Day5:The Anthropocene epoch:scie

Humanity’s impact on the Earth is now so profound(意义深远的) that a new geological epoch(地质时期）– the Anthropocene – needs to be declared, according to an official expert group who presented the recommendation to the International Geological Congress（国际地质大会）in Cape Town on Monday.

The new epoch should begin about 1950, the experts said, and was likely to be defined(肯定) by the radioactive elements（放射性元素）dispersed across the planet by nuclear bomb（核弹）tests, although an array of （一大堆）other signals, including plastic pollution, soot from power stations, concrete, and even the bones left by the global proliferation（全球繁衍） of the domestic chicken（驯养的鸡） were now under consideration（在考虑之中）.

The current epoch, the Holocene, is the 12,000 years of stable climate since the last ice age during which all human civilisation developed. But the striking acceleration（显著的增加）since the mid-20th century of carbon dioxide emissions（二氧化碳排放量） and sea level rise, the global mass extinction of species（物种灭绝）, and the transformation of land by deforestation（森林开伐） and development mark the end of （标志...的结束）that slice of geological time, the experts argue. The Earth is so profoundly changed that the Holocene must give way to（让位于）the Anthropocene.

“The significance of the Anthropocene is that it sets a different trajectory（轨迹） for the Earth system, of which we of course are part,” said Prof Jan Zalasiewicz, a geologist at the University of Leicester and chair of the Working Group on the Anthropocene (WGA), which started work in 2009.

“If our recommendation is accepted, the Anthropocene will have started just a little before I was born,” he said. “We have lived most of our lives in something called the Anthropocene and are just realising the scale and permanence of the change.”

Prof Colin Waters,principal geologist（首席地质学家）at the British Geological Survey and WGA secretary, said: “Being able to pinpoint an interval of time is saying something about how we have had an incredible impact on the environment of our planet. The concept of the Anthropocene manages to pull all these ideas of environmental change together.”

Prof Chris Rapley, a climate scientist at University College London and former director of the Science Museum in London said: “The Anthropocene marks a new period in which our collective activities（集体的活动）dominate the planetary machinery.

“Since the planet is our life support system – we are essentially the crew of a largish（大的）spaceship – interference with its functioning at this level and on this scale is highly significant. If you or I were crew on a smaller spacecraft, it would be unthinkable to interfere with the systems that provide us with air, water, fodder and climate control. But the shift into the Anthropocene tells us that we are playing with fire（玩火）, a potentially reckless mode of behaviour which we are likely to come to regret unless we get a grip（控制住自己）on the situation.” Rapley is not part of the WGA.

Martin Rees, the astronomer royal and former president of the Royal Society, said that the dawn of the Anthropocene was a significant moment. “The darkest prognosis（预测） for the next millennium is that bio, cyber or environmental catastrophes could foreclose（阻止） humanity’s immense potential, leaving a depleted biosphere,” he said.

But Lord Rees added that there is also cause for optimism. “Human societies could navigate these threats, achieve a sustainable future, and inaugurate eras of post-human evolution even more marvellous than what’s led to us. （学习一下动词搭配）The dawn of the Anthropocene epoch would then mark a one-off（一次性）transformation from a natural world to one where humans jumpstart the transition to electronic (and potentially immortal) entities, that transcend our limitations and eventually spread their influence far beyond the Earth.”

The evidence of humanity’s impact on the planet is overwhelming, but the changes are very recent in geological terms, where an epoch usually spans（持续）tens of millions of years. “One criticism of the Anthropocene as geology is that it is very short,” said Zalasiewicz. “Our response is that many of the changes are irreversible.”

To define a new geological epoch, a signal must be found that occurs globally and will be incorporated into（被收录进） deposits in the future geological record.For example, the extinction of the dinosaurs 66m years ago at the end of the Cretaceous epoch is defined by a “golden spike” in sediments around the world of the metal iridium, which was dispersed from the meteorite that collided with Earth to end the dinosaur age.

For the Anthropocene, the best candidate for such a golden spike are radioactive elements from nuclear bomb tests, which were blown into the stratosphere（喷发进平流层）before settling down to Earth. “The radionuclides are probably the sharpest – they really come on with a bang,” said Zalasiewicz. “But we are spoiled for choice. There are so many signals.”

