Global Climate Report - May 1998
Global Surface Mean Temperature Anomalies(1880-1998)
The period January-May 1998 was the warmest since reliable records began in 1880, according to the National Climatic Data Center. This conclusion was based on separately examining land data using the Global Historical Climatology Network; ocean data using the National Centers for Environmental Prediction (NCEP) - Reynolds Sea Surface Temperatures (SST) blended with the NOAA Comprehensive Ocean-Atmosphere Data Set (COADS) and the United Kingdom Meteorological Office Historical SST analysis; and a global surface temperature index that combines the ocean and land data. The final anomaly may change due to inclusion of new areas of the world not currently represented. Each of the anomaly time series is based on its respective 1880-1998 mean.
...the land data anomalies...
...the ocean data anomalies...
...the land and ocean data combined into an anomaly index ...
...all in degrees C with respect to long-term (1880-1998) mean.
The average annual temperature of the globe is about 59 deg. F (15 deg. C). That value can be added to global anomalies to approximate absolute temperatures. Anomalies (also called departures from average) are used because they describe more accurately climatic variability over large areas than the absolute temperatures do and they give a frame of reference that allows for easier interpretation of the numbers. For example, a summer month over a large area may be cooler than average, both at a mountain top and in a nearby valley, but the absolute temperatures may be quite different at the two locations. The use of anomalies in this case will show that temperatures for both locations were below average. For these reasons, it is the anomalies that are computed for large-area summaries (like a hemisphere or the globe), not the temperature itself.
An analogy using human body temperatures might help to explain why anomalies are useful. The "average" human body temperature is about 98.6 deg. F. But some people have lower normal temperatures than others. For someone with a normal body temperature of 98.0 deg. F., a temperature reading of 98.6 deg. would indicate a fever. The use of 98.6 deg. alone would not indicate this condition to most people but the use of an anomaly or departure from the normal body temperature for this person (+0.6 deg. F) would indicate the feverish condition.
Intergovernmental Panel on Climate Change
GCRIO: Global Warming and Climate Change
UNFCCC: Convention on Climate Change