Translate

Tuesday, May 9, 2017

John Christy and the Tropical Tropospheric Hot Spot, Part 1

The outline for this post is as follows:
  1. Introduction
  2. Summary of the Objections
  3. Elaboration on the Objections
  4. References
This is part 1 of a two part series; part 2 is available here. If you want the "tl;dr" for this post, then I suggest reading sections 1 and 2. Alternatively, if you are familiar with John Christy's congressional testimony and you know what the "tropical tropospheric hot spot" is, then simply skip ahead to section 2.

Each numbered point in section 2 corresponds with a numbered portion of section 3. So there is no need to read this entire post; instead, you can see which numbered point you find interesting in section 2, and then for further details you can skip to the corresponding numbered portion in section 3.

This is the "main version" of this post, which means that this post lacks of most 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.  Introduction



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, above that is the mesosphere, and above the mesosphere is the thermosphere. 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 into the troposphere. 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, reducing the magnitude of the tropospheric lapse rate.

Thus the tropical troposphere will behave somewhat like a moist adiabat, in which the rate of warming increases with increasing height in response to water vapor condensing from moist, vapor-saturated air; so the 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 troposphere. This tropical amplification of tropospheric warming is known as the tropical tropospheric hot spot. Though climate scientists have discussed this tropospheric temperature amplification since at least the 1970s up to 2012 (before some of the more recent tropospheric amplification data was published), the term "hot spot" largely originated in climate "skeptic" websites, as opposed to in mainstream scientific sources.

The climate scientist John Christy co-authored a report in which he argues that there is no evidence of a hot spot. I will critique the first edition of Christy et al.'s report in this post; in part 2 I will critique the second edition. In his report, Christy defines the hot spot as follows:
  • statistically significant warming in the tropical upper troposphere, lower troposphere, and Earth's surface
  • greater warming in the tropical upper troposphere vs. the lower troposphere
  • greater warming in the tropical lower troposphere vs. the Earth's surface
Christy also links the hot spot to the magnitude of tropospheric warming. I will address that issue in my post "John Christy, Climate Models, and Long-term Tropospheric Warming".

According to Christy and his report co-authors Wallace III and D'Aleo, the hot spot is expected to be a fingerprint of global warming caused by carbon dioxide (CO2). So the lack of a hot spot, according to Christy et al. rebuts the US Environmental Protection Agency's claim that CO2 is a pollutant that causes dangerous global warming; this claim has been re-iterated on a number of climate "skeptic" websites, in an attempt to cast doubt on CO2 being a significant cause of recent global warming. So the hot spot has become a major "skeptic" argument against the science on anthropogenic climate change; the argument has been taken up by "skeptics" such as Richard Lindzen, Roy Spencer, S. Fred Singer, Roger Pielke Sr., Judith Curry, Steve McIntyre, Anthony Watts, Christopher Monckton, Tom Nelson, and David Evans. Christy et al. are some of the most prominent purveyors of this "skeptic" argument.

In their report, Christy et al. also argue that El Niño (also known as the El Niño phase of ENSO, the El Niño-Southern Oscillation) and the Sun, not CO2, are responsible for most of the global warming trend observed since the 1970s. This may initially make sense, since even young children know that the Sun warms the Earth. Furthermore, scientists know that during the El Niño phase of ENSO, both the oceans' surface and the troposphere warm. Christy also cites this report on other occasions to justify his position on environmental regulation. The report/blogpost is not peer-reviewed, and the report, as it stands, lacks much merit. So let's go over some of the issues with the report.


2. Summary of the Objections



There are a number of problems with the Christy et al.'s report. Competent peer review likely would have corrected these problems. So it is quite telling that Christy et al. did not first get their report peer reviewed, before citing the report to Congress. Their report seems less about providing credible research to informed, skeptical scientists, and more about politically influencing uninformed, credulous politicians. So let's take Christy et al.'s advice:

"Given the potential significance of this research, it is appropriate to question everything about it [7, page 66]."


Below is a summary of some of the report's deficiencies:


1) Christy et al. do not address much of the scientific research showing the hot spot, even though Christy et al. claim that there is no evidence of a hot spot. Christy himself presents evidence of a hot spot, thereby contradicting the conclusion of the report he co-authored. The report provides no sound argument against the existence of the hot spot, especially since Christy et al. do not directly compare the rates of upper tropospheric warming, lower tropospheric warming, and surface warming.

2) Christy et al. incorrectly state that the hot spot is claimed to be a fingerprint/signature of CO2-induced global warming. In reality, scientists know that surface warming caused by other factors (such as increased solar activity) would also result in a hot spot. Christy et al.'s proposed solar-induced and ENSO-induced ocean warming should result in a hot spot. Yet Christy et al. claim there is no hot spot. So Christy et al. position may be internally inconsistent.

3) Christy et al. discount CO2 as a cause of post-1970s global warming, even though they do not discuss data showing radiative forcing (meaning "change in energy per unit area") from CO2. This forcing may be indicative of a warming effect from CO2. CO2 might also indirectly warm the atmosphere by increasing the frequency or intensity of El Niño events. If this is the case, then Christy et al. are incorrect when they claim that ENSO, not CO2, is responsible for much of the post-1970s global warming.

4) Christy et al. claim that there is no statistically valid evidence showing that CO2 significantly contributed to post-1970s temperature changes. However, Christy et al. overlook one of the fingerprints/signatures of CO2-induced global warming: stratospheric cooling. Long-term stratospheric cooling has occurred. CO2 also explains much of the observed mesospheric and thermospheric cooling. This cooling is not easily explained by Christy et al.'s proposed mechanisms of solar forcing and/or ENSO, which may be why Christy et al. conveniently overlooked this cooling.

5) Christy et al. use a cumulative ENSO index that makes little sense in terms of atmospheric physics, undermining their report's central argument. A more plausible ENSO index undermines Christy et al.'s contention that ENSO caused most of the recent global warming, consistent with Christy's own co-authored research and in contradiction to Christy et al.'s claims.

