Winter's Last Gasp | Opinion

Last month, Wisconsin ended its warmest winter ever recorded and second-shortest lake-ice season. As a result my colleagues and I have been fielding a raft of media inquiries. Is it true that sugar maple sap has begun flowing?  What birds are you seeing right now? What's the effect of this early thaw on walleye? When did the ice fishermen stop fishing? (And, in classic Wisconsin fashion, our winter warmth was followed by a brief late-March snowstorm—a good example of short-term weather events intersecting with longer-term climate change.)

These are all good questions, and they're all good examples of the early ways by which species are responding to changing climates, warmer winters, and earlier springs. I'm grateful to reporters for sharing these stories. For all climate impacts are local, and many people first notice climate change in their backyard.

Yet, as a scientist who studies how species responded to the climate changes accompanying the end of the last ice age, I'm concerned that these questions miss the bigger ones: is this the end of winter in the northern U.S. as we know it? How will the end of winter affect our cherished landscapes, the species that live in them, and the human livelihoods that depend on winter?

For we can expect winters to continue warming and the snow season to shorten as long as we burn fossil fuels and release greenhouse gases into the atmosphere. By 2080, for scenarios of high greenhouse gas emissions, spring snow cover is projected to decrease by 50 percent across the Northern Hemisphere. Southern Wisconsin's best contemporary climate analogue in this future is Kansas City.

One response might be: so what? Isn't it pleasant having less snow to shovel and fewer early mornings scraping ice off our cars? An answer, of course, is that climate change affects all of us. The weakening of winter in Wisconsin also means lethally hot summers in Texas, intensified hurricanes in Florida, and devastating mega-fires in California. Overall, Wisconsin is fortunate: because we were colder to begin with than elsewhere in the US, our state is less exposed to climate change. Already people are choosing to move to Wisconsin as a refuge from the risks of global warming.

But Wisconsin is not immune to climate change, and we, too, will be transformed. Every school child in Wisconsin knows that we were once covered by the mile-high Laurentide Ice Sheet, as recently as 15,000 years ago, and that the lakes and dells of Wisconsin are legacies of that vanished ice. Perhaps fewer know that this unimaginably massive ice sheet was melted by a 6 degrees C (11 degrees F) rise in global mean temperature, which is similar to the warming expected later this century if fossil fuels are burned unchecked. For a first essential lesson from the past is that small temperature rises have big effects when they happen globally.

And it's not just ice. This past 6 degrees C warming gives scientists a remarkable model system to study, at scale, how species are affected by climate change. A second lesson from the past is that species and ecosystems are highly sensitive to climate change: when climates change, species move and ecosystems transform.

For example, at the last ice age, when ice covered my city of Madison, Wisconsin, all spruce trees grew south of us, as far south as southeast Georgia. Today, of course, spruce grows mostly north of us, covering northern Wisconsin and Canada. Hence, this past 6 degrees C global warming drove a nearly complete die-off of spruce across their former range and a rapid expansion elsewhere. For, when climates change, even trees move, as some populations die off and others expand.

Wisconsin ecosystems were transformed multiple times by the climate changes accompanying the end the last ice age. For example, the sediments at Devils Lake, Wisconsin, record four major climate-driven ecosystem transformations over the past 15,000 years, from spruce-dominated parklands to northern pine forests to forests of temperate oaks and elms, to oak savannas. All these transformations are well within human history in the Americas and were driven by rising temperatures and changing patterns of rainfall. We thus can expect that the changes we see now are just precursors to the transformations to come.

The good news is that nearly all species survived the last 6 degrees C warming, except for the big animals—mammoths, mastodons, ground sloths, and many more—killed off by a combination of changing climates, climate-driven changes in habitat, and human pressure. In eastern North America, only one plant species—Critchfield's spruce—is known to have gone extinct. So, the third lesson from the past is that many species have a high adaptive capacity to changing climates, as long as they have time to adapt and aren't heavily impacted by other stressors.

This last lesson gives grounds for pragmatic optimism because it means that we can help species survive today's changing climates. We can protect the natural diverse microhabitats that were good refuges for species during past global warmings. We can modify environments locally to cool them, e.g. by planting shade trees by rivers to protect cold-water fish. We can identify at-risk species and move individuals into newly favorable regions. To prevent future catastrophic fires, we can prescribe fires to thin forests after a century of fire suppression and overgrowth. There are many solutions to slow down climate change and many to help species adapt, but we must take the concern seriously, and we must act.

So, yes, the sap is running early in the sugar maples this year, but if we take no action, then someday we can expect many fewer sugar maples in Wisconsin. And less ice fishing, snowmobiling, and all the other recreations on which our northern economies depend. For winter is not coming, Wisconsin, and we must prepare.

Jack Williams is a professor of geography at the University of Wisconsin–Madison. His research areas include paleoecology, paleoclimatology and global climate change.

The views expressed in this article are the writer's own.