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Condensed Isle Royale Climate Table

This page is part of the project: Isle Royale

This table is a synthesis of a larger body of work that can be found here.

  Trend Historical Climate Description

Projected Mid-21st Century Change

Projected end of 21st Century Change

Confidence in Scientific Understanding (high, medium, low)
Temperature + Warming temperatures have been observed in the Great Lakes region.  Although parts of the northwest have warmed the most, Isle Royale has likely1 warmed less quickly than surrounding mainland areas due to the modifying effects of Lake Superior.2 3 The most warming was during the cold season.

1.9 to 3.6ºC (3.42 to 6.48ºF) (warmer emission sceario)4 5

(mean-1std dev to mean+2 std dev)

3.8 to 7.1ºC (6.84 to 12.78ºF)/century (warmer emission sceario)5

(mean-1std dev to mean+2 std dev)

High confidence in the direction of change (increasing);Confidence is for global and regional projections. Because of the micro climate at Isle Royale, confidence for local change is lower. Lower confidence in the magnitude of change.
Precipitation + Precipitation has generally increased over the Midwest.1 However, precipitation over Lake Superior (including Isle Royale) during the warm-season is generally less than the mainland.6 5% to 15% increase over the Midwest4 9% to 31% increase over the Midwest Medium confidence that precipitation will increase;7  Less confident of the magnitude of change.
Snow likely - Regionally, snowfall has decreased, but there are not enough records on Isle Royale to draw any local conclusions.8 6  Spring snow melt is occurring earlier. snow days reduced by 25% for MI4 snow days reduced in half for MI Low
Frost-Free Growing Season Length + The growing season across the Midwest is increasing days on both sides of the warm-season.  Isle Royale's growing season may be extended further into Autumn as water temperatures remain warm for a longer period of time.9

Last spring freeze 1 week earlier

+20-25 frost-free days in growing season

Last spring freeze 2 weeks earlier

+29 frost-free days in growing season

High confidence that there will be more warm days, less confidence that those warm days will occur consecutively in the growing season.
Extreme Temperature Events no change to + Anomalous Midwest warm events in the 1930s cause the trend to appear as if no change has occurred, but there is a positive warming trend for the past 30 years.  There is not as much change in extreme heat during the warm-season, rather, there are less cold events during the cold-season. 

days with temperatures >=95°F will increase by less than 5 days

heat waves are not expected to increase in the northern Midwest

days with temperatures <=32°F will decrease (about 15 less days)

heat waves will increase across the Midwest (up to 25 heat waves by end-of-century)

days with temperatures <=32°F will decrease (about 30 less days)

There is high confidence that there will be less cold days.
Extreme Precipitation Events + The intensity of extreme precipitaiton events has increased.  Severe drought is less common in northeast MN, near Isle Royale, than other parts of MN.  November is historically when the most extreme snow events have occurred over Lake Superior. up to +30% increase in number of days with greater than 1 inch precipitation for parts of the northern Midwest region 20% increase in days where precip > 2.5cm; decrease of 4 to 6 days where precip < 3mm MediumMost increases have been observed in lake-effect precipitation, which is not as prevelant at Isle Royale.
Wind likely + Winds over Lake Superior have generally increased.  Extreme wind events in November have historically caused strong wind storms that impact shipping on Lake Superior as well as ice formation in general.  Strong winds can break up ice or prevent ice from forming.  likely no change in extreme wind events wind events more extreme than the historical envelope will likely not develop until the end of the century Low
Lake Levels No Change There has been a shift to an earlier spring maximum, but historically, lake levels have fluctuated. -0.6m to 0.4m change4 -0.6m to 0.9m change Low-Medium
Lake Temperatures +

Warming lake temperatures, especially in the fall, cause a delay in ice formation.  Greater ice-free periods will allow more moisture flux to the atmosphere that may impact precipitation/snowfall.10

+2ºC (3.6ºF) in warm-season water temperatures.

Water temperatures will reach 10ºC (50ºF) in the spring 10-12 days earlier.  Water temperatures will cool to 10ºC in the fall 15-17 days later.

+4.6ºC to 6.7ºC (8.28ºF to 12.06ºF) increase in warm-season water temperature.

Water temperatures will reach 10ºC (50ºF) in the spring 26-39 days earlier.  Water temperatures will cool to 10ºC in the fall 36-52 days later.

Lake Ice Cover - Ice cover has dramatically decreased.  Ice cover is greatest during late winter/early spring, but the existence of a solid ice bridge between Isle Royale and the mainland rarely forms.12 Days with ice is expected to decrease by 12 to 47 days (1950-1995 ranged from 77 to 115 days of ice cover) Days with ice is expected to decrease by 37 to 81 days (1950-1995 ranged from 77 to 115 days of ice cover) High13
Arctic Oscillation Wildcard It is difficult to predict the mode of the Arctic Oscillation (AO), and one extreme negative mode can be followed by an extreme positive mode.  The modes determine the type of weather that is experienced: warmer and dryer air, soil is dry (+ phase); or colder and wetter conditions, wet soil (- phase).  The AO is primarily a wintertime variable (DJFM)14     Low
Weather "Blocking" Patterns Wildcard Warming air temperatures in the Arctic may impact the direction storms travel and how quickly they pass a region.  Severe weather (conditions for flood, drought, hot, or cold spells) may persist for longer periods of time.     Medium
ENSO Wildcard Although El Niño events and strong La Niña events are associated with lower ice cover on the Great Lakes, interferences between ENSO and NAO can complicate the overall outcome of ice cover.     Low


  • 1. a. b. Isle Royale does not have a continuous record of observations, and very few cold-season observations, so we can not have high confidence in the historical trends.
  • 2. Island temperatures are generally cooler (warmer) than the mainland in summer (winter) because they are insolated by the relatively cold (warm) waters of Lake Superior
  • 3. GCMs do not simulate the Great Lakes, so they will not capture the important lake-induced influences on local air temperatures for Isle Royale. 
  • 4. a. b. c. d. mid century projections were determined by taking 1/2 of the end-of-century change.  For mid-century change the choice of emission scenario has little impact on the outcome, and the after mid-century rates of change are expected to increase)
  • 5. a. b. these values are based on gridded downscaled CMIP3 projections that do not include any local information for Isle Royale or Lake Superior in general.
  • 6. a. b. GCMs do not simulate precipitation processes well, especially warm-season convective precipitation and lake-effect precipitation (including snowfall).
  • 7. Confidence is for global and regional projections.  Because of the micro climate at Isle Royale, confidence for local change is lower.
  • 8. The weather stations nearest Isle Royale do not pass quality control tests or they are located within the lake-effect zone that is not characteristic of Isle Royale.
  • 9. GCMs have difficulty simulating precipitation mechanisms at the warm-season/cold-season transitions (too dry in September and too wet in May)
  • 10. The lakes are not simulated in GCMs so future lake temperature information will likely need to come from regional or hydrologic models.
  • 11. Water temperatures will be influenced by the rate of warming air temperatures.
  • 12. The lakes are not simulated in GCMs so future lake ice information will likely need to come from regional or hydrologic models.
  • 13. Loss of ice cover will be related to warming air temperatures.
  • 14. GCMs do not simulate the AO consistently, which produces different outcomes for regional weather predictions.  There is evidence to suggest that the NAO will intensify its positive phase with increasing greenhouse gas concentrations