6) Christy et al. attribute much of the recent global warming to the Sun, even though they do not discuss much of the scientific evidence against the Sun being a cause of most of the recent global warming. A more plausible use of TSI undermines Christy et al.'s contention that the Sun caused much of the recent global warming.

7) Christy et al. make unjustified references to "alarmist" scientists, despite there being multiple studies debunking canards regarding "alarmist" scientists.


Let's examine each of these objections in turn.


3. Elaboration on the Objections



3.1 Avoiding evidence of the hot spot and failing to provide evidence against the hot spot's existence


Christy et al. do not address much of the scientific research showing a post-1970s hot spot, even though Christy et al. claim that there is no evidence of a post-1970s hot spot. This evidence includes some of the same lines of evidence Christy has drawn on in his other work: weather balloon (radiosonde) thermal wind and temperature data, temperature re-analyses, and satellite data. Earlier research found reduced tropical tropospheric temperature amplification or virtually no amplification, due to artificial biases in the tropospheric temperature record. These biases are known as heterogeneities, and include factors such as equipment erroneously reading stratospheric temperature as tropospheric temperature. Later research used homogenization to correct for these heterogeneities, resulting in greater tropical tropospheric temperature amplification for both radiosonde and satellite temperature records, along with re-analyses. However, some research generated a hot spot that exceeded model-based projections (see section 3.1 of "John Christy, Climate Models, and Long-term Tropospheric Warming" for more on heterogeneities and homogenization).

Christy all but concedes the hot spot's existence in the following diagram from his 2017 congressional testimony [6, page 9]:

Figure 1: Comparison of tropical warming trend to model-based projections of the warming trend. The red and blue lines indicate projected warming from models used by the United Nations Intergovernmental Panel on Climate Change (IPCC); the effects of extra greenhouse gases (GHGs, such as CO2) are included in the red line projections, but not in the blue line projections. The gray lines indicate the observed warming trends [6].

Figure 1 is deeply flawed, as I discuss in "Myth: Santer et al. Show that Climate Models are Very Flawed" and "John Christy, Climate Models, and Long-term Tropospheric Warming". But suppose one takes figure 1 at face value. Then the observations (the gray lines) in figure 1 show greater warming in the tropical troposphere vs. nearer to the Earth's surface, though the tropospheric temperature amplification is relatively slight. And Christy's co-authored research also shows greater warming in the upper troposphere than at the surface. So Christy's own figure and research shows a hot spot, thereby debunking the conclusion of Christy et al.'s report. One might think Christy would try to reconcile this inconsistency. He does not. Instead, on the page following figure 1, Christy cites the second edition of Christy et al.'s hot spot report/blogpost in which Christy et al. claim there is no evidence of the hot spot. Thus Christy cites the hot spot report, right after citing evidence that debunks the reportAnd Christy brazenly does this in his testimony to the US Congress. To borrow Christy's own words: This took guts. It took guts for Christy to blatantly cite false claims to Congress, right after he all but showed that said claims were false using figure 1. So to borrow another quote from the "skeptic" Judith Curry [11]: I thought that there would be consequences for lying during congressional testimony. I guess not.

Despite the hot spot in figure 1, Christy's University of Alabama in Huntsville (UAH) satellite data analysis does not show a post-1970s hot spot, as stated by Christy's UAH collaborator Roy Spencer. This stands in contrast to three other independent satellite data analyses that do show a hot spot. Figure 2 illustrates this point by comparing the surface warming trend from HadCRUT4 and tropospheric warming trends generated by research groups at UAH, Remote Sensing Systems (RSS), the National Oceanic and Atmospheric Administration Center for Satellite Applications and Research (NOAA/STAR), and the University of Washington (UW) [3]:


Figure 2: 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 [3]. 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.

To avoid the tropospheric amplification shown in figure 2, Christy sometimes sidesteps mid- to upper tropospheric warming in favor of lower tropospheric warming. I instead focus on mid- to upper tropospheric amplification for at least three reasons:


  • Only RSS and UAH generate satellite analyses of tropical lower tropospheric temperature (another satellite research group generates lower tropospheric temperature analyses, though this group is rarely cited). In contrast, at least 5 research groups produce satellite analyses of tropical mid- to upper tropospheric temperature. More groups means a greater chance for a group to catch and correct the mistakes made by another group. Christy should be aware of this since the RSS team addressed mistakes made by Christy's UAH research team (for more on this, see section 3.1 "John Christy, Climate Models, and Long-term Tropospheric Warming"). Yet Christy often opts for lower tropospheric temperatures that have less opportunity for correction, instead of mid- to upper tropospheric temperature that have more chance for correction. That choice does not makes much sense, which is one reason why I focus on mid- to upper tropospheric temperatures.
  • If Christy were trying to test for amplification, then Christy should look where the amplification is most pronounced and thus most likely to be found. And as I discussed in the introduction, mid- to upper tropospheric amplification is expected be greater, and thus more pronounced, than lower tropospheric amplification. Christy himself admits this. So Christy should be looking in the mid- to upper troposphere for amplification, not just the lower troposphere. This is what I choose to do as well.
  • Christy points out that stratospheric temperature contaminates measurements of mid- to upper tropospheric temperature. To avoid this contamination, Christy examines lower tropospheric temperature. However, stratospheric contamination does not justify ignoring mid- to upper tropospheric analyses, since 5 research groups use 3 different methods to adequately account for stratospheric contamination. Furthermore, the lower tropospheric temperature analyses Christy uses come with their own technical hurdles. Therefore lower tropospheric analyses may not be inherently more reliable than mid- to upper tropospheric analyses. Finally, Christy himself sometimes cites mid- to upper tropospheric analyses, when he thinks the analyses are convenient for his position; he often does so without accounting for stratospheric contamination. So if Christy supports his position by citing contaminated mid- to upper tropospheric analyses, then I would be justified in citing corrected analyses in response.

