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What Are Cooling Degree Days?

Cooling degree days provide a measure to relate the outdoor temperature to the energy demand to cool a building. Primarily applicable during the spring through fall, more cooling degree days means that a building needs to use more energy to adequately cool the interior to comfortable conditions. The number of seasonal cooling degree days has been generally increasing in New Jersey since 1900 and is projected to increase through 2100 with both moderate and high increases in greenhouse gas emissions. Higher emission projections generate more seasonal mean cooling degree days compared to moderate emissions through 2100. This trend will increase the energy demands for cooling, potentially stressing energy generation and utility infrastructure.

About the Data

With this tool, the simulated mean seasonal cooling degree days can be viewed for the whole of New Jersey over historical and future time periods. Note the historical period of 1991-2020 is climate model data and not based solely on observed temperature measurements. The future time periods are simulated using greenhouse gas emissions scenarios RCP 4.5 (moderate emissions) and RCP 8.5 (high emissions). The modeled data have been extracted from the U.S. National Oceanic and Atmospheric Administration’s Applied Climate Information System and localized to New Jersey.

Cooling Degree Days (CDD)

Cooling degree days (CDD) are way to quantify the intensity of cooling needs over a period of time, presented above at a seasonal scale. The CDD for a single day are calculated as the mean daily temperature minus 65°F. So, a daily mean temperature of 75°F yields 10 CDD, providing a metric that can be related to energy demand to cool the building back to 65°F. 65°F is used as a basis for the CDD calculation because it is an outdoor temperature generally regarded as comfortable. In this tool, the CDD for each day is totaled across a season, and negative CDD values are not incorporated because CDD are only meant to quantify the time and intensity of outdoor temperatures above 65°F. The winter season in New Jersey does not typically require cooling as mean daily temperatures rarely exceed 65°F.Mean Cooling Degree Days are simulated using a grid over the state. The mean cooling degree days for the selected season are computed at each grid cell. The statewide seasonal mean is calculated by averaging these grid cell values over the whole state. The 10th through 90th percentile range of simulated seasonal CDD values is represented by the light blue shading.

Data Sources
The Applied Climate Information System (ACIS) was developed and is maintained by the NOAA Regional Climate Centers (RCCs). It was designed to manage the complex flow of information from climate data collectors to the end users of climate data information. The main purpose of ACIS is to alleviate the burden of climate information management for people who use climate information to make management decisions.

Climate Scenarios

RCPs or Representative Concentration Pathways (RCPs) are narrative descriptions about how global greenhouse gas emissions (such as carbon dioxide and methane), air pollutants, and land use may change over the 21st century. These narratives are guided by expert analysis of current emission trends and changes over the next century due to economic and industrial growth, adoption of alternative energy sources (such as wind and solar), and societal shifts. The two RCP scenarios presented here are RCP 4.5 (moderate) and RCP 8.5 (high), which represent an intermediate and high emission scenario, respectively.

RCP 4.5 (moderate)
In RCP 4.5,global greenhouse gas emissions peak near 2035-2040 and are reduced substantially by 2080-2100. This reduction is mainly dependent on changes in energy generation, an increase in energy efficiencies, and a decrease in overall energy usage. Of the energy that is used, the proportion of energy generated from fossil fuels decline and other sources (such as renewables and nuclear energy) increase. By 2100, the global surface temperatures increase by 1.8 (1.1-2.6 likely range) degrees C relative to 1986-2005.

RCP 8.5 (high)
RCP 8.5 assumes continued and expanding greenhouse gas emissions throughout the century without any significant emissions reduction or climate policy. The scenario depicts a rapidly growing global population with slow economic development that results in high energy demand with little progress in energy efficiency and alternative energy sources. To meet this demand, traditional fossil fuel (such as coal) usage increases. By 2100, the global surface temperatures increase by 3.7 (2.6-4.8 likely range) degrees C relative to 1986-2005.