ScienceDaily: About 4 to 5 million years ago, the Earth was warmer than today. Now that greenhouse gas pollution has the planet's temperature rising again, researchers want to know more about why this early Pliocene period was so warm, with the hopes of improving future climate predictions. A new study in the journal Nature concludes that it is difficult to model the exact conditions behind the pattern of warming in the early Pliocene. None of the proposed mechanisms -- from high carbon dioxide levels to changes...