Christian Science Monitor: About 4.5 million years ago, during the early Pliocene period (3 to 5 million years ago), temperatures on Earth were some 3 to 4 degrees C (5.4 to 7.2 degrees F.) higher in the tropics, and perhaps 10 degrees C (18 degrees F.) warmer near the poles. To get that much warming, current climate models have to pump up atmospheric carbon dioxide levels to between 500 and 600 parts per million — about twice the preindustrial level of 280 ppm. We’re currently around 387 ppm. And, …