but not as much as today's emissions are expected to warm it in the future).
At the start of his 2003 lecture, Ruddiman announced that he would present four “outrageous propositions.” So far, we've discussed three:
1. Several thousand years ago, atmospheric levels of methane and carbon dioxide started an upswing that is contrary to their normal cycles.
2. These changes were caused by puny, preindustrial humans.
3. Humans have had twice as much effect on climate as was previously believed. (The unrecognized half was before the Industrial Revolution. The other half is modern.)
His fourth claim is the true kicker. “The most in-your-face statement I can make is that humans stopped a glaciation,” he said. “And I think there's a strong case that can be made for that."
A 1.4 degrees warming may not sound like much, but (as with all climate-change scenarios) the effects are magnified at high latitudes. They're strong enough, he argued, that climate models show that if people hadn't irrigated rice and cut down so many trees, huge areas of North America would see mean annual temperature decreases of 5 degrees F to 7 degrees F. The result would be year-round snow cover in Canada's Baffin Island, and eleven-month winters in the Labrador highlands: the two areas from which prior glaciations appear to have originated.
If anything, the models Ruddiman used to calculate these effects may have understated the imp
act. That's because they weren't sophisticated enough to take account of climate-driven changes in arctic vegetation.
By comparing old photos with present landscapes, researchers in Alaska have noted that today's warming trend has produced significant vegetation shifts, most notably a dramatic increase in the prevalence of woody shrubs in lands that previously were tundra. This changes the amount of solar heating. In an article published in the September 7, 2005 issue of the Journal of Geophysical Research—Biogeosciences, Matthew Sturm of the U.S. Army Cold Regions Research and Engineering Laboratory and colleagues reported that the dark branches of these shrubs, protruding above the snow, absorb a lot more sunlight than do low-lying tundra grasses. In his experimental plots, Sturm discovered that spring melting began several weeks earlier in shrubby regions than in unbroken tundra.[10]
[Footnote 10: The effect is complicated by the fact that patchy snow may linger in shaded areas beneath the densest vegetation. Still, increasing shrubbiness substantially increases overall solar heating.]
During a cooling period, the same factors would work in reverse. Rather than expanding their range, shrubs would retreat. The same would happen to evergreen forests, whose dark needles also absorb a lot of sunlight. These vegetation shifts would amplify the effects found by Ruddiman's climate model, quite possibly by enough to produce incipient glaciers in Labrador, as well as Baffin Island. Whether these glaciers would now be spreading south remains an open question—and a fruitful topic for an alternate history story.
* * * *
Latest Cores
It will be years before scientists can be sure whether Ruddiman's theory is correct. Shortly after he unveiled it, I talked to Ralph Keeling, a professor of geochemistry at Scripps Institution of Oceanography in La Jolla.
"At some level,” Keeling said, “it seems inevitable that early agriculture would have had an impact on the atmosphere. The question is simply how big.” But he added that confirmation of Ruddiman's theory would require the drilling of ice cores going back more than 400,000 years.
The problem was that the Antarctic ice cores on which Ruddiman was relying only went back far enough to capture the three most recent repetitions of the 100,000-year cycle. And (in one of those Murphy's Law, “of course” realities) the one just before that turns out to be the one in which the orbital parameters were most akin to today's. If that earlier era showed the same methane and CO2 anomalies we see in the current cycle, then the cause is natural and Ruddiman's theory goes down the drain.
Luckily, it turned out that there were a few feet at the bottom of the old cores that hadn't previously been studied due to difficulties in figuring out their age. That problem was resolved while Ruddiman was writing his book, allowing the chronology of methane and carbon dioxide levels to be pushed back just barely far enough to get him the information he needed. The result: a slight change in the numbers, but confirmation of his overall hypothesis.
This brings us full circle to Weiss's theory of the early Mesopotamian civilizations. The 6200 BC cold snap—called the 8.2 Kya event by geophysicists—was an anomaly that had nothing directly to do with global climate change. But it did much to boost farming-fed civilizations by creating that ancient world's most powerful kingdoms. And that, in turn, instituted a long-term shift in climate.
Except during the cold snap, the Earth was warmer then than now, but steadily cooling. Since then, we've had one of the most stable climate periods in the last several million years. Anthropologists have long pointed to this as a fortuitous circumstance that helped prevent civilization from being erased by the next major climate change. But if Ruddiman is correct, this stability wasn't the result of some nicely timed Earth process, but rather the result of two offsetting factors: the Earth's slow, natural cooling, and the human-caused buildup in greenhouse gases. Thus, while the Earth was trying to enter a new ice age, it did not, and except for a few minor blips such as the Little Ice Age and the drought that may have toppled Egypt's Old Kingdom, nothing truly untoward happened for 8,000 years.
