Now or never. Before I can second guess myself I accelerate into a full blown sprint. My feet pushing off the coarse concrete with each step until nothing stands beneath them. For a fraction of a second I feel nothing. I close my eyes as I hit the water.

Pulling myself above the waves I turn around to stare at the Chicago skyline before swimming to the ladder.

A couple stares at me in bewilderment, shaking their heads, and then continuing their stroll.

To be fair - it was early October.

Yet the water was still nearly 60 degrees.

Having swam in Lake Michigan over 100 days in 2024 I, and many of my friends, have noticed that the lake has felt noticeably warmer than prior years.

As an avid lake jumper I already know what I consider "warm" diverges from societal norms, so could my observations be skewed?

As a power user of the National Weather Service’s “Southern Lake Michigan Water Temperatures” daily reports, it only seemed natural to leverage their historic database (supported by Iowa State University). 

Focusing on temperatures collected near the one of Chicago’s Water Intake Cribs (located about two miles from shore), I analyzed individual daily temperatures from 2008-2024. 

For each day of the year I calculated the average temperature and standard deviation in this time frame (ie what was the average water temperature for all Dec 25ths in this timeframe and standard deviation from this average). Standard deviation is the average distance of each value in a data set from the mean. In datasets where most of the data is clustered around the mean and there is minimal variance, the standard deviation is small, whereas in datasets where it is widely spread out it is larger. Assuming that a dataset has a normal distribution, ~68% of the data should fall within 1 standard deviation above and below the mean. Thus a datapoint being either larger than the mean + 1 standard deviation or less than mean - 1 standard deviation is a notable diverge from the mean, even if not necessarily itself statistically significant.

Leveraging this I calculated the number of days in each calendar year that fell one standard deviation above or below the average temperature for that date. Years with a higher number of days falling one standard deviation above the average temperature for those respective dates would indicate the lake was noticeably warmer for periods of that year than normal seasonality would account for, and the inverse being true for years with a higher number of days falling one standard deviation below the average temperature.

Table 1 shows the number of days each year that fell within 1 standard deviation, were more than 1 standard deviation above, or more than 1 standard deviation below the average temperature for that day. Figure 1 graphs the 7 day rolling average temperatures for 2022, 2023, and 2024 and whether they fall within +/- 1 standard deviation range of a date’s average temperature.

Although there is always a large degree of fluctuation, outside of a few outliers generally most years have fewer than 50 days with temperatures above 1 standard deviation of that day’s average temperature. All but 5 have fewer than 75 days with a temperature above 1 standard deviation of a date’s respective average temperature. Similarly all but 3 years have fewer than 75 days below 1 standard deviation of that a respective date’s average temperature, and all but 5 have more than 20 days falling below that range.

As of November 1st, 2024 has already had 95 days where the water temperature near Chicago was measured above 1 standard deviation of that date’s average temperature since 2008, and only 1 date with a temperature below 1 standard deviation of its average.

Only 2012 and 2021 surpass 2024 in terms of number of days with higher than normal average temperatures (143 and 174 respectively).

Notably when looking at Figure 1 it becomes apparent that many of the dates with higher temperatures actually occurred earlier in the year (February - March, late April - May) rather than during the summer months. Compared to 2023 this allowed water temperatures to rise faster, essentially being at May temperatures in April, and April in June. Water temperatures also have held on stronger than 2023, being at the upper end of the +/- 1 standard deviation range since September.

Naturally one would think that this would be solely an indication of current temperatures, however looking at Chicago average air temperatures in recent years compared to data going back to 1990 (Figure 2), this trend only partially holds up.

Although 2024 is consistently above the +1 standard deviation mark for each month, 2022 (which also had relatively high but within +1 standard deviation Fall water temperatures) was decently within the +/- 1 standard deviation range for air temperatures throughout the year. Even when water temperatures were well below -1 standard deviation in 2022, air temperatures were essentially average for most months during that time.

Air temperature is a major driving factor of lake water surface temperature, but also the temperature of water sources (lakes, rivers, ice coverage), the amount of solar radiation received (negatively impacted by ice coverage and clouds), and other factors.

This is not just a trend localized to Chicago’s coastline. When looking at the each of the Great Lakes as a whole over a longer time horizon (1990-2024) the same trend can be observed (Figures 3-7, Tables 2-6). 2024 has had ~200 days where the average lake water temperature in each of the Great Lakes was above 1 standard deviation of that day’s average temperature since 1990. This represents almost the entire second half of winter and first half of spring (February - mid-May) as well as late summer - early autumn (September - October). 

Air temperatures are clearly not the sole culprits for the recent rise in Great Lakes water temperatures. One of the few easy to analyze is winter ice coverage thanks to the resources available from National Oceanic and Atmospheric Administration’s (NOAA) CoastWatch Great Lakes’ Ice Coverage data. 

Graphs and data related to all of the Great Lakes can be found on their website, however leveraging Lake Michigan as an example, pulling NOAA’s CoastWatch Ice Coverage reports for 2022, 2023, and 2024 (Figures 10-12) helps complete the picture. 

