Sarah Kaplan, The Washington Post: About 700 million years ago, Earth turned into a snowball. The polar ice sheets expanded until they engulfed the globe. The oceans turned to slush. The vast expanses of ice and snow reflected the sun's light back into space, exacerbating the endless winter. Temporary relief came in the form of massive volcanic eruptions, which spewed carbon dioxide into the atmosphere and triggered a period of global warming. But that, too, spiraled out of control. Earth became a greenhouse — its oceans hot enough to cook their inhabitants, its blighted landscape further ravaged by floods. Then, suddenly, something about the shifting continents or ash-darkened skies prompted the planet to cool again. The snowball returned.
Andrew Kenny, physicist and mechanical engineer: This article discusses CO2 levels and global temperatures over the last 550 million years, roughly the period of multi-celled life. CO2 has averaged about 2,000 ppm over this time but with huge fluctuations. Temperatures by contrast have been remarkably steady except for three periods of cold (when there was ice at the poles) and a period of semi cold.
Sebastian Luening, Geologist, Paleontologist, publisher of kaltesonne.de: The website: kaltesonne.de brings very interesting articles in German and English together for a consistent discussion on the history of Earth's climate. This article shows that several thousand years ago Greenland had less ice than today. Principal components analysis reveals two dominant Holocene trends, one with early Holocene warmth followed by cooling in the middle Holocene, the other with a broader period of warmth in the middle Holocene followed by cooling in the late Holocene. The temperature decrease from the warmest to the coolest portions of the Holocene is 3.0 ± 1.0 °C. The Greenland Ice Sheet retracted to its minimum extent between 5 and 3 thousands of years ago, consistent with many sites from around Greenland depicting a switch from warm to cool conditions around that time.
Doug Hurst is a retired RAAF navigator with a long-term interest in weather and climate. In writing this article he was assisted by: Ex-RAN engineers Colin Davidson and Peter Bobroff; epidemiologist Dr Judy Ryan; IT specialist Mike O’Ceirin; medical practitioner Dr Patrick Purcell; public servant and military historian Peter Edgar; polymath Peter Kemmis; biochemist Maureen Hanisch; novelist and poet Alan Gould; and geologists Aert Driessen and Dr Howard Brady, author of Mirrors and Mazes, a guide through the climate change debate.
Simon Armitage, Royal Holloway University, Charlie Brislow, University of London, Nick Drake, Kings College, London, UK.: North Africa was wetter 15,000–5,000 years ago than today. We reconstruct the lake-level history of Lake Mega-Chad, when it was the largest African lake, and demonstrate that this humid period ended abruptly 5,000 years ago, indicating that the African monsoon exhibits a nonlinear response to insolation forcing. The northern basin of Lake Mega-Chad, currently the world’s greatest dust source, became dry around 1,000 years ago.