The Earth's atmosphere recorded the huge decline in the population of the Western Hemisphere in the 150 years following the arrival of Columbus from Spain in 1492. Soldiers, officials, settlers and merchants from Eurasia and slaves from Africa unwittingly introduced common diseases such as smallpox (天花)，measles (麻疹)and influenza to which the inhabitants of the Americas possessed no immunity. Scholarly estimates of the total number of deaths from disease vary widely, but the number may have exceeded 50 million and certainly wiped out 75% or more of native Americans. This rapid depopulation of the hemisphere allowed forests to grow in former farmlands. By 1610 the growth of all those trees had sucked enough carbon dioxide (CO2) out of the sky to cause a drop of at least seven parts per million in atmospheric concentrations of the most prominent greenhouse gas and start a little ice age.
Based on that dramatic shift, Simon Lewis and Mark Maslin, British ecologists working at University College London, believe 1610 should be considered the starting date of a new geologic epoch currently under discussion among earth scientists: the Anthropocene, or recent age of humanity. Lewis and Maslin dub the decrease in atmospheric carbon dioxide the ’’Orbis Spike" from the Latin for world, because since 1492 human civilization has progressively globalized. In a paper published this month in Nature, they argue that human impacts on the planet have been dramatic enough to warrant formal recognition of the Anthropocene epoch and that the Orbis Spike should serve as the marker of its opening.
The Anthropocene is not a new idea. As far back as the 18th century the first scientific attempt to lay out a chronology of Earth’s geologic history ended with a human epoch. By the 19th century the idea was commonplace, appearing as the Anthropozoic ("human life rocks") or the '*Era of Man" in geology textbooks. But by the middle of the 20th century, the idea of the Holocene (the term means "entirely recent" in Greek designates the most recent period, dating from 11,700 years ago, when the glaciers (冰河)of the last ice age receded) had come to dominate, recognizing humans as an important element of the current epoch, but not the defining one.
That idea is no longer adequate, according to scientists ranging from geologists to climatologists. Human impacts have simply grown too large: some scientists point to the flood of nitrogen (N) released into the world by the invention of the Haber-Bosch process
for wresting the vital nutrient from the air to support agriculture, others emphasize the fact that modem people now move more earth and stone than all the world’s rivers put together.
Researchers have advanced an array of proposals for when this putative new epoch might have begun. Some link it to the start of the mass extinction of large mammals （卩甫孚L 动物）such as woolly mammoths and giant kangaroos some 50,000 years ago or the advent of agriculture around 10,000 years ago. Others say the Anthropocene is much more recent and to the beginning of the uptick in atmospheric CO2 concentrations after the invention of an effective coal-burning steam engine.
The most prominent current proposal connects the dawn of the Anthropocene to that of the nuclear age: long-lobed radionuclide （放射性同位素）leave a long-lived record in the rock. The boom in human population and consumption of everything from copper to maize （玉米）after 1950 or so, known as the "Great Acceleration", roughly coincides with this nuclear marker. So does the advent of plastics and other remnants of industrial society, dubbed ”technofossils" by Jan Zalasiewicz of the University of Leicester, the geologist who heads the group that is pushing for addition of the Anthropocene to the standard geologic time scale. The radionuclides can then serve as what geologists call a Global Stratotype Section and Point, more commonly known as a “golden spike.” Perhaps the most famous such golden spike is the thin layer of iridium（Ir） a found in pock exposed near El Kef, Tunisia; it pinpoints the asteroid impact that terminated the age of the dinosaurs and ended the Cretaceous about 65 million years ago.
Lewis and Maslin reject this radionuclide spike because it is not tied to a *'world- changing event"--at least not yet—although it is a clear signal in the rock. On the other hand, their Orbis Spike in 1610 reflects both the most recent CO2 low point and the redistribution of plants and animals around the world associated with the Age of Discovery and the rise of world empires, a true changing of the world. Much like the golden spike that marks the end of the dinosaurs, the proposed Orbis Spike itself would be tied to the low point of atmospheric CO? concentrations around 1610, as recorded in ice cores, where tiny trapped bubbles betray past atmospheres. Further geologic evidence will come from the appearance of maize pollen in sediment cores taken in Europe and Asia at that time, among other indicators that will complement the CO2 record. Therefore, scientists looking at ice cores, mud or even rock will find this epochal shift in the future.
The CO2 drop coincides with what climatologists call the Little Ice Age. That cooling event may have been tied to regenerated forests and other plants growing on some 50 million hectares of land abandoned by humans after the mass death brought on by disease and warfare, Lewis and Maslin suggest. And it wasn't just the death of millions of aboriginal Americans. The enslavement （or death） of many millions of Africans for labor in the new lands may also have added to the climate impact. The population of the regions of western Africa most affected by the New World slave trade did not begin to recover until the end of the 19th century. In other words, from 1600 to 1900 areas of that region may have been regrowing forest, enough to reduce atmospheric CO?, just like the regrowth of the Amazon and the great North American woods, although this hypothesis remains in some dispute.
However delimited, the new designation would mean we are living in a new
Anthropocene epoch, part of the Quaternary period, which started more 2.5 million years ago with the advent of the cyclical growth and retreat of massive glaciers. The Quaternary is part of the Cenozoic ("recent life”)era that began 66 million years ago and is, in turn, part of the Phanerozoic ("revealed life") eon, which started 541 million years ago and encompasses all of complex life that has ever lived on this planet. In the end, the Anthropocene might supplant the Holocene. "It is designated an epoch, unlike other interglacial, because back in the 18th century geologists thought humans were a very recent species, arriving via divine intervention or evolving on Earth in the Holocene,*' Lewis argues, but scientists now know Homo sapiens arose more than200,000 years ago in the Pleistocene epoch. "Humans are a Pleistocene species, so... calling the Holocene an epoch is a relic of the past." Maslin suggests downgrading the Holocene to a stag within the Pleistocene, like other interglacial spans in the geologic record. But Zalasiewicz disagrees with this bid to get rid of the Holocene. *'I don’t see the need," he says, "systematic tracing of a Holocene-Anthropocene boundary globally would be illuminating in all sorts of ways."
1.The scientists cited in this article agree that humans have become a major factor in the history of Earth. As the article highlights, what they disagree on is（ ）
2.European penetration of the New World in the century and a half after 1492 led to（ ）
3.The decline in atmospheric carbon dioxide that reached its low point in or around 1610 was（ ）
4.The scientific hypothesis that humanity can alter the history of Earth in highly significant ways（ ）
5.The Orbis Spike proposed by Lewis and Maslin would be most reliably located in（ ）
6.Radionuclides can be likened to iridium（ ）
7.Some scientists think that radionuclides are the key to defining the Anthropocene because（ ）
8.Professor Zalasiewicz probably favors retaining the distinction between the Holocene and the Anthropocene because tracing its boundary（ ）
