Average Annual Number of Growing Degree Days from Global Climate Model Data

Average Annual Number of Growing Degree Days from Global Climate Model Data

This tool is excerpted from Chesapeake Bay Watershed Climate Impacts Summary and Outlook for Summer 2021.

Tool Background

Growing Degree Days (GDDs) measure the number of degrees that accumulate over a given year, as a metric of the heat available for various needs (i.e. crops, insects) and are an important metric for agricultural management.1 GDDs help estimate when different plants and insects will arrive at various life stages, which is useful for determining when to plant crops and when to protect plants from various pests.2 Generally, a larger number of GDDs indicates warmer growing conditions.3 Research has shown that GDDs are increasing across the continental United States, which may reduce crop yields across the United States, particularly for corn, soybean, sorghum and wheat.4

GDDs can be calculated in a number of ways, but for this analysis we use the following equation:

Growing Degree Days Gained = Average Daily Temperature – Baseline Temperature

with a baseline temperature of 50 degrees F. This number is calculated for each day of the year and each positive number is added together to get the total annual GDDs. If GDDs are negative for a given day, which occurs when the average daily temperature is below 50 degrees F, it is not included in the annual total. To calculate the GDDs for a specific crop, a different baseline temperature is usually selected. For example, a baseline temperature of 40 degrees F is typically used for wheat, barley or rye, while 50 degrees F is used for corn, sorghum or rice.5

The number of GDDs varies with geography, with more southern or lower elevation areas typically having more GDDs in a given year and northern or higher elevation areas having fewer GDDs.

Increases in GDDs over time indicate either more days with average temperatures above 50 degrees F, or an increase in the average temperatures on days that were already above 50 degrees F, or some combination of the two. Therefore, increases in GDDs could potentially indicate an increase in the length of the growing season.

Changes in the accumulation of GDDs over the course of a year can also shift the timing of various crops and insect cycles. This could create a situation where crops reach vulnerable stages in their development at a time of year where frosts are still possible or when precipitation is less available.

Key Findings

  • Across all observed historic observations and the future modeled time periods, the southeastern portion of the region (Eastern Maryland and Central and Eastern Virginia) experiences the greatest number of GDDs.
  • The average annual number of GDDs is projected to continue to increase in the future, for both the low and high emissions scenarios (Figure 5).
  • In a high emissions future, portions of Eastern Virginia and Coastal Maryland may experience increases in average annual GDDs greater than 40 percent (Figure 5).

How to Use the Tool

Selecting Time Periods and Future-Emissions Scenarios
Use the slider to the right of the maps to adjust the 30-year period used to calculate the average annual number of growing degree days. Users can also select the future-emissions scenario (Low or High Emissions).

Viewing Variability Within a Location
Hover or tap over a point of interest. A window will pop up that displays changes in growing degree days by decade. You can also use the Geography filter to the right of the map to zoom into a location of interest.

Technical Notes

NOTE: Localized Constructed Analogs (LOCA) is a downscaled climate data product available at 1/16-degree (6-km) resolution over the continental United States. LOCA data sets6 include the 32 climate models available in the Coupled Model Intercomparison Project 5 (CMIP5) archive, for two future greenhouse gas concentration trajectories: a low-emissions future, Representative Concentration Pathway (RCP) 4.57; and a high-emissions future, RCP 8.58. For this study, we used LOCA data over the Chesapeake Bay watershed from 1991–2100 (or 2099 for some models). Access LOCA datasets and learn more about the methodology.

Data were processed by the Northeast Regional Climate Center to calculate the average annual growing degree days. In this analysis, growing degree days are calculated for each day using the following formula: Growing Degree Days Gained = Average Daily Temperature – Baseline Temperature.

In our calculations, the baseline temperature is 50 degrees F. Positive numbers from each day of the year are added together to create a total Growing Degree Days number for the year. For the future projected data, we plot the average of this number over a 30-year period and use 1991–2020 to represent the historic period. Note that the final future period is only 20 years long, due to availability of climate model data. These values were calculated for a low-emissions future (RCP 4.5)9 and a high-emissions future (RCP 8.5)10. A weighted average was provided by the Northeast Regional Climate Center to average across climate models for each grid cell in the LOCA data set.11 The LOCA data sets were masked to the boundaries of the Mid-Atlantic region.

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