Taking Michigan’s Temperature

An Interview with Michigan’s Official Climatologist

Andresen

In the face of a public debate that’s generating more heat than light, MichiganScience sat down with Jeffrey A. Andresen to discuss Michigan’s climate history.

Jeffrey Andresen has been the official state climatologist since 2001. Operating from within Michigan State University’s Department of Geography, the state climatologist conducts research, disseminates weather and climate information to the public, and administers the Michigan Automated Weather Network, which records weather data from 48 monitors statewide.

Green
Cartoon by Henry Payne

Real-time weather information
from the network is available online
at www.agweather.geo.msu.edu/.

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MichiganScience:
What can you tell us about the history of climate events in Michigan?

Dr. Andresen:
In terms of the climate in the geological time frame — thousands to millions of years before present — there is relatively little known other than that the climate has varied dramatically here. Michigan has experienced periods of glaciations, sheets of ice, as recently as 15,000 years ago, and there have also been periods of relatively warm, tropical-type climate.

Chart - click to enlarge

MichiganScience:
What do you use to determine climate patterns?

Dr. Andresen:
Prior to the initial use of weather instruments about 200 years ago, one must use so-called paleoclimate records, such as ice cores, sediment cores, pollen counts and tree ring patterns. For many of these records, you need a reference series that you believe in so that you can calibrate what you see in a tree ring or in concentrations of pollen. I have colleagues who work with sand dunes and sediment cores from ponds and lakes. They’ve been able to piece together some idea of past climate in the state, but it’s very, very sketchy other than knowing that there have been periods of relative difference. The only thing we know with a lot of confidence is that there have been wide swings from one extreme to another and everything in between.

Our state stone, the Petoskey, is fossilized coral. It doesn’t seem right for coral to be present in Michigan because coral requires a warm, saltwater-type environment. The Petoskey dates from the Devonian Period of the Paleozoic Era approximately 100 to 400 million years ago. The theory is that there was a very large and shallow, very warm and salty water body in the general vicinity that supported this kind of life. So, the conclusion is that the climate has been very, very different.

MichiganScience:
How wide has the fluctuation been between warm and cold over the past 15,000 years?

Dr. Andresen:
That’s difficult to say. Between the end of the last glaciation and the present, one has to believe climate has warmed several degrees Celsius. But that’s a guess. I don’t think I’ve ever seen anyone put specific numbers or have estimates on the temperatures, but it is safe to assume that the climate was significantly colder than it is now.

Chart - click to enlarge

MichiganScience:
What about the period of relative warming during the Middle Ages? Is there any data to suggest that had any impact on Michigan’s climate?

Dr. Andresen:
We have relatively little data from this period.

There are some stories relative to the Vikings in the North Atlantic and the colonization of Greenland. The Vikings colonized many areas of the North Atlantic region beginning in the 7th and 8th centuries, which was a relatively warm period in at least this part of the world. As it warmed up, their agriculture-based food production system worked even in more severe Northern climates. They spread out to the British Isles, Iceland, Greenland and even to North America. There’s a lot of good evidence on the extensive colonies they built in Greenland. Greenland, at that point in time, between 1100 and 1300, was warmer — certainly more than it has been since. It allowed them to take some of their agricultural system, including sheep, goats and dairy cattle from Scandinavia and other parts of Europe. Then the climate began to cool down again. The Norse were agrarians, while the Inuit natives fished and hunted. When the Norse could not produce fodder for their animals, they essentially just disappeared from those parts of the world. The records that were kept by the Norse just basically stopped.

MichiganScience:
Would it be safe to surmise that the same warming period in Greenland also impacted North America

Dr. Andresen:
Perhaps. Sometimes climate in different regions behaves independently. But, at least in the North Atlantic, it’s fairly widely thought, at least in that part of the world between 900 and 1200, that those were relatively warm centuries. After all, the Norse called Labrador "the garden of paradise for the world." Given the evidence we do have, I’d say there’ve been some very significant swings. In the last 1,000 years, there’s much more confidence.

MichiganScience:
How sudden was this cooling following the warmer temperatures after the 1300s?