In addition to showing amplification in the mid- to upper troposphere, figure 2 also 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 the 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. 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 the oceans than above the land, since the oceans provide a readier source of water that can evaporate, condense in the upper troposphere, and thus produce the tropospheric hot spot.

The aforementioned explanations debunk Christy and Christy's co-author D'Aleo, since Christy and D'Aleo falsely insinuate that the hot spot should appear above land, if the land surface record does not contain a warm bias due to heterogeneities (in section 3.5 of part 1 of "Christopher Monckton and Projecting Future Global Warming", I discuss how scientists know that heterogeneities due to urbanization are not significantly biasing land surface temperature records).

Figure 2 also shows that UAH is the outlier among the four satellite data analyses, because UAH is the only analysis that does not show greater warming in the mid- to upper troposphere vs. the surface. This discrepancy is likely due to errors with UAH's homogenization method, as I discuss in section 3.1 of "John Christy, Climate Models, and Long-term Tropospheric Warming". Temperature re-analyses and radiosonde data tend to support the greater RSS/NOAA/UW warming trends, as opposed to the UAH's lesser warming trend (for more on this, see section 3.2 of "John Christy, Climate Models, and Long-term Tropospheric Warming"). Thus Christy's UAH analysis likely under-estimates tropical mid- to upper tropospheric warming, which may explain why Christy et al. claim there is no tropical hot spot. And Christy et al. may not have brought up other research showing the hot spot, because doing so would bring attention to the discrepancy between Christy's UAH research and the research published by other scientists.

The UAH analysis creates a further problem for Christy: Christy attributes much of the recent global warming to the Sun and the warm El Niño phase of ENSO. Warming from the Sun and ENSO should cause a hot spot (as I discuss in section 3.2), yet Christy's UAH analysis shows no hot spot. So Christy's solar warming and ENSO-warming hypotheses further undermine the credibility of his UAH data analysis. Or his UAH analysis undermines the credibility of his solar warming and ENSO-warming hypotheses.

So let's recap this section by summarizing some of the relevant evidence on the hot spot:

  • 3 out of 4 satellite analyses (or 4 out of 5, if one includes an earlier satellite analysis that was cited by Christy in his published work) show tropical tropospheric amplification, as shown in figure 2. Christy et. al do not include mid- to upper tropospheric temperature from the NOAA analysis nor from an earlier satellite analysis; ignoring these analyses allows Christy et al. to avoid some satellite evidence of the hot spot. Christy has cited these two analyses on some occasions and ignored them on other occasions. Given this fact and the fact that Christy chose the temperature analyses used by Christy et al., it is unsurprising that Christy et al. would ignore some analyses that inconvenience Christy's position. Christy's UAH analysis is the only satellite analysis that supports Christy et al.'s claim of no post-1970s hot spot; UAH's result should be taken with a grain of salt, given UAH's history of incorrect tropospheric results based on UAH's faulty homogenization (for more on this, see sections 3.1 and 3.2 of "John Christy, Climate Models, and Long-term Tropospheric Warming").

  • 4 out of 5 radiosonde analyses show greater warming in the tropical upper troposphere (at an atmospheric pressure level of around 300 hPa, as shown in figure 3) than near Earth's surface. Christy et al. focus on radiosonde temperature at 150 hPa and 200 hPa (equivalent to 150 mb and 200 mb, respectively). This may cause them to miss the tropospheric amplification at 300 hPa. Christy is presumably at fault here since Christy chose the radiosonde temperature records in question. This is a rather odd mistake for Christy et al. to make, since Christy's co-authored research states that the model-predicted amplification is highest at 300 hPa and that the observed amplification is greater at 300 hPa than at 200 hPa. In any event, the Hadley Centre's radiosonde analysis HadAT2 is the only radiosonde analysis that does not show a hot spot over certain time periods; this may explain why some "skeptics" rely on HadAT. One should treat this HadAT2 result with skepticism, since the HadAT team strongly recommends that researchers not rely on just the HadAT data-set. Instead, the HadAT team recommend that researchers use other radiosonde data-sets as well, along with the RSS satellite analysis, to determine if their result in robust. Furthermore, the HadAT2 analysis does show the hot spot over a number of multi-decadal time periods. The other radiosonde analyses and the RSS analysis show a post-1970s hot spot. So the lack of a hot (over some time periods) in HadAT2 is not a robust result.

  • 4 out of 5 re-analyses show greater warming in the tropical upper troposphere (at an atmospheric pressure level of around 300 hPa) than near Earth's surface. Christy et al. largely ignore the re-analyses, presumably on Christy's advice, and instead focus on radiosondes and satellite data. Evading re-analyses not only allows Christy et al. to ignore evidence inconvenient for their position, but the evasion also conflicts with Christy's subsequent use of re-analyses in his congressional testimony and in his peer-reviewed work. The National Centers for Environmental Prediction (NCEP-2) re-analysis shows greater warming in the lower troposphere than in the upper troposphere. This NCEP-2 trend should be taken with a grain of salt, since the NCEP re-analysis has a history of under-estimating tropospheric warming; this may explain why the "skeptic" Anthony Watts gladly trumpets the NCEP re-analysis. Other re-analyses, such as the European Centre for Medium-Range Weather Forecasts Interim (ERA-I) re-analysis, 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 using another re-analysis instead of NCEP-2. ERA-I shows greater warming near the surface than in the lower troposphere. This is because ERA-I under-estimates the rate of lower tropospheric warming, as acknowledged by the ERA-I team.

Thus the hot spot likely exists, based on the preponderance of the evidence. This runs contrary to Christy et al.'s claim that there is no hot spot. To make matters worse for Christy et al., tropical precipitation/convection patterns provide further evidence of a post-1970s moist-adiabatic lapse rate reduction, indicative of a post-1970s hot spot. In addition, short-term warming produces a short-term hot spot with a decreased lapse rate. And in the distant past, the lapse rate likely decreased in response to global warming.

