Friday, July 21, 2017

Myth: The Tropospheric Hot Spot does not Exist

The outline for this post is as follows:
  1. The Myth and Its Flaw
  2. Context and Analysis
  3. Posts Providing Further Information and Analysis
  4. References

This is the "main version" of this post, which means that this post lacks most of my references and citations. If you would like a more comprehensive version with all the references and citations, then please go to the "+References" version of this post.

References are cited as follows: "[#]", with "#" corresponding to the reference number given in the References section at the end of this post.

1.  The Myth and Its Flaw

There is little-to-no evidence of a tropical tropospheric hot spot, or the scientific evidence argues against the hot spot's existence.

Proponents of this myth include David Evans, Stefan Molyneux, Judith Curry, Richard Lindzen, S. Fred Singer, Christopher Monckton, Anthony Watts, John Christy, Roger Pielke Sr., Jospeh D'Aleo, James Wallace III, Steve McIntyre, Ross McKitrick, The Daily Caller, Paul Homewood, Tom Nelson, Roy Spencer, and a number of blogs including WattsUpWithThat.

So this myth's bandwagon includes some of the most prominent "skeptics" of the mainstream scientific consensus on man-made climate change. This group of "skeptics" includes Curry and Christy, two of the four witnesses who testified in a climate science Congressional hearing held earlier this year.

The myth's flaw: tropical precipitation/convection patterns and paleoclimate data provide evidence of the hot spot, as do the majority of satellite analyses, radiosonde analyses, and re-analyses. So the preponderance of the evidence shows that hot spot exists, contrary to the myth.

2. Context and Analysis

Earth's atmosphere contains multiple layers. The layer closest to the Earth's surface air is known as the troposphere. Tropospheric temperature decreases with increasing height; the rate of decrease is known as the tropospheric lapse rate

Climate models and basic physical theory predict that warming at Earth's surface will cause more water to evaporate, especially over tropical oceans. This evaporation increases the amount of water vapor in the air, since warmer air can hold more water vapor. The vapor-rich air then rises into the troposphere by convection. The water vapor subsequently condenses with increasing tropospheric height, since tropospheric temperature and pressure decreases with increasing height.

Condensation of water vapor releases some of the energy that went into evaporating the water; this is known as release of latent heat. So water vapor condensation causes more warming of the lower troposphere and even more warming of the upper troposphere. This latent heat release shrinks the rate at which tropospheric temperature decreases with increasing height; therefore latent heat release reduces the magnitude of the tropospheric lapse rate, as depicted in figure 1:


Figure 1: A diagram of tropical tropospheric warming reducing the magnitude of the tropospheric lapse rate (adapted from Crok, Strengers, and Verheggen [5, page 3]). The horizontal dimension represents temperature, with temperature increasing as one goes to further right. The vertical dimension represents altitude in the troposphere, with altitude increasing as one goes further up from Earth's surface at the black line. The blue line represents the tropical temperature profile before warming, while the red line represents the tropical temperature profile after warming. Latent heat release causes more warming with increasing height, leading in the red line being steeper than the blue line. As a result, there is less of a temperature decrease with increasing height after tropical warming. Thus the lapse rate's magnitude is greater for the blue line than for the red line, indicative of a lapse rate reduction in response to tropical warming.

So the tropical troposphere should behave somewhat like a moist adiabat, in which the rate of warming increases with increasing height in response to water vapor condensing from vapor-saturated air. 

The aforementioned tropical warming amplification is called the tropical tropospheric hot spot by many myth proponents. So the hot spot I will discuss relates to amplification of warming as one goes from the tropical surface to higher in the tropical troposphere. This is different from the question of whether the amount or magnitude of observed tropospheric warming matches the amount of warming projected by climate models; I address the "magnitude" issue in "Myth: Santer et al. Show That Climate Models are Very Flawed". So, for the sake of argument, I will take the advice of myth proponent John Christy and not compare observations to models:

"Section III. Research Design
Unlike some research in this area, this research does not attempt to evaluate the existence of the THS [tropical hot spot] in the real world by using the climate models. This would constitute a well-known error in mathematics and econometrics [...] [6, page 14]."

