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Common Sense Climate Index

Climate Index Scale

The value 0 is assigned to the average of each climate indicator (such as temperature) for the period of "climatology", which we take as 1951-1980 (the era during which "baby-boomers" grew up). Positive values are used for an indicator change of the sense expected in a warming climate, while negative values are associated with cooling.

The scale for the Index is based on the interannual (year-to-year) "standard deviation" during the period of climatology, 1951-1980. Specifically, the Index value +1 is one standard deviation above the 1951-80 mean. The standard deviation is a measure of the typical year-to-year fluctuation of a quantity. Mathematically, if T´ is the average of T over the period 1951-80, then the standard deviation of T during that period is

Standard Deviation = [Sum ((Ti - T´)2) / 30]1/2

where the Sum is taken over all 30 years (i=1951 to i=1980).

The index for each year is then calculated by subtracting the mean value of the indicator from the value of the indicator for the year in question, and then dividing this number, which is called the anomaly, by the standard deviation. Thus, if the anomaly is equal to the standard deviation, the index is 1, if it is twice the standard deviation, the index is 2, and so on. A positive temperature anomaly corresponds to a positive index. On the other hand, a positive heating degree day anomaly, indicating more heat required than normal, corresponds to a negative index.

Our hypothesis is that a value of +1 for a single quantity in the Index is generally large enough to be noticeable, because a value that large would normally (that is, in 1951-1980) occur only about 15% of the time. Thus if the summer were warm enough to yield an index of +1 or more, people who had been living at that location for a long time would tend to agree that it was an unusually warm summer.

A given climate indicator is expected to occasionally attain values of +1 or more, even if no long term climate change is occurring. But if the index achieves and consistently maintains such a value, it may be an indication of a long-term climate change.

As an example of a climate indicator let us consider heating degree days in New York City. Heating degree days are calculated as the number of degrees that the daily mean temperature falls below 65°F; heating degree days are accumulated over the entire heating season. Figure 1 shows that the mean annual number of heating degree days at Central Park was 4950 in 1951-1980, and the standard deviation was 300 degree days. Thus a year with 4650 or fewer heating degree days normally would occur only about one out of every 6 or 7 years.

In 1990 and in 1991 the heating degree days were about 800 to 1000 less than normal, that is, about three standard deviations, corresponding to a climate index value of about +3. This fluctuation would be noticeable as heating fuel requirements were probably reduced about 20 percent and heating costs even more as the price of fuel generally declines with reduced demand.

However, within the period 1951-present there is no clear evidence of long-term change, as heating degree days rebounded almost to normal in 1992. Also note that in the early 1950's there were several successive warm years (small heating degree days), comparable to the warmth in the 1990's.

In the following pages we first show the Climate Index for New York City (Central Park), including a graph of the component indices which were averaged to obtain the composite Climate Index. We then show the data used for the individual component indices.

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