|Fig.1. Greenland Ice Sheet (Credit: Christine Zenino, 2009)|
Researchers at Stockholm University, Plymouth University, and the Natural History Museum, London (NHM) may have solved one of the most enduring puzzles from the last Ice Age. Around 12,880 years ago, the Northern Hemisphere experienced a sharp change in temperatures that culminated in a decrease of about 5℃ over 400 years. This event, called the Younger Dryas, is well documented in environmental records, but the cause had been largely unknown. In a study recently published in the journal Nature Communications, the researchers brought together three types of collections to address this mystery.
Locked in Ice and DirtThe previous culprit to blame for the Younger Dryas had been the melting of the Laurentian Ice Sheet in North America.Theoretically, ice sheet melting pours an enormous amount of freshwater into the North Atlantic Ocean, effectively shutting down the process that carries warm water towards Europe by the Gulf Stream. This event would cause an abrupt cooling of the North Atlantic and other parts of the Northern Hemisphere. The recorded climatic fluxes in ice core records, however, did not match with models for a freshwater flood from North America around 13,000 years ago.
Instead, lead author Francesco Muschitiello at Stockholm University and his colleagues think that the melting of the Fennoscandian Ice Sheet in Scandinavia caused the cold snap. His team turned to sediment cores from an ancient lake in Sweden and ice cores from Greenland. The sediment held moisture-sensitive compounds that Muschitiello et al. used to understand changes in the hydroclimate, which they then calibrated with ancient temperatures recorded in ice cores.
Additionally, they examined the sediment cores for fossilized insects, known as midges, which act as proxies for temperature increase or decrease. NHM researcher Steve Brooks developed a “midge thermometer” that proved to be an accurate indicator of changing climatic conditions. The abundance and types of midge species in lakes are directly related to the local summer air temperatures. As midge larvae grow, they shed their heads, which are then preserved in sediment and can be compared to a natural history collection for identification.
The Past and Future ClimateBrooks is not the only scientist to use insects as temperature gauges. A recent study in the Journal of Animal Ecology was the product of an 18-year, full-season collection of insects from the rooftop of the Natural History Museum of Denmark. During the nearly two-decade experiment, around 250,000 insects representing 1,543 different species of moths and beetles were collected. Analysis of the collections showed turnover in the local insect community as temperature increased, pushing out more cold-adapted species. Insects that depended upon a single food source were hit particularly hard and forced to move as their food shifted north. Insect collections, like this recent one in Denmark or like those in London, allow scientists to directly connect climate change to shifts in species assembly and abundance.
Between insect collections and sediment or ice core records, these studies create a more nuanced model of how rapid fluxes in the climate affect animals and the landscape. Ultimately, researchers attribute an ancient replacement of Northern Hemisphere’s forests with permafrost and even a possible decline of human populations to the Younger Dryas. Today, rising sea levels threaten not just coastal habitats and wetlands but also millions of people living in areas vulnerable to flooding. Accurate climate models based on ancient and modern data are necessary to understand what the future may hold and how to respond to such dangers.
Muschitiello, F., et al. (2015, November 17). Fennoscandian freshwater control on Greenland hydroclimate shifts at the onset of the Younger Dryas. Nature Communications, vol. 6. doi: 10.1038/ncomms9939
The midge thermometer. (2015). Natural History Museum, London News. Retrieved from: http://www.nhm.ac.uk/our-science/our-work/origins-evolution-and-futures/the-midge-thermometer.html
Thomsen, P. F., et al. (2015). Resource specialists lead local insect community turnover associated with temperature - analysis of an 18-year full-seasonal record of moths and beetles. Journal of Animal Ecology. doi: 10.1111/1365-2656.12452