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 B. explain why levels of methane in Earth's atmosphere began to rise approximately 5,000 years ago C. suggest that Earth's climate would have become even warmer without the advent of human agricultural activities D. identify 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 E. support 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