Part 5—Create a Temperature Anomaly Map

Step 1 –
Create a Spatial Join betweeen the CCSM grid and Temperture layers

At this point, it is convenient to perform a Spatial Join of the point shapefiles with the CCSM global polygon shapefile. The CCSM polygon shapefile and additional documentation are available from the NCAR GIS Initiative Climate Change Portal ( This site may be offline. ) . The polygon layer was derived using the 4 corner coordinates, based on latitude and longitude, for each grid cell of the CCSM output data. This creates a rectangular grid (referred to as a Gaussian grid) often used in scientific modeling on a sphere. The polygon file is in the same CCSM-defined projection as the CCSM climate change data - a Geographic Coordinate System on a perfect sphere with a radius of 6371.22 km. With this polygon layer, the attributes of the climate change data (which are downloaded as a point shapefile from the Climate Change Portal) can be appended to the corresponding polygon to get an accurate spatial distribution of the modeled climate data. Each point is the centroid of the corresponding polygon cell.

  1. Either download the CCSM Global Polygons from NCAR GIS Initiative Climate Change Portal ( This site may be offline. ) or use the already downloaded shapefile CCSM Global Polygons (Zip Archive 963kB Jan10 09) and add it to your project file.
  2. Perform a spatial join between the CCSM polygons and the tas_JJA_1980_1999_20th_Century_Experiment using the CCSM polygon layer as the target. Name the Output Feature Class CCSMpolys_tas_JJA_1980_1999_join. Remove all the monthly temperature fields from the from the join, leaving only the CCSM polygon fields and the JJA_20_avg field from tas_JJA_1980_1999_20th_Century_Experiment.
  3. Perform a second spatial join between CCSMpolys_tas_JJA_1980_1999_join and tas_JJA_2021_2040_Scenario_A2. Name the Output Feature Class tas_JJA_anomalies2021_2040_1980_1999. Remove all fields except the fields contained in CCSMpolys_tas_JJA_1980_1999_join and the JJA_A2_avg field from tas_JJA_2021_2040_Scenario_A2.
  4. Open the attribute table of tas_JJA_anomalies2021_2040_1980_1999 and confirm that the JJA_20_avg and JJA_A2_avg fields have be joined to the CCSM polygon shapefile.

Step 2 –
Create a Temperature Anomaly Map between 20th Century Experiment and A2 Scenarios

Now that you have joined the two 20-year averages for both 1980-1999 and 2021-2040 to the CCSM polygon layer you are now ready for the last step: Calculating the difference (anomaly) between the JJA temperature of the present-day climate simulation to the temperatures projected for 2030 using the A2 emission scenario. This anomaly map will highlight the areas around the globe that will experience either air temperature increases or decreases in 2030 relative to 1990.

  1. If necessary, open the attribute table of tas_JJA_anomalies2021_2040_1980_1999
  2. Add a new field to this table, naming it anomaly. Make it type: Double.
  3. Open the field calculator for the anomaly field and subtract JJA_20_avg from JJA_A2_avg.
  4. Confirm that the difference was calculated and then close the attribute table.
  5. Returning to the the layer list turn off all layers except for the continental outlines and tas_JJA_anomalies2021_2040_1980_1999. If necessary, reorder the layers so the continental outlines is on top.
  6. Change the symbology of the tas_JJA_anomalies2021_2040_1980_1999 to a graduated color based on the anomaly field. Create a legend that ranges from -1.0 to 3.5 degrees. Experiment with different color ramps and number of classes to determine which shows the temperature anomalies most effectively. Be sure to modify the the sample size as before to ensure all the data are being used.
  7. Examine your anomaly map, which should look like the one below. Identify 2-3 regions which may experience the greatest temperature increase by 2030 according to the A2 Scenario as compared to the 1980-1999 20th Century Experiment Scenario.
    anomaly map part 3
    anomaly map legend
    Consider the following questions:
    • Are most of these warming regions over land or over water?
    • Which regions does the model show might actually experience a temperature decrease?
If you had difficulty completing the steps above, download the following folder containing completed project file with data at the end of Part 5. NCAR Climate Change Part 5 complete (Zip Archive 27.4MB Feb3 09)