Another Attempt To Find The Tropical Hotspot!

By Paul Homewood  

 

In his latest rant against deniers, David Holub  referred to an article in the Guardian which was, even by their standards, particularly misleading. 

 

http://www.theguardian.com/environment/climate-consensus-97-per-cent/2015/mar/25/one-satellite-data-set-is-underestimating-global-warming

 

Written by the notoriously unreliable John Abraham, it says:

 

A very important study was just published in the Journal of Climate a few days ago. This paper, in my mind, makes a major step toward reconciling differences in satellite temperature records of the mid-troposphere region. As before, it is found that the scientists (and politicians) who have cast doubt on global warming in the past are shown to be outliers because of bias in their results.

It is known that there is a problem because there are multiple groups that create satellite temperature records. For instance, NOAA, Remote Sensing Systems (RSS), and the University of Alabama Huntsville (UAH). The problem is, their results don’t agree with each other. In particular, the UAH team, led by Dr. John Christy and Dr. Roy Spencer (who have discounted the importance and occurrence of climate change for years) present results that differ quite a bit from the others. In fact, in the current paper, it is stated that “Despite using the same basic radiometer measurements, tropical TMT trend differences between these groups differ by a factor of three.”

An important aspect to this issue is that for many reasons, it is expected that the tropospheric temperatures in the tropics will warm more than surface temperatures. This is called “tropospheric amplification.” According to two satellite groups, there is in fact such amplification. According to the UAH team, there is no amplification. The presence or absence of amplification is often used by some skeptics to discount the importance of global warming.

Why are there differences? Well that’s part of what this paper tries to answer.

Each of the teams try to deal with and correct for various satellite errors. For instance, sometimes the diurnal cycle effect is removed using temperatures from climate models. The RSS and NOAA teams “apply a drift correction based on the diurnal cycle from a GCM (global climate model) whereas UAH produces a microwave sounding unit mid-tropospheric temperature diurnal correction based on temperature comparisons between three co-orbiting satellites … UAH does not yet correct the diurnal drift for satellites carrying Advanced Microwave Sounding Units because they attempt to use these satellites during periods when the diurnal drift is small.”

The present paper presents a calibration scheme that allowed them to obtain a diurnal correction from the satellite measurements themselves, in particular, by solving for a common diurnal cycle correction using temperatures from all available satellites.

As the authors state in the paper, their new results agree with the two groups that show more warming. They disagree with UAH. As the authors state,

In general, our trends corrected with a GCM and trends corrected with our observationally derived diurnal cycle correction are similar to trends from NOAA and RSS … the UAH ocean trend is notably lower than trends from the other datasets.

They also discovered that the results from RSS, NOAA, and the new study all show tropical amplification and are in agreement with the expected amplification from climate models. They state, “There is no significant discrepancy between observations and models for lapse rate change between the surface and the full troposphere.”

 

 

The first thing to highlight is that the paper is talking about mid tropospheric temperatures, rather than the lower tropospheric ones which we continually compare to surface temperatures. The latter go up to around 2km in the atmosphere.

The headline, deliberately in my view, does not make this clear. As we know, many people don’t get far beyond the headline, and certainly won’t know the significance between the two layers. As a result, they will have taken home the message that satellite temperatures cannot be relied on, and that the surface data must therefore be right.

The article then compounds this by talking about “the UAH team, led by Dr. John Christy and Dr. Roy Spencer (who have discounted the importance and occurrence of climate change for years)”, leaving the impression that UAH are some sort of outlier.

As we know, of course, UAH and RSS data for the lower troposphere, TLT, follow each other very closely since 1979, and both have been rapidly diverging from the surface data in recent years.

 

The actual topic of the article is the tropospheric hotspot, which climate models say should exist. As Marcel Crok explains:

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.

 

Measurements from both satellites and radiosondes have repeatedly shown that the hotspot does not exist.

 

http://jonova.s3.amazonaws.com/graphs/hot-spot/hot-spot-model-predicted.gif

 

 

This has of course been a huge embarrassment for both the models and their supporters, and naturally the topic is a hot one, which has been well covered elsewhere. Earlier this year, UNSW researchers claimed that the hotspot did exist after all, but only after retrospectively “adjusting” radiosonde data!

This paper by Bo-Chedley talks in the Abstract of using a homogenized TMT dataset. Alarm bells should start ringing when we hear that term.

 

However, it is worth taking a look at the RSS graphs themselves, which show things are not quite as the Guardian boldly claims:

First, the graphs for the Tropics, lower and mid troposphere.

 

http://images.remss.com/msu/msu_time_series.html

   

We can see that:

1) The TLT trend is higher than the TMT, opposite to what the models would predict.

2) Both show a flatlining since 1998.

3) Both also show little change between 1979 and 1997, followed by a step change brought about by the El Nino of 1998.

 

Recall that for the tropics, where most of the moist is, the amplification should be about 1.4 times the warming at the surface. Also remember that GISS surface data has been diverging from the RSS TLT data at a rapid rate of knots since 1998. Therefore it is apparent that the RSS data does not support the theory that Tropical mid-tropospheric temperatures are rising faster than the surface ones, (or at least not as reported by GISS!).

 

http://www.woodfortrees.org/plot/gistemp/from:1998/plot/gistemp/from:1998/trend/plot/rss/from:1998/plot/rss/from:1998/trend

 

 

FOOTNOTE

For reference, the UAH TMT trend for the Tropics since 1979 is 0.06C per decade, just slightly below RSS.

http://vortex.nsstc.uah.edu/data/msu/v6.0beta/tmt/uahncdc_mt_6.0beta3.txt

 

GISS surface trend for the Tropics is much higher at 1.07C per decade.

http://data.giss.nasa.gov/gistemp/tabledata_v3/ZonAnn.Ts+dSST.txt