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.

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 is known as the "hot spot". The myth states that the hot spot is a fingerprint of anthropogenic (human-caused), CO2-induced global warming.

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, 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 CO2 emissions.

The myth's flaw: the hot spot is not a fingerprint of CO2-induced global warming, since the hot spot occurs with any sufficiently large surface warming in the tropics, especially warming of the tropical oceans.

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.

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. So water vapor condensation causes more warming of the lower troposphere and even more warming of the upper troposphere.

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 moist air saturated with water vapor. 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 tropospheric amplification, in areas that behave quite unlike a moist adiabat. For instance, some of the strongest warming on Earth occurs in deserts, even though this CO2-induced surface warming comes with relatively little upper tropospheric warming.

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 carbon dioxide (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 1: 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 30N and 30S. 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 1, 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 1 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 1. This is made clear in figure 2 below, which comes from an earlier 2001 IPCC report:

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 30SThe 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].

It thus makes little sense to say the hot spot fingerprint of CO2-induced global warming. Fingerprints distinguish one cause of warming from another cause of warming. For example, strong stratospheric cooling is a fingerprint that occurs with CO2-induced warming, while the sun would not account for most of the observed stratospheric cooling (see figure 2 above). In contrast to strong stratospheric cooling, the hot spot is not a fingerprint since the hot spot occurs with any large warming of Earth's surface in the tropics (especially the surface of the ocean), as long as the warming triggers the latent heat release discussed above. A hot spot even occurs with short-term surface warming that is not caused by CO2. The following source makes this clear:

"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]."

Even Roy Spencer, a research colleague of myth proponent John Christy, admits this, 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, and the climate models they cite. Oh well.

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"