Science Briefs

A Common Sense Climate Index: Is Climate Changing Noticeably?

It seems a scientist is a man that can find out anything, and nobody in the world has any way of proving he really found out anything or not. — Will Rogers, as cited in a Utah newspaper, August 1989

Global warming has long been predicted to result from increasing greenhouse gases in the atmosphere. Global surface air temperature has indeed increased in the past century, but at a rate less than 0.1°C/decade. Record global temperatures have been achieved several times in the 1980s and 1990s, but a new record often exceeds the old record by only a few hundredths of a degree. What relevance, if any, do such small temperature changes have to most people?

Figures A, B

Figures A and B: Composite temperature index for New York (La Guardia) based on (A) seasonal mean temperature and (B) heating and cooling degree days.

A popular scientific activity is to try to "detect" (mathematically) significant climate change. But the practical detection issue is this: when will global warming be large enough to be obvious to most people? It is common for people to perceive the latest climate fluctuation as long-term climate change. But it is just such misinterpretations that make it desirable to have quantitative measures of practical climate change.

We propose a climate index that is intended to be a simple measure of the degree, if any, to which practical climate change is occurring. It also illustrates natural climate variability, thus revealing how difficult it is to reliably perceive a change of quantities that are naturally "noisy" or chaotic. Our aim is to help people judge whether or not climate fluctuations are a significant indication of change and to provide improved understanding of climate variability.

The index is derived from temperature and precipitation measurements. The average value of the climate index is zero for the period of climatology, which we take as 1951-1980, a time when many of today's adults grew up. The scale for the index is the interannual standard deviation during that period. The standard deviation is a measure of the typical year-to-year fluctuation of the given quantity. A value +1 (or -1) is great enough to be noticeable, because a value that large or larger would normally (that is, in the period 1951-1980) occur only about 15% of the time. For example, if the summer is warm enough to yield an index of +1 or greater at a given place, most people who had been living at that location for a long time would tend to agree that it was a "hot" summer.

Our contention that a persistent climate index of +1 or greater represents a noticeable climate change is presented as a hypothesis, because people's perceptions are a sociological matter. But it is a testable hypothesis. We find that there are regions in Alaska and Siberia where the index is approaching unity, and thus surveys of people's perceptions could be carried out.

The climate index will occasionally attain a value of +1 or more, even if no long-term climate change is occurring. But if such an index value is achieved and maintained, it will signify substantial long-term climate change. Using the concept of climate dice, a persistent change of the climate index by +1 would represent a sufficient "loading" of the climate dice to be noticeable to most people.

As an example of our temperature index, we show results for New York (La Guardia Airport). Figure A gives the mean temperature for each season. The component of the climate index based on seasonal-mean temperature is the mean of the indices for the four seasons. The largest index for seasonal temperature occurred in 1991. That year spring was remarkably warm on the United States East Coast; for example, cherry trees blossomed in early March in Washington, D.C. The unusual warmth was obvious to New York residents, as both the winter and spring were about 5°F above normal. But temperatures dropped back to normal the next year, in fact slightly below the 1951-1980 average.

Figures C, D

Figures C and D: (C) Composite temperature index for New York (La Guardia) based on frequency of unusually hot summer days and cold winter days (D) The net temperature index.

Figure B shows the second component of the temperature index, based on heating and cooling degree days. Heating degree days are calculated as the number of degrees that the daily mean temperature falls below 65°F accumulated over the entire heating season. Heating degree days less than normal give a positive contribution to the temperature index, while cooling degree days, based on temperatures above 65°F, give a positive contribution if they are greater than normal. In New York the index for heating and cooling days has been high for the past decade, but not much higher than in the 1950s.

Figure C shows the component of the temperature index based on the number of days when the temperature exceeds a level local inhabitants are likely to consider as "hot" or "cold". We define a hot day as one that occurred only 10 times per year, on the average, during the period 1951- 1980, which yields 91°F or higher as the definition of a hot day in New York (and 15°F or less as a cold day). There were 26 "hot" days in New York in 1991, but in 1992 the number fell back to seven, i.e., less than the long-term mean. There is no obvious trend in the frequency of "hot" or "cold" days in New York during the past 50 years.

The composite temperature index for New York, the mean of the three components, is shown in Figure D. The largest index occurred in 1991 with a value greater than 2. The unusual warmth of 1991 was obvious to the lay person, with record spring warmth, anomalies of more than 20% in heating and cooling degree days, a large number of hot days and few cold days. If such warmth continued, most "baby boomers", who grew up during the period of climatology, 1951-1980, would agree on the existence of noticeable climate change. However, the temperature index fell back to near zero in 1992. Overall, New York has been warm during the past decade, but not much more so than in the 1950s.

Although the long-term change of the climate index is small for New York, global maps of the index (see Hansen et al. 1998) show that the Eastern United States is not representative of the world. In most regions of the world the climate index is positive, and in two areas (Alaska and Siberia), it has already reached values signifying noticeable climate change. If projection for warming in the next several years based on climate models (Hansen et al. 1997) are correct, the areas with noticeable warmth will expand considerably and thus many people may soon agree that climate change is occurring.

We are preparing datasets that will allow the climate indices to be provided over our web site for thousands of locations around the world. The data will be updated annually to allow monitoring of practical climate change. A preliminary version of the Common Sense Climate Index, based on temperature data, is now available.


Hansen, J., et al. 1997. Forcings and chaos in interannual to decadal climate change. J. Geophys. Res. 102, 25679-25720.

Hansen, J., M. Sato, J. Glascoe and R. Ruedy 1998. Common sense climate index: Is climate changing noticeably? Proc. Natl. Acad. Sci. 95, 4113-4120.