But in case you are interested in what the models show, figure 2 depicts a modeled hot spot (amplification of warming with increasing height in the tropics) in response to warming caused by increased solar activity or in response to warming caused by increased carbon dioxide (CO2):

Figure 2: ECHAM3/LSG model (European Center/Hamburg Model 3 / Large Scale Geostrophic coupled atmosphere-ocean climate model) simulation of the atmospheric response to (a) increased solar forcing (from increased solar output) and (b) increased CO2 forcing (from increased CO2 levels). Colored areas indicate significant responses, with darker blues indicating cooling and darker reds indicating warming. The horizontal axis represents latitude, with the tropics being between roughly 30N and 30S. The vertical axis represents altitude, with decreasing atmospheric pressure as altitude increases. The tropical troposphere lies below 150hPa, while the tropical stratosphere is above 70hPa. Tropical tropospheric warming increases with height in both panels a and b, indicating that the hot spot forms in response to both solar-induced warming and CO2-induced warming. In contrast, strong tropical stratospheric cooling comes with CO2-induced warming, but not solar-induced warming. This figure is taken from a 2001 report of the United Nations Intergovernmental Panel on Climate Change (IPCC) [4, page 707].

Myth proponents claim that scientific evidence argues against the hot spot's existence, or that there is little-to-no evidence of the hot spot's existence. Many myth proponents then falsely claim that the absence of the hot spot is evidence against CO2-induced global warming, as I discussed in "Myth: The Tropospheric Hot Spot is a Fingerprint of CO2-induced Warming". But the proponents' argument fails, since multiple lines of evidence show the hot spot exists. Let's go through some of these lines of evidence, which are as follows:

  1. Tropical precipitation and convection patterns
  2. Evidence from climate change in the distant past (paleoclimate evidence)
  3. Satellite-based temperature records
  4. Measurements from weather balloons (radiosondes)
  5. Temperature re-analyses

(The last three sources have been cited by many myth proponents, so I do not seen how they could object to my citing those sources as well.)

During the period of post-1970s global warming, tropical precipitation and convection patterns changed in a way indicative of moist-adiabatic lapse rate reduction, as shown in a 2010 paper. A subsequent 2011 paper disputed this finding, though a 2017 paper supported the results from the 2010 paper. Land-based records also suggest that warming increased with increasing height in the recent past, though these results are more tentative, especially in the tropics. Furthermore, paleoclimate research indicates that the tropical lapse rate likely decreased during tropical warming in the distant past, while the lapse rate increased in response to tropical cooling. So paleoclimate data and recent tropical precipitation/convection patterns provide two independent lines of evidence showing a hot spot during tropical warming.

Satellite-based temperature measurements provide another independent line of evidence for the hot spot. Scientists can infer tropospheric temperature from satellite data. At least six satellite research groups generate satellite-based tropospheric temperature records, using different data analysis methods and different corrections for known artifacts/errors in the data; these corrections are known as homogenization. The following six groups generate homogenized, satellite-based tropospheric temperature records:

  • a group at the University of Washington (UW)
  • a group at the National Oceanic and Atmospheric Administration Center for Satellite Applications and Research (NOAA/STAR or NOAA)
  • a group at Remote Sensing Systems (RSS)
  • Vinnikov et al. at the University of Maryland (UMD); myth proponents John Christy and S. Fred Singer cited UMD's analysis, so it would be worthwhile to see if the UMD analysis supports the "no hot spot" myth
  • a group at the University of Alabama in Huntsville (UAH)
  • Weng and Zou at the University of Maryland

Weng and Zou produce a temperature record showing the hot spot. However, other climate scientists do not often cite the Weng and Zou temperature record, and I have some doubts about the veracity of the Weng and Zou analysis. So let's aside their analysis. That leaves us with five satellite-based tropospheric temperature records.

4 out of 5 satellite analyses show the hot spot, with greater warming in the tropical mid- to upper troposphere than at Earth's surface. Specifically: the UW, NOAA/STAR, RSS, and UMD analyses show the hot spot. Figure 3 shows four of these five analyses, with two versions of the UW analysis:

Figure 3: HadCRUT4 tropical surface warming trends and tropical mid- to upper tropospheric warming trends (in K per decade) above the land, oceans, and both land and oceans from 1979 - 2012. Tropospheric warming trends are from UW, NOAA, RSS, and UAH satellite data analyses. UW(GCM) and UW use different methods for processing the satellite data. The value in parentheses is the ratio of the tropospheric warming to the surface warming for a given tropospheric temperature trend [1]. The RSS tropospheric warming trend is spuriously low due to an error in homogenization. The RSS team later corrected this error. This resulted in a RSS tropical mid- to upper tropospheric warming trend that is between the NOAA trend and the UW trend.