In summary, since Christy et al. claim there is no evidence of a post-1970s hot spot with a reduced tropical tropospheric lapse rate, then they need to square their claim with:
  1. research showing a post-1970s hot spot (Christy et al. would at least need to mention that there was research that purports to show a post-1970s hot spot)
  2. tropical precipitation/convection observations indicative of a post-1970s hot spot
  3. evidence that longer-term warming in the distant past reduced the tropospheric lapse rate
  4. evidence of shorter-term hot spots with a short-term lapse rate reduction

Yet Christy et al. do none of this. They do mention a paper from Sherwood and Nishant, without mentioning that this paper found the hot spot. They likely would not have gotten away with this if their "report"/blogpost passed competent peer review by a reputable scientific journal. This is because competent reviewers make a paper's author(s) address at least some of the evidence that might conflict with the paper's conclusions. It is therefore rather telling that Christy et al.'s "report" is not peer reviewed, since this allows Christy et al. to blissfully ignore any scientific research that might counter their claims. Much of this opposing research was published before Christy et al.'s August 2016 hot spot report, so Christy et al. could have easily addressed this research in their report. But they chose not to, in contrast to other scientists who cite previously published tropospheric amplification research. How convenient.

At this point, one might expect Christy et al. to provide some evidence against the existence of the hot spot. For example, they should measure the tropical rates of mid- to upper tropospheric warming, lower tropospheric warming, and surface warming. Then they should compare these rates to see if mid- to upper tropospheric warming is greater than lower tropospheric warming, and to see if lower tropospheric warming is greater than surface warming. This would let them know whether there is a hot spot, as per their definition of the hot spot and as per the test they set for themselves. Yet Christy et al. do not directly compare the tropical rates of mid- to upper tropospheric warming, lower tropospheric warming, and surface warming. So Christy et al. do not perform the tests needed to check for the hot spot's existence. This, in combination with the aforementioned evidence of the hot spot's existence, undermines Christy et al.'s claim that there is no hot spot.

(Some tropospheric amplification also occurs globally, particularly over oceans. Amplification also occurs in regions outside of the tropics, as would be expected of global warming caused by greenhouse gases such as CO2. However, some regions of the globe do not show this tropospheric amplification. So tropical or global tropospheric amplification does not imply amplification in every region of the world, just as global warming does not imply that every region will warm.
Many higher elevation land areas also show elevated warming trends or elevation-dependent amplification, consistent with trends observed in the distant past. Some of the mechanisms that cause in elevation-dependent amplification also cause the tropospheric amplification that occurs in air far above land, though not all the mechanisms are shared between the two forms of amplification. Some regions and data-sets, however, do not display elevation-dependent amplification and the temperature data from higher elevations must be appropriately analyzed so as not to exaggerate the rate of amplification.)

3.2 Misidentifying a fingerprint/signature of CO2-induced global warming


Christy et al. state that the hot spot is claimed to be a fingerprint or signature of global warming caused by greenhouse gases; Christy has insinuated as much when he enters the political or scientific realm. However, this is not an accurate representation of the claims made by the mainstream scientific community; many scientists and non-scientists (including Christy's UAH collaborator Roy Spencer) know that surface warming caused by other factors would also result in a hot spot, as long as the troposphere behaves somewhat like a moist adiabat. For example, short-term, non-anthropogenic warming events produce a short-term hot spot. And increased solar activity would warm the tropical oceans, leading to a hot spot. Since the hot spot would occur with any surface warming, the hot spot is not a signature/fingerprint that distinguishes CO2-induced warming from other types of global warming. Christy likely knows this since, according to Bart Verheggen, Christy wrote as much in an email to Verheggen.

Yet Christy et al. state that the hot spot is claimed to be a fingerprint/signature of CO2-induced warming and they cite another source as supporting this claim. This source states that warming results in a lapse rate reduction. But nowhere does this source claim that the hot spot would occur with CO2-induced warming, but not with some other forms of global warming, such as solar-induced warming. Thus Christy et al.'s were incorrect when they said their cited source claimed that the hot spot is a fingerprint/signature of CO2-induced warming.

If a multi-decadal hot spot does not exist, then there are two main explanations for this lack of a hot spot:
  1. There is no surface warming, and thus no warming for the troposphere to amplify.
  2. The tropical troposphere does not behave like a moist adiabat.
Option 1 fails since there is clear evidence of multi-decadal surface warming for both land and oceans. There are also other signs of warming, such as sea level rise resulting from melting ice and thermal expansion of water, increased hurricane intensity, and increased water vapor levels (see section 3.7 of "John Christy, Climate Models, and Long-term Tropospheric Warming" for more on this), among other metrics. These and other lines of evidence debunk option 1. That leaves option 2. If option 2 is right, then the magnitude of the tropospheric lapse rate did not significantly decrease. This implies greater global warming, since lapse rate reduction serves as a negative feedback that limits global warming, though some researchers dispute this point. Thus surface warming with no hot spot implies less negative feedback and greater warming, contrary to the claims made by some critics of mainstream climate science.

Some critics of mainstream climate science attempt to introduce an option 3, by claiming that the hot spot is a fingerprint of CO2-induced global warming in particular; Christy et al. rely on option 3 in their discussion of the hot spot. So if option 3 were true, then the lack of a hot spot would argue against CO2 being a significant cause of recent global warming. Option 3 and the hot spot have therefore become talking points on many "skeptic" websites, where critics doubt the impact of anthropogenic CO2 on recent global temperatures. These critics sometimes support option 3 by (intentionally or unintentionally) misrepresenting a figure made by the United Nations Intergovernmental Panel on Climate Change (IPCC). The hot spot in the IPCC figure is not specific to CO2-induced warming. Instead, the pronounced hot spot appears in the CO2 portion of the figure because increased forcing from CO2 caused most of the recent global warming. If increased forcing from the Sun had instead caused most of the recent global warming, then there would be a pronounced hot spot in the solar portion of the IPCC's figure.

