“ Average Global Temperature”? Do You Know What It Represents?

The warmers are saying that we cannot let the global temperature increase by more than 2ºC. According to them, really bad things will happen if we exceed that number. The 2ºC must be another of those “tipping points” that we have heard so much of in recent years. Most all of which have come and gone with out any noticeable effect.

What is the global temperature? You almost never hear it expressed like a typical temperature reading you get each day from the weather stations.   The only place I can find a description stated as a typical temperature reading is from the World Meteorological Organization saying that the average global temperature, between 1961 and 1990, was 14ºC (57.2ºF). But I find nothing more recent. One reason is that an agreement around a specific temperature is difficult to come by.

At the South Pole, the highest temperature ever recorded was 12.3º C (9.9ºF).  Singapore’s record low was 19.4ºC (66.9ºF). So where is the average?   An average global temperature does not exist in the real world.  I will discuss near the end of this posting.

Anomalies are used instead.  A long-term average is used for reference and the temperature differences from that are termed anomalies.   Positive anomalies and negative anomalies are increases or decreases, respectively, from this long-term reference. Further these changes are very small. If you made a chart plotting a series of numbers of slowly warming normal temperature readings say:14.0ºC, 14. 01ºC, 14.01ºC, 14.02ºC, 14.02ºC, 14.03ºC,  you probably would not be able to visibly detect any change.   How frightening would that be?  Everything looks more dramatic when you plot the anomalies (0.01, 0.02, 0.o2, etc.).

Temperatures from around the globe are submitted to the National Oceanic and Atmospheric Administration (NOAA) and combined to create an average anomaly.   Much of the surface of the Earth has no temperature measuring or reporting devices. Roughly three quarters of the Earth’s surface are oceans. The temperatures used are often estimates.

The following are two examples of how NOAA’s National Climatic Data Center (NCDC) copes with these issues:

“Absolute estimates of global average surface temperature are difficult to compile for several reasons. Some regions have few temperature measurement stations (e.g., the Sahara Desert) and interpolation must be made over large, data-sparse regions. In mountainous areas, most observations come from the inhabited valleys, so the effect of elevation on a region’s average temperature must be considered as well. For example, a summer month over an area may be cooler than average, both at a mountain top and in a nearby valley, but the absolute temperatures will be quite different at the two locations. The use of anomalies in this case will show that temperatures for both locations were below average.

Using reference values computed on smaller [more local] scales over the same time period establishes a baseline from which anomalies are calculated. This effectively normalizes the data so they can be compared and combined to more accurately represent temperature patterns with respect to what is normal for different places within a region.

For these reasons, large-area summaries incorporate anomalies, not the temperature itself. Anomalies more accurately describe climate variability over larger areas than absolute temperatures do, and they give a frame of reference that allows more meaningful comparisons between locations and more accurate calculations of temperature trends.”

And :

“How is the average global temperature anomaly time-series calculated?

The global time series is produced from the Smith and Reynolds blended land and ocean data set (Smith et al., 2008). This data set consists of monthly average temperature anomalies on a 5° x 5° grid across land and ocean surfaces. These grid boxes are then averaged to provide an average global temperature anomaly. An area-weighted scheme is used to reflect the reality that the boxes are smaller near the poles and larger near the equator. Global-average anomalies are calculated on a monthly and annual time scale. Average temperature anomalies are also available for land and ocean surfaces separately, and the Northern and Southern Hemispheres separately. The global and hemispheric anomalies are provided with respect to the period 1901-2000, the 20th century average.”

No doubt the task to derive the average global temperature is a major challenge. The text makes it clear that a lot of estimated temperatures are used. For example at the equator, each degree of latitude is about equal to 70 miles. So a grid of 5ºx5º grid is 350 miles wide by 350 miles high and has an area of approximately 122,500 square miles (317,273 square kilometers) of the Earth’s surface. That is about the size of Italy, or Poland or Norway.  Lots of opportunities to infill with estimated temperatures.

According to John Goetz, the USA is 6.6% of the Earth surface but provides 39% of the data used to calculate the global temperature anomaly. That certainly is a bias built into the final result.

Furthermore temperature is a intensive property, meaning, for example,  it is not dependent on the volume of air it is measuring. It is not like height or weight which are extensive properties. If you bring a gallon of water to the boiling point, it will read 100C. If you bring a half gallon of water to the boiling point it will read 100C. It will take half the heat to bring the half gallon to the boiling point. The heat required , an extensive property, is dependent on volume of the water but for the temperature, an intensive property, the volume is immaterial. So averaging temperatures from point A with temperatures from point B is not meaningful. The Washington Times carried Tom Harris’ posting “Deceptive Record Claims”saying:

“In the final analysis, it is no more meaningful to calculate an average temperature for a whole planet than it is to calculate the average telephone number in the Washington D.C. phone book. Temperature, like viscosity and density, and of course phone numbers, is not something that can be meaningfully averaged. “Global temperature” does not exist.

The product, average global temperature, has some value but it is probably too imprecise to use as a basis for developing world policy.

Manipulating raw temperature readings happens routinely.   Is it justified? The next posting is a look at the way the temperature data is massaged.

cbdakota