Baselines and Trends

October 20, 2011

When scientists make statements about changes in climate they compare the magnitude of recent observations to conditions in the past. In the early 1970s, the the World Meteorological Organization (WMO) defined a climatological baseline as a 30-year "normal" period. It was believed that 30 years was a long enough period for the natural variability of the atmosphere to be averaged out, and thus whichever 30 year period was selected would not make a substantial difference.

This has not been the case for the Arctic. Large natural changes can occur over 30-year periods and there is a recent persistent upward trend in temperature and downward trend in sea ice extent (Fig. A2 in the Temperature and Clouds essay). Several plots in the Arctic Report Card are for the first time calculated from a new baseline period of 1981 through 2010. The magnitude of these temperature anomalies over the Arctic are about 1.0 °C less than those calculated from a baseline of data from 1971 through 2000.

Thus, readers of all sections of the Report Card should be aware of what baseline period or trend interval is used to assess the magnitude of recent changes. Figure A2 shows that even if recent years are included in the baseline, the year 2011 remains a warm year. More information can be found at http://www.ncdc.noaa.gov/oa/climate/normals/usnormals.html.