21 March 2014, by Tamera Jones
A striking drop in solar activity was probably responsible for a long spell of harsh winters in northern Europe from around 1400 to 1800 known as the Little Ice Age, say scientists.
The findings mean an imminent quiet period in the Sun’s activity predicted by some scientists could lead to severe winters in Britain.
Researchers found that lower solar activity can drastically affect the climate in the North Atlantic, encouraging high pressure blocking systems to develop.
These blocking systems end up changing the course of prevailing westerly winds, stopping warm winds from the tropics getting to Europe. This leads to cold winters setting in, exactly like those of 2010 and 2013.
Researchers have linked low solar activity to the Little Ice Age before. But this is the first time anyone has shown how a drop in the Sun’s energy can lead to changes in the climate around the North Atlantic. The findings are published in Nature Geoscience.
‘Our study concludes that although the temperature changes expected from future solar activity are much smaller than the warming from human carbon dioxide emissions, regional climate variability associated with the effects of solar output on the ocean and atmosphere should be taken into account when making future climate projections,’ says Dr Paola Moffa-Sánchez of Cardiff University, who led the study.
The researchers used a sediment core taken from the ocean floor south of Iceland to investigate how the Gulf Stream differed from today. They did this by analysing the chemical composition of the shells of fossilised microorganism called foraminifera within the core.
‘We found big and abrupt changes in temperature and salt concentrations in the sediment core, which matched changes in solar activity.’
Dr Paola Moffa-Sánchez, Cardiff University
Moffa-Sánchez and her colleagues looked in this region because the currents here, which are part of the surface waters of the so-called ocean conveyor belt, are important for the whole of the Earth’s climate.
The conveyor belt is a system of ocean currents, which move heat around in the world’s oceans. In the Atlantic Ocean, warm upper-ocean water travels north, to the high northern latitudes, where it loses heat to the atmosphere. It’s this process that keeps the UK relatively mild in winter compared to other countries at similar latitudes. This water cools then sinks and returns southwards at great depth.
‘We found big and abrupt changes in temperature and salt concentrations in the sediment core, which matched changes in solar activity,’ says Moffa-Sánchez.
‘Our measurements show that when solar activity was low, sea water south of Iceland was colder and fresher, rather than warm and salty. The ocean changes and the atmosphere probably both react to changes in the Sun’s energy output and develop feedbacks between the ocean and atmosphere. If you have a colder ocean current and a high pressure system during solar minima, the heat transport to Europe is going to be reduced leading to the cold spells recorded in Europe,’ she adds.
The researchers tested their results using a computer simulation of the climate and found exactly the same results.
Other scientists have suggested that higher levels of volcanic activity during the Little Ice Age led to colder winters. But Moffa-Sánchez separated the lower solar activity and volcanic activity in the climate model to test this idea.
They found that the Sun’s energy levels had a much bigger part to play in the climate during the last 1000 years than volcanic activity did.
‘When you see an increase in volcanism, this coincides with solar minima, so it’s hard to separate the two and say which contributed most to the ocean changes we record. But when we separated the volcanic activity and changes in the Sun¿s energy ouput in our model, we found that the Sun played a dominant role in the ocean and atmospheric changes over the last 1000 years,’ she explains.
This isn’t the first time scientists have linked the Sun’s energy output to high pressure blocking systems in the North Atlantic. Other studies have shown how solar activity affects UK winter temperatures. A period of low solar activity, called the Maunder Minimum, has been linked to cold temperatures in the so-called Central England Temperature record.
Paola Moffa-Sánchez, Andreas Born, Ian R. Hall, David J. R. Thornalley & Stephen Barker, Solar forcing of North Atlantic surface temperature and salinity over the past millennium, Nature Geoscience, published online 9 March 2014, doi:10.1038/ngeo2094