Thus the IPCC's figure does not support option 3's claim that the hot spot is a specific fingerprint of CO2-induced warming. There are several other reasons for thinking that the IPCC does not support option 3. These reasons include:
  • The IPCC does not claim that a long-term lapse rate reduction is specifically a fingerprint of CO2-induced surface warming, as opposed to surface warming caused by the Sun.
  • The IPCC claims that lapse rate reduction occurs because the tropical atmosphere behaves somewhat like a moist adiabat. If this were the case then similar moist adiabatic processes would also cause a hot spot from solar-induced warming and other forms of surface warming.
  • Climate models (including models used by the IPCC) show lapse rate reduction in response to solar-induced surface warming.
  • In its discussion of the lapse rate, the IPCC cites research showing lapse rate reduction in response to non-anthropogenic warming caused by factors such as ENSO. This cited research includes Santer et al. 2005, along with Hegerl and Wallace 2002.

So the IPCC's figure does not support option 3, and option 3 fails since surface warming caused by non-CO2 factors would also result in a hot spot. For example, a 2% increase in solar irradiance (which would warm the Earth's surface about as much as would a doubling of atmospheric CO2 levels) would cause amplified tropospheric warming with increasing height. Shorter-term increases in solar irradiance would also cause short-term temperature tropospheric warming amplification. Furthermore, the warm El Niño phase of ENSO causes a short-term hot spot.

This creates another problem for Christy et al., since they attribute much of the recent global warming to solar activity and ENSO, while claiming that there is no evidence of a hot spot and while Christy's UAH analysis shows no hot spot. Yet solar-induced and ENSO-induced surface warming should elicit a hot spot, as illustrated in climate models, basic physical theory, evidence of a shorter-term hot spots, decreased lapse rate with global warming in the distant past, and Verheggen's account of Christy's position. So not only do Christy et al. misidentify a fingerprint of CO2-induced global warming, but Christy may be contradicting himself. Christy is not the only critic to contradict himself in this way: Richard Lindzen has at times implied that the hot spot is a fingerprint of greenhouse-gas-induced warming and that the hot spot can occur via moist adiabatic processes in the absence of greenhouse-gas-induced warming.

But suppose that, for the sake of argument, one accepts option 3. Thus one accepts that the hot spot is a fingerprint of significant greenhouse-gas-induced global warming in particular. This implies that post-1970s greenhouse-gas-induced global warming occurred, since a post-1970s hot spot exists (as I discussed in section 3.1). So advocates of option 3 are in a bind:
  • Option 3 advocates could drop option 3 by admitting that the hot spot is not a fingerprint of greenhouse-gas-induced global warming in particular. But they likely do not want to do that either, because they would then need to admit that they misrepresented basic climatology and admit that scientists were correct when these scientists corrected said misrepresentations.
  • Option 3 proponents could refuse to accept the hot spot's existence, despite strong evidence that the hot spot exists. This refusal could involve explicitly denying the hot spot's existence, intentionally avoiding evidence of the hot spot, misrepresenting scientific research on the hot spot, etc.
The aforementioned bind explains why some "skeptics" refuse to accept the hot spot's existence. Christy et al. resolve this bind by settling on the last option: they refuse to accept the hot spot's existence. Therefore the bind helps explain why Christy et al. wrote a blogpost/report denying that the hot spot exists. That is what happens when ideologically-motivated "skeptics" misidentify a fingerprint of greenhouse-gas-induced global warming.

3.3 Disregarding evidence of CO2's effect on radiative forcing, temperature, and (possibly) ENSO


Christy et al. discount CO2 as a cause of post-1970s global warming, even though they do not discuss data showing radiative forcing from CO2 within specific infrared wavelengths. This forcing represents one fingerprint of CO2-induced warming since this forcing translates into a warming effect; the radiative forcing's warming effect is proportional to climate sensitivity. Multiple lines of evidence support a relatively high climate sensitivity, implying that CO2 significantly affected post-1970s global temperature (for further discussion, see the introduction to part 1 of "Christopher Monckton and Projecting Future Global Warming"). Even a low estimate of climate sensitivity would involve CO2 causing much of the recent global warming. So the evidence on climate sensitivity rebuts Christy et al.'s claim that CO2 did not have a statistically significant impact of global temperature. Christy et al. do not address any of this evidence on climate sensitivity and thus Christy et al. do not address one line of evidence against their position.

Nor do Christy et al. discuss evidence showing that forcing from anthropogenic CO2 contributes substantially to Earth's energy balance (the amount of energy the Earth takes up vs. the amount of energy the Earth releases). Ironically, CO2 might also indirectly affect atmospheric temperatures, by increasing the frequency of El Niño warming events. If this is the case, then Christy et al. are mistaken when they claim that ENSO, not CO2, is responsible for much of the global warming trend observed since the 1970s.  And even if CO2 did not influence the frequency/intensity of El Niño warming events, the observed radiative forcing from CO2 still debunks Christy et al. claim that there is no evidence of CO2 significant impact Earth's temperature.

3.4 Ignoring the stratospheric, mesospheric and thermospheric signature of CO2-induced global warming


Christy et al. note that, based on their data analysis, there is no statistically valid evidence showing that CO2 impacted long-term, post-1970s temperature trends. However, Christy et al. conveniently overlook one of the fingerprints/signatures of CO2-induced global warming: stratospheric cooling. During the 1960s, 1970s, and 1980s, scientists predicted that CO2-induced global warming would produce stratospheric cooling. Ozone depletion also contributes to stratospheric cooling, with some researchers arguing that ozone depletion contributes more to the cooling than does CO2. 

There has been stratospheric cooling, with much of this cooling attributable to CO2. Take, for instance, the following figure [4] from a paper cited by Christy et al. in their hot spot report:


Figure 3: Tropical temperature 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 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 [4].

Figure 3 shows a clear tropical hot spot, with surface warming, greater warming in the upper troposphere, and then cooling in the stratosphere. This is the atmospheric temperature pattern one expects from CO2-induced global warming in an atmosphere that behaves somewhat like a moist adiobat.

