Is the Tropospheric Hotspot a Problem for Climate Science?

Since warm air holds more moisture, any rise in surface temperatures, regardless of the cause of that rise, will cause more evaporation and thus more water vapor in the atmosphere. This means that a rise in surface temperatures decreases the “lapse rate” - the rate of cooling as altitude increases. The lapse rate is slower at the equator than at the poles - the rate at the equator is about half that of the subtropics. Because of this, it’s been predicted that there should be a tropospheric “hot spot” in the tropics. Climate models predict this because there is good reason to expect it, and this is true regardless of what is causing the warming. It should be there whether warming is caused by an increase in TSI or an increase in GHGs. Finding that hotspot would not mean that we have detected an anthropogenic signature of warming due to GHGs; it rather would mean that we understand how surface warming affects lapse rates. Not finding the hotspot means either: 1) we have more to learn about how surface warming affects lapse rates or 2) we don’t have good data for the troposphere in the tropics. It does not imply that there is a hole or flaw in theory that predicts (and finds) that the increase in GHGs will cause surface warming.

To illustrate how similar the "hotspot" will look with if warming is caused by the Sun or by GHGs, I copied model results of both.[2] The top diagram above shows the modeled effects of doubled CO2 while the bottom shows the effects of a 2% increase in solar forcing. As you can see, the "hotspot" looks the same in both. The biggest difference between these the cooling of the stratosphere with GHG warming. If warming is caused by an increase in solar forcing, the stratosphere doesn't cool. 

Top: 2xCO2
Bottom: +2% Solar Forcing

At this point in time, satellite data alone cannot conclusively detect this hotspot in the tropics, since UAH and RSS give us different answers to this question. However, this does not necessarily mean it’s not there. A report from the U.S. Climate Change Science program (co-authored by Christy) concluded that the most likely explanation for any model vs observation discrepancy is due uncertainty in satellite measurements: 

Over the last three to five decades, global surface temperature records show increases of about +0.15ºC per decade. Explaining atmospheric and surface trends therefore demands relative accuracies of a few hundredths of a degree C per decade in global time series of both surface and upper-air observations. As this and subsequent chapters will show, the effects of instrumental biases on the global time series are significantly larger than a few hundredths of a degree for the upper-air data, though the global surface temperature compilations do appear to reach this level of accuracy in recent decades (Folland et al., 2001b). These biases, especially those of the upper air, must therefore be understood and quantified rather precisely (see section 3 below). For this fundamental reason, reliable assessment of lapse rate changes remains a considerable challenge.[1]

At the same time, since 2008, scientists have been able to use the correlation between temperature and wind shear allows scientists to at least indirectly detect a hot spot. Allen 2008 derived “estimates of temperature trends for the upper troposphere to the lower stratosphere since 1970. Over the period of observations, we find a maximum warming trend of 0.65±0.47 K per decade near the 200 hPa pressure level, below the tropical tropopause. Warming patterns are consistent with model predictions except for small discrepancies close to the tropopause.”[3] 

Sherwood 2015 Shows the "Hotspot" forming in the Tropics

These results were confirmed in 2015 by a more recent study, which concluded: 

“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 (Po-Chedley et al 2015) 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.”[4]

You can see the trends in the upper troposphere warming greater rates than the surface for the tropics consistent with what we would expect from sound meteorological principles. In other words, the tropospheric hotspot appears to be a nonissue for climate science.

Update, 11/9/2024

Just today I found a graph from the ERA reanalysis showing the progression of warming at altitudes from 1000 to 10hPa. You can clearly see both the cooling stratosphere with warming troposphere, but you can also see the darkest red over the last decade or so occurs in the upper troposphere. In other words, it does appear that the hotspot is emerging in the upper troposphere essentially as expected.


References:

[1] “Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences”
https://downloads.globalchange.gov/sap/sap1-1/sap1-1-final-all.pdf

[2] "Tropical tropospheric trends." https://www.realclimate.org/index.php/archives/2007/12/tropical-troposphere-trends/

[3] Allen, R., Sherwood, S. Warming maximum in the tropical upper troposphere deduced from thermal winds. Nature Geosci 1, 399–403 (2008). https://doi.org/10.1038/ngeo208
https://www.researchgate.net/publication/228630146_Warming_maximum_in_the_tropical_upper_troposphere_deduced_from_thermal_winds

[4] Steven C Sherwood and Nidhi Nishant (2015) Atmospheric changes through 2012 as shown by iteratively homogenized radiosonde temperature and wind data (IUKv2). Environ. Res. Lett. 10 054007.
https://iopscience.iop.org/article/10.1088/1748-9326/10/5/054007/meta