The Increase in Global Temperature: What it Does and Does Not Tell Us

The global temperature record is often used to support the claim that the Earth is warming at a rate that is “reasonably consistent” with predictions for warming due to the buildup of greenhouse gases. This paper examines how the Earth’s temperature is taken, examines the reliability of those measurements, and highlights several factors that affect temperature trends and variation. No matter how many confounding variables are brought to the table, many argue that the rise in greenhouse gases is or soon will dominate all other factors that affect the distribution of energy in the atmosphere, and will drive the global temperature further upward. If reported temperature continues to rise, some scientists will inevitably claim that the empirical evidence is highly supportive of that theory. However, it is important to recognize that association or statistical correlation is not causation. But major problems remain with the above assessment. The warming rate in the last three decades is not significantly different from the warming rate in the early 20th century (1915-45). Temperature records are far from perfect and contain biases from urbanization, distribution of measurement stations, instrument changes, time-ofobservation, assorted problems in measuring temperatures in ocean areas, and so on. These factors could introduce a total bias of 0.2-0.3°C, or about one-third of the observed warming. Even if we accept that the warming as real, there is a strong argument that approximately half of the warming—the portion that took place in the early 20th century—was a natural recovery from the Little Ice Age. There are many non-greenhouse factors at work on temperature, and it is very difficult to isolate the signal related to the buildup of human produced greenhouse gases. Finally, we need to understand why the trends in surface and lower tropospheric temperature differ to be able to explain the roles of the various climate system forcings. Consequently, strong evidence suggests that there was a systematic underestimation of temperature. In other words, the Earth was probably warmer in the past than our records indicate and, therefore, the change in temperature that we now observe is not as great as it appears. This conclusion is important for at least two reasons. First, it provides further evidence that modeled estimates of the CO2 influence on temperature are overstated, which means that the projections of future temperature increases are as well. Second, it demonstrates the high priority that should be assigned to improving the global observation system. More accurate data are essential for setting wise policy and for developing better models.