Cretaceous Era

THE CRETACEOUS ERA spanned the time period from 144 to 65 million years ago. It was the final epoch of the dinosaurs. It ended when the dinosaurs became extinct. At its height, the Cretaceous was a period of great warmth. The poles were ice-free, and warm ocean currents spread from the equator to the poles. The concentration of carbon dioxide was higher than it is today, causing a greenhouse effect. The abundance of plants was not enough to lower the amount of carbon dioxide in the atmosphere. The vigor of their growth implies that the Cretaceous climate was warm and wet, although, curiously, rainfall in the tropics was not heavy enough to support rainforests. Plants grew as far north as the Arctic Circle, proving that high latitudes were far warmer than they are today. The climate cooled, however, and rainfall diminished during the Late Cretaceous. The Cretaceous climate reached its nadir 65 million years ago, when a meteor impacted earth, ejecting debris, dust, and ash into the atmosphere, blocking out sunlight and cooling the Earth.

The sun was not the reason the Cretaceous era had a warmer climate than today; it produced 1–2 percent less heat. Earth was warmer during the Cretaceous era because the atmosphere contained 3–6 times more carbon dioxide than the current era. Carbon dioxide formed from the decay of large amounts of dead plants. Moreover, the Cretaceous, particularly the mid-Cretaceous, was a period of extreme [Page 289]volcanism, with the eruption of volcanoes releasing carbon dioxide into the atmosphere. The weathering of carbonaceous rocks also liberated carbon dioxide. All of this carbon dioxide created a greenhouse effect, in which carbon dioxide trapped sunlight as heat, warming the atmosphere.

A reservoir of heat, the ocean displayed the consequences of the greenhouse effect in its warmth, particularly in the tropics. During the Cretaceous Era, tropical waters were between 82–111 degrees F (28–44 degrees C). At 30 degrees latitude, ocean temperatures dipped to 68 degrees F (20 degrees C). At 60 degrees latitude, ocean temperatures were 54 degrees F (12 degrees C), and at 90 degrees latitude, temperatures were a cold 40 degrees F (4.5 degrees C). Polar temperatures, thus, though cold, were nonetheless above freezing and the poles did not have ice, at least not during the Late Cretaceous, though clima-tologists are less certain about the earlier periods of the Cretaceous. As is true today, the Cretaceous era oceans were cooler at lower depths, yet even at great depths they were warmer than today s oceans.

Plants absorbed carbon dioxide during the Cretaceous, though they may not have absorbed as much of the gas as they do today, accounting for the greenhouse effect during the Cretaceous. The tropics were drier during the Cretaceous than they are today, and so did not support a rainforest. The absence of a rainforest may have meant that Cretaceous plants were not numerous enough and did not grow vigorously enough to absorb as much carbon dioxide as today. Although this may have been true of the tropics, high latitudes had forests where today there is only tundra. These forests must have taken carbon dioxide out of the atmosphere, but on balance, plants did not remove carbon dioxide in sufficient quantities during the Cretaceous to reduce the greenhouse effect.

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