Lake Superior is Changing. Fast. Lakes like Superior appear to be responding more quickly to climate change than we previously suspected. Our current hypothesis as to why this is occurring has to do with the simultaneous decline in the amount of ice found on these lakes. It's a pretty cool story. I'll also note that many, if not most, of these trend observations have been made before (i.e. increase in temperature, decrease in ice); all we've done is snap them together into a big picture. This research has recently been accepted for publication at Geophyiscal Research Letters, and was performed by myself and my colleague Steve Colman.
The observation that started this was based on a very long-running measurement of data near the St. Mary's Falls hydroelectric power plant in Sault Ste Marie, in the St. Mary's river, which drains Lake Superior into the rest of the Great Lakes. At this location, a daily temperature measurement has been made since 1906! It's one of the longest directly measured water temperature records anywhere in the world.
In this figure, the light blue line represents the annual summer (July-September) average temperature anomaly, where the anomaly represents how different it is from the long-term average (i.e. if the anomaly value is -2C, it means that that year was 2C cooler than the long-term average). The heavy blue line represents 10-year averages of this data. The red data is from the Goddard Institute for Space Sciences Air Temperature database; this is the curve you see when you read an article on global warming- it's the globally averaged surface air temperature (anomaly, again). The really striking thing here is that the long-term trend in Superior is so much stronger than the global average. Well, we know that the upper midwest is warming more rapidly than the global average, but not this much more rapidly. To better understand what was going on, we decided to focus on the last 25-30 years, for which much more (and much better) data is available.
We gathered data from three buoys (operated by NOAA's National Data Buoy Center),in Lake Superior, in the Western, Central, and Eastern Basins (shown below as 45006, 45001, and 45004).
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Go ahead- you can grab it too- it's free to anybody who cares to download it. These buoys measure water temperature, air temperature, wind velocity, sometimes waves, sometimes humidity. I care about water temperature, though it's worth noting that there are interesting trends in many of the other fields too.
Before we look at long term trends, here's a typical annual cycle of temperature at the western buoy:
In mid-April, the buoy is placed in the water (they are typically pulled out in the winter to avoid ice damage). The heat going into the water has to heat up a lot of water, so the temperature changes slowly. Fresh water has the bizarre property that it's density (essentially, how heavy a fixed volume of fluid is) has a maximum at around 4C. This means that if you spot 4C water at the surface, you know the density has to be pretty uniform throughout the water column, and the water column mixes. In this example, the overturn occurs around the beginning of July. Once the water warms up beyond 4C, the lake develops stratification, where warmer water sits on top of colder water. This is an important date, because after this occurs, heat put in to the surface of the lake is distributed over the top, say, 30 meters instead of the entire lake- the surface temperature can increase quickly, as we see here. Some time in August, the lake attains its maximum temperature, in this example a bit over 19C. After this, it starts cooling, both by losing heat to the atmosphere, but also by large storm events mixing cold water from the lower layer into the surface layer, as we observe in this example around mid-September. In November, the buoy is removed from the water.
Now let's look at some long-term trends:
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This figure shows summer (July-September) averaged values for water and air temperature (first row), wind speed (middle row) and the date of the spring overturn at the three buoys. It's clear from these graphs that (1) air and water temperatures are increasing quickly (about 1 degree Celsius, or ~2F, every decade), wind speeds are increasing, and the date of the spring overturn is becoming earlier (about 1/2 day per year).
Yes, there's a lot of scatter- especially noteworthy is the data from 1998, when a huge el nino caused a very early overturn and very high summer water and air temperatures. THere is plenty of variability from year to year- it's a complex system! The point is that on average, things are getting warmer, overturns are becoming earlier.
Here are the some other rates of change- regional air temperature, again taken from the Goddard Institute for Space Science global air temperature database. (Again, if you're so inclined, go and download the data yourself- it's free). To produce this regional average, I found all of the stations I could within a 500km radius of the middle of Lake Superior. The regional air temperature is increasing about 0.5C per decade- about HALF as fast as the Lake Superior summer water temperature! What's going on?
The next panel (B) shows the average amount of ice on Lake Superior from 1978-present. The average is over both space and time (between 1 December and 1 May) and the original data can be found here. It shows the ice going away over time, again with huge amounts of interannual variability.
Ice plays an important role in the thermal cycle of the Great Lakes, as well as in oceanic regions like the Arctic Ocean. Ice is good at reflecting light, so that if there's lots of ice, it's difficult to absorb sunlight, which is the primary source of energy for heating up the lake in the spring. Take the ice away, and it's a lot easier to warm up the lake (or the Arctic).
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Now, here's the kicker: when you look at the date of the start of the stratified season as a function of that year's average ice cover (the top row in this figure; each dot represents a different year from 1979-2005), you see that the more ice cover you have, the later the start of the stratified season. In the same vein, the more ice you have over the winter, the cooler the following summer is going to be (the bottom row). By delaying the start of the stratified season, ice essentially shrinks the amount of time that the lake has to heat up in the spring and summer. In fact, winter ice cover is the primary indicator of how warm the lake will be the following summer- in other words, if you have a lot of ice, but a warm summer, the lake will still be cool that year.
Here's a figure that summarizes these relationships:
Here I've plotted the water temperature (averaged carefully between the three stations) and the regional air temperature on the top graph, and ice cover on the bottom. You can see that in warm air temperature years (i.e. 1983, 1998, or 2005) we get warm water; in low ice years (1983, 1987, 1995, 1998-2000, 2002) we get warm temperatures- if both are true (i.e. 1983, 1998) we get really warm temperatures. In cool years (i.e. 1992-1993) we get cool temperatures, and in high-ice years (i.e. 1979, 1994, 1996, 2003) we get cool water temperatures. It doesn't work perfectly- this is a massively complicated system. But it explains what sets water temperatures to first order.
To conclude, summer temperatures in Lake Superior (and Huron and Michigan, by the way) are increasing due to two separate but related trends: summer air temperatures are increasing, and winter ice cover is decreasing. Both of these effects add to produce the observed response of around a degree C per decade increase in Lake Superior water temperatures.
Press references:
As it Happens, CBC (the first piece in the second half)
Minnesota Public Radio
One of a zillion copies of an AP article
Here and Now, on Wisconsin Public TV (go to about 7:40)
