West Michigan Climate Model: Change threatens local apple crop

Introduction

Lake Michigan has a significant effect on downwind weather and climate, regulating temperature and supplying precipitation. This “lake effect” is strongest within the first 30-50 miles inland, creating a microclimate along Michigan state’s west coast suitable for fruit agriculture. For this reason, Michigan is host to Midwestern vineyards and is one of the nation’s largest apple exporters.

The following diagram visualizes West Michigan’s climate model. Climatic warming drives top-down change within the system, impacting the assumed goal of apple production.

Biennial balancing feedback loop

Before explaining how the lake effect supports apple development, we must establish the lake’s biyearly temperature cycle, which is represented in the model as a circuit between “cool lake temperature in the winter/spring,” “warm lake temperature in the fall/winter,” and “lake ice cover.” 

A fundamental concept to understand for regional climate dynamics is “thermal lag” experienced by water. Relative to air temperature, Lake Michigan is fairly cold in the spring and warm in the fall, demonstrated by the fact that water temperature in April is 18*F colder than October, and in June it is 15*F colder than August, despite these months having similar air temperatures. This is due to water’s high specific heat capacity, meaning that it changes temperature much more slowly than the atmosphere does; it takes a lot of heat to warm water up, and it takes a while to release all that heat. So while air temperatures have already risen in the spring, the water still has “thermal memory” of the winter and is relatively cold. Accordingly in the fall, months of summer warming have swayed the water’s “thermal memory” in the other direction, being relatively warm compared to the air. This phenomenon is called “thermal lag,” based on the amount of time it takes an object to warm up once exposed to heat, and the amount of time it takes for heat to diffuse away when the object’s environment is colder.

Water’s thermal memory plays an important part in the lake’s biyearly balancing feedback loop. Primarily, water temperature early in the year positively correlates with late-year water temperature, where if water is extra cold in the spring then it won’t get as warm in the summer and fall, and if water is extra warm in the spring then it will stay extra warm throughout the summer and fall. This is represented in the diagram as cold winter/spring water temperature having a negative correlation with warm water temperature in the subsequent fall/winter. A simple conclusion from this correlation is that excess warming will cause a runaway temperature effect, where warm water one year leads to even warmer temperatures the next, however there are various mechanisms to prevent this. 

The primary means of regulation is lake ice formation, preventing a temperature runaway in either direction. In summary, lake ice is created by warm water and then blocks the absorption of solar energy, making the lake colder. When lake temp is cold, lake ice is not formed, allowing for solar heating and making the lake warmer. Counterintuitively, warm lake water creates a wider extent of lake ice in the winter due to what’s called “evaporative cooling.” When the water is warm during the fall and winter, more of it can evaporate into the atmosphere, taking heat away from the water’s surface. With less heat at the surface, more ice then forms on top of the lake despite having a higher initial temperature. Ice is important due to its high albedo, meaning that it reflects sunlight back into the sky rather than absorbing its energy as heat. This reflectivity prevents any warming during the winter, thus reducing lake temperatures and leading to a cold spring season. Of course as previously stated, low spring temperatures entail low fall temperatures, so the next winter there will be less heat in the water than the previous year. With a lower temperature there will be less evaporation and less evaporative cooling, meaning that less lake ice forms than previous. Less ice cover allows the water to be warmed throughout the winter by solar radiation, thus leading to a warm spring season. In this way, a warm lake causes the next year to be colder, and a cold lake causes the next year to be warmer, mediated by the amount of lake ice cover. As represented by a three variable circuit in the model, this biyearly cycle is a balancing feedback loop that regulates and stabilizes temperatures, being an important component to local climate patterns.

Precipitation

Each of these variables affects precipitation as well, where warm water years produce above-average precipitation, and cold water years produce less. Mentioned previously, warm water evaporates more readily due to having more heat, causing more rain or snowfall downwind. Despite not being included in the model, having warm spring and summer water temperature does increase precipitation, though it is in fall and winter months when warm water temperature supports a large production of snow. In the winter when cold air moves across the warm lake, the evaporation of water accumulates as snowclouds, leading to what’s called “lake effect snow.” As per evaporative cooling, this forms lake ice which then blocks further evaporation. In the model, these relationships are represented through a positive correlation between precipitation and warm water temperature in the fall and winter, although the resultant sea ice then has a negative correlation with precipitation by preventing evaporation. 

Climatic warming will increase the temperature of Lake Michigan, given warmer air temperatures supply more heat to the water year-round. With higher lake temperature, we expect to see greater evaporation and therefore a greater amount of precipitation in western Michigan. More evaporation also means more evaporative cooling, although it is shortsighed to say that this would actually increase lake ice cover in the winter months. Increased evaporative cooling will be offset by faster melting rates caused by increased air and water temperatures, thus reducing the ice cover faster. Despite usually acting as thermal insulation to prevent additional heat exchange, having less ice cover means that sunlight can continue warming the water throughout the winter, thus breaking the established regulatory feedback system. Whereas ice cover usually transitions a warm-water year into a cold-water year, a lack of ice means continued warming into the following year. Less ice also leaves more surface area for evaporation during the winter, and in conjunction with warmer water temperature, this leads to bigger, more frequent snowstorms. The resulting meltwater is good for watering crops, but extreme snowfall is also dangerous for humans due to the disruption of transportation, services, or power lines. 

Premature warming and freezing events

Besides precipitation, water bodies are largely influential on climate due to their impact on atmospheric temperature. Having a high heat capacity, Lake Michigan is able to suck up or release a lot of it to the atmosphere, thus reducing temperature extremes. In the springtime when the water is cold, it can remove a lot of heat from warm air masses, thus cooling them down. This mitigates early spring warming events that prematurely initiate the growth of fruit, given that subsequent frosts would ruin apple flower bud development. Complimentary, in the fall when the water is warm, it can release a lot of heat to cold air masses, warming them up. This then mitigates fall freeze events which would ruin the fully-developed apples prior to harvesting. In this way, the lengthening of the cold and warm seasons is crucial for successful crop development, preventing mismatched phenology. 

Although climatic warming would appear to mitigate premature freezing events due to higher air temperatures, the issue of jet stream instability actually increases their likelihood. Climate change has been more severely affecting the temperature of polar regions, causing them to warm up faster than the temperate and torrid zones. The latitudinal temperature gradient is therefore less pronounced between the polar and mid-latitude air masses, which slows the polar vortex and weakens the stability of the polar front. As the Earth continues to warm, this instability will lead to the intrusion of mid-latitude air masses northward and polar air masses southward, causing more frequent stochastic warming and freezing events in the spring and fall, respectively. This has already been a problem for apple farmers in recent years, where crops have been ruined due to these premature temperature extremes.