GLOBAL AVERAGE TEMPERATURE

In media articles about "global warming" there are descriptions of "global average temperature" which is said to be increasing a few degrees.

This statement, unfortunately, is complete nonsense and quite confusing to the public.

It permits comedians like Dennis Miller to make hilarious jokes about global warming (on the Daily Show, a few days ago, for example).

There is no such thing as "the global average temperature". Such a concept is meaningless. (Some accounts do not even specify whether the change is supposed to be Fahrenheit or Centigrade.)

Since temperatures can vary twenty degrees or so (Fahrenheit) over the course of a day, a hundred degrees or more in one spot over the course of a year, two hundred or more on any one day around the surface of the globe (including bubbling geysers, for example), and by more than that in 3D when you include the range from the bottom of the ocean to the top of the atmosphere and occasional lava flows over the course of a year, it is really hard for the average person to relate to a 2 or 4 or 8 degree average temperature increase, if there really were such a thing.

Actually, what one can say is that there are many global average temperatures (and each one would show some kind of change from year to year.

For example, one could place thousands of sensors scattered around the globe and measure the temperature at local noon each day -- Or local midnight. One could average those temperatures over a year and get an annual temperature average by location; or average over all the sensors to get a daily global average temperature; or do both and get an annual global average temperature. Obviously the average temperature calculated from the noon measurements would be different from the average temperature calculated from the midnight measurements.

Similarly measurements taken over the oceans would give different results from measurements in barren land areas which would be different from measurements based in cities. And measurements taken on the surface would be different from measurements under the oceans or measurements of atmospheric temperatures at different altitudes.

Statements that "the earth is warming" need to take into account the variety of ways that these measurements could be analyzed statistically. A convincing argument would specify exactly which quantity was actually being measured and averaged. And then it would need to relate that to a reliable model of how that particular average is correlated to environmental change.

In practice, the actual measurements are usually one of three kinds. First, there are the measurements of temperature sensors scattered over the earth as described above. Using older measurements and connecting them to newer measurements for long-term studies of temperature changes can be problematic, because the older measurements are sparce, often biased by where they were located (eg cities), and hard to calibrate for accuracy and consistency with current measurements. But newer measurements (when used consistently and corrected for bias), over the last 30 years or so, seem to give global average temperatures which fluctuate less than half a degree centigrade from year to year, and may be accurate to a tenth of a degree when plotting a five year moving average. Thirty year trends using this kind of measurement show changes (a gradual increase) which are much greater than the fluctuations.

Second, there are historical measurements made by inference, for example by studying the composition of gasses stored in packed ice (eg. near the poles). Using these measurements in a consistent way to provide an "average annual temperature" gives a correlation between these temperature measurements and climate over long periods of time. These measurements show changes of around ten degrees centigrade or less from ice ages to periods of warm, virtually ice-free earth.

Third there are satellite measurements. These can measure temperature inferred from radiation, and may be looking at temperature at different altitudes from ground based sensors. These measurements of earth can be similar to measurements of temperature on nearby planets -- Venus and Mars -- allowing connections to be made between planetary science and earth science.

It would be helpful to the discussion if the media always used descriptions of things that could be understood and verified.

While global climactic modeling is still primitive, not precise, not always accurate, and not reliable for any interesting time spans, there is much that is well understood and certain facts that are undeniable.

The temperature of the earth, defined using almost any measurement consistently (eg. the average over many sensors spaced more or less evenly around the globe), depends heavily on the difference between the amount of energy from the sun that reaches us, less the energy we radiate back into space.

The more carbon dioxide there is in the atmosphere, the less energy we radiate. So more CO2 in the atmosphere tends to raise the temperature of the earth. (A substantial fraction of the CO2 in the atmosphere is due to the burning of fossil fuels by humans, especially humans in developed countries.)

Ice reflects energy back into space without absorbing it, and water absorbs energy efficiently without reflecting it. This gives an important positive feedback. So when H2O freezes and forms ice it tends to further lower the temperature of earth, and conversely when ice melts that tends to allow more heating of earth. As a consequence, once freezing starts, or melting starts, the process can accelerate.

Other factors that affect the temperature are heat from inside the earth released by volcanoes and other processes; changes in the energy absorption and radiation of the atmosphere due to chemicals, particles from volcanic emissions and cloud cover; the circulation of hot and cold water in the three dimensional oceans; the pattern of radiation and atmospheric circulation due to changes in the earth's movement with respect to the sun; and the amount of energy radiated from the sun to the earth.

The New Yorker is running a three part series on global warming (Part II is in the May 2, 2005 issue). It describes very real, measurable, changes in the ice mass at the poles. As ice melts, the oceans rise, and the energy dynamics of the earth change.

A substantial amount of the CO2 (and other gasses) in the atmosphere responsible for trapping some of the solar energy appears to be caused by human use of fossil fuels. Continuing to increase the CO2 leads to increasing instability in the environment according to the best climactic models.

Even without fully understanding the cause of the melting of ice on earth, or knowing what actions humans could take to prevent it, it should be obvious that a change in the energy dynamic of the earth, and melting ice causing a rising level of the ocean, will have dramatic consequences on the lives of many people on earth.

Even if we are not prepared to prevent a rise in earth's temperature, or to stop the ice from melting, we will need to deal, possibly quite soon (within years or a few decades at most), with the consequences of changing weather patterns and coastal flooding.

Part II of the New Yorker article deals with evidence of how severe the effect of a changing climate has been on some of the great civilizations of the past.