Over at the Climate Dialogue website we start with what could become a very interesting discussion about the so-called tropical hot spot. Climate models show amplified warming high in the tropical troposphere due to greenhouse forcing. However data from satellites and weather balloons don’t show much amplification. What to make of this?
Have the models been ‘falsified’ as critics say or are the errors in the data so large that we cannot conclude much at all? And does it matter if there is no hot spot?
The (missing) tropical hot spot is one of the long-standing controversies in climate science. In 2008 two papers were published, one by a few scientists critical of the IPCC view (Douglass, Christy, Pearson and Singer) and one by Ben Santer and sixteen other scientists. We have participants from both papers. John Christy is the ‘representative’ from the first paper and Steven Sherwood and Carl Mears are ‘representatives’ of the second paper.
Below I repost the introduction that we – the editors of Climate Dialogue – prepared as the basis for the discussion. Feel free to post it on your own blog with a link to the discussion atclimatedialogue.org.
The (missing) hot spot in the tropics
Based on theoretical considerations and simulations with General Circulation Models (GCMs), it is expected that any warming at the surface will be amplified in the upper troposphere. The reason for this is quite simple. More warming at the surface means more evaporation and more convection. Higher in the troposphere the (extra) water vapour condenses and heat is released. Calculations with GCMs show that the lower troposphere warms about 1.2 times faster than the surface. For the tropics, where most of the moist is, the amplification is larger, about 1.4.
This change in thermal structure of the troposphere is known as the lapse rate feedback. It is a negative feedback, i.e. attenuating the surface temperature response due to whatever cause, since the additional condensation heat in the upper air results in more radiative heat loss.
IPCC published the following figure in its latest report (AR4) in 2007:
Source: http://www.ipcc.ch/publications_and_data/ar4/wg1/en/figure-9-1.html (based on Santer 2003)
The figure shows the response of the atmosphere to different forcings in a GCM. As one can see, over the past century, the greenhouse forcing was expected to dominate all other forcings. The expected warming is highest in the tropical troposphere, dubbed the tropical hot spot.
The discrepancy between the strength of the hot spot in the models and the observations has been a controversial topic in climate science for almost 25 years. The controversy [i] goes all the way back to the first paper of Roy Spencer and John Christy [ii] about their UAH tropospheric temperature dataset in the early nineties. At the time their data didn’t show warming of the troposphere. Later a second group (Carl Mears and Frank Wentz of RSS) joined in, using the same satellite data to convert them into a time series of the tropospheric temperature. Several corrections, e.g. for the orbital changes of the satellite, were made in the course of years with a warming trend as a result. However the controversy remains because the tropical troposphere is still showing a smaller amplification of the surface warming which is contrary to expectations.