Past climates: Slower changes
Natural processes including changes in solar output, plant evolution, the gradual drift of the continents, slow changes in ocean currents, and long-lasting volcanic eruptions have led to past climate changes over timescales of millions of years. Changes in atmospheric carbon dioxide levels through photosynthesis or chemical weathering have often played an important part.
‘Snowball Earth’ periods
Scientists think there were periods over 600 million years ago when almost the entire Earth was covered in ice. These ‘Snowball Earths’ may have been caused by low solar intensity, continental configurations, reduced carbon dioxide (CO2) levels or a combination of factors. There is no certainty about this because reconstructing such ancient climates is extremely difficult. Some evidence suggests that after the ice had spread, large amounts of CO2 – possibly from volcanic eruptions – accumulated in the Earth’s atmosphere. High CO2 is usually associated with high temperatures, but CO2 doesn’t generate heat – it only traps heat. If almost all sunlight was reflected by ice, the Earth wouldn’t give off much heat for CO2 to absorb.
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An artist's impression of the Earth during a snowball period
Image: www.celesiamotherlode.net
Plant evolution during the Carboniferous period
Nearly 400 million years ago, levels of carbon dioxide (CO2) in the atmosphere were higher than today and global temperature was warmer. Over millions of years, many new plant species evolved. The plants absorbed CO2 from the atmosphere through photosynthesis, storing the carbon and releasing the oxygen back into the atmosphere. The resulting high oxygen levels caused the size of animals to increase – fossils show metre-long scorpions and seagull-sized dragonflies. Because CO2 is a greenhouse gas, the reduction in CO2 cooled the Earth down. Scientists estimate that global temperature fell about 10 °C over about 100 million years, from around 375 to 275 million years ago – known as the Carboniferous period.
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An artist's impression of continental positions during the late Carboniferous period
Image: Dr Ron Blakey / creativecommons.org
The volcanic eruption that formed the Siberian Traps
Today most volcanoes erupt briefly with explosive force, ejecting ash and debris which blocks out sunlight and cools the Earth. But some volcanoes – flood basalts – spew out lava continually for a long time. In one example, about 251 million years ago, a flood basalt eruption lasted about a million years and covered an area nearly the size of Europe in lava, forming a large expanse of volcanic rock called the Siberian Traps. As lava spreads out, it releases trapped gases, including carbon dioxide. The Siberian Traps lava emitted huge amounts of carbon dioxide which stayed in the atmosphere for some time after the eruption ended. Scientists think this increased global temperatures by about 6 °C over a couple of million years.
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Volcanic lava spread out over thousands of square kilometres to form the Siberian Traps
Image: Hawaii Volcano Observatory (DAS)
The rise of the Himalayas
The continents drift around the Earth, moving about 3 cm per year. Scientists calculate that around 50 million years ago the Indian subcontinent ploughed into Asia, gradually forming the Himalayan mountain range. The exposure of large rock surfaces to the air caused an increase in chemical weathering – which removes carbon dioxide from the atmosphere – cooling the planet. The rise of this towering mountain range also encouraged glacier formation, since temperatures decrease with altitude in the lower atmosphere. White ice and snow spreading across darker land cooled the planet by reflecting more sunlight. The Himalayas are still rising, affecting atmospheric circulation patterns and influencing the monsoons.
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The Himalayas seen from Annapurna Base Camp in Nepal
Image: Ryszard Stelmachowicz
Drifting continents and the freezing of the Antarctic
The slow drift of the continents can affect the climate by altering the flow of ocean currents, which carry heat around the globe. The presence or absence of land at the poles also influences the accumulation of polar ice, since ice forms more readily on land than on water. Around 33 million years ago Antarctica broke away from Australia and South America, leaving the Southern Ocean in-between. Over time, the presence of a large land mass at the South Pole, isolated by a cold ocean current flowing around it, enabled the Antarctic ice sheet to form. Scientists estimate that temperatures dropped by about 5 °C over tens of thousands of years.
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The separation of Antarctica from Australia and South America allowed a vast ice sheet to form
Image: Dave Pape / NASA
Pliocene warm period
Around three million years ago – during a period known as the Pliocene – atmospheric carbon dioxide (CO2) levels were higher than today and the Earth was warmer. In addition to increased atmospheric CO2, scientists think stronger ocean currents also contributed to the warmer conditions by carrying more heat to the poles, which melted some polar ice. Positive feedbacks also played an important part – ice melt leads to less sunlight being reflected and more being absorbed, magnifying the warming. This period saw the evolution of some of our first human-like ancestors who walked on two legs. Scientists have estimated that temperatures increased about 2–3 °C over hundreds of thousands of years before falling again as CO2 levels decreased.
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Artist's impression of animals during the Pliocene, including a giant sloth
Image: Carl Buell
Profile: Dr. Jane Francis
Jane Francis is a palaeoclimatologist who studies past climates. Her research shows that the Earth was warm enough for forests to grow in polar regions millions of years ago. ‘This research can help us understand how our climate might change in future as our world gets warmer.’ Spending her time at Leeds University and out in the field in Antarctica, Jane collects fossils to understand what vegetation was growing millions of years ago. ‘Using these fossils I’ve been able to reconstruct what polar forests looked like millions of years ago – a bit like a detective, piecing the story together from bits of evidence.’
