Cretaceous Period

The Cretaceous period 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 icefree, 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 period had a warmer climate than today; it produced 1–2 percent less heat. Earth was warmer during the Cretaceous period because the atmosphere contained 3–6 times more carbon dioxide than in 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 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 period, tropical waters were between 82 and 111 degrees F (28 and 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 climatologists are less certain about the earlier periods of the Cretaceous. As is true today, the Cretaceous period oceans were cooler at lower depths, yet even at great depths they were warmer than today’s oceans.

Secrets in Plants

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.

Plants reveal that the Cretaceous had a tropical climate, even at high latitudes. The plant Heilungia grew in Alaska. Although it is extinct, its relatives grow in Mexico and the Caribbean, suggesting that Alaska had a tropical climate during the Cretaceous. Temperatures must have varied little, staying around 80 degrees F (27 degrees C) year-round at high latitude. Rainfall must have exceeded 80 in. (203 cm) a year.

The fact that Alaska had swamps during the Cretaceous also suggests that rainfall was abundant. Moreover, plants grew in profusion and density in Alaska, implying high rainfall and warm temperatures. Tree rings were wide, suggesting a warm, wet climate. Conifers grew at lower latitudes, implying that the climate was less moist and that the climate became drier toward the equator, the opposite of conditions today. 

In Siberia, plants grew in the Arctic Circle. Summer temperatures averaged 70 degrees F (21 degrees C), whereas winter temperatures averaged only 43 degrees F (6 degrees C). Arctic Siberia therefore had seasons, with summer 27 degrees F (15 degrees C) warmer than in winter. The difference between summer and winter temperatures during the Cretaceous is less than the seasonal difference today at high latitudes. The climate was warm enough this far north to permit plant growth for more than seven months each year.

Autumn was brief, with the transition from summer to winter coming rapidly. South of Siberia, in what is Czechoslovakia today, temperatures averaged 68 degrees F (20 degrees C). Summer temperatures averaged 82 degrees F (28 degrees C), whereas winter temperatures averaged 52 degrees F (11 degrees C).

Enigma

The climate of the Late Cretaceous is an enigma. The oceans remained warm and the poles ice-free, but on land, temperatures and rainfall decreased. The final blow came at the end of the Cretaceous 65 million years ago, when a meteor hit Earth, ejecting debris into the atmosphere and blocking out sunlight, cooling the Earth. The meteor impact ended the Cretaceous, temperatures continued to decrease, falling low enough to cause ice ages, the last of which ended only 10,000 years ago. As the concentration of carbon dioxide diminished and ice collected at the poles, tundra replaced forests at high latitudes. Rainfall increased in the tropics, giving rise to the rainforests.