However, some data-sets do not show statistically significant cooling for the past two decades or so, particularly in the lower stratosphere. The decreased rate of stratospheric cooling in the past two decades is very likely due to the Montreal Protocol, an international treaty that led to reduced anthropogenic emissions of ozone-depleting chlorofluorocarbons (CFCs). The Montreal Protocol resulted in a slow recovery of stratospheric ozone levels. This ozone recovery partially offset CO2-induced cooling of the stratosphere, in line with the scientific predictions made in the 1970s and 1980s.

The El Niño phase of ENSO also causes stratospheric cooling, though ENSO likely does not account for most of the long-term stratospheric cooling. Furthermore, solar forcing does not explain most of the long-term stratospheric cooling. Christy et al. conveniently overlook how this stratospheric cooling conflicts with their solar-induced warming hypothesis. This is rather surprising since this issue was pointed out in a report co-authored by Christy. In contrast, CO2 and ozone depletion explain this lower stratospheric cooling rather well. CO2 also accounts for much cooling higher in the stratosphere; this mid- to upper stratospheric cooling has persisted over the past two decades, despite the more pronounced decrease in the rate of lower stratospheric cooling. Furthermore, CO2 explains much of the cooling above the stratosphere, in the mesosphere and thermosphere. So stratospheric, mesospheric, and thermospheric cooling debunk Christy et al.'s claim that there is no evidence of CO2 significantly impacting recent temperature trends. And Christy et al. have yet to show that ENSO can account for most of the recent, long-term stratospheric cooling.

3.5 Using implausible physical indices


So there is evidence that the hot spot exists (see section 3.1), Christy et al. do not perform the tests needed to find the hot spot (see section 3.1), and there is evidence that CO2 significantly impacted post-1970s atmospheric temperature (see sections 3.3 and 3.4). Why then do Christy et al. claim otherwise? To make their case, Christy et al. generate a cumulative multivariate ENSO index and cumulative total solar irradiance (cumulative MEI and cumulative TSI, respectively) to represent ENSO and the Sun, respectively. They then subtract these indices out of various temperature records, thereby removing warming while correcting for TSI and ENSO. Since there is no significant tropospheric warming left in these corrected records, then there is no tropospheric hot spot, according to Christy et al.'s reasoning. Their reasoning makes no sense, since showing no TSI-corrected ENSO-corrected hot spot does not show that there is no hot spot before correction for TSI and ENSO; so even if their TSI and ENSO corrections are correct, the hot spot could still exist. Thus Christy et al. do not do the tests needed for investigating the hot spot's existence, as I discussed in section 3.1.

Anyway, Christy et al. then reason that since the ENSO-corrected, TSI-corrected temperature trend is flat, then there is no evidence left that CO2 significantly affected temperature. This temperature correction also allows Christy et al. to attribute most of the post-1970s global warming to ENSO and the Sun.

Christy also explains that using a cumulative TSI generates the same results as a cumulative MEI, so a cumulative MEI is not required. Using the cumulative TSI has its own problems, as I explain in sections 3.4 and 3.6. In any event, Christy et al.'s central arguments hinge on their cumulative MEI and cumulative TSI; if either of these indices are deeply flawed, then Christy et al.'s "report"/blogpost fails. Unfortunately, both indices are physically implausible, as I will show later in this section.

To help provide context to Christy et al.'s use of a cumulative ENSO index, Christy cites other sources that use an ENSO index:

"The explanatory variables are those that have been known for decades such as indices of El Nino-Southern Oscillation (ENSO), volcanic activity, and a solar activity (e.g. see Christy and McNider, 1994, “Satellite greenhouse signal”, Nature, 367, 27Jan). [One of the ENSO explanatory variables was the accumulated MEI (Multivariate ENSO Index, see https://www.esrl.noaa.gov/psd/enso/mei/) in which the index was summed through time to provide an indication of its accumulated impact. This “accumulated-MEI” was shown to be a potential factor in global temperatures by Spencer and Braswell, 2014 (“The role of ENSO in global ocean temperature changes during 1955-2011 simulated with a 1D climate model”, APJ.Atmos.Sci. 50(2), 229-237, DOI:10.1007/s13143-014-001-z.) [6, page 10]."

But these sources actually hurt Christy et al.'s case. To see why, note that in one of these sources ("Satellite greenhouse signal") Christy used a non-cumulative ENSO index [226]. The NOAA website Christy links to also uses a non-cumulative MEI taken from another paper. So neither source supports Christy et al.'s use of a cumulative ENSO index. That leaves Christy's last cited source: Spencer and Braswell. Spencer and Braswell use the same non-cumulative MEI cited on the NOAA website and presented in the NOAA's original source. Despite this fact, Christy's congressional testimony insinuates that Spencer and Braswell use a cumulative ENSO index. So to once borrow a quote from the "skeptic" Judith Curry [11]: I thought that there would be consequences for lying during congressional testimony. I guess not.

So, despite Christy's claims, none of his listed sources justify Christy et al.'s use of a cumulative MEI, since all of Christy's sources use a non-cumulative MEI. At best, Christy et al. could cite some unpublished, non-peer-reviewed reports on the relationship between cumulative indices and global temperature. But Christy may have realized that such citations would not persuade an informed audience. This may be why Christy cited more reputable sources, such as the NOAA and peer-reviewed publications.

But Christy cannot appeal to the NOAA's non-cumulative MEI, since the non-cumulative MEI is not the same as Christy et al.'s cumulative MEI. A similar point applies to Christy et al.'s cumulative TSI: the cumulative TSI is not the same as a non-cumulative TSI, so Christy cannot rely sources that use a non-cumulative TSI. This is made clear in the following four figures:

Figure 4: Cumulative TSI and cumulative MEI used by Christy et al. [7, page 18].