The last year that had significantly below 1 standard deviation water temperatures (2022) had slightly above normal ice coverage during much of the winter. Combined with relatively average temperatures in the Great Lakes Basin that year may have contributed to a delayed warming of the lakes, as the ice served to block solar radiation from receiving additional warmth during the winter months.

2023 and 2024 on the other had significantly below average levels of ice, and during the winter of 2024 Lake Michigan was ice free by early March. This thus enabled the “warming season” for the lake to begin much earlier than in 2022, helping to give water temperature a kickstart to head towards their summer seasonal highs.

In the short term this may indicate we should expect lower than normal average water temperatures in 2025 - as significantly higher water temperatures in the fall may lead to more evaporation if air temperatures drop to below freezing in early winter, which may lead to more winter precipitation and corresponding ice coverage (“Great Lakes Ice Coverage | GLISA”). This can then to lower average water temperatures the following year, leading to less evaporation in the fall, resulting in less precipitation in the winter, starting the multi-year cycle over again.

Longer term however we can expect 2024 to become more the rule than the expectation. As noted by GLISA (Great Lakes Integrated Sciences and Assessments) ice coverage of the Great Lakes has been declining since they started measuring coverage in 1973. Between projections of continual declining lake ice coverage and rising air temperatures thanks to global warming, this will inevitably lead to a continued rise in average lake temperatures - even as some semblance of the multi-year ice coverage cycles may remain.

Although beneficial for human enjoyment of the Great Lakes, warming temperatures come with serious drawbacks. 

Reduced winter ice coverage may lead to increased lake effect snow / freezing rain, causing strain on transportation in coastal communities along the lake through the winter months. 

Water temperatures rising earlier in the year results in more significant and prolonged thermal stratification in the lakes. This, combined with rising temperatures and nutrient discharge, can lead to an increase in quantity and severity of algal blooms which can devastate local marine life. (GLISA). Beyond algal blooms, rising water temperatures will directly negatively impact many of the local cold water fish and plant species that have evolved to the conditions of the Great Lakes are outcompeted by warm water species (including non-native invasives). 

Beyond the tremendous potential loss of biodiversity and life, impacts on local fisheries will inevitably negatively impact the Great Lakes fishing industry which generates over $7 billion in annual revenue and supports over 75,000 jobs. (Great Lakes Fishery Commission).

The major driver of Great Lakes' warming is human derived climate change, and without intentional national and international action to address this then any local efforts will be fighting an upstream battle.

However there are actions that can be taken to mitigate some of the symptoms of rising Great Lakes temperatures.

1. Reducing algal blooms by minimizing fertilizer runoff through stricter controls on agricultural practices and reducing residential application: Although warming waters increases the risk of algal blooms, their intensity and frequency can be limited if excess nutrient build up is mitigated. 

2. Proactive management of local fish populations and spawning habitats by state and federal agencies: Whether through population supplementation via fish stocking, rehabilitating and protecting fish spawning environments in rivers and wetlands, or other means - negative impacts to local fish species can be managed to avoid their complete eradication from our lakes.

3. Combatting invasive fish and plant species through limiting additional introduction and controlled removal: Native flora and fauna are not just at risk from the warming waters themselves, but from being outcompeted by other species that evolved in warmer water ecosystems. Maintaining and expanding programs to reduce the introduction of additional invasive species through regulated cargo ship ballast discharges, guarding connections to other water ways such as the Mississippi River, and working on programs to remove non-native species that have already been introduced all can reduce this threat to local wildlife.

For more information on what can be done to protect the Great Lakes, there are a number of great organizations such as Alliance for the Great Lakes that you can either donate to or get involved with to help protect our Great Lakes for future generations of avid lake jumpers to enjoy.

“Climate Impacts | GLISA.” n.d. GLISA. Accessed November 3, 2024. https://glisa.umich.edu/resources-tools/climate-impacts/

GLISA. n.d. “Algal Blooms | GLISA.” GLISA. Accessed November 3, 2024. https://glisa.umich.edu/resources-tools/climate-impacts/algal-blooms/

Great Lakes Fishery Commission. n.d. “The Fishery.” Great Lakes Fishery Commission. Accessed November 3, 2024. https://www.glfc.org/the-fishery.php

“Great Lakes Ice Coverage | GLISA.” n.d. GLISA. Accessed November 3, 2024. https://glisa.umich.edu/resources-tools/climate-impacts/great-lakes-ice-coverage/

National Oceanic and atmospheric Administration. n.d. “Average Surface Water Temperature (GLSEA).” Coastwatch Great Lakes. Accessed November 3, 2024. https://coastwatch.glerl.noaa.gov/statistics/average-surface-water-temperature-glsea/

National Weather Service. n.d. “Climate at a Glance | National Time Series.” National Weather Service. https://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/national/time-series/110/tavg/1/0/1990-2024?base_prd=true&begbaseyear=1901&endbaseyear=2000

National Weather Service. n.d. “Other Marine Products | Chicago.” National Weather Service. https://mesonet.agron.iastate.edu/wx/afos/list.phtml?by=cccc&source=LOT&pil=AFD&year=2024&month=6&day=28&year2=2024&month2=6&day2=28&view=grid&order=asc