Read more about the research at UW Today.
What is the impact of this finding?
Computer models (global circulation models or climate models) that simulate the Earths changes from increased greenhouse gases predict increases in the Earths atmospheric temperature (troposphere) that are larger than those at the surface. For this reason, scientists are trying to determine the relative changes between the surface and the troposphere, to see if this relation holds in the real world.
Some have claimed that tropospheric temperature observations do not agree with climate models, particularly those observations from the University of Alabama in Huntsville. This paper shows that there is a bias in the Alabama product that reduces its estimate for warming in the troposphere. After correcting for this bias, UW researchers estimate that the mid-tropospheric trend in the Alabama observation is about twice as large. This helps to reconcile long-standing differences with other records, including the one developed by Remote Sensing Systems, and reduces the discrepancy with climate models.
What is the troposphere and why is it important?
The troposphere is the lowest 10 miles or so of the atmosphere (in the tropics) and it is the region of the atmosphere in which climate models predict a great deal of warming as a result of human emissions of greenhouse gases. In fact, scientists expect that the tropical troposphere should warm faster than the surface.
How do you measure the temperature of the troposphere?
Microwave sounding units on satellites sense microwave emissions from molecular oxygen. Scientists are able to infer temperature from those microwave emissions and then derive a temperature for various deep layers of the atmosphere. The observation from the mid-tropospheric channel of the microwave sounding units is used to create a full tropospheric product that is used for comparisons with climate models. Scientists typically do not want to use the mid-tropospheric product directly because some of the emissions come from the layer of the atmosphere above the troposphere (stratosphere), which is cooling as a result of global warming. Because the microwave sounding unit mid-tropospheric channel senses both the warming of the troposphere and the cooling of the stratosphere, scientists generally subtract out the stratospheric (cooling) influence to obtain the tropospheric warming.
How do Po-Chedley and Fu detect the bias?
Radiosondes (weather balloons) are used as an independent check for the microwave sounding unit. Scientists test to see if the satellite measurements of the Earth temperature are effected by the satellite calibration itself. For unknown reasons, the calibration of the various satellites changes between laboratory testing on Earth and its orbit in space. The satellite calibration uses the temperature of the satellite to determine the Earth temperature. If the measured Earth temperature always goes up when the satellite temperature goes down, researchers can conclude that the satellite calibration is influencing the satellites measurement of the Earth temperature. UW researchers found that the temperature in the Alabama-Huntsville observation went down when the NOAA-9 satellite temperature went up when comparing the Alabama data to five weather balloon datasets. Since NOAA-9 warms throughout its lifetime, this introduces a spurious cooling into the satellite measurement, which subsequently affects the entire 30-plus year record.
Because the weather balloon measurement is independent of the temperature of the satellite (which warms and cools based on the angle in which the sun hits it), the UW researchers were able to conclude that the Alabama measurements of Earth temperature for one of its satellites (NOAA-9) were affected by the satellite temperature itself. This indicates that the Alabama research mis-calibrated the NOAA-9 satellite.
Why is the Alabama-Huntsville report different?
The 30-year record of satellite atmospheric temperatures is actually based on more than a dozen individual satellites (the number varies for each team that creates a long-term time series). Each of these satellites needs to account for individual biases while being incorporated into the long-term record. The bias UW focused on is the calibration of the satellite.
As a conceptual example, imagine that the actual temperature of your oven is always 5 percent too hot compared to what an oven thermometer records (or 20 F too warm when the oven dial reads 400 F). That means that you would need to set the oven at 20 F lower than what your oven dial says. This is analogous to the microwave sounding unit – for some reason the calibration changes from pre-launch measurements on Earth to its actual measurements in orbit and it needs to be adjusted.
The teams that make these datasets decide how much to adjust the satellites by comparing satellites that are orbiting at the same time. If one satellites Earth temperature measurements are too warm or cold compared to another satellite over time, the scientists can determine the correct calibration coefficients. This is similar to determining how much your oven is too warm or cold using an oven thermometer. Importantly, the Alabama report uses specific overlaps to determine these calibration coefficients whereas Remote Sensing Systems and NOAA use all possible overlaps.
In the past, this procedural difference was considered a structural uncertainty, because both methods seemed reasonable. This study indicates that the procedure used by the Alabama researchers leaves in a residual bias and that the calibration coefficients were not adequately determined using the satellite overlaps they selected.
This work does not resolve all of the discrepancies between the different satellite teams. Important differences in the tropics remain and it will be interesting to see how these differences are resolved in the coming years.
How exactly does this study change the temperature report?
To put this all in perspective: The surface temperature in the tropics has increased by about 0.21 F per decade since 1979. Alabama-Huntsville, Remote Sensing Systems, and NOAA show 0.05 F, 0.18 F and 0.24 F per decade warming, respectively. Removing the influence of the cooling stratosphere, the troposphere is warming by 0.13 F, 0.26 F, and 0.33 F per decade for Alabama, Remote Sensing Systems, and NOAA, respectively. When we correct for the bias in Alabama, the warming increases to about 0.21 F per decade, which is much closer to the expectations from climate models.