Other spikes being considered as evidence of the onset of the Anthropocene include the tough, unburned carbon spheres emitted by power stations. “The Earth has been smoked, with signals very clearly around the world in the mid-20th century,” said Zalasiewicz.

Other candidates include plastic pollution, aluminium and concrete particles, and high levels of nitrogen and phosphate in soils,derived from（来自...）artificial fertilisers. Although the world is currently seeing only the sixth mass extinction of species（第六次物种大灭绝）in the 700m-year history of complex life on Earth, this is unlikely to provide a useful golden spikeas the animals are by definition very rare and rarely dispersed worldwide.

In contrast（相反）, some species have with human help spread rapidly across the world. The domestic chicken is a serious contender to be a fossil that defines the Anthropocene for future geologists. “Since the mid-20th century, it has become the world’s most common bird. It has been fossilised in thousands of landfill sites and on street corners around the world,” said Zalasiewicz. “It is also a much bigger bird with a different skeleton than its prewar ancestor.”

The 35 scientists on the WGA – who voted 30 to three in favour of formally designating the Anthropocene, with two abstentions（弃权）– will now spend the next two to three years determining which signals are the strongest and sharpest.

Crucially, they must also decide a location which will define the start of （标志着...的开始）the Anthropocene. Geological divisions are not defined by dates but by a specific boundary between layers of rock or, in the case of the Holocene, a boundary between two ice layers in a core taken from Greenland and now stored in Denmark.

The scientists are focusing on sites where annual layers are formed and are investigating mud sediments off the coast（沿岸）of Santa Barbara in California and the Ernesto cave in northern Italy, where stalactites and stalagmites accrete annual rings. Lake sediments, ice cores from Antarctica, corals, tree rings and even layers of rubbish in landfill sites are also being considered.

Once the data has been assembled, it will be formally submitted to the stratigraphic authorities and the Anthropocene could be officially adopted within a few years. “If we were very lucky and someone came forward with, say, a core from a classic example of laminated sediments（由薄片叠成的沉淀物）in a deep marine environment, I think three years is possibly viable,” said Zalasiewicz.

This would be lightning speed for such a geological decision, which in the past would have taken decades and even centuries to make.The term Anthropocene was coined（发明） only in 2000, by the Nobel prize-winning scientist Paul Crutzen, who believes the name change is overdue.He said in 2011: “This name change stresses theenormity （巨大的）of humanity’s responsibility as stewards of the Earth.”

Crutzen also identified in 2007 what he called the “great acceleration” of human impacts on the planet from the mid-20th century.

Despite the WGA’s expert recommendation, the declaration of the Anthropocene is not yet a foregone（预知的）conclusion. “Our stratigraphic colleagues are very protective of the geological time scale. They see it very rightly as the backbone of geology and they do not amend it lightly,” said Zalasiewicz. “But I think we can prepare a pretty good case.”

Rapley also said there was a strong case: “It is highly appropriate that geologists should pay formal attention to a change in the signal within sedimentary rock layers that will be clearly apparent to future generations of geologists for as long as they exist.The ‘great acceleration’ constitutes a strong, detectable and incontrovertible（无可辩驳的） signal.”

Evidence of the Anthropocene

Human activity has:

• Pushed extinction rates of animals and plants far above the long-term average. The Earth is on course to（将要）see 75% of speciesbecome extinct in the next few centuries if current trends continue.

• Increased levels of climate-warming CO2 in the atmosphere at the fastest rate for 66m years, with fossil-fuel burning pushing levels from 280 parts per million before the industrial revolution to 400ppm and rising today.

• Put so much plastic in our waterways and oceans that microplastic particles are now virtually ubiquitous（这个词之前遇到过哦，还记得吗。遍布的，到处都是的）, and plastics will likely leave identifiable fossil records for future generations to discover.

• Doubled the nitrogen and phosphorous in our soils in the past century with fertiliser use. This is likely to be the largest impact on the nitrogen cycle in 2.5bn years.

• Left a permanent layer of airborne particulates in sediment and glacial ice such as black carbon from fossil fuel burning.