Earth鈥檚 climate isn鈥檛 easy to cool down 鈥� especially if you want it cold enough for ice caps to form. According to a model of the geological forces shaping the climate over the past 420 million years, reaching such 鈥渋ce house鈥� temperatures requires a combination of rapid rock erosion removing carbon dioxide from the atmosphere and less CO2-releasing volcanic activity.
Despite record high temperatures on brief human timescales today, Earth鈥檚 climate is currently colder than usual. 鈥淲hen you look back in the geological past and you try to figure out how hot the Earth has been in its history, you find a lot of evidence of warm climates,鈥� says at the University of Leeds, UK. 鈥淵ou don鈥檛 find much evidence for glaciation.鈥�
The relatively rare periods where it was cold enough to form permanent ice caps at the poles 鈥� called 鈥渋ce house climates鈥� 鈥� are associated with low levels of CO2 in the atmosphere reducing the greenhouse effect. But there are different explanations for what drove these periods of low CO2.
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Some researchers have proposed they were due to reductions in CO2 released by volcanoes; others have suggested they were caused by increased rates of erosion of rock 鈥� a process known as weathering that involves chemical reactions that remove CO2 from the atmosphere. Weathering rates might increase when continents collide to make long mountain chains that are then subject to erosion, or when continental drift drags landmasses into regions 鈥� such as the tropics 鈥� where higher rainfall rates and temperatures can accelerate erosion. Changes in vegetation cover can also increase weathering rates.
To determine whether any of these geological cooling processes spurred the ice houses of the deep past, Mills and his colleagues developed an Earth system model to explain changes in CO2 levels in the atmosphere over the past 420 million years 鈥� a stretch of time spanning most of the Phanerozoic Eon, which has seen several separate ice houses.
A key difference from previous models was that they accounted for the changing position of the continents. 鈥淣obody has been able to do this in a three-dimensional system,鈥� says Mills.
The researchers found that the model鈥檚 results best matched the observed geological records of temperature, CO2 levels and the position of ice caps when they included both the effects of rock weathering and changes in outgassing from volcanoes. This suggests neither factor can explain transitions to ice house climates on its own, says Mills. 鈥淚t means that everyone was right a little bit.鈥�
However, the study鈥檚 conclusion that multiple factors are required to spur an ice house is far from the final word on the subject, says at the University of California, Davis. 鈥淲e鈥檝e all understood for decades that it is more than one process,鈥� she says. 鈥淏y definition that is the Earth system. This paper doesn鈥檛 actually give us a solution.鈥�
Part of the problem is that the model captures changes over tens of millions of years at best, so can鈥檛 explain shorter-term changes in the climate, says Monta帽ez. She also says that it relies on an out-of-date record of changes in CO2. The model also doesn鈥檛 account for the dispersal and evolution of plants during the Phanerozoic, which has affected the rate of rock weathering. 鈥淭hey need to add the vegetation, because it has a huge impact,鈥� she says.
More detailed modelling could shed light on the much more rapid pace of climate change in response to human emissions of CO2, and what the future holds for the ice house climate the planet has maintained for the past 34 million years. But these new results capture changes on far too long a timescale to be directly relevant to today, says at the University of Leeds, one of the study鈥檚 authors.
Still, the fact that ice house climates can rapidly come to an end is a warning of what can happen in the Earth system, says Merdith. 鈥淲e should be careful with the assumption that if we reverse rising CO2 things will return to what they were in the pre-industrial age.鈥�
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