NOAA succinctly defines climate change as “a long-term shift in the statistics of the weather (including its averages).” Climatologists generally rely on 30-year averages of climate elements, updated at the beginning of each new decade—called climate normals—as a basis for describing the statistics of a location’s climate. Climate data exhibit year-to-year fluctuations above or below these long-term averages. This variability occurs whether or not climate change is taking place.

With these ups and downs continually occurring, how do we know when climate change has actually taken place? Statistical testing can reveal whether or not differences in two groups of data are likely to be due to random variability or to other factors. If the comparison of two non-overlapping 30-year climate data sets from the same location determines they are statistically different, more than random chance is involved and it can be assumed climate change has taken place.

Use this application to comprehensively explore the question “Climate variability or climate change?” The app allows users to change parameters (Average Temperature, Maximum Temperature, Minimum Temperature, Precipitation, Cooling Degree Days, Heating Degree Days), times scales (Annual, 1-Month, Seasonal), and locations (Contiguous U.S., Climate Region, State, Climate Division, City).

Background information

NOAA’s National Centers for Environmental Information (NCEI) and the Education Program of the American Meteorological Society (AMS) have collaborated to make it possible to easily test for climate change on scales from local to national levels in the U.S.

• NOAA’s Climate at a Glance (NOAA CAG) website delivers data from hundreds of locations across the U.S. on a variety of climate elements on different time scales.
• AMS Education Program’s Climate at a Glance Application (AMS CAGApp) draws selected data from NOAA CAG, enabling users to evaluate data for a particular climate element at a location for two 30-year periods. Evidence of a statistically significant difference between the two data samples is interpreted to be consistent with climate change.

The AMS CAGApp works by embodying data accessible via NOAA CAG in a way that allows the electronic retrieval of the total historical record of a given parameter (e.g., average annual temperature) from a chosen location as described in the NOAA CAG Data Information section. The user selects two non-overlapping 30-year periods starting on 1901, 1911, 1921, 1931, 1941, 1951, 1961, 1971, or 1981 as the length of the data record permits. Means and Standard Deviations are calculated for each of the two sample periods. Then, assuming independent samples, the user's t value is automatically calculated. The calculated t value is compared to the two-tail t value of a 95% level of confidence (corresponding to the Intergovernmental Panel on Climate Change likelihood terminology of “Extremely Likely”). The AMS CAGApp then states whether or not the statisical test results are consistent with climate change.

Further testing can be achieved by returning the AMS CAGApp to its default settings by clicking on the browser’s refresh (reload) icon or changing to new individual settings values.