Common Sense Climate Index
Discussion
This Climate Index is a simple measure of the degree (if any) to which practical climate change is occurring. The Index also illustrates natural climate variability, thus helping to show the difficulty in detecting a change of quantities that are naturally "noisy" or chaotic.
Global warming in the 20th century amounts to only about 0.75°C (1.5°F). When a new global record temperature is set, it may exceed the previous record by only a few hundredths of a degree. What relevance, if any, do such small temperature changes have to most people?
A simple measure is needed of the degree to which practically noticeable climate change is occurring. Because global warming has been predicted to result from increasing human-made greenhouse gases, it is useful to have a handy measure that allows us to keep tabs on whether noticeable climate change is actually occurring.
Our Common Sense Climate Index is a composite index of climate quantities that are noticeable to the lay person. Positive values of the index refer to changes that would be associated with warming, and negative values with cooling.
The index is defined locally, because that is how climate is experienced. An informative global summary of the index is provided in the section Global Maps of Climate Index. This includes a global map of the Climate Index for the most recent year's data, and the average index for the most recent five years. We also show bargraphs of the average index for the United States and for the world (see Bargraphs of U.S. and World Average Climate Index). The global maps and the U.S. and world bargraphs are based on the homogenized data.
Our index is analogous to the United States Greenhouse Climate Response Index of Karl et al. (1995). We employ different climate indicators that we hope are easily understood by the lay person and we try to define a scale that will reveal when change should be noticeable above the level of natural climate variability.
Our expectation that a persistent climate index of 1 or greater represents a noticeable change is at present a hypothesis, but it is a testable hypothesis. There are regions, especially in Alaska and Siberia, where the index is near unity, and thus surveys of people's opinions could be carried out.
The index is based on daily observations in the case of cities for which daily data is readily available. Other locations use only monthly mean data, which restricts the analysis to a small number of climate indicators. However, examination of results for cities with daily data shows that the index based on monthly data is usually similar to the composite index based on all climate indicators including daily data.
The climate data used to compute our index are obtained from NOAA National Climate Data Center, with the primary source being the unadjusted records of Peterson and Vose (1997) (see the GHCN web page). Our calculations of the index are described by Hansen et al. (1998).
Climate Change Indicators in the Index
The Climate Index is the mean of several climate change indicators. These indicators, such as the frequency of extreme temperatures and heating degree days, are quantities that tend to be noticed by people and have economic significance.
The following lists those indicators currently included in the Index for cities with daily data. The Index is based on the four seasonal mean temperatures for cities with only monthly data.
- Seasonal mean temperatures (four seasons)
- Degree days (heating season, cooling season)
- Frequency of extreme temperatures: ("hot" summer days, "cold" winter days)
- Record daily temperatures (record highs, record lows)
In forming the Composite Index we give each of the major categories (for example Degree Days) equal weight. Within each category the subcategories (for example, Heating Degree Days and Cooling Degree Days) receive equal weight.
Qualifications and Limitations
Our expectation that an observant person can notice a change of climate when the Climate Index reaches and consistently maintains a value of 1 or more is only a hypothesis, because people's perceptions of change are a sociological matter. And even a change of that magnitude will be still commonly exceeded by day-to-day weather variations. Thus it would be useful to test the perceptions of people in locations where the index has changed substantially. (See the accompanying page on Climate Index Scale for further information of calculating this value.)
Many of the climate records of individual stations contain errors due to a variety of causes. These errors are not expected to be large enough to alter systematically the calculated large-scale regional or global climate change. But results at any specific location should be treated with appropriate caution.
Our adjustments to correct climate time series for urban change effects is of course imperfect. Adjustments are based on "homogenizing" the urban stations long-term change with that of nearby rural and small town locations. This "homogenization" will tend to make the adjusted station's result more representative of the region than of the actual station location.
Data Availability
Our data are only available on a station by station basis. To retrieve the data, go to the Climate Index for Individual Stations page and click on the world map near the desired location. Then select a station from the list and click on its name.
Monthly mean temperature data is available for about 5,000 weather stations around the world. An exhaustive list of daily data is available for more than 250 stations around the U.S.
We welcome any information in flaws that may be found in our station data or other suggestions regarding our climate index.
