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Climate, Attribution and Lake Superior's Productivity

2017 wasn't an easy weather year on Earth. Southern Europe's blistering summer, for example, included a heatwave so deadly it was nicknamed Lucifer. In a recent analysis1, scientists reported that the chances of getting a heatwave in southern Europe increased by at least a factor of four since 1900 due to changes in Earth’s climate.1 In other words, people in Spain a century ago could expect three-day heatwaves once every 20 years; now they can expect them every five.

Hurricanes, too, wrought death and destruction, particularly in September, which the NOAA National Hurricane Center reports was the most intense month for Atlantic Ocean tropical storms and hurricanes on record achieving an accumulated cyclone energy measure that soared about 3.5 times over average Septembers from 1981 to 2010.2 NOAA scientists at the Geophysical Fluid Dynamics Laboratory are cautious in their approach to determining what portion of hurricane intensity can be attributed to human-induced climate change. However, even these scientists project that human activities that have already added greenhouse gases to the atmosphere will likely cause tropical cyclones globally to be more intense by 2 to 11 percent by the end of the century.3

Advances in understanding the science behind extreme events has enabled climate scientists to become increasingly able to attribute particular events to climate change over the past decade. The National Academy of Sciences published a report4 documenting these advances and the science of extreme event attribution,
which seeks to tease out the influence of human-caused climate change from other factors, such as natural sources of variability like El Niño.

Some of Earth's systems more measurably reflect a changing climate than do the
frequency of extreme events. Lake Superior is one of these systems. Through research partially funded by Minnesota Sea Grant, scientists with the University of Minnesota Duluth's Large Lakes Observatory discovered that an unprecedented increase in Lake Superior’s algae abundance has occurred over the last century. They attribute this new productivity to warming because of human activities in a recent issue of Nature Communications.5 The surge of algae production in Lake Superior is a specific example of how the Earth is reflecting a changing climate. "Even abrupt climate changes like those experienced in the Medieval Climate Anomaly [950-1250 AD] or the Little Ice Age [1400-1900 AD] did not influence primary production in Lake Superior like what we’re seeing now," said Molly O'Beirne, lead author of the article.

O'Beirne and co-authors explain that natural weathering likely caused a slow, steady increase in primary production since the last ice age due to iron and
phosphorous being released from the rocks. They found the current rate of algae growth coincides with warmer surface water temperatures and longer periods of summer stratification, in which a layer of relatively warm water rests on top of cold water. Extended stratification is related to diminishing amounts of winter ice cover. The Nature Communications article is available online.5 For a printed copy, contact Minnesota Sea Grant (seagr@d.umn.edu; 218-726-8106) and ask for JR631.

1Cullen, H., van Oldenborgh, G.J., Karoly, D., Otto, F., and van Aalst, M., 2017. Euro-Mediterranean Heat – Summer 2017. World Weather Attribution.

2NOAA National Hurricane Center. 2017. Monthly Atlantic Tropical Weather Summary, Oct. 1.

3NOAA Geophysical Fluid Dynamics Laboratory. 2017. Global Warming and Hurricanes: An Overview of Current Research Results. Last Revised: Aug. 30, 2017.

4National Academies of Sciences, Engineering, and Medicine. 2016. Attribution of Extreme Weather Events in the Context of Climate Change. Washington, DC: The National Academies Press.

5O’Beirne, M.D., Werne, J.P, Hecky, R.E., Johnson, T.C., Katsev, S. and Reavie, E.D. 2017. Anthropogenic climate change has altered primary productivity in Lake Superior. Nature Communications.


By Sharon Moen
January 2018

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