Figure 5: Non-cumulative MEI. The red peaks represent El Niño events and the blue troughs represent La Niña events. The relative magnitudes of the peaks and troughs are proportional to the strength of corresponding El Niño and La Niña events [8]. See figure 6 for a representation of some of the uncertainties involved in calculating the magnitude El Niño and La Niña events.


Figure 6: Non-cumulative ENSO index (Niño3.4) based on ocean temperature in the east central equatorial Pacific, up to the year 2016. Niño3.4 is one of several non-cumulative indexes used in generating the non-cumulative MEI shown in figure 5. Red lines indicate the Niño3.4 index values, while the black region represents uncertainty at the 95% confidence level [9].


Figure 7: Non-cumulative TSI from the NOAA's Climate Data Record (CDR), based on satellite observations. The peaks and troughs represent the 11 year solar cycle. Data sources are SORCE TIM (Solar Radiation and Climate Experiment, with Total Irradiance Monitor), ACRIM (Active Cavity Radiometer Irradiance Monitor) and PMOD (Physikalisch-Meteorologisches Observatorium Davos) [10].

Christy et al.'s cumulative MEI (figure 4, red line) is clearly not equivalent to the non-cumulative ENSO indices in figures 5 and 6. Nor is Christy et al.'s cumulative TSI (figure 4, blue line) equivalent to the non-cumulative ENSO index in figure 7.

To support their cumulative TSI index, Christy et al. cite a 1993 paper. There are at least three problems with their citation:
  1. Christy et al.'s cited source does not use a cumulative TSI. 
  2. Christy et al.'s source suggests that solar irradiance and Northern hemisphere temperature diverged in recent decades ("recent decades" before 1993) in a way consistent with CO2-induced global warming. The source's suggestion does not fit well with Christy et al.'s claim that the Sun, not CO2, is responsible for much of the post-1970s global warming trend.
  3. Other researchers have shown that non-cumulative TSI decreased or remained relatively flat since the 1970s, as measured from the maximum TSI during the 11 year solar cycle (see figure 7). So a solar irradiance measured by a non-cumulative TSI would not explain post-1970s global warming. This places the onus on Christy et al. to explain why one should reject the non-cumulative TSI results in favor of a cumulative TSI. Christy et al. did not meet this burden.

To make matters worse, Christy et al. remain obscure about how they generate their ad hoc, cumulative TSI and cumulative ENSO index. Sou offers insight into how these indices may have been generated:

"How or why anyone would distort [the Multivariate ENSO Index] into a cumulative chart is anyone's guess. [...] It took me a while to figure out what [Christy et al.] did. [...]
1. Take an annual average (why on earth?). 
2. Add the second year's data to the first. 
3. For each subsequent year, add the current year to the sum of all prior years. [...] 
The current solar cycle is the weakest it's been for ages. Despite this, this pack of ratbags did the same thing with TSI that they did with [the Multivariate ENSO Index]. They developed what they called the "cumulative TSI" [2]."

And the scientist Timothy Osborn notes that Christy et al.'s cumulative indices are physically implausible:

"The statistical models used in this report co-authored by Christy are flawed and thus do not support his statement that Mother Nature can cause the observed warming trends on her own. They suggest that these trends arise from cumulative anomalies of ENSO or cumulative anomalies of solar irradiance, but offer no compelling physical basis for this hypothesis. There is no consideration that a warmer climate would cause increased loss of radiative energy to space, nor that a cooler climate would decrease the emissions of radiation to space. This effect is necessary to explain how the Earth’s climate has remained within a relatively small range of mean temperatures for much of the Earth’s geological history, yet their hypothesis assumes that a step up in temperature (due, e.g. to an El Nino event) would be sustained even after the El Nino had dissipated, because they use the cumulative ENSO index. Furthermore, a cumulative variable must have a physically-defined baseline from which the anomalies are defined, which has not been done in this case. Otherwise, the baseline can be changed to produce an entirely arbitrary upward or downward trend in the cumulative variable, and thus support a false claim that the cumulative variable can “cause” a trend in the climate [1]."

Similar comments have been made by other climate scientists. Neither Osborn nor Sou support their claims with evidence from peer-reviewed publications, so their statements here should be taken with a grain of salt. However, Sou's summary of the cumulative indices matches Christy's claim that the non-cumulative indices were summed through time in order to generate the cumulative indices. Osborn is also right in his contention that Earth has remained within a ~10K to ~11K range for global mean temperatures (and ~17K for Antarctic temperatures) for much of Earth's history, and that a warmer planet loses more energy through outgoing radiation. Furthermore, the warm El Niño phase of ENSO results in Earth radiating more energy into space than Earth takes up; this is largely because El Niño increases cloud cover and these clouds reflect the solar radiation Earth would otherwise absorb. This compensates for less emission of radiation by clouds during El Niño. Christy acknowledges this point, at least when it comes to clouds limiting global warming caused by greenhouse gases such as CO2. Yet, seemingly, Christy conveniently does not use this reasoning to show similar cloud-based limits on global warming caused by the Sun or ENSO.

So it is true that Earth releases energy from ENSO, in accordance with Osborn's reasoning: the El Niño phase of ENSO warms the surface air and the troposphere, then the El Niño passes, and the Earth transfers the excess surface and tropospheric energy into space. So El-Niño-induced warming is very transient and does not account for most of the post-1970s global warming trend. In contrast to a temporary El Niño, CO2 remains for much longer, warming the Earth by absorbing radiation released by Earth's surface. This warming compensates for the energy that a warmer Earth releases into space. Thus CO2 can cause long-term global warming, as CO2 has done in the past.

