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Scientific consensus is that humans first began to have a warming effect on Earth`s climate within the past century, after coal-burning factories, power plants, and motor vehicles began releasing carbon dioxide (CO2) and other greenhouse gases in significant quantities into the air. However, evidence suggests that human agricultural activities may have had such an effect much earlier:concentrations of CO2 started rising about 8000 years ago, even though natural trends indicate they should have been dropping; methane levels rose similarly about 3,000 years later. Without these rises, however, current temperatures in northern parts of North America and Europe would be cooler by three to four degrees Celsius-enough to inhibit agriculture-and an ice age would probably have begun several thousand years ago in northeastern Canada.

The author mentions "natural trends" most likely in order to

A propose a possible explanation for why current temperatures in North America and Europe are not as low as they might otherwise be
Bexplain why levels of methane in Earth's atmosphere began to rise approximately 5,000 years ago
Csuggest that Earth`s climate would have become even warmer without the advent of human agricultural activities
Didentify factors that may have contributed to the rise of greenhouse gases in Earth`s atmosphere approximately 8,000 years ago and again some 3,000 years later
Esupport the claim that human activity may have played a role in the rise in CO2 and methane levels in the atmosphere thousands of years ago

E显示答案

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