Past climates: Faster changes
Dramatic events such as meteorite impacts, massive floods and sudden releases of carbon have led to past changes in climate over timescales ranging from thousands of years to just decades. Short-lived volcanic eruptions and variations in the Sun’s output have led to less dramatic climate changes over timescales from a few years to a few decades.
Meteorite impact (about 65 million years ago)
Scientists have found a crater that was left by a meteorite about 65 million years ago. The meteorite was probably 10–15 km wide and slammed into the Earth faster than a bullet, carving out a deep hole in our planet’s surface. The force of the impact blasted huge amounts of gas, dust and aerosols into the atmosphere. Scientists think this may have blocked out enough sunlight to cause global cooling of more than 5 °C in a decade or less. At around the same time, the dinosaurs – after walking the Earth for nearly 200 million years – died out, along with about half of all plant and animal species.
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Artist's impression of a meteorite on a collision course with Earth
Image: Triff / Shutterstock
Ancient greenhouse-driven warming
A vast release of carbon caused a substantial rise in global temperature about 56 million years ago, a period known as the Palaeocene-Eocene Thermal Maximum. Scientists think the carbon release may have originated from underwater volcanoes or from methane gas trapped in crystal-like cages (clathrates) on the ocean floor, which may have been disturbed by the collapse of ground at the edges of the continents. In the atmosphere, carbon is in the form of carbon dioxide and methane – greenhouse gases that affect the Earth’s temperature. Scientists studying climate proxies dating from this period have calculated that the huge increase in greenhouse gases caused global warming of about 6 °C over 10,000 years or so.
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Clathrates are underwater crystalline structures containing methane gas
Image: Professor Ross Chapman, University of Victoria, British Columbia, Canada
Younger Dryas cooling (about 13,000 years ago)
As the Earth emerged from the last ice age about 13,000 years ago, the warming trend was interrupted by a temporary cooling. The melting of the ice sheet covering North America had left behind an enormous lake – Agassiz. Today’s Great Lakes are all that remains of Lake Agassiz, which once held more fresh water than all present-day lakes combined. Scientists think Agassiz may have been held in place by an ice dam which broke as the ice melted, allowing part of the lake to flood into the Arctic Ocean. The rapid influx of fresh water would have disrupted ocean currents in the north Atlantic, causing global cooling of about 0.6 °C over a few decades.
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Scientists think a massive flood of meltwater triggered the Younger Dryas cooling
Image: PSD Photography / Shutterstock
Medieval warm period (around 900–1300)
During medieval times – between about 900 and 1300 – some parts of the world experienced a prolonged period of higher temperatures. Greenland saw pronounced warming and there is some evidence of higher temperatures in medieval Europe and Asia, but it’s unclear whether this effect was global or confined to regions of the northern hemisphere. Scientists think a slight increase in the Sun’s output may have triggered this ‘medieval warm period’. The Sun sometimes goes through periods of increased or reduced activity, leading to small variations in the amount of solar energy reaching the Earth.
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Satellite image of the Sun showing a solar flare
Image: SOHO (ESA & NASA)
Little ice age (around 1350–1850)
Between about 1350 and 1850, many regions of the northern hemisphere experienced a prolonged period of lower temperatures that became known as the ‘little ice age’. Norse communities abandoned their settlements in Greenland and returned south. Ice-skating on frozen rivers was common in European cities owing to colder winters. There’s some evidence of cooling in the southern hemisphere, although it’s unclear how widespread this effect was. Sunspot records from this time show a period of low solar activity. Scientists think the combined effect of an increase in volcanic eruptions – blocking out some sunlight – and a slight drop in the Sun’s output may have caused cooling of about 0.8 °C over hundreds of years.
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Artist's impression of a frost fair during the Little Ice Age
Image: Thomas Wyke
Year without a summer (1816)
From 1812 to 1815, Mount Tambora in Indonesia erupted several times. The most explosive eruption, in April 1815, ejected huge amounts of ash, debris and aerosol particles into the atmosphere. The aerosols blocked out so much sunlight that the following year – 1816 – became known as the ‘year without a summer’. Global temperatures fell by about 0.5 °C over a year or so and 1810–1819 was the coldest recorded decade in the past 500 years. The sudden drop in temperature led to widespread crop failures and famine. About 60,000 people died in the global aftermath of the Tambora eruption.
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Mount Tambora Volcano, Sumbawa Island, Indonesia
Image: NASA /JSC
Profile: Dr. Joanna Morgan
Sixty-five million years ago a large asteroid hit the Earth, resulting in a dramatic decrease in the Earth’s temperature, which triggered the extinction of a number of species including the dinosaurs. Working in the heat of the Mexican jungle, geophysicist Joanna Morgan aims to understand what happened all those millions of years ago. According to research, the likely causes of the temperature drop are a global blackout and cooling, owing to the large volumes of dust and sulphur the impact threw up. ‘Using geophysical data and computer modelling we can calculate the amount of material released into the atmosphere, and begin to understand how this changed the Earth’s climate’.