Figure 3 shows amplification above the tropical oceans and the sum of land+oceans, but not above tropical land. This result is not surprising. To see why, first note that as warm surface air rises to the troposphere, the warm air mixes by convection. This transfers heat from some of the warmer air that rose from the land to some of the nearby, less warm air that rose from the oceans. Thus convection makes the tropospheric warming rates above the tropical land more similar to the warming rate above the nearby tropical oceans (figure 5 below shows this more clearly, with tropospheric warming become more similar above different regions as one gets higher in the troposphere). And since land surface warming should be greater than ocean surface warming, the similar tropospheric warming rates above tropical land and oceans would imply a lower amplification ratio above land than above oceans. One would also expect a greater amplification ratio above oceans than above land, since oceans provide a readier source of water that can evaporate, condense in the upper troposphere, and thus produce the tropospheric hot spot. Thus one can explain why figure 3 shows greater tropospheric warming amplification above the tropical oceans vs. above the tropical land.

Figure 3 also indicates that UAH is the only analysis lacking a hot spot. This is not surprising since:
  1. UAH has a long history of under-estimating tropospheric warming due to UAH's faulty homogenization.
  2. Other scientists have critiqued UAH's homogenization methods.
  3. UAH's satellite-based temperature analyses often diverge from analyses made by other research groups.
UAH is thus the odd analysis in the bunch, and likely the least credible analysis. So UAH's data analysis serves as (at best) a very weak argument for the "no hot spot" myth. As noted by the authors of figure 3:

"Our amplification factor over land is reduced because of enhanced land surface warming relative to sea surface warming [...]. All of the MSU/AMSU [microwave sounding unit for determining atmospheric temperature] datasets demonstrate tropical tropospheric amplification, except UAH [emphasis added] [1, page 2285]."

Rather tellingly, UAH scientists John Christy and Roy Spencer have been some of the most vocal defenders of the "no hot spot" myth; Spencer and Christy may propagate the myth as a means of defending their flawed UAH analysis. In fact, myth proponents such as Judith Curry, Anthony Watts, and The Daily Caller use Christy and Spencer's claims to prop up the "no hot spot" myth. One might wonder why these myth proponents focus on the outlier UAH team, as opposed to the other satellite research groups mentioned...

Anyway, in addition to mid- to upper tropospheric temperature, UAH and RSS also produce satellite-based estimates of lower tropospheric temperature. NOAA/STAR and UW do not produce a lower tropospheric temperature analysis. Consistent with UAH's status as the outlier group, UAH's analysis does not show amplified mid- to upper tropospheric warming relative to lower tropospheric warming; in contrast, RSS' analysis does show amplification. So once again the flawed UAH analysis is the only satellite-based temperature record supporting the "no hot spot myth", in contrast to four other records debunking the myth.

Alongside satellites, scientists also use weather balloons (radiosondes) to measure tropospheric temperature. Radiosonde temperature records can be calculated from measurements of wind or from temperature sensor readings; this provides two methods for confirming temperature trends. There are at least five homogenized radiosonde temperature records, which are as follows:

  • Radiosonde Observation Correction using Reanalysis (RAOBCORE) from the University of Vienna
  • Radiosonde Innovation Composite Homogenization (RICH) from the University of Vienna
  • Iterative Universal Kriging (IUKv2) from the University of New South Wales
  • Radiosonde Atmospheric Temperature Products for Assessing Climate (RATPAC) from the NOAA
  • Hadley Center Radiosonde Temperature (HadAT2) from the United Kingdom Met Office

4 out of 5 radiosonde analyses show the hot spot, with greater warming in the tropical upper troposphere than near Earth's surface. Upper tropospheric amplification occurs at an atmospheric pressure level of around 300hPa, as illustrated in figure 4 using the IUKv2 record:

Figure 4: Tropical warming/cooling trend versus height for weather balloon measurements. Pressure decreases from the Earth's surface (near the bottom of the y-axis) to the troposphere to the stratosphere (near the top of the y-axis). The tropical troposphere lies below 150hPa. The green line extends to the beginning of satellite era of atmospheric temperature measurements (1979), while the purple line extends to before the satellite era. The inset legend for circles, diamonds, etc. indicates different temperature trends given different data analysis choices. The blue line roughly indicates the warming pattern expected for a moist adiabat [2].

Of the 5 radiosonde analyses, HadAT2 is outlier; HadAT2 shows the hot spot over some multi-decadal time periods, but not over other multi-decadal periods. This may explain why myth proponent David Evans relies so heavily on HadAT analyses, to the exclusion of other analyses. One should treat the HadAT2 result with skepticism, however, since the HadAT research group strongly recommends that researchers not rely on just the HadAT2 data-set. Instead, the HadAT team recommends that researchers use other radiosonde data-sets, along with the RSS satellite analysis, to ensure that their result is robust. Since the hot spot appears in the other radiosonde analyses and the RSS analysis, HadAT2's lack of a hot spot (over some time periods) is not a robust result.

