Tuesday, July 18, 2017

Myth: The Tropospheric Hot Spot is a Fingerprint of CO2-induced Warming

This post is part of a series addressing issues related to the hot spot. The other parts of this series are listed in the "Myths about the Hot Spot" section of the "Quick Scientific Debunking" page.

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.

The following twitter thread summarizes some of the main points in this blogpost:

1.  The Myth and Its Flaw

Climate models predict that in moist tropical areas, a region of the lower atmosphere will warm more than Earth's surface. This region of greater warming is known as the "hot spot". The myth states that the hot spot is a specific sign, or fingerprint, of anthropogenic (human-caused) carbon-dioxide-induced global warming.

Proponents of this myth include David Evans, Stefan Molyneux, Judith Curry, Richard Lindzen, S. Fred Singer, Nicola Scafetta, Christopher Monckton, Anthony Watts, John Christy, Roy Spencer, Roger Pielke Sr., Craig Idso, Joseph D'Aleo, James Wallace III, Steve McIntyre, Ross McKitrick, William Happer, Tim Ball, Don Easterbrook, Anthony Lupo, Luboš Motl, the "lukewarmer" Lucia Liljegren, The Daily Caller, Paul Homewood, Tom Nelson, Vladan Ducić, and a number of blogs including WattsUpWithThat.

Christy, D'Aleo, Wallace III, and an organization known as ICECAP, use the myth to attack the US Environmental Protection Agency's attempts to regulate carbon dioxide (CO2) emissions.

The myth's flaw: the hot spot is not a fingerprint for attributing global warming to increased CO2, since the hot spot occurs with any sufficiently large surface warming in the tropics, especially warming of the tropical oceans. The scientific community has known this since at least 1975. This conclusion is supported by climate models, an understanding of the mechanism via which the hot spot forms, and observations of the hot spot forming in response to short-term non-CO2-induced warming.

2. Context and Analysis

Earth's atmosphere contains multiple layers. The layer closest to the Earth's surface air is known as the troposphere. Above the troposphere is the stratosphere. 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 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 energy release causes further warming. So water vapor condensation causes more warming of the lower troposphere and even more warming of the upper troposphere. This shrinks the rate at which tropospheric temperature decreases with increasing height, thereby reducing 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], though originally taken from a climate science textbook [8]). The horizontal dimension represents temperature, with temperature increasing as one goes from left to 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. Water vapor condensation causes more warming with increasing height, leading to 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.

To borrow an analogy from the climate researcher Mark Richardson: this energy transfer from the near-surface to higher in the troposphere is similar to sweating, in which sweat evaporates on a person's skin and then condenses elsewhere, cooling the skin and transferring body heat to the site of condensation. Thus the tropical troposphere will behave like a moist adiabat, in which the rate of warming increases with increasing height in response to water vapor condensing from water-vapor-saturated, moist air. So this amplified tropospheric warming has more to do with heat released by condensing water vapor (also known as release of latent heat), and less to do with CO2 emitting radiation into the atmosphere

CO2-induced warming can occur without this tropospheric amplification, particularly in areas that behave quite unlike a moist adiabat. For instance, increased CO2 causes surface warming in the Arctic and deserts. Yet these regions lack both tropospheric amplification in the upper troposphere and amplification of warming with increasing elevation. This makes sense since these regions are dissimilar to a moist adiabat, consistent with climate model results and basic physical theory.

It is at this point that the myth comes in. The aforementioned tropical amplification is called the tropical tropospheric hot spot by many myth proponents. Myth defenders claim the hot spot is expected to be a sign/fingerprint of global warming caused by CO2. Many myth proponents support this claim by (intentionally or unintentionally) misrepresenting the following 2007 figure made by the United Nations Intergovernmental Panel on Climate Change (IPCC):

Figure 2: Atmospheric temperature change in K/century from 1890 to 1999 in the PCM (Parallel Climate Model) in response to (a) increased solar forcing (from increased solar output), (b) volcanic activity, (c) well-mixed greenhouse gases such as CO2, (d) tropospheric and stratosopheric ozone changes, (e) sulfate aerosols, and (f) the sum of all the aforementioned factors. Darker blues represent cooling and darker reds represent warming. The horizontal axis indicates latitude, with the tropics being between roughly 30°N and 30°S. The vertical axis indicates altitude, with decreasing atmospheric pressure as altitude increases [1, page 675]. The tropical troposphere lies below 150hPa, while the tropical stratosphere is above 70hPa [2]. Tropical tropospheric warming increases with height in panel f, indicative of the hot spot. This figure is taken from a 2007 report of the United Nations Intergovernmental Panel on Climate Change (IPCC) [1, page 675].

A number of myth proponents claim that since the hot spot most clearly appears in the greenhouse gas portion of figure 2, then the hot spot is a specific sign of greenhouse-gas-induced (and thus CO2-induced) global warming. Yet myth defenders are wrong on this point; the hot spot in figure 2 is not specific to CO2-induced warming. Instead, the pronounced hot spot appears in the CO2 portion of the figure because CO2 levels increased enough that CO2 caused most of the recent global warming. If instead solar output increased enough to cause most of the recent global warming, then there would be a pronounced hot spot in the solar portion of the figure 2. This is made clear in figure 3 below, which comes from an earlier 2001 IPCC report:

Figure 3: 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 30°N and 30°SThe vertical axis represents altitude, with decreasing atmospheric pressure as altitude increases [3, page 707]. The tropical troposphere lies below 150hPa, while the tropical stratosphere is above 70hPa [2]. 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) [3, page 707], the report took the image from a 1997 paper.

