Paleobiologist Anthony D. Barnosky and his colleagues reached this conclusion after review of studies of the extensive large mammal, or megafauna, extinctions that occurred in the late Pleistocene, when animals such as mammoths and mastodons, the saber-toothed cat, ground sloths and native American horses and camels went extinct.
In the forensic quest for who done it, many have pointed fingers squarely at humans.
But in a review appearing in the Oct. 1 issue of Science, Barnosky and his colleagues conclude that climate change also played a big role in driving these extinctions.
Barnosky's colleagues in the study are Paul Koch, professor of earth sciences at UC Santa Cruz; Scott Wing, a paleobotanist in the Department of Paleobiology at the Smithsonian Institution's National Museum of Natural History; UC Berkeley graduate student Alan Shabel; and recent UC Berkeley Ph.D. Bob Feranec, now a post-doctoral fellow at Stanford University.
"There's been a lot of talk about people causing the extinction of the megafauna by killing everything they saw, like a blitzkrieg," said Barnosky. "But if you look at all the evidence, it's clear that while humans had a major role in these extinctions, in many cases climate change was a key part of the recipe.
"Humans and climate change were the one-two punch that drove extinction between 50,000 and 10,000 years ago, and the same thing is happening in a major way today."
Because climate change is occurring more rapidly today than even in the late Pleistocene, when the majority of megafauna went extinct, serious consequences for many large animal species that weathered the Pleistocene extinction could be just down the road, Barnosky said. And the impact could be even greater today because of impoverished large animal populations and surging populations of humans taking over former large-animal habitats.
"Human activities today, combined with climate change, probably are going to result in inevitable extinction of many more species and unpredictable ecosystem changes," he said.
The authors' warnings are based on a review of previous studies of Pleistocene animal extinctions around the world, from Australia to Europe to North America. The Pleistocene, a period starting about 1.8 million years ago, was a time of glacial comings and goings, with more than 20 cycles of cooling and warming that concluded only about 10,000 years ago with the end of the last ice age.
In previous studies of animal remains layered in caves in the American West, Barnosky has found that during some of the last few glacial/interglacial cycles between 1 million and 600,000 years ago, the number of small, medium and large mammals in a given community remained fairly stable, though different species may have filled the various ecosystem niches. In the late Pleistocene, however, something happened to make the number of large mammals nosedive continent-wide.
This new analysis of archeological, climatic, ecological and simulation studies shows that these extinctions happened around the world. Of more than 150 genera of megafauna - that is, animals weighing more than 44 kilograms (97 pounds) - living on Earth 50,000 years ago, at least 97 were extinct by 10,000 years ago. If you look at localized extinctions instead of global extinctions, 121 genera disappeared from at least one continent.
Those blaming humans ascribe the extinctions to human hunting, either through overkill - hunting that could have led to extinction in about 1,500 years - or through a "blitzkrieg" of hunting that could have knocked off a species in less than 500 years. Another suggestion, dubbed "sitzkrieg" after the term for a "sitting" war that shows slow or no progress, is that humans caused extinction through long-term habitat alteration.
Barnosky and his colleagues found sparse evidence outside Australia that humans were the sole cause of extinction. Data are sketchy for the Australian continent, Barnosky cautioned, but little climate change was going on at the time of extinction between 40,000 and 50,000 years ago. However, humans were certainly on the scene, and some scientists think that fires set by humans had as much to do with extinction as direct hunting. Over a few thousand years, Barnosky said, an extended sitzkrieg may have led to the extinction of large mammals such as kangaroos, wombats, the marsupial lion (Thylacoleo carnifex) and the largest ever marsupial, the 2 1/2-ton Diprotodon.
Elsewhere, human activities combined to a greater or lesser degree with climate change to lead to extinctions. In Europe and parts of Asia, mammals such as the giant Irish deer or Irish elk died out broadly toward the end of the late Pliestocene, in some areas before humans were present. Earlier, though, warm-adapted megafauna such as straight-tusked elephants (Palaeoloxodon) and hippos, which were abundant during preceding interglacials, disappeared with the cooling of the last ice age, starting around 45,000 years ago and persisting up to the height of the glacial period 20,000 years ago.
"This is a very clear case of climate-caused extinction without the significant input of humans," Barnosky said.
Similarly, in Alaska and the Yukon, the disappearances of short-faced bears, such as the grizzly-like Arctodus simus, the largest land carnivore ever to inhabit North America; mammoths; and two horse species occurred before apparent signs of human contact.
A second pulse of climate-caused extinctions began in Europe and Asia about 12,000 years ago as cold-adapted animals - the wooly rhino and the mammoth - died out with warming temperatures. But it's possible, Barnosky said, that the rise of modern humans - Homo sapiens sapiens - with their broad variety of tools and diverse diet, negatively impacted these animals to an extent not seen in Europe with earlier human species, such as Homo erectus and Neanderthals.
North America, in particular, is an example of a place where humans speeded the process of climate-caused extinction, in many cases by overkill. Evidence of mammoth kills date from near the first appearance of stone spearheads made by the human Clovis civilization 11,400 years ago. Only mammoths and mastodons have been found with incontrovertible evidence that they were killed by humans in North America, though human artifacts have been found in association with extinct megafauna fossils on all continents, including Africa.
Over a period of, at most, 1,500 years, following the appearance of Clovis-style hunters, camels and horses, rhinos and peccaries, short-faced bears and saber-toothed tigers, as well as the armadillo-like glyptodonts and the giant ground sloths (Megatheriadae), all disappeared from the North American continent.
"Humans and climate change came together at exactly the same time" to lead to these great megafauna extinctions, Barnosky said.
The case in South America is still muddled, he noted, but there, too, human incursions combined with climate change possibly coincided with the departure of large mammals, such as a variety of armadillos and llama- and camel-like animals, in a case similar to that in North America. In Africa, as well, it is unclear why any large mammals went extinct, since humans arose in concert with these animals and, by some arguments, they should have been in balance with one another. There, as in South America, the uncertainty comes from lack of data.
In these fairly recent large-mammal extinctions, Barnosky sees lessons for the future.
"Humans tend to impact the bigger animals, with the smaller animals as collateral damage," he said. "Climate change is just the opposite - it affects the little guy first and then, through them, the big guys. Today, we see humans taking out the bigger animals and climate change affecting the smaller animals, so we can expect to see some pretty dramatic changes in the ecosystem."
One major problem today is that, because of human encroachment, there are no refuges for animals that might want to relocate because of climate change.
"One thing we can do, as conservationists, is to create and connect natural areas" to allow animals to move around, he added. "Because species can no longer do this by themselves, maybe the solution is to do it for them."
The work was supported by the National Science Foundation.