Dr. Andresen:
Probably between 100 and 150 years.

MichiganScience:
That seems pretty rapid for such a significant change.

Dr. Andresen:
It is. One of the ironic aspects of the recent warm up and melting of glacial ice in this part of the world is the realization that the remnants of a lot of these settlements and areas that were colonized are visible again for the first time in centuries. There were several thousand Europeans living on the southwest and southeast coasts of Greenland at the beginning of the last millenium.

MichiganScience:
Isn’t the period following the warming in the Middle Ages called the “Little Ice Age”?

Dr. Andresen:
Yes, and the beginning of that was what doomed the Norse colonies in Greenland. Europe was very cold for a long time, up until the 1870s, which was the end of the Little Ice Age.

MichiganScience:
What caused the climate shift?

Dr. Andresen:
It’s difficult to say. Climatologists look at the global ocean conveyor belt — sometimes referred to as the thermohaline — and the movement of the Gulf Stream. That’s a key to climate in the North Atlantic. The Gulf Stream is one of the major ways the atmosphere and ocean move excess heat from the lower latitudes up into the Arctic and the Northern Hemisphere. Given that heat transport, Northern and Western Europe are much warmer climatologically than they would be otherwise. We know for a fact that at different times in Earth’s history, the Gulf Stream — instead of moving northeast from tropical sections of the Western Atlantic up into the Arctic — veered more directly east toward Iberia and northwest Africa. And when that happens, it cuts off heat to the North Atlantic and it can trigger the onset of glaciations in areas such as Greenland, Northeastern North America and Scandinavia.

Chart - click to enlarge

MichiganScience:
What other factors caused these climate shifts?

Dr. Andresen:
There are three minor but still important imperfections of the Earth’s elliptical orbit. A mathematician by the name of Milutan Milankovich postulated that each of these factors had cycles that lasted from 22,000 to 400,000 years. Occasionally, all three — changes in tilt, the wobble rate and changes in the elliptical orbit around the sun — caused the Earth to be either closer, more directly in the sun’s rays, or the opposite. That likely is a major piece of the onsets and ends of glaciations and other climate changes. There have also been meteor impacts, which is what many scientists believe brought about the demise of the dinosaurs and the end of the Cretaceous Period due to the rapid cooling of the Earth under a shroud of dust in the atmosphere. For similar physical reasons, major volcanic eruptions are also thought to have led to at least temporary changes in climate in the Earth’s past. Following the eruption of Mount Pinatubo in the Philippines in 1991, for example, global mean temperatures dropped on average almost 2 degrees Fahrenheit.

If you go back farther in time than that, you also have to consider continental drift, which could be another factor. We have plates on the Earth’s surface, on the semi-molten crust below the mantle, that have been drifting and crunching together. That, in turn, has an impact on the atmosphere above it. So, depending on where the continents are situated on the globe, there’s another significant impact on the climate.

MichiganScience:
When did weather records for Michigan originate?

Dr. Andresen:
We have some records that go back to the middle of the 19th century here, but these are very, very few in number and sketchy in detail. What little we do have, though, is interesting. The instruments suggest that many of the extreme minimum temperatures in our observational climate record that were reported were set in the 1880s and 1870s. It’s not really a fair comparison, though, because the exposure of those instruments was different than it is now. One could infer that it was possibly even colder than that because the instruments used to record the temperatures were on the tops of roofs, which is typically a warmer setting.

MichiganScience:
Tell us about the 1930s, when there was a significant increase in temperature.

Dr. Andresen:
The ‘30s, in our region, was especially significant because it was the driest decade since the beginning of the instrumental record. The 1930s, though, was a very demanding decade weather- and climate-wise. Especially 1934 and 1936, which were two of the worst droughts in the Midwest on record. Those are two benchmark years. Ironically, the 1936 drought followed one of the most severe winters, causing horrible, horrible conditions, with documented incidents of snowdrifts covering houses. Many of the record maximum temperatures in Michigan were set in either 1934 or 1936. The all-time state record of 112 degrees Fahrenheit was set in 1934 and still stands after 70 years. If you look around the Midwest, a lot of extreme record temperatures were set in one of those two years.

