The average global temperature during the last Ice Age 20,000 years ago was 7.7 degrees centigrade, scientists have revealed.
The temperature of the planet during the Last Glacial Maximum (LGM), has been ‘nailed down’ thanks to ancient plankton fossils.
Knowing exactly how cold it was during the LGM can be used to calculate what is known as ‘climate sensitivity’, or how much the earth’s temperature changes in response to carbon in the atmosphere.
Lead author Associate Professor Dr Jessica Tierney at the University of Arizona said: ‘We have a lot of data about this time period because it has been studied for so long.
‘But one question science has long wanted answers to is simple: How cold was the ice age?’
Huge glaciers covering half of North and South America, Europe and parts of Asia, meant only fauna and flora adapted to the cold survived.
Data was collected from ocean plankton fossils and used to determine the sea-surface temperature during the ice age.
The fossil-data was then combined with climate model simulations of the LGM using a weather forecasting technique called data assimilation.
Dr Tierney said: ‘What happens in a weather office is they measure the temperature, pressure, humidity and use these measurements to update a forecasting model and predict the weather.
‘Here, we use the Boulder, Colorado-based National Center for Atmospheric Research climate model to produce a hindcast of the LGM, and then we update this hindcast with the actual data to predict what the climate was like.’
The average temperature during the ice age was six degrees celsius (11F) cooler than today, the researchers found.
In the 20th century, the planet’s average temperature was 14 degrees celsius (57F), but during the ice age it was 7.7 degrees celsius (46F).
Maps were created by the team to illustrate how temperatures varied from region to region.
Dr Tierney said: ‘In your own personal experience that might not sound like a big difference, but, in fact, it’s a huge change.
‘In North America and Europe, the most northern parts were covered in ice and were extremely cold.”
‘But the biggest cooling was in high latitudes, such as the Arctic, where it was about 14 C (25 F) colder than today.’
Building on their new discovery, the team calculated the planet’s sensitivity to carbon in the atmosphere.
Everytime the amount of carbon in the atmosphere doubles, the planet’s temperature should increase by 3.4 degrees celsius (6.1F), the researchers found.
During the ice age atmospheric carbon dioxide levels were very low, around 180 parts per million.
While before the industrial revolution they had grown to 280 parts per million, today they have reached 415 parts per million.
Dr Tierney said: ‘The Paris Agreement wanted to keep global warming to no larger than 2.7 F (1.5 C) over pre-industrial levels, but with carbon dioxide levels increasing the way they are, it would be extremely difficult to avoid more than 3.6 F (2 C) of warming.
‘We already have about one degree celsius (2 F) under our belt, but the less warm we get the better, because the Earth system really does respond to changes in carbon dioxide.’
The team is hoping to use the same technique to examine the climatic conditions during warm periods in the earth’s past.
Dr Tierney said: ‘If we can reconstruct past warm climates, then we can start to answer important questions about how the Earth reacts to really high carbon dioxide levels, and improve our understanding of what future climate change might hold.’
The findings were published in the journal Nature.