One of the most common "climate skeptic" arguments has to do with the magnitude of warmth compared with other periods in the past. "It was a lot warmer when the dinosaurs were around!" "There was more carbon dioxide in the atmosphere in the past - this is a natural cycle!"
These statements betray a fundamental ignorance about how the earth system works, and has worked in the past. They are true to a point; yes, the earth was warmer during the age of the dinosaurs, and there was most certainly more carbon dioxide in the atmosphere. So why is the current warming anything different than a part of the Earth's natural cycle?
Consider the Cretaceous Period, about 145-65 million years ago. The dinosaurs still roamed the Earth, and were about to bite the dust in the extinction that geologists use to label the break between the Mesozoic and Cenozoic eras (we live in the Cenozoic). By this time, the earth's continents were finally separating from Pangaea
, the supercontinent centered over the equator. Europe and North America were still attached, and Gondwana (composed of South America, Africa, Australia, India, and Antarctica) hadn't yet drifted apart. At the beginning of the Cretaceus, the Rockies, Andes, Himalayas, and Sierra Nevada Mountains hadn't formed (though there were some mountains in the American West - thanks to tectonite
for the correction!). The Rockies, Andes, and Himalays would begin their uplift at the end of the Cretaceous, and the Sierra Nevadas would begin forming in the Jurassic. A shallow inland sea covered most of inland North America, from the Appalachian Mountains to the east (believed to then be as high as the Andes, or even the Himalayas), and the rolling foothills where the Rockies would eventually uplift in the west. Sea levels were much higher because there was no polar ice yet; as much as a third of the Earth's land mass was under water (see some nice maps of Cretaceous landmasses here
The climate was most certainly much warmer than today, with tropical oceans being 9-12 degrees (C) warmer than present, and deep oceans as much as 15-20 degrees warmer. The Tethys Sea, which separated the diverging Laurasian (North American and Asia) and Gondwana supercontinents, connected the tropical oceans. Land masses hadn't drifted as far north as they are today, and the extensive oceans mediated the climate. Tropical vegetation prevailed in most regions, and fossils of palm trees have been found in modern-day Alaska. Even as North America moved close to its present-day position by the end of the Cretaceous, the poles remained warm; the Edmontosaurus
lived so far north
it would have to have migrated with seasonal darkness in the northern hemisphere!
We're currently living in the Quaternary Period
; for the last two and a half million years, we've seen regular cycles of ice ages (100,000 years of ice, and 10,000 of interglacial warmth in between). Even though the continents were close to their present position during the late Cretaceous, the climate was significantly warmer. We know ice ages are caused by a combination of cycles in the Earth's tilt and orbit over tens of thousands of years, but these certainly haven't changed during Earth's history. So why, then, do we have ice ages today, but not during the Cretaceous?
The position of the continents certainly helped mediate the climate, but it took more than that to cool the earth enough for permanent ice to form at the poles. Several important changes in the millions of years following the Cretaceous are responsible for this shift:
1. Mountain uplift
. The Himalayas and the Tibetan Plateau in particular had a major impact on global cooling after the Cretaceous, forming as a result of the Indian subcontinent colliding into Asia. High mountains change prevailing wind and Jet Stream patterns, blocking warm and moist ocean air from reaching the interior of the continents.
2. Mountain-building and atmospheric carbon
. When mountains uplift, they expose more rock to the air, allowing for greater rates of weathering. Carbon dioxide dissolved in rainwater reacts with minerals in rock, forming carbonates. These carbonates get washed to sea, where they settle to the sea-floor and are "trapped" from the atmosphere. The formation of mountains after the Cretaceous actually helped sequester carbon, lowering the amount of carbon dioxide in the atmosphere!
3. The closing of the Isthmus of Panama
. When North and South American were connected during the Great American Interchange, the tropical sea currents that connected the widening Atlantic with the Pacific were closed off. The Atlantic in particular cooled significantly, now that ocean currents were diverted from tropical warmth.
4. The opening of the Drake Passage
. When South America and Antarctica finally broke apart, this allowed oceans to circulate the South Pole and created a new zone where cold oceanic deepwater is formed (deepwater is cold surface water that sinks to the deep ocean and helps drive oceanic conveyor belts).
The dinosaurs weren't the only part of our planet to disappear during the Cretaceous; temperatures were warmer, and carbon dioxide levels were greater than present, but these were due to a distinct set of characteristics that no longer exist on planet Earth
. It took tens of millions of years for the current Earth system to evolve, and the system that has existed for the last 2.4 million years is dramatically different than the past. To compare the age of the dinosaurs to the modern climate system is ludicrous, but it betrays a common misunderstanding of the changing Earth system.