Which came first, the warmer temperatures or the increased carbon dioxide in the atmosphere? This has been one of the perplexing questions in studies of past climate. A new study was released last week which shows that the changes in carbon dioxide in past ice ages was due to a combination of colder sea surface temperatures and iron-carrying dust blowing off of southern hemisphere continents. The colder ocean waters are much more efficient at absorbing atmospheric CO2 than warm water, so CO2 levels in the atmosphere dropped quickly when ocean temperatures were cold. Iron dust encourages the growth of phytoplankton, which suck up CO2 and deposit it deep in the ocean. You can read more at Physics.org here.