As this CO2-induced warming continues, eventually Earth reaches an equilibrium state that can be much warmer than contemporary times; in this equilibrium state, the Earth's release of energy into space equals the energy entering Earth from space and CO2-induced warming ceases. So CO2-induced warming does not necessarily imply a runaway global warming scenario where Earth continues to warm in an irreversible way; Earth's warming and cooling patterns differ from the runaway warming that occurred on Venus. Runaway warming on Earth will not occur for at least another billion years. Thus one can explain why CO2 causes long-term non-runaway warming and ENSO causes short-term warming, even if Earth has mechanisms that eventually stop both CO2-induced warming and El-Niño-induced warming. Christy et al. run afoul of these points by using a cumulative ENSO index that assumes energy from El Niño will accumulate in the troposphere and thereby result in a long-term warming trend.

In contrast to Christy et al., other researchers (including Christy himself) have used a physically plausible, non-cumulative ENSO index. This index allows for El-Niño-induced tropospheric warming to readily dissipate into space, in contrast to Christy et al. cumulative ENSO index. Researchers have found that when one uses this non-cumulative ENSO index, ENSO does not account for most the recent global warming, though some researchers attribute much of the tropospheric warming to ENSO. Similarly, using a non-cumulative TSI reveals that TSI does not account for most of the recent global warming. The following figure shows how much of the surface warming remains, even after ENSO and TSI are factored out [5]:


Figure 8: (a) Global surface temperature trend after correcting for TSI, ENSO, and volcanic aerosols. The upper-left, boxed inset depicts a measurement of the Atlantic multi-decadal oscillation (AMO), a cycle that affects ocean temperatures. (b) Global surface temperature trend after correcting for the AMO, TSI, ENSO, and volcanic aerosols [5]. 
It unclear whether the AMO is an independent cause of ocean warming vs. the AMO being a type of ocean warming caused by other factors. There is also some dispute over whether the AMO impacts temperature as strongly as is shown panel (b). Some sources attribute much of the recent warming to the AMO, while other sources argue that the AMO does not account for much of the recent warming. In either case, greenhouse gases such as CO2 substantially contributed to recent global warming.

Moreover, based on the ENSO index, the 1997-1998 El Niño was about as strong as the 1982-1983 El Niño in terms of warming of the ocean surface. Yet Earth's surface was warmer in 1997-1998 than in 1982-1983, while the mid-troposphere was warmer in 1997-1998 than in 1982-1983. Thus, significant global warming remains even after ENSO is factored out. Christy supports this point in research he co-authored and cited; this research uses a non-cumulative ENSO index, as opposed to Christy et al.'s cumulative MEI. So Christy shows a tropospheric warming trend even after correction for ENSO, thereby undermining Christy et al.'s claim that ENSO caused much of the recent global warming. These points support Osborn's contention that Christy et al. do not provide a compelling physical basis for thinking that ENSO caused most of the long-term global warming.

3.6 Disregarding evidence that the Sun did not cause most of the recent global warming


Christy et al. attribute much of the post-1970s global warming to the Sun, even though they do not discuss much of the scientific evidence against the Sun being a cause of most of the recent global warming. For example, solar activity does not adequately explain recent stratospheric cooling, solar output has not correlated well with recent global warming, significant global warming remains even after correcting for total solar irradiance, and the relationship between solar output and global warming fails a number of statistical tests and model-based tests.

3.7 Unwarranted references to "alarmis[m]"


Christy et al., for no legitimate reason, call scientists "alarmist". These hyperbolic references lack merit and are standard denialist rhetoric.

(Note: "Denialist" is a fairly well-defined term in science and philosophy, with AIDS denialists being one of the standard examples of a denialist group. Denialism is not the same as scientific skepticism, though many denialists try to portray themselves as being skeptics. Everytime I placed the words "skeptic" in quotation marks in this article, I really meant "denialist".)

For example, many denialists claim that climate scientists are "alarmists" who exaggerate anthropogenic climate change and its effects. This charge fails since scientists, both at the IPCC and elsewhere, tend to under-estimate the effects of climate change, as opposed to exaggerating them. In the case of scientists at the IPCC, their under-estimation of trends likely results from critics (often denialists themselves) applying undue pressure to mainstream scientists. This contradicts the denialists' charge of IPCC "alarmism". This point is often ignored by critics of mainstream climate science. Instead these critics carefully cherry-pick particular instances in which the models used by the IPCC over-estimated a given trend, while disregarding instances in which the models under-estimated a trend. The critics can then use this cherry-picking to falsely paint climate scientists as being "alarmist", just as Christy et al. did.

And even though climate scientists could justifiably use "alarmist" language, the tone of IPCC scientists tends to be more tentative and less "alarmist," with the IPCC paying proper attention to how to talk about uncertainty. Many mainstream climate scientists also avoid defending hyperbolic notions such "[imminent] runaway global warming" (as I discussed in section  3.5) and "catastrophic anthropogenic global warming", despite denialist claims to the contrary. This again runs contrary to unwarranted charges about scientists being "alarmist," as does the fact that climate scientists often correct exaggerated media stories on anthropogenic climate change. So Christy et al. should consider retracting their charge of "alarmis[m]," until they can provide a sound basis for this charge.


4. References


  1. http://climatefeedback.org/scientists-reactions-us-house-science-committee-hearing-climate-science/
  2. http://blog.hotwhopper.com/2016/09/barmy-bloopers-from-john-christy-and-co.html
  3. "Removing diurnal cycle contamination in satellite-derived tropospheric temperatures: understanding tropical tropospheric trend discrepancies"
  4. "Atmospheric changes through 2012 as shown by iteratively homogenized radiosonde temperature and wind data (IUKv2)"
  5. "Deducing multidecadal anthropogenic global warming trends using multiple regression analysis"
  6. "U.S. House Committee on Science, Space & Technology, 29 Mar 2017, Testimony of John R. Christy"
  7. "On the Existence of a “Tropical Hot Spot" & The Validity of EPA’s CO2 Endangerment Finding"
  8. https://www.esrl.noaa.gov/psd/enso/mei/ (accessed June 12, 2017)
  9. "Ranking the strongest ENSO events while incorporating SST uncertainty"
  10. "A solar irradiance climate data record"
  11. https://judithcurry.com/2017/03/31/deniers-lies-and-politics/