June 4, 2011 — andyextance

Known as a scourge of Europe through the middle ages, the mighty Vikings disappeared from their Greenland settlements in the face of abruptly changing temperatures. In doing so, they followed the example of the Saqqaq people, whose Greenland existence also ceased in a period of rapid climate change. Climate is probably only one of many factors that led to these upheavals, warns William D’Andrea of University of Massachusetts, Amherst, one of the researchers who has made this link. However, studying past climate changes should help us better anticipate how climate may change in the future.

“Climate is a major factor that influences societies and cultures,” D’Andrea told Simple Climate, “but there are other factors that are social in origin and involve the way that cultures adapt. There have been very large changes in Earth’s climate system both over the last 10,000 and 100,000 years. Now, the globe as a whole is getting warmer and it’s because of greenhouse gases in the atmosphere in very large part. But what we really want to know is how will the places that we’re living in change. We can’t begin to do that unless we understand how the climate system has responded to changes in the past.”

It was the search for that in-depth climate knowledge that originally drove D’Andrea and his colleagues from Brown University, in Providence, Rhode Island, University of Nebraska, Lincoln, and Loughborough University, UK, to Greenland. “The goal of the paleoclimate community is to generate climate records from all over the world,” he explained. “No single site is going to be able to inform us about the climate system. We had identified western Greenland as a site that was interesting to work in because there weren’t many records from there.”

High in saturates or unsaturates

Historical climate records in Greenland had previously been taken from columns of ice drilled from glaciers that have existed for many centuries. However, they don’t necessarily represent temperatures in west Greenland, which is not always covered in ice. Unable to use ice records, D’Andrea and Brown’s Yongsong Huang found that the beds of two lakes, Braya Sø and Lake E, contained large amounts of fats called alkenones. These compounds are produced by algae, and their composition varies between being more unsaturated – more like margarine – or saturated – like butter.

As temperatures rise, algae produce more saturated alkenones, ultimately depositing them in sediment at the bottom of the water body where the algae live. Scientists studying ocean temperatures have exploited the ratio between saturated and unsaturated alkenones in sediment records since the mid-1980s in a technique known as “alkenone paleothermometry”. Yet to date, few have exploited it in lakes. “This is one of the first applications of alkenone paleothermometry to lake settings,” D’Andrea said. “A lot of the work that we did – and this took place over a series of years – was monitoring the lakes to understand those dynamics and evaluate whether or not we could even use this approach in lake systems.”

D’Andrea and colleagues interpreted the alkenone saturation in the lakes in a Proceedings of the National Academy of Sciences of the USA paper published Tuesday. The researchers found that Braya Sø and Lake E sediment provided a record reaching back 5,600 years, with temperatures in the surrounding region varying by up to 5.5ºC in that time. They could show that when the Vikings established small communities along the west Greenland coast in the 980s, the weather was relatively mild, similar to today. Around 1100 AD temperatures dropped 4ºC in 80 years and stayed cold, likely hampering farming and sea trade, until Vikings abandoned Greenland in the 1300s and 1400s.

What can my grandchildren expect?

Similarly, the Saqqaq departed around 2,800 years ago, at the end of a 200 year period when temperatures fell 4 °C. However, this was also the time that another civilisation, known as the Dorset people, who had technology better suited to cold, like snow shoes and ice knives, came to Greenland. Having endured centuries of rapidly fluctuating temperatures, they too left Greenland around 50 BC, during another cool period – although the link to climate change here is less clear. “The major cultural transitions were coincident with large changes in lakewater temperature,” D’Andrea summarised.

As well as revealing climate’s impact on people living nearby, the scientists’ temperature record allowed them to draw links with others collected in Ireland. These show cooler periods in Ireland when Greenland is warm. “It implies that there is some forcing mechanism that’s controlling these temperature variations which is having an opposite impact on the temperatures of western Greenland and Ireland,” D’Andrea noted. “Exactly what does that imply is a really interesting question. The North Atlantic Oscillation causes this temperature see-saw to occur, but as we understand it it occurs over much shorter timescales, over a period of a few years to a few decades. It’s not correct to say that we must be seeing the North Atlantic Oscillation over this timescale as well, but it is suggestive of that. What will help us tease that out is to develop climate reconstructions from additional sites that are also sensitive to the North Atlantic Oscillation.”

That’s now just what D’Andrea is doing, with sites in south-eastern Greenland, Ellesmere Island in Canada and Norway. Together these studies will help fill in the paleoclimate picture that is needed for improved understanding of the local effects of today’s climate changes. “What kind of storm frequency, what kind of temperature changes, what kind of seasonality shifts can my children and my grandchildren expect if they’re living in New York versus living in Australia, versus living in California, or Ireland, or Siberia, or Bangladesh?” D’Andrea asked. “If we can understand the system better then we will be better able to predict how it will respond to any type of forcing, whether that comes from our own greenhouse gas emissions or whether it comes from changes in natural forcing mechanisms.”