The orthodoxy has been that as a consequence of greenhouse gas emissions global temperatures will continue to increase. But what is really meant by global temperatures? According to James Hansen from NASA’s Goddard Institute for Space Studies even defining surface air temperature is not easy:
“I doubt that there is a general agreement how to answer this question [of what is surface air temperature]. Even at the same location, the temperature near the ground may be very different from the temperature 5 ft above the ground and different again from 10 ft or 50 ft above the ground. Particularly in the presence of vegetation (say in a rain forest), the temperature above the vegetation may be very different from the temperature below the top of the vegetation. A reasonable suggestion might be to use the average temperature of the first 50 ft of air either above ground or above the top of the vegetation. To measure SAT we have to agree on what it is and, as far as I know, no such standard has been suggested or generally adopted. Even if the 50 ft standard were adopted, I cannot imagine that a weather station would build a 50 ft stack of thermometers to be able to find the true SAT at its location.”
He is also ambiguous when it comes to daily mean surface air temperatures:
“Again, there is no universally accepted correct answer. Should we note the temperature every 6 hours and report the mean, should we do it every 2 hours, hourly, have a machine record it every second, or simply take the average of the highest and lowest temperature of the day ? On some days the various methods may lead to drastically different results.”
Read more here: http://data.giss.nasa.gov/gistemp/abs_temp.html




Jennifer Marohasy BSc PhD has worked in industry and government. She is currently researching a novel technique for long-range weather forecasting funded by the B. Macfie Family Foundation.