Deep-time and glacial paleoclimate puzzles

6 problems

• What causes abrupt Dansgaard-Oeschger warmings during glacial periods?

  Open

  During the last ice age Greenland repeatedly warmed by 8 to 16 degrees within a few decades, then slowly cooled again, about 25 times. The trigger of these jumps, called Dansgaard-Oeschger events, is unknown.

• What kept early Mars warm enough for rivers and lakes?

  Contested

  About 3.7 billion years ago Mars had river valleys and lakes even though it is farther from a then fainter Sun. Carbon dioxide and water vapour alone do not seem able to warm it enough, so the question is what else did.

• How did early Earth stay unfrozen under a fainter young Sun?

  Contested

  Four billion years ago the Sun gave about 25 percent less light, enough to freeze today's Earth solid, yet rocks show liquid water. Something, such as extra greenhouse gases or a darker surface, must have made up the difference.

• What lowered atmospheric $\mathrm{CO_2}$ by about $90\,\mathrm{ppm}$ in glacial times?

  Open

  Air bubbles in Antarctic ice show carbon dioxide fell from about 280 to about 190 parts per million during each ice age and rose again afterwards. The ocean must have stored the missing carbon, but the mix of processes that did this is still debated.

• Why did glacial cycles switch from 41-kyr to 100-kyr periods?

  Contested

  Before about a million years ago ice ages came every 41 thousand years, matching a wobble in Earth's tilt, then shifted to bigger cycles every 100 thousand years. Earth's orbit did not change at that time, so the cause must be inside the climate system.

• What started and ended the Neoproterozoic Snowball Earth glaciations?

  Open

  Around 700 and 640 million years ago ice seems to have reached the equator, covering nearly the whole planet. It is unclear what pushed Earth into this state and how it escaped, since white ice reflects most sunlight.