Commentary

The Current Wisdom: Atmospheric Temperture Trends Measured by Satellite for the Last Third of a Century

The Current Wisdom is a series of monthly articles in which Senior Fellow Patrick J. Michaels reviews interesting items on global warming in the scientific literature that may not have received the media attention that they deserved, or have been misinterpreted in the popular press.

Traditionally The Current Wisdom only comments on science appearing in the refereed, peer-reviewed literature, or that has been peer-screened prior to presentation at a scientific congress. In this case we are building off of a summary of that literature published about satellite-sensed temperatures, the only truly global record that there is, and the only one that cannot suffer from the problem of urban (warming) bias.

Prior to April, 2011, issues of this Wisdom, which began in 2010, are available at our blog Cato@Liberty (www.cato-at-liberty.org/).


It has been 33 years now that satellites have carried Microwave Sounding Units (MSU) capable of measuring the temperature of the lower levels of the atmosphere. These instruments take advantage of the fact that the vibration of the diatomic bond in common oxygen (O2) is proportional to temperature.

The pioneering work of taking the raw MSU observations and developing a temperature record that could be used in climate studies was first done by Roy Spencer and John Christy, then of NASA and University of Alabama-Huntsville, whose joint paper in Science magazine back in 1990 introduced the world to this novel atmospheric temperature dataset. Spencer and Christy have continued to extend and improve this important work ever since.

One of the great characteristics of the satellite MSU measurements is that they do not suffer from many of the ills that thermometric measurements taken from the earth’s surface often do. Since the satellite observations are from a bulk layer of the atmosphere, they are not influenced by near-surface properties that confound the measurements of surface-based thermometers, including such issues as local landscape changes (such as urbanization and deforestation) and uneven spatial coverage.

However, there are also some problems that satellite measurements share with surface measurements, including changing instruments as the MSU sensor has a limited lifetime, so there have been many different ones taking measurements over the years.

There is also the problem changing instrument locations. The satellites orbit at an altitude in which there are still traces of the atmosphere, which drag on the satellites resulting in lower altitudes.

As with surface measurements, these issues must be carefully worked through in order to produce a climatologically useful temperature history. After all, much like the network of surface thermometers, the initial MSU instruments were developed to provide temperature measurements to improve daily weather forecasts, rather than as long-term climate monitors.

The painstaking work of Spencer and Christy (and other scientists subsequent to their initial publication), has made the MSU temperature record one the most important tools we have in the study of how the earth’s climate has been changing in recent decades and how the observed changes compare with climate model projections.

The images below show the character of the temporal (Figure 1) and spatial (Figure 2) temperature trends from the earth’s lower atmosphere as compiled by Spencer and Christy from December 1978 through November 2011 — a period spanning 33 years, or nearly a third of a century.

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Figure 1. Satellite-derived global lower tropospheric temperature departures from the recent thirty-year average, December 1978-November 2011 (source: University of Alabama in Huntsville Earth System Science Center, http://nsstc.uah.edu/climate/)

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Figure 2. Satellite-derived global lower tropospheric linear temperature trends, December 1978-November 2011 (source: (source: University of Alabama in Huntsville Easrth System Science Center, http://nsstc.uah.edu/climate/)

Some general observations:

• The linear trend through the 33-yr global record of temperatures in the earth’s lower atmosphere (Figure 1) is 0.138°C/decade.

• The warming trend increases from south to north. The trend in the Southern Hemisphere is 0.078°C/decade. The trend in the Northern Hemisphere is 0.197°C/decade. Substantial portions of the high southern latitudes show little warming or actual cooling.

• The high northern latitudes are warming the at the greatest rate.

• The global rate of warming is near the low end of the climate model projected rate of warming over the last 33 years.

• The rate of warming in the tropical latitudes has been much less than climate model projections of that warming rate.

Take some time to study and consider both Figure 1 and Figure 2 above, for they are the best records of how the earth’s temperatures have changed during the period of the most rapid increase in the anthropogenic emissions of greenhouse gases and aerosols. Bear in mind that not all of what you see is a direct response to those emissions. For instance, volcanic eruptions and the oscillations of El Niño/La Niña have a noticeable and marked impact. But the general picture gives you a pretty good idea of the character of the earth’s temperature change — a character that more than likely will be continued into at least the near future (the next several decades) and likely throughout much of the coming century.

References:
Spencer, R. W., and J. R Christy, 1990. Precise Measurement of Global Temperature Trends from Satellites. Science, 247, 1558-1562.

Spencer, R.W., and J.R. Christy (1992). Precision and Radiosonde Validation of Satellite Gridpoint Temperature Analyses. Journal of Climate, 5, 858-866.

Patrick J. Michaels is a Senior Fellow in Environmental Studies at the Cato Institute.