For science-fictional world builders, this raises all kinds of interesting questions. What if the two rates of change hadn't been so nicely balanced? Could the Romans have coped with an ice age? What would have happened if the world had been in a warming trend when farming was discovered, rather than a cooling one? Then, rather than offsetting, the two factors might have reinforced each other—and melting Antarctic and Greenlandic glaciers might have forced many low-lying civilizations to continually seek higher ground.
Now, the human factor is overwhelmingly powerful. As of 2006, the carbon dioxide level has overshot anything the Earth has seen since the dinosaurs and is heading off into what Ruddiman calls “terra incognita."
What exactly this entails is open for debate, but in the final chapters of his book, Ruddiman poses an interesting argument. The next few centuries might be a bit warm, he suggests, but eventually we'll run out of coal and oil. Soon enough (geologically speaking) the atmosphere will start purging itself of the extra greenhouse gases ... and we'll still be in the present orbital cycle...
Get the book. It's one of the most intriguing climate hypotheses to come along in years.
Copyright (c) 2007 Richard A. Lovett
[Back to Table of Contents]
DON'T KILL THE MESSENGER by Kim Zimring
Sometimes packaging is everything....
The sign said, “Found: Small Gray Alien with Velvet Eyes. If Lost, please Call...” and then it gave a local number.
Dr. Albert Finchi considered this as he looked up at the flyer roughly stapled to the telephone pole. He had spent his life at SETI waiting for some sign of sentient life, had listened to a hundred thousand beeps and pings of possibility, and had lately decided that he was fated to retire unsuccessful.
He tilted his head and considered the photo at the bottom of the flyer. It showed something small and odd, that was true, but the picture wasn't good. It was a prank, most likely, or possibly an ugly cat.
But what was a single phone call when his whole career had been built on a slim-to-nothing chance? He took the flyer down, flipped his cell phone open, and called the number.
It rang; the voice on the other end was male, older sounding, and somewhat thin and reedy. Yes, there was an alien, the voice said. He had found it in the park, crying. He gave it milk and cat food, but it wouldn't eat. No, he didn't have it anymore. A Mrs. Everett had come by and adopted it.
Dr. Finchi found himself lying. Generally speaking, it was out of the ordinary for him and he certainly felt bad, but still, if there was any chance at all ... He found himself saying things like “my alien” and “absolutely irreplaceable” until the voice coughed up an address.
He arrived at Mrs. Everett's a few minutes ahead of the TV cameras and several hours ahead of the government.
It weighed about two pounds. It was gray and slightly fuzzy; it was burbly and cute with large and softly textured eyes, and it was quite unmistakably an alien.
Finchi couldn't resist picking it up, even as sensible precautions about quarantines flashed through his head. Nothing had happened to Mrs. Everett, though, and she had had it for over a day, and the man who had found it was obviously alive and talking too.
The media loved it. It sat in his arms and cooed up at him, and in the end he was glad he'd ignored those fears, partly because the alien turned out not to have so much as a flea upon it, and partly because by the time the government took over, he was too well-established as the Resident Expert on Aliens to get completely kicked to the curb.
Six months later he was still excited, even though the linguists were confounded. The best and the brightest, from all around the world, and not one of them could get the alien to say so much as “hi” in its own or any other language. A few months after that and the consensus seemed to be that it didn't have the capacity for language, which puzzled Finchi.
They had found the creature's (tiny) spaceship, after all, and it was clearly the work of a technological
ly advanced race, although they were at a loss to explain why it was completely automated. Still, it wasn't the type of thing that was likely to have been built by a species with no ability to communicate.
In any case, after that they stopped trying to obtain informed consent and just popped the alien into a CT scanner, which it bore with warbling good grace.
It was a good thing they didn't start with an MRI, Finchi thought when he saw the results. There was something inside the little alien, something egg-shaped and metallic, something that beeped softly if you listened on the right frequency.
Something clicked for him. Finchi called a meeting, the first he'd personally put together, and he showed them slides about the Artifact Hypothesis. Robert Freitas had proposed it, saying that maybe SETI was wrong, that they shouldn't be looking for electromagnetic signals when they searched for sentient life. Said that maybe they should be looking for things, physical objects, and the most recent research said that the object wouldn't have to be that big. You could pack a world of information inside a small package, like a metal egg for instance.
With that, the people in charge decided to cut it open. The consensus now seemed to be that the alien was nonsentient; it would peep and burr and snuggle, but it was never going to talk, not their language or any other. It fit with what they had seen of the automated vessel, after all—mice and dogs and chimps had gone into space, but they didn't fly the ship.
Finchi wasn't so sure that this was a good idea. It just wasn't making sense to him—why would the intelligent aliens put their artifact inside a living creature? Plus, the beastie seemed like, well, like such a pet to him. It couldn't talk, but it certainly liked company.
He was there when they took it to a veterinary operating room, nice and sterile at least, and he watched as they hooked up the monitors that tracked the beeps and pings of the egg inside it.
Analog SFF, April 2007 Page 9
