According to a recent paper, human actions may have caused Earth's climate to warm much earlier than previously expected. In an article to be published in Geophysical Research Letters, and widely reported in the media, around 15,000 years ago, early hunters were a major factor in driving mammoths to extinction. Supposedly, this die-off had the side effect of heating up the planet. This is an interesting conjecture, since a letter just published in Nature Geocience reaches the opposite conclusion regarding climate and the mammoths' decline. This mammoth confusion illustrates the uncertain and even contradictory evidence that abounds in climate science.
In a new study, “Biophysical feedbacks between the Pleistocene mega-fauna extinction and climate: The first human-induced global warming?,” Chris Doughty, Adam Wolf, and Chris Field—all from the Carnegie Institution for Science—present an hypothesis explaining how neolithic hunters triggered global warming thousands of years before the invention of agriculture. You might recognize Field as the co-chair of the IPCC working group on impacts, adaptation and vulnerability. Here he participates in what The Economist called “some serious boffinry.”
Supposedly, the demise of leaf-chomping woolly mammoths at the hands of Homo sapiens contributed to the spread of dwarf birch trees in and around the Arctic. This proliferation of previously suppressed birch trees darkened the largely barren, reflective landscape and accelerated temperature rise across the polar north.
The northward spread of vegetation affected the climate because of the albedo effect: white snow and ice was replaced with darker land surfaces that absorbed more sunlight and created a self-repeating warming cycle. In the scenario proposed by Doughty et al., the process would have added to natural climate change, making it harder for mammoths to cope, and helping the birch spread further.
A human getting ready to change the climate.
“A lot of people still think that people are unable to affect the climate even now, even when there are more than 6 billion people,” says lead author Doughty, “even when we had populations orders of magnitude smaller than we do now, we still had a big impact.” Of course, the end of the last glacial period was already under way when the extinction of woolly mammoths began. The deglaciation was marked by a worldwide rise in temperatures and the dramatic retreat of glaciers that once covered much of the Northern Hemisphere, but this did not happen in a smooth upward shift in temperature.
In a letter to the Nature Geoscience, Felisa A. Smith, Scott M. Elliott and S. Kathleen Lyons comment on the same human impact on the large animals of the late Ice Age. In “Methane emissions from extinct megafauna,” Smith et al. reach a very different conclusion than Doughty and colleagues. Herbivores produce methane as a by-product of fermentation during digestion. Today, enteric emissions by domestic livestock are an important contributor to greenhouse gas concentrations, representing ~20% of annual methane emissions. Given that methane is ~30 times more potenet a GHG than CO2, gas from large wild animals could have a significant impact on climate. According to the authors:
About 13,400 years ago, the Americas were heavily populated with large-bodied herbivores such as mammoths, camelids and giant ground sloths; the megaherbivore assemblage was richer than in present-day Africa. However, by 11,500 years ago and within 1,000 years of the arrival of humans in the New World, 80% of these large-bodied mammals were extinct. The eradication of megafauna had marked effects on terrestrial communities, including changes in vegetative structure and reorganization of food webs. Here, we suggest that the extinction also had profound effects on methane emissions and atmospheric methane concentrations, with potential implications for abrupt climate change during the Younger Dryas cold event.
Smith et al. calculated the amount of methane that would have been released by the large herbivores around the time humans arrived in America and found that the elimination of those creatures at the hands of paleo-hunters could have led directly to global cooling. Supporting their hypothesis, the letter authors note that ice-core records of methane concentration reveal an abrupt drop at the onset of the Younger Dryas cold event, about 12,800 years ago. The drop seems to be in sync with the extinction of New World megafauna (see the figure below).
Not that the impact of either scenario would have been that dramatic. “We're not saying this was a big effect,” Chris Field of the pro-warming study said, “about 0.2 degrees C (0.36 degrees F) of regional warming is the part that is likely due to humans.” They qualify their results, saying that “the point of the paper isn't that this is a big effect. But it's a human effect.”
Even so, Field and colleagues argue, the evidence of an even earlier human-made global climate impact suggests the Anthropocene could have started much earlier. The Anthropocene is the controversial name some scientists give to the time of humanity's overwhelming impact on nature (see “A Brave New Epoch?”). The authors' results, “suggest the human influence on climate began even earlier than previously believed, and that the onset of the Anthropocene should be extended back many thousands of years.”
