As we step into a new decade, it’s a good time to reflect upon our collective actions as humans and their impacts on our only home, Earth. In Madison, Wisconsin the freeze and thaw data of Lake Mendota has been recorded for over 150 years. In 1855 the lake froze on December 18th, while this past season the lake froze on January 3rd, 2021. Many factors influence when the lake will freeze for any given year. However, some interesting trends pop out when we look at all the data at once. This article describes how and why the ice cover on Lake Mendota is changing.
Elevated atmospheric CO₂
Let’s revisit the years before 1850, the time of the Industrial Revolution. The levels of carbon dioxide (CO₂) in the atmosphere pre-1850 were steady at around 280 parts per million (ppm), well below the safe concentration of 350 ppm (Fig. 1). We cruised past this safe concentration around 1990. In 1850 came the second phase of the industrial revolution, which ushered in the use of petroleum as a new energy source. We started burning vast amounts of underground carbon in the form of coal and other fossil fuels, skyrocketing CO₂ in the atmosphere.
Weather versus climate
For our planet, CO₂ at the right levels keeps us at a comfortable temperature while too much leads to rising global temperatures, resulting in devastating climate changes. This is analogous to holiday candy for our bodies; a moderate amount is great and enjoyable, however too much can lead to serious issues like obesity and diabetes.
This increase in temperature isn’t depicted by a straight line, i.e., we find some winters colder than others and some warmer. For example, 2020 was the warmest year on record over land, coinciding with Lake Mendota freezing on January 3rd, 2021, later than normal. That is indicative of the weather that year. When we try to understand climate, we must account for the average changes over long periods of time, e.g., a few decades. In other words, the climate is the average weather across many years.
Chemistry teaches us that water freezes at 32 °F. Anything above that and ice starts to melt, thus the difference between water and ice is a difference of just 1 °F. Using data from the Global Historical Climatology Network, the Madison winter has warmed, on average, 2.4 °F since 1940 (Fig. 2). The warming is even more pronounced in winter than in summer. That temperature change can influence our environment in subtle, yet impactful, ways.
Changes in freeze and thaw date
The freeze and thaw date of Lake Mendota has been observed for over 165 years. The original definition for frozen was determined by whether or not one can row a boat from Picnic Point to Maple Bluff. Keeping this original definition throughout the years ensures a consistent record. However, due to this definition, the lake may not be 100% ice-covered on this date. Although we have the technology to make a more accurate measurement, we don’t want to ruin the integrity of this +165 year record and introduce any biases that could arise from changing the definition of “frozen.”
The data over the last 165 years shows the ice-on date creeping later in the season and the ice-off date pushed earlier (Fig. 3). These trends are not unique to Lake Mendota but are observed on many Northern Hemisphere lakes.
Two dates stick out in these time series. On January 30th, 1932 was when the lake froze unusually late and a record warm autumn likely contributed to the late freeze. The other date is February 27th, 1998, which is when the lake opened up during the 1997–1998 winter season. This is unusually early. The 1997–1998 El Niño may have been a factor since an El Niño event is associated with warmer and drier winters in the Midwest. El Niño’s are naturally occurring and happen roughly every 3–5 years.
These two events point to how weather and natural climate variability can influence Lake Mendota. However, weather and natural cycles can not explain the overall trend. The reason the ice-on date is shifting later and the ice-off date is shifting earlier is because the global temperature is increasing, and increasing faster in winter than in summer (Fig. 2).
Ice cover on Lake Mendota
Using the example of Lake Mendota, it is possible to observe the impact long-term increases in temperature have had on local ice fishermen. The ice duration ice cover on Lake Mendota is decreasing at a rate of about 2 days per decade (Fig. 4). This doesn’t sound like a lot, but given 165 years this translates into 33 fewer days of ice cover. Illustrating the effects of global warming can be observed not only on the global scale but on a local scale as well.
These changes in ice cover are a result of humans changing the climate's thermostat by adding CO₂ to the atmosphere. Global problems are difficult to solve, but not impossible. The ozone hole, discovered in the 1980s, is nearly closed in great part due to the 1987 Montreal Protocol, and acid rain no longer plagues us due to the enactment of cap-and-trade policies. This demonstrates a clear role for government policies in resolving environmental issues. To bring us back down to safe CO₂ levels, global economies need to decarbonize sooner rather than later. While the best time to act has passed, the second-best time is now.