The level of carbon dioxide in the atmosphere today is probably higher than it has ever been in the past 3 million years. This increase in the level of carbon dioxide and greenhouse gas could bring temperatures not seen over that entire timepan, according to new research.
The study used computer modeling to examine changes in climate during the Quaternary period, which started around 2.59 million years ago and continues into today. Over that period, Earth has undergone a number of changes, but none so fast as those seen today, said study author Matteo Willeit, a postdoctoral climate researcher at the Potsdam Institute for Climate Impact Research. [Photographic Proof of Climate Change: Time-Lapse Images of Retreating Glaciers]
"To get a climate warmer than the present, you basically have to go back to a different geological period," Willeit told Live Science.
3 million years of climate
The Quaternary period began with and a period of glaciation, when ice sheets were stolen from Greenland to cover much of North America and Northern Europe. At first, these glaciers advanced and retreated on a 41
But between 1.25 million and 0.7 million years ago, these glacial and interglacial cycles stretched out, re-occurring every 100,000 years or so, and a phenomenon called the mid-Pleistocene transition because of the epoch in which it occurred. Willeit and his team used an advanced computer simulation of the quaternary to try to answer that question. Models are only as good as the parameters included, and this includes a lot: atmospheric conditions, ocean conditions, vegetation, global carbon, dust and ice sheets.
How things have changed
The team found that for 41,000-year glacial cycles to change to 100,000- year cycles, two things had to happen: Carbon dioxide in the atmosphere had to decline, and glaciers had to scour away a layer of sediment called the regolith. [Images: Greenland’s Gorgeous Glaciers]
Carbon dioxide may have declined for different reasons, Willeit said, such as a decrease in the greenhouse gas spewing from volcanoes, or changes in the rate of weather of rocks, which would lead to more carbon being locked up in sediments carried to the bottom of the sea. Less carbon in the atmosphere meant that less heat was trapped, so the climate would have cooled to the point where large ice sheets could form more easily
Geological processes provided the crucial second ingredient for longer glacial cycles. When continents are ice-free for long periods of time, they acquire a top-layer of ground-up, unconsolidated rock called regolith.
Ice that forms on this regolith tends to be less stable than the ice that forms on the firm bedrock, Willeit said (imagine the moon's thick dust layer is a regolith) difference in stability between a ball bearing and a flat table top. Similarly, regolith-based ice sheets flow faster and stay thinner than ice does. When changes in Earth's orbit alter the amount of heat that hits the Earth's surface, ice sheets are particularly prone to melting
But glaciers also bulldozze regolith away, pushing the dusty stuff into their glacial edges. This glacial scouring re-expands the bedrock; after a few glacial cycles in the early Quaternary, the bedrock would have been exposed, giving newly formed ice sheets a firmer place to anchor, Willeit said. These resilient ice sheets, plus a cooler climate, resulted in the longer glacial cycles seen around a million years ago.
Climate then and now
These findings are important for understanding the conditions that determine whether places such as Chicago or New York City are liveable or are covered in a mile of ice. But they are also useful for framing today's climate change, Willeit said. [8 Ways Global Warming Is Already Changing the World]
Records of atmospheric carbon that existed about 800,000 years ago have to be reconstructed rather than measured directly from ice cores, so estimates of the amount of carbon in the atmosphere have varied. Willeit and his team's modeling research suggests that carbon dioxide was below 400 parts per million for the entire Quaternary period. In the late Pliocene, around 2.5 million years ago, the average global temperatures were temporarily about 2.7 degrees Fahrenheit (1.5 degrees Celsius) higher than the average before the widespread use of fossil fuels, Willeit's model showed. Those ancient temperatures currently hold the record for the highest in the Quaternary period.
But that could soon change. Already, the globe is 2.1 degrees F (1.2 degrees C) warmer than the pre-industrial average. The 2016 Paris Agreement would limit the warming to 2.7 F (1.4 C), matching the climate of 2.5 million years ago. If the world can not manage that limit and head to 3.6 degrees F (2 degrees C), the previous international goal, it will be the hottest global average seen in this geological period
"Our study puts this into perspective, "Willeit said. "It clearly shows that even if you look at past climates over very long timescales, what we are doing now in terms of climate change is something big and very fast compared to what happened in the past."
The findings will be