Human beings are in the process of dramatically reshaping the Earth’s ecosystems. As far back as the 19th century, some scientists have noted that the current era is defined mainly by the impact of human activity. Now, there is an emerging consensus among Earth scientists that we have indeed entered a new period of geological time, the Anthropocene epoch.

Scientists who study the history of the Earth usually divide geological time according to major changes to the biology and climate of the Earth. For instance, the ancient Cambrian period, some 500 million years ago, is distinguished by a sudden explosion in the diversity of life, including the emergence of the ancestors of many modern species. More recently, the Pleistocene epoch, which ended about ten thousand years ago, is notable for the glaciers that swept over much of the Earth. The new Anthropocene epoch would be distinguished from all earlier times in Earth’s history by the dramatic impacts of human activity on the Earth.

Though Earth scientists debate exactly when the Anthropocene began, there is a clear consensus that human changes to the environment are real and extreme. For one, many life forms have become, and are becoming, extinct as a result of human activity. For this reason, some paleontologists argue that the human impacts of the Anthropocene began at the end of the last Ice Age, around ten thousand years ago. The fossil record indicates that around that time, many large animals, like woolly mammoths and giant sloths, went extinct shortly after humans arrived in their ranges. Their sudden disappearance suggests that habitat destruction and overhunting by humans may have contributed to their demise. Indeed, many large animals, like elephants and gorillas, are endangered for those same reasons today.

The pace of human-caused extinctions has only increased in the past several hundred years. The growth and spread of human populations, caused by advances in seafaring technology and agriculture, has led to overexploitation of fragile ecosystems, introduction of invasive species, and pollution, causing many extinctions. Scientists have estimated the rate of extinction by studying the fossil record, monitoring existing species, and using statistical models to estimate the number of undiscovered species that have been lost. Estimates vary, but most scientists believe that diverse species are going extinct at hundreds or thousands of times the natural rate. The International Union for the Conservation of Nature, for instance, has found that, of species surveyed on its “Red List,” about a fifth of all mammals and reptiles and nearly a third of amphibians are in danger of extinction.
 
This ongoing, rapid loss of species has been described as a mass extinction, as severe as the event that wiped out the dinosaurs 65 million years ago. To some ecologists, this steep decline in biodiversity suggests that the Anthropocene epoch began in the 17th and 18th centuries, when the rate of extinction shot up dramatically.

Human activity is also altering the climate as a whole. Since the Industrial Revolution of the 18th and 19th centuries, humans have significantly altered the atmosphere by mining and burning fossil fuels such as coal, oil, and natural gas. Some byproducts of the use of these fuels, like carbon dioxide, are greenhouse gases that trap solar energy on Earth. To assess the impact of these greenhouse gases on the Earth, scientists have had to investigate the history of the Earth’s climate. Ice cores, samples of ice layers that have trapped atmospheric chemicals over time, have supplied scientists with millennia of year-by-year information about greenhouse gas concentrations and atmospheric temperature. Evidence from ice cores clearly show that the Industrial Revolution brought about a sudden jump in carbon dioxide in the atmosphere, along with an increase in temperatures. A scientific consensus exists that this ongoing rise in temperatures has resulted in warming of the oceans, rising sea levels, and more frequent extreme weather events. Thus, some climatologists propose that the Anthropocene’s onset occurred with the Industrial Revolution and its effects on Earth’s atmosphere.

Whenever the Anthropocene is judged to have begun, its impact is undeniable. Human activity has changed the face of the planet; the global ecosystem has been and is being reshaped, the composition of the atmosphere has been altered, and even weather patterns are changing in response to human activity. The consequences of these changes will affect life on Earth for millions of years to come, leaving a mark of human activity that may well outlive humanity itself.

1. What is the main purpose of the passage?
(A) respond  to  controversial  claims  made  by  rival scientists.
(B) argue for potential solutions to the problems posed by climate change.
(C) describe human impacts on the Earth’s environment.
(D) account for recent changes in global biodiversity.

2. What is the author’s tone of writing in this passage?
(A) a dejected defeatist.
(B) a concerned observer.
(C) a jaded skeptic.
(D) an uncertain specialist.

3. What is the purpose of the 2nd paragraph?
(A) explain how scientists divide geological time.
(B) provide a broad description of the Earth’s history.
(C) describe the origins of the majority of the Earth’s biodiversity.
(D) compare  the current  geological  epoch  to  the Cambrian period.

4. Based on the passage, which choice best describes the relationship between carbon dioxide and ice cores?
(A) Carbon dioxide destroys ice cores, leading to a loss of a source of information.
(B) Ice  cores  remove  carbon  dioxide  from  the atmosphere, reducing its effects on the climate.
(C) Carbon dioxide is extracted from ice cores and used to fuel industrial processes.
(D) Ice  cores  can  be  studied  to  track  changes in atmospheric carbon dioxide levels.

5. The passage most strongly suggests that
(A) some  life  forms are  going  extinct  before  being discovered by humans.
(B) all  extinctions  currently  taking  place  result from human activity.
(C) modern extinctions are destroying the biodiversity generated in the Cambrian period.
(D) the recent increase in the extinction rate occurred as a result of human-caused climate change.