Normally 6°C wouldn’t be very warm – but in the Norwegian islands of Svalbard it’s a sultry modern summer, unlike anything seen for at least 1,800 years. That’s what sediments taken from an Arctic lake have told William D’Andrea from Columbia University in New York and a US team. It’s even warmer than a medieval warm period when parts of the northern half of the planet were as hot as, or hotter, than today. And while the record they’ve made reflects just this one site, it adds to the picture showing how unique today’s climate is. It’s also another step towards understanding how climate has changed through history, William told me.
Climate dynamics are extremely complex, and cooling in some locations can happen at the same time as warming in others, or increased precipitation in some places along with drought in other places,” he said. “These are the fingerprints we are trying to map and understand by generating such reconstructions.”
The fingerprints slowly become clearer as scientists collect more historical records, often as tubes of ice drilled from glaciers, or of mud and rock drilled from sea and lake beds. The tubes, or cores, cut through layers of mud or ice built up year after year. Scientists can then use fossils and chemicals to date and work out what conditions were like when they were laid down.
One method, called alkenone paleothermometry, uses the amount of two different types of fat built up in algae fossils to show what the temperature was like. Certain algae produce unsaturated fats – more like margarine – or saturated ones – like butter – making more saturated fats at higher temperatures. When the algae die, they fall to the bottom of the water where they live, building a fatty temperature record. “It has been used in the oceans for 20 years, but its usefulness for reconstructing lake water temperature has only been demonstrated more recently,” William said. He has been pushing its use in lakes, and previously used it to try and understand temperature changes around the times Vikings and ancient Eskimo cultures disappeared from Greenland.
Armed with a tool to take to new sites, William was keen to go to Svalbard, thanks to its unusual place in the Arctic, north of Norway. “Svalbard is an interesting place from a climate perspective,” he said. “It sits at the junction of physically distinct ocean currents and atmospheric air masses and right in the pathway of major conduits of heat and moisture transfer from the Atlantic sector to the Arctic. That means that it is subject to intense climate changes when and if any or all of these processes vary through time. A sensitive site like Svalbard can be very informative.”
In August 2012, William and his teammates ventured to Kongressvatnet lake, in western Svalbard to take their cores. As well as being new places to collect temperature data, lakes also let them use a well-tested way to date these data. That method relies on grains of glass spewed out by volcanoes hundreds of miles away. In their case the dates of three eruptions in Iceland in 170, 1104 and 1362 are now well known. To pin them down, scientists can measure the chemical signature of the volcanic ash to link it with a volcanic eruption. However, Svalbard is the furthest north this test has been used. “This shows using minute shards of volcanic ash from the sediments can be used effectively to date sediment cores at High Arctic locations, quite far from volcanoes,” William said.
Medieval Lukewarm Period and Tiny Little Ice Age
In a paper published last month in the research journal Geology, the scientists first showed that alkenone paleothermometry could match temperatures recorded over the last century at Longyearbyen, Svalbard’s capital, 40 kilometres away from the lake. “The technique we are using does a good job of reconstructing the known changes in air temperature over the past 100 years,” William said. Then, looking further back, they found that modern temperatures on Svalbard hadn’t previously been seen in their record. Summers since 1987 on Svalbard have also been 2-2.5°C hotter than they were there during warmest parts of the Medieval Warm Period. Another discovery they made concerned a colder period in 18th and 19th centuries, known as the Little Ice Age. On Svalbard this period wasn’t particularly cold, even though that’s when glaciers on Svalbard reached the furthest they have in recent times. “The glacier advances that culminated between 100-200 years ago on Svalbard were probably due to increased snowfall in the winter, not colder temperature in the summer,” William explained.
William is using his method to look at more sites around the Arctic, as well as developing ways to get more information, in more detail, from lake sediments. He underlines that only by bringing data together from multiple sites can the big climate picture be seen. He points to a 2009 paper in the research journal Science by Darrell Kaufman, which showed a cooling of the Arctic until the 20th century, when it has warmed rapidly. “This work on its own doesn’t give us the full history of Arctic temperature,” he explained. “From our record, we can say that the recent warming on western Svalbard surpasses the summer warmth of Medieval times. However, we can only say with conviction that the Arctic is now warmer than it was during Medieval times, when the Vikings settled Greenland and explored the North Atlantic, by looking to the numerous temperature reconstructions throughout the Arctic produced by the scientific community.”
William J. D’Andrea, David A. Vaillencourt, Nicholas L. Balascio, Al Werner, Steven R. Roof, Michael Retelle and Raymond S. Bradley (2012). Mild Little Ice Age and unprecedented recent warmth in an 1800 year lake sediment record from Svalbard Geology DOI: 10.1130/G33365.1