My money’s on January 11. Well, not really – it’s a low-stakes wager (no money). Here in Wisconsin, it’s an annual contest. Guess when Lake Mendota will freeze and if you guess correctly, your name goes into the hat with other correct-guessers for a drawing of a $1000 prize. You can play too if you are U.S. resident. Click here! Your name likely gets captured for a mailing list, but the Clean Lakes Alliance is a worthy non-profit that works for the health of lakes, streams, and wetlands in the Yahara watershed.
January is an informed guess. Mendota is my home lake and I get a passing glance of it nearly every day. Our big urban campus has roughly 4 miles of shoreline and I’ve walked every inch. Wisconsin’s State Climatology Office has recorded dates of ice-on and ice-off for Mentoda since the 1850s and the data are easily found online. Before making my prediction, I glanced at the data and thought about the warm fall we’ve had.
The contest is good civic-minded fun but there’s foreboding behind the cheery boosterism. On Saturday morning, I wondered about my prediction and downloaded the data into a spreadsheet for a closer look. A simple analysis suggested that there has been a long-term decline in first-ice since the 1850s and that, on average, Mendota sees its first ice roughly a day later for each decade that has passed (the dashed line).
I knew this, of course. Mendota is, famously, one of the most-studied lakes in the world. It’s one of 514 lakes in the northern hemisphere with long-term data on ice cover. And ongoing analyses of these lakes are another in a long list of empirical observations of a warming climate. World-wide, northern lakes are icing up later in the year and becoming ice-free earlier in the year. These trends appear to be driven primarily by changes in mean air temperatures and secondarily by variation in the size, depth, and structure of the lakes.* In Mendota’s case, duration of average annual ice cover has declined by over a month since the 1860s.
Yesterday was day 2 of a cold snap and I photographed ice starting to form. I had business on the other end of campus, and I walked the lakeshore path. The juvenile loon that was stopped-over for a few weeks was gone. This was her first migration, recharging her batteries with Mendota fish. Imagine her delight when her ancient internal autopilot guides her to some rich and humid waterway near the Gulf of Mexico (where she’s never been).
I had townie mallards and geese and pugnacious coots and the ever-reliable muskrats for company despite the raw wind out of the north. A bobbing raft of northern shovelers were filtering the surface for invertebrates just beyond the ice that formed overnight. They’ll likely be in the Caribbean in a few weeks. I am betting that by January 11, I’ll be walking in winter stillness where they are now, and they’ll be somewhere else.
Ice dynamics profoundly affect the ecology of our northern lakes and we scientists trail along trying to understand before the opportunity to understand is gone.
My collaborators and I are studying how ice-cover influences the mammals living on Great Lakes islands. We have already shown that mammal communities in the Apostle Islands (Lake Superior) differ depending on how isolated individual islands are from one another and from the mainland. Some part of that story reflects winter ice cover and the willingness or ability of some species to cross the ice. With ice cover declining in Lake Superior as well, those communities will change and the degree to which Great Lakes islands can function as biological refugia in the face of the climate crisis will diminish.
For our planet, humanity is making very much a high stakes wager about the drivers of changing ice dynamics. Our data will be a baseline for when ice on Superior becomes a rarity. That’s what we do now – document the changes and mechanisms, think about mitigations, worry.
Yearly I find myself standing on the shore to let the tensions of a busy fall semester follow my imagination out over the foam stringers and the dark water. Ice can surprise you, sometimes it freezes suddenly and clear and you can skate effortlessly over an invisible window on the underwater plants – small bubbles literally frozen in place. Sometimes it freezes halfheartedly only to be broken in the morning chop – sounding like a great chorus of muffled gentle wind chimes on the windward shore. Sometimes it sings like a whale song when the sun sets, and the temperatures drop.
We entwine our spiritual rhythms with earthly rhythms. With our earthly realities changing, will our spiritual realities change too? Can we do any less? Our hemisphere tilts farther into the dark, and cold introspection becomes tactile. My advent draws more naturally with the spiritual gravity of open water nearby. Any day now, open water will be gone. Iced over and covered with snow (at least for now). And then, it too, will be something to long for again.
*Sharma, S., Blagrave, K., Magnuson, J.J. et al. Widespread loss of lake ice around the Northern Hemisphere in a warming world. Nat. Clim. Chang. 9, 227–231 (2019). https://doi.org/10.1038/s41558-018-0393-5