## Observational Data

The source of observed temperature data was 176 weather stations measuring daily maximum temperature, minimum temperature, and precipitation during 1950-2006 in and around Wisconsin, from the National Weather Service’s Cooperative Observer Program. This data was interpolated to an 8-km grid (Serbin and Kucharik, 2009). Daily average temperature was estimated by averaging the daily maximum and minimum temperatures. Trends were estimated using the slopes of linear regression fits for the entire 1950-2006 time series.

## Model Projection Data

The climate output that was analyzed was produced by fourteen global climate models from the Coupled Model Intercomparison Project Phase 3 (CMIP3), a critical source of data to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4). Results are based on three emission scenarios, A2, A1B, and B1, in the order of greatest greenhouse gas emissions to least emissions by the end of the century.

The coarse climate projections were downscaled to a 0.1° latitude x 0.1° longitude grid over Wisconsin and debiased against observed temperature and precipitation from station observations within the National Weather Service’s Cooperative Observer Program. By interpolating and debiasing probability distribution functions and their attributes, a realistic representation of the variance and extremes of temperature and precipitation was achieved, in addition to a realistic representation of the means of both variables. In computing the projected changes in a variable, the difference in the mean of that variable was computed from 1961-2000 to either 2046-2065 (mid-21st century) or 2081-2100 (late 21st century)

#### Cooling degree days

Cooling degree days (CDDs) for a single day are computed here as the number of degrees Fahrenheit that the daily mean temperature exceeds 65°F. So, two days with daily mean temperatures of 75°F and 50°F have 10 and 0 CDDs, respectively. For annual total CDDs, the CDDs for each day are summed.

#### Heating degree days

Heating degree days (HDDs) for a single day are computed here as the number of degrees Fahrenheit that the daily mean temperature is below 65°F. So, two days with daily mean temperatures of 50°F and 75°F have 15 and 0 HDDs, respectively. For annual total HDDs, the HDDs for each day are summed.

#### Freezing degree days

Freezing degree days (FDDs) for a single day are computed here as the number of degrees Fahrenheit that the daily mean temperature is below 32°F. So, two days with daily mean temperatures of 20°F and 40°F have 12 and 0 FDDs, respectively. For annual total FDDs, the FDDs for each day are summed.

#### Growing degree days

Growing degree days (GDDs) for a single day are computed here as the number of degrees Fahrenheit that the daily mean temperature exceeds 50°F. So, two days with daily mean temperatures of 75°F and 40°F have 25 and 0 GDDs, respectively. For annual total GDDs, the GDDs for each day are summed.

#### Daily temperature range

The daily (or diurnal) temperature range is defined here as the difference between the daily high temperature and daily low temperature in degrees Fahrenheit. So, a day with a high temperature of 70°F and low temperature of 40°F has a daily temperature range of 30°F.

#### Length of the growing season

The length of the growing season is defined here as the number of days between the last spring freeze and first autumn freeze, where a freeze is when the daily low temperature drops below 32°F.

#### First autumn freeze date

The first autumn freeze is defined as the first day, after mid-summer, with a low temperature below 32°F.

#### Last spring freeze date

The last spring freeze is defined as the last day, between January 1 and mid-summer, with a low temperature below 32°F.

#### Spring onset date

The date of spring onset is defined here as the first day of the year in which the average high temperature of that day and the four prior days averages at least 50°F and no subsequent five-day period, from then until mid-summer, has an average high temperature below 43°F.

#### Frequency of 1” precipitation events

The frequency of 1” precipitation events is defined here as the number of days per year with precipitation of at least one inch.

#### Frequency of 2” precipitation events

The frequency of 2” precipitation events is defined here as the number of days per year with precipitation of at least two inches.

#### Frequency of 3” precipitation events

The frequency of 3” precipitation events is defined here as the number of days per year with precipitation of at least three inches.

#### Spread in Monthly Precipitation Projections

Each line represents a different global climate model’s projection of monthly precipitation change for Wisconsin. The mean and median projections are shown in red and blue, respectively. The percentage of the models showing an increase in precipitation is shown below each month.

#### 10th and 90th Percentiles of Annual Average Temperature Changes

The number of climate models analyzed was 11 for the A2 scenario and 14 for the A1B and B1 scenarios. The range of their projections of change in annual average temperature is represented by the 10% percentile (weak warming) and 90% percentile (strong warming), based on gaussian kernel density estimation.

#### 10th and 90th Percentiles of Annual Average Precipitation Changes

The number of climate models analyzed was 11 for the A2 scenario and 14 for the A1B and B1 scenarios. The range of their projections of change in annual average precipitation is represented by the 10th percentile (drying or weak moistening) and 90th percentile (substantial moistening), based on gaussian kernel density estimation.