There was a 300 year period that ended in 1850 when it was incredibly cold on Earth, specifically in North America, Europe and Asia. During this time, ice expanded, glaciers covered huge parts of the Alps, Alaska, and Norway and it was colder than people were used to, according to NASA. This time period was called the Little Ice Age and now researchers are worried the Earth might be headed for another mini ice age, Sky News reported Wednesday.

The concerns stem from mathematical predictions about the sun’s magnetic activity in the coming years that researchers have done. During the last Little Ice Age, it was so cold in London that the massive river that runs through the city, the river Thames, actually froze over, accounts from that time period show.

What causes a mini or little ice age?

The cause of these small ice ages is a decrease in the sun’s activity. This doesn’t mean the sun isn’t shining, it simply means the activity of sunspots or the magnetic activity is lower than it has been and the sun is shining a tiny bit less bright. The actual drop in temperature that happened centuries ago was due to a change in the ozone in the atmosphere. When the sun gave off less energy in the form of ultraviolet radiation, less ozone formed which then had a domino effect on the other climate factors and processes on Earth, according to NASA. Climate models reflected these changes when researchers ran them with the change in radiation from the sun.

This is why researchers who predict another decrease in the sun’s activity are worried that such an ice age might happen again. But there is a chance that the Earth won’t get hit with a mini ice age, and that’s because of climate change, professor Valentina Zharkova of Northumbria University told Sky News. If it does happen though, Zharkova sees it as an opportunity to sort out some of the climate change problems. "I hope global warming will be overridden by this effect, giving humankind and the Earth 30 years to sort out our pollution," she told Sky News.

But even if this does happen as she predicts, she also expects the sun to regain its regular levels of solar activity a few decades later, meaning things would heat up again.

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