Radiosonde data and satellite data, along with other data sources, are also incorporated into temperature re-analyses. These re-analyses help generate a broader picture of global temperature using a wealth of data. A number of myth proponents (including Judith Curry, John Christy, Roger Pielke Sr., and Anthony Watts) cite tropospheric warming trends from re-analyses. So it would be worthwhile to see if the re-analyses support the "no hot spot" myth.

There are at least six re-analysis groupings, with each grouping having different re-analysis versions and updates. The groupings are as follows:

  • European Centre for Medium-Range Weather Forecasts Interim re-analysis (ERA-I)
  • Modern-Era Retrospective analysis for Research and Applications (MERRA)
  • National Centers for Environmental Prediction / Climate Forecast System Re-analysis (CFSR or NCEP/CFSR)
  • Japan Meteorological Agency Re-analysis (JRA)
  • National Centers for Environmental Prediction / National Center for Atmospheric Research re-analysis (NCEP, NCEP-2, or NCEP/NCAR)
  • 20th Century Re-analysis (20CR)

20CR only incorporates surface data, and thus may not be very useful for examining mid- to upper tropospheric warming trends. So let's set aside 20CR. That leaves us with five re-analysis groupings.

4 out of 5 re-analyses show the hot spot, with greater warming in the tropical upper troposphere than near Earth's surface. Upper tropospheric amplification occurs at an atmospheric pressure level of around 300hPa, as depicted in figure 5 for three of the re-analyses:

Figure 5: Tropospheric warming trend for the entire tropics (blue line; from 30°N to 30°S), tropical land (green line; from 30°N to 30°S), and the Sahara desert (red line). The Sahara's red line is not relevant for our purposes, since the Sahara is too arid to behave like a moist adiabat. The horizontal axis represents the warming trend in K per 34 years. The vertical axis represents altitude, with decreasing atmospheric pressure as altitude increases [3].

As illustrated in figure 5, NCEP-2 shows greater warming in the lower tropical troposphere than in the upper troposphere. This NCEP-2 trend should be taken with a grain of salt, since the NCEP-2 re-analysis has a history of under-estimating tropospheric warming. Other re-analyses, such as MERRA and ERA-I (see figure 5), tend to have more upper tropospheric warming than NCEP-2 and tend to perform better than NCEP when it comes to representing atmospheric phenomena. So one should consider relying on another re-analysis instead of NCEP-2 (which is a problem for myth proponent Anthony Watts, who relies on the NCEP-2 re-analysis). In contrast to NCEP-2, ERA-I shows greater warming near the tropical surface than in the lower troposphere (see figure 5). This is because ERA-I under-estimates the rate of lower tropospheric warming, as acknowledged by the ERA-I research team.

ERA-I is particularly damaging to the position of myth proponent Judith Curry, since Curry defends the "no hot spot" myth, even though Curry lauds the ERA-I analysis that shows tropical tropospheric amplification. Yet Curry (to my knowledge) does not acknowledge the tropical tropospheric amplification in ERA-I. Myth defender John Christy has an even worse conundrum since he defends the "no hot spot" myth, even though he cites re-analyses, satellites analyses, and radiosonde analyses that show the hot spot. The hot spot even appears in a figure made by Christy. So Christy is aware that the hot spot exists, but he claims that it does not exist (I discuss this more in "Myth: John Christy Thinks There is No Evidence of the Hot Spot"). Thus one of the most cited proponents of the "no hot spot" myth, is well-aware that the myth is nonsense. Amazing.

3. Posts Providing Further Information and Analysis

4. References

  1. "Removing diurnal cycle contamination in satellite-derived tropospheric temperatures: understanding tropical tropospheric trend discrepancies"
  2. "Atmospheric changes through 2012 as shown by iteratively homogenized radiosonde temperature and wind data (IUKv2)"
  3. "Detection and analysis of an amplified warming of the Sahara Desert"
  4. "Climate change 2001: The scientific basis; Chapter 12: Detection of climate change and attribution of causes"
  5. "Extended Summary of the Climate Dialogue on the (missing) tropical hot spot"
  6. "On the Existence of a “Tropical Hot Spot" & The Validity of EPA’s CO2 Endangerment Finding"