US and Australian scientists think they know why glaciers existed at the Earth's equator 600 million years ago and why the Earth's polar regions were warmer then than they are today. The researchers at Pennsylvania State University and the University of Adelaide say that the reason was that the Earth used to have a pronounced tilt up to 55 degrees, compared to 23.5 degrees today. They believe that the expansion and contraction of giant ice sheets around the equator might have caused a positive feedback loop to the position of the Earth's gravitational tilt (Nature 396 453).
Geologists have known for some time that ice sheets existed at the equator between 2, 400 and 2, 200 million years ago and then again between 820 and 550 million years ago. Their favourite theory to explain these observations is the ‘snowball’ effect, which says that glaciers crept towards the equator from the poles. However, there is a problem with this theory: if glaciers moved below 25-30 degrees latitude, a runway glacier effect would cause giant ice sheets to form and turn the Earth into a giant frozen ball. According to their calculations, this would cause the oceans to freeze over to a depth of 1 km, and make all life on the planet extinct.
Instead, Darren Williams, James Kasting and Lawerence Frakes argue that if the poles were ice-free, and received most of the sunlight because of a high Earth tilt, the equator would naturally develop glaciers. The tilt angle is important as it defines the amount of sunlight each region of the Earth receives. If the Earth’s tilt was 55 degrees, then large areas around the globe would have experienced the long summer days and long winter nights that are normally associated with high lattitude areas such as Alaska. It would also mean that the poles would be the warmest places on Earth.
The researchers speculate that the Earth might have been highly oblique for most of its history, up to 550 million years ago. However, geologists know that by 430 million years ago the tilt was much lower, closer to its present-day value. They suggest that oblateness – the deviation of the Earth’s shape from being a perfect sphere – participated in a feedback loop to change the Earth’s tilt in under 100 million years. If the location of the continents encouraged the formation of large ice sheets, glaciation would have altered the way that mass is distributed on the Earth’s surface, encouraging the decline of the Earth’s tilt.
Other evidence for an earlier 55 degree tilt is the now higher-than-expected (5°) inclination of the Moon’s orbit, the extinction of a large number of species at 600 million years ago, and temperate fossils found in Antarctica.
