MACROSCOPE

# Will AC Put a Chill on the Global Energy Supply?

# The Cooling Calculation

To gain a greater understanding of potential future air-conditioning needs in each of the countries around the world, I conducted an analysis that takes into account two primary factors: a country’s local climate and the size of its population.

The climate is represented through a measure known as *cooling degree days*, which provides an index of the energy demand required to cool indoor spaces. The cooling-degree-days index is calculated by subtracting 18 from the mean daily outdoor temperature in degrees Celsius and summing up only positive values over a fixed period, such as an entire year. The selection of 18 degrees as the base outdoor temperature accounts for the additional heat generated by occupants and their activities, resulting in an average indoor temperature of 21 degrees—typical room temperature—when it is 18 degrees outdoors. For an average outdoor temperature higher than 18 degrees, most buildings require cooling to maintain a 21-degree indoor temperature.

My calculations used a pair of underlying variables, the annual cooling degree days for each country (weighted by the geographical distribution of the population) and the country’s total population (using figures for 2010). Examples of the values of these two variables are shown in the graphs at right.

The specific calculations involved two steps: deriving *annual* *person cooling degree days*—an index of the potential demand for energy for cooling—as a product of these two variables, and then normalizing the product with respect to the corresponding value in the United States. This index estimates what the energy usage would be in each country if air-conditioning became as prevalent there as it is in the United States. Examples of the final results are shown in the graphs above. The analysis was performed for the 170 countries that were common to both sets of underlying data.

The results indicate that of the top 25 countries in terms of person cooling degree days, 14 are in Asia, 7 are in Africa, and 2 each are in North and South America. The United States ranks as number 9, despite the fact that it has the coolest climate of these 25 countries (meaning it has the lowest number of cooling degree days before multiplying by population).

EMAIL TO A FRIEND :

**Of Possible Interest**

**Letters to the Editors**: Traffic Lights Near and Far

**Engineering**: Traffic Signals, Dilemma Zones, and Red-Light Cameras

**Perspective**: Taking the Long View on Sexism in Science