MichiganScience:
How does that jibe with claims that the highest recorded temperatures have occurred within the last 10 years?

Dr. Andresen:
It’s the difference between weather and climate. Climate is the long-term average of weather conditions over extended periods such as decades. Weather describes environmental conditions over shorter time periods, including such events as storms, hurricanes, droughts. The rule about what is classified weather and what is classified climate, according to international convention, is that a 30-year time period marks a climate period.

While many of those extremes occured in the 1930s, when you average over longer periods in time, the years in the most recent decade come out looking warmer — mainly because the minimum temperatures in the cold season have increased — and those temperatures have taken the annual averages up. So, the numbers you’re hearing about recently are the mean average temperatures for the entire year.

When we look at weather extremes, such as heavy rain events and hail storms, in Michigan, I have not seen any evidence of a change in the frequency of extremes. There isn’t much of a real trend. There’s a little more of a tendency for heavy rain events, but it’s small. In terms of extreme maximum temperatures, I don’t see much of a change. The overall averages of those temperatures have increased during nighttime hours and during the winter months.

One probably would make the argument that those extremes are what really matter because of the impact they have on humans, society and ecosystems. We should be concerned about any trends, especially the occurrence of extremes. Right now, I don’t see a big change or trend in extreme events. We’re going to be running an update of an earlier study soon that investigates trends of extremes and will be checking carefully for any recent changes.

MichiganScience:
February was blisteringly cold this year, wasn’t it?

Dr. Andresen:
Average temperatures in the state generally ranged from 4 and 8 degrees Fahrenheit below average for February. That was the largest negative departure we’ve seen in some time.

MichiganScience:
What do you attribute that to?

Dr. Andresen:
Physically, it’s because of the pattern of the jet stream — where it goes and how it flows around the hemisphere. From mid-November through the middle of January, we had a jet stream pattern across North America — with relatively cold conditions to the north and warm conditions to the south — that was further North than it normally is. This also kept the main storm track to our north in Canada, and most of the air masses moving into Michigan were of relatively mild Pacific origin, leading to many 50 degree days. Then in mid-January, there was a big change. We saw a big ridge develop over western North America, with a big trough over the East. Then, one after another, air masses from the Arctic and even Siberia came across the Pole, and the jet stream literally steered those frigid air masses down into the lower 48 states. This gave us our cold outbreak, and it was really pretty serious and significantly cold. Eight degrees below or above normal temperatures is a pretty big departure.

One of the more interesting aspects of the climate system is that we typically don’t see one jet stream pattern persist more than several weeks. That would most likely either be in the middle of the winter or the summer, which are the most stable seasons. During spring and fall, with a seasonal transition going on, you almost never see anything last this long. The worst, coldest winters are when we’re stuck in a cold pattern for a long time. Or, we see extended warm periods. This winter, we’ve seen both with incredibly mild conditions early and followed by very cold weather. All the warm water from the Pacific dissipated. It’s been replaced with cold water that rises with anomalously strong trade winds to create La Niña, which we haven’t seen in some time.

It probably is part of the natural variability of the system. We probably can’t attribute it to one particular cause. There’s a lot of variability, which is why it’s difficult to discern what’s natural and what’s not.

MichiganScience:
What does the future hold for Michigan’s climate?

Dr. Andresen:
Michigan has become warmer over the last three decades, but the temperatures now are still within the range of the natural variability that we’ve observed over the last 100 or 120 years. They are currently at the high-end of that range. Much of that warming has occurred in the winter and the spring seasons — and a good deal of that has occurred at nighttime, which is partially related to increases in cloudiness. Changes in summer have not been nearly as evident. At the same time, it’s also become somewhat wetter. The benchmark dry decade in Michigan was the 1930s, and it’s been wetter ever since. There is some indication in the last 10 years that the increasing trend in precipitation may have leveled off somewhat. That may be temporary, but then again it may not.

Future projections of climate in our region during the next century from most recent global climate simulations suggest conditions generally warmer and wetter than those of recent decades. Whether or not the general agreement of these projections with observations during the past few decades is coincidental is not clear — only time will tell.