Smith et al., are a bit more assertive, stating “the 185 to 245 ppbv methane drop observed at the Younger Dryas stadial is associated with a temperature shift of 9 to 12 °C.” But like all good scientists, they too are hedging their bets: “The attribution and magnitude of the Younger Dryas temperature shift, however, remain unclear. Nevertheless, our calculations suggest that decreased methane emissions caused by the extinction of the New World megafauna could have played a role in the Younger Dryas cooling event.”
Did the spread of darker vegetation, made possible to the elimination of browsing mammoths and other large herbivores, cause global warming or did the elimination of enteric methane emissions, also due to the demise of ice age megafauna, cause the abrupt cooling of the Younger Dryas? Interestingly there is a chance that both hypotheses are correct.
Given that methane has a relatively short lifespan in the atmosphere, on the order of 20 years, it is possible that the cooling effect postulated by Smith et al. could have taken place fairly rapidly, leading to cooling over the short term. The change in albedo due to changing vegetation could have take longer to transpire, contributing to the general warming trend after the Little Dryas cold snap. Of course, both ideas may be wrong or the magnitude of their impact on climate negligible.
There is overwhelming evidence that the Little Dryas was caused by a disruption of the Atlantic thermohaline circulation, probably due to a massive influx of glacial melt water from the North American ice sheet. The trigger for this was most probably changes in insolation due to Earth's orbital cycles. And, given that thawing permafrost soils release large volumes of methane, CO2 and nitrous oxide (see “High nitrous oxide production from thawing permafrost,” in the May 2010, Nature Geocience), the lack of digestive emanations from the departed megafauna may have been quickly compensated for.
So who is correct? Sorry, no consensus has been reached—and herein lies an important lesson for all those following the anthropogenic global warming controversy.
Was the climate altered by lack of grazing or lack of gas?
This blog previously reported on the unsettled science regarding the extinction of the dinosaurs at the end of the Cretaceous Period, some 65 million years ago. In “Chicxulub Redux: A Lesson In How Science Works,” it was revealed that scientists still cannot agree about what killed the dinosaurs and caused one of the greatest mass extinctions in Earth's history. One might conclude that 65 million years was a long time ago and that some confusion is to be expected. But, remember, this was the most recent extinction event, and it is well documented in the rock record. Still, scientists cannot come to agreement on the events that transpired.
The events of the deglaciation that led to the Holocene warming are much more recent, lying only 10-15 thousand years in the past. Yet, much like the controversy surrounding the dinosaur extinction, scientists cannot agree what happened during the most recent transition from glacial to interglacial conditions. Was it caused in part by the demise of the mammoths and other megafauna? It isn't even certain that humans were primarily responsible for the extinction of the mammoths and their companions. If science cannot tell us what happened in the recent past, events that led to our current temperate climate, why should be believe predictions of future conditions 100 or a 1000 years from now?
As these examples show, two teams of scientists can study the same chain of events and then publish scholarly papers in respected journals in which diametrically opposed conclusions are reached. The facts regarding anthropogenic global warming are at least as contradictory and unclear. This is why the proclamations by the IPCC should be taken with a small grain of salt. The simple truth is that science, and climate science in particular, does not know all the answers. An honest scientist will admit that progress means going from being wrong to being less wrong, and that little in science is really settled.
Scientists do not agree about the extinction of the dinosaurs 65 million years ago, what effect the disappearance of the late ice age megafauna had on climate 12,000 years ago, or even how CO2 is sequestered in Earth's oceans today. What triggered the end of the last glacial period is hotly debated in scientific circles, yet we are told there is unanimity of opinion, a consensus with regard to climate change.
“Don't blame me, I'm outa' here.”
Scientists are a bunch of bickering, willful and self-deluding individuals, just like the rest of us. If you have any doubt, witness the attempts to belittle skeptics by AGW believers. Climate science has devolved into the politics of personal destruction and the primary reason the debate has grown so nasty is because the science is so scanty, the evidence so lacking.
There are no authoritative answers regarding the dinosaurs' downfall or the disappearance of late ice age megafauna and how that event affected climate. And to blame our ancient ancestors for global warming or cooling more than 10,000 years ago smacks of hubris and self-deception. If climate science cannot accurately identify the causes of climate change in the past there is no chance it can predict the future. As a scientist, I will wait for more convincing evidence about the dinosaurs, the mammoths and global warming.
Be safe, enjoy the interglacial and stay skeptical.
[ Thanks to Pat Kerr, one of our Canadian readers, for suggesting this topic for an article. I appreciate feedback on our books and the web site, and welcome suggestions for future article topics. ]