Figure 2a illustrates this point as well. In figure 2a, bright yellow appears in the tropics (30N to 30S) at around 250hPa, in contrast to the greenish-yellow in the tropics at around 1000hPa. Thus increased solar output results in more tropical upper tropospheric warming than tropical near-surface warming, indicative of a hot spot. So climate models (including models used by the IPCC) show a hot spot in response to solar-induced surface warming.

This model-based result addresses the issue whether or not the hot spot is a fingerprint of CO2-induced global warming. Fingerprints distinguish one cause of warming from another cause of warming, as noted by the IPCC. For example, strong stratospheric cooling is a fingerprint that occurs with CO2-induced warming, while increased solar output would not account for most of the observed stratospheric cooling (see figures 2 and 3 above). Scientists have observed this, and other, fingerprints of CO2-induced warming, as I discuss in "Myth: The Sun Caused Recent Global Warming and the Tropical Stratosphere Warmed" and "Myth: Attributing Warming to CO2 Involves the Fallaciously Inferring Causation from a Mere Correlation". Some individuals called this pattern of stratospheric cooling with tropospheric warming a piece of "smoking gun" evidence of CO2-induced human-caused climate change. Unfortunately, the myth proponent Judith Curry misrepresents this statement by claiming that it was the tropospheric hot spot that was meant to be the smoking gun.

In contrast to strong stratospheric cooling, the hot spot is not a fingerprint for causal attribution of warming, since the hot spot occurs with any large warming of Earth's surface in the tropics (especially warming of the ocean surface), as long as the warming triggers the latent heat release discussed above. For instance, a hot spot forms in response to short-term surface warming that is not caused by CO2. The following source makes this clear, as do others:

"In the tropics, moist thermodynamic processes amplify surface warming [...]. Such tropical amplification occurs for any surface warming; it is not a unique signature of greenhouse gas (GHG)-induced warming, as has been incorrectly claimed (Christy 2015) [4, page 383]."

Ben Santer, the lead author of this quote, was also the lead author for the research used to make the IPCC image shown in figure 2. He also served as the contributing author to the chapter in which figure 2 appeared. So given Santer's aforementioned words and his involvement with figure 2, figure 2 most likely was not meant to show that the hot spot is a fingerprint that distinguishes CO2 from other causes of warming.

Other portions of the IPCC's report fit with this assessment. For example, the IPCC cites research on short-term hot spot formation in response to non-CO2-induced warming. The IPCC cites this research in the same 2007 report from which figure 2 was taken. In fact, on the page before figure 2, the IPCC cites the 1997 source of figure 3 when discussing the atmospheric response to increased solar output. Thus it makes no sense to claim that the solar-induced hot spot in the 2001 figure 3 remains incompatible with the 2007 IPCC report from which figure 2 is taken.

And in this 2007 report, the IPCC also states that lapse rate reduction occurs because the tropical atmosphere behaves somewhat like a moist adiabat. So even in the 2007 report many myth proponents misrepresent, the IPCC acknowledges that tropical tropospheric amplification also occurs in response to non-CO2-induced warming due to the tropics behaving somewhat like a moist adiabat. This is consistent with the solar-induced hot spot depicted in figure 3 above from an earlier 2001 IPCC report.

Moreover, since at least 1975 the scientific community has known that warming from increased solar radiation would cause the hot spot, and known the moist adiabatic basis of the hot spot. As Manabe and Wetherald note in their seminal 1975 paper on model-based projection of solar-induced global warming:

"In lower latitudes of the model, the warming is greater in the upper troposphere (~336 mb) than near the surface. Qualitatively, the same feature was present in Fig. 4b of MW75 ["MW75" is a 1975 paper that presented a model-based projection of CO2-induced warming [7]] and results from the predominance of moist convection which adjusts temperatures in a vertical column toward the moist adiabatic lapse rate. Since the moist adiabatic lapse rate decreases with increasing temperature, the temperature of the upper troposphere increases more than that of the lower troposphere in lower latitudes where moist convection predominates [6, page 2048]."

Myth advocates are therefore decades behind on the relevant science. Thus myth proponents, not the IPCC, bear the blame for myth proponents misrepresenting the IPCC's 2007 figure as being support for the proponents' myth.

Even Roy Spencer, a research colleague of myth proponent John Christy, mentions the convection-based hot spot mechanism discussed above. He also admits that the hot spot is not a fingerprint of CO2-induced warming (Spencer also contradicts himself by saying it is a fingerprint, that there are not any fingerprints, and that greenhouse gases cause stratospheric cooling), though Spencer does not get much else right with respect to climate science.

Thus the hot spot is not very useful for distinguishing different causes of warming. A number of myth proponents acknowledge this point and accept that CO2-independent warming mechanisms would also cause the hot spot (ex: Evans, Lindzen, Christy, and Curry). Yet these proponents defend the myth anyway, even though the myth contradicts basic physics, their own position, the climate models they cite, and observations of a hot spot from short-term non-CO2-induced warming. Oh well.

(In "John Christy and Atmospheric Temperature Trends", I discuss other examples of the myth proponent John Christy misrepresenting climate science in politically expedient ways)

3. Posts Providing Further Information and Analysis

4. References

  1. "Climate change 2007: The physical science basis; Chapter 9: Understanding and attributing climate change"
  2. "Tropical Tropopause Layer" [doi:10.1029/2008RG000267]
  3. "Climate change 2001: The scientific basis; Chapter 12: Detection of climate change and attribution of causes"
  4. "Comparing tropospheric warming in climate models and satellite data"
  5. "Extended Summary of the Climate Dialogue on the (missing) tropical hot spot"
  6. "The effects of changing the solar constant on the climate of a general circulation model"
  7. "The effects of doubling the CO2 concentration on the climate of a general circulation model"
  8. ("4.2.1 Water vapour and lapse rate feedbacks" ;

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