Comparing trends in vegetation from the last few thousand years to recent trends in vegetation can also help scientists determine whether human activities have had significant impacts on ecosystems.
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Because of their unique shapes, scientists can then take a core sample of the sediment layers and determine what kinds of plants were growing at the time the sediment was deposited.
Knowing what types of plants were growing in the area allows the scientists to make inferences about the climate at that time by using knowledge about modern and historical distributions of plants in relation to climate.
(Prices for fresh produce have increased since the 1980s.) Washington had succeeded in eliminating food as a political issue—an objective dear to most governments at least since the time of the French Revolution.
But although cheap food is good politics, it turns out there are significant costs—to the environment, to public health, to the public purse, even to the culture—and as these became impossible to ignore in recent years, food has come back into view.