Note: The original title of this post was “A Global Warming Fingerprint Confirmed: Upper Troposphere Warming” because I was thinking that upper troposphere warming was a fingerprint. John Cook contacted me to let me know that he didn’t think it was. The reason it is not is that more than one thing can cause upper tropospheric warming, not just AGW. However, it does turn out to be more complicated than that. Various people claiming that a lack of UT warming was evidence of no warming have now been shown wrong, but even a lack of warming is not, if you will, an anti-AGW fingerprint. In the end it turns out to be a very complicated phenomenon and it would probably take me five blog posts to adequately related the conversation I’ve had over the last 24 hours with various climate scientists about the details.
John Abraham will be having something on this in the Guardian very soon, I’ll put a link here. An now, back to the original post:
Global warming is real, and caused by the release of human generated greenhouse gas pollution. We can measure the greenhouse gas concentrations (mainly CO2) and we can measure surface warming and upper ocean warming. But global warming should have a number of additional indicators, predicted by modeling or other aspects of climate science, and identifying those indicators both confirms the overall idea of global warming and helps us understand its effects.
The troposphere is the lower layer of the Earth’s atmosphere, where about three quarters of the air and most of the water vapor resides, and it is about 7 to 20 kilometers thick, thickest in the tropics and thinest near the poles. Climate models and thermodynmaic calculus predict that the upper troposphere in the tropics should experience warming. Temperatures at this altitude are measured using radiosonde technology. This is a package of instruments sent aloft on a weather baloon. But there are a lot of problems with the data. Instruments that measure temperature are ideally situated in one spot and properly enclosed. Such instruments dangling off the end of a balloon flying through the air are subject to heating and cooling from various uncontrolled effects, such as sunlight (vs. not), air movements, etc. Given the low quality of the data, it has been hard to observe upper troposphere temperatures.
Satellite measurement of temperature variation in the troposphere are not as useful as one might hope, because those instruments tend to average out temperatures over a much larger area of air than suitable for measuring the expected warming.
The new study, Atmospheric changes through 2012 as shown by iteratively homogenized radiosonde temperature and wind data (IUKv2), by Steven Sherwood and Nidhi Nishant, takes a new approach. From the Abstract:
We present an updated version of the radiosonde dataset homogenized by Iterative Universal Kriging (IUKv2)…This method, in effect, performs a multiple linear regression of the data onto a structural model that includes both natural variability, trends, and time-changing instrument biases, thereby avoiding estimation biases inherent in traditional homogenization methods. One modification now enables homogenized winds to be provided for the first time. ..Temperature trends in the updated data show three noteworthy features. First, tropical warming is equally strong over both the 1959–2012 and 1979–2012 periods, increasing smoothly and almost moist-adiabatically from the surface (where it is roughly 0.14 K/decade) to 300 hPa (where it is about 0.25 K/decade over both periods), a pattern very close to that in climate model predictions. This contradicts suggestions that atmospheric warming has slowed in recent decades or that it has not kept up with that at the surface. Second, as shown in previous studies, tropospheric warming does not reach quite as high in the tropics and subtropics as predicted in typical models. Third, cooling has slackened in the stratosphere such that linear trends since 1979 are about half as strong as reported earlier for shorter periods.
This graphic shows the relative warming of the upper troposphere in the tropics:
This is not the first study showing this, but rather, a significant update clarifying the observations.
Several modifications have been introduced, which have not had a large effect on estimated long-term trends in temperature but have enabled us to present a homogenized wind dataset in addition to that for temperature.
The warming patterns shown in the revised dataset are similar to those shown in the original study except that expected patterns now appear somewhat more clearly. These include a near-moist-adiabatic profile of tropical warming with a peak warming rate of 0.25–0.3 K/decade near 300 hPa since either 1959 or 1979. This is interesting given that (a) many studies have reported less-than-expected tropospheric warming, and (b) there has been a slowing of ocean surface warming in the last 15 years in the tropics. We support the findings of other recent studies … that reports of weak tropospheric warming have likely been due to flaws in calibration and other problems and that warming patterns have proceeded in the way expected from models. Moreover our data do not show any slowdown of tropical atmospheric warming since 1998/99, an interesting finding that deserves further scrutiny using other datasets
Another outcome of this research is the observation of increased winds in the Southern Hemisphere. This increase may be related to Ozone depletion at the southern end of the planet, a phenomenon that may also account for increasing extent of winter sea ice around Antarctica. Quoted in PhysOrg, author Steve Sherwood notes, “I am very interested in these wind speed increases and whether they may have also played some role in slowing down the warming at the surface of the ocean.”
John Abraham has also written up this research, here.
Since we were talking about “fingerprints” I thought you might like to watch this video by John Cook discussing global warming fingerprints. The video is from the MOOC “Denial101x Making Sense of Climate Science Denial”