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Monday, July 17, 2017

Myth: El Niño Caused Post-1997 Global Warming

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



El Niño caused post-1997 global warming. Thus the recent warming is natural, as opposed to anthropogenic or caused by humans.

Purveyors of this myth include Roy Spencer, the Daily Caller, The Competitive Enterprise Institute, Leo Goldstein, Marc Morano, and David Wojick. Spencer seems to be the primary purveyor of this myth, since other myth proponents typically cite Spencer.

The myth's flaw: Post-1997 global warming remains, even after one corrects for El Niño.



2. Context and Analysis



During El Niño (also known as the El Niño phase of ENSO, the El Niño Southern Oscillation) the oceans' surface warms, as does a region of the lower atmosphere known as the troposphere. The strength of El Niño can be measured using the multivariate ENSO index, or MEI. Spencer should be aware of this, since Spencer employed the MEI in his published research.

MEI-corrected temperatures still show post-1997 global warming, as shown in published research and unpublished analyses. Thus El Niño did not cause the post-1997 global warming. Even Spencer's colleague John Christy admits that ENSO did not cause the post-1997 tropospheric warming trend (though Christy gets little else right when it comes to climate science). Figure 1 depicts some MEI-corrected temperature trends:


Figure 1: Relative temperature from 1979 - 2010 for three surface temperature records (GISS, NCDC and CRU) and two lower tropospheric temperature records (RSS and UAH), after correction for ENSO (using MEI), volcanic effects, and changes in solar irradiance [1].

In fact, the 2015/2016 El Niño was weaker than (or about as strong as, in some regions) the 1997/1998 El Niño, even though 2015/2016 was warmer than 1997/1998, as shown in "Myth: No Global Warming for Two Decades". Figure 2 below depicts this warming using the mid- to upper tropospheric warming that Spencer commented on, while figure 3 displays the MEI and figure 4 depicts one regional component of the MEI:


Figure 2: Near global mid- to upper tropospheric temperature averaged for the UAH, NOAA, and RSS satellite-based analyses [2].  


Figure 3: Multivariate ENSO index (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 [3]. See figure 4 for a representation of some of the uncertainties involved in calculating the magnitude El Niño and La Niña events.


Figure 4: 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 [4].

So given this evidence, why do Spencer and his followers defend their myth? Well, they may be (intentionally or unintentionally) conflating the following two claims:
  • El Niño contributed to 2015/2016 being warmer than 2014
  • El Niño contributed to 2015/2016 being warmer than 1997/1998

The first claim is true, since El Niño conditions were stronger in 2015/2016 than 2014. However, since the 2015/2016 El Niño was weaker than the 1997/1998 El Niño (see figure 3), then the second claim is false. It is this second claim that Spencer defends when he claims that the past two decades of warming was natural. So Spencer is defending a false myth.

To put this another way, the distinction between the above two bullet points is analogous to the distinction between:
  • Earth's 24-hour rotation cycle contributed to late afternoon in summer being warmer than midnight in summer in Canada
  • Earth's 24-hour rotation cycle contributed to late afternoon in summer being warmer than noon (with the Sun at its zenith) in winter in Canada

The first claim is true, since Earth's shorter-term, 24-hour rotation cycle causes Canada to receive more solar energy at during late afternoon than during midnight. However, the second point fails, since Earth's 24-hour rotation cycle would cause a late afternoon day to be cooler than noon. Instead Earth's axial tilt relative to the Sun, not Earth's 24-hour rotation cycle, explains the longer-term, multi-month warming from the winter to summer.

Analogously, El Niño contributes to shorter-term warming on an inter-annual time scale, while other factors contribute to longer-term warming on a multi-decadal time-scale. If you're curious about what those "other factors" are, then see section 3.7 of "John Christy Fails to Show that Climate Models Exaggerate CO2-induced Warming" and "Myth: The Sun Caused Recent Global Warming and the Tropical Stratosphere Warmed".

In response to what I've said, a critic might claim that the energy from El Niño accumulates over the long-term. So, for instance, the 1997/1998 El Niño expelled a large amount of energy that persisted for 10+ years, explaining why the post-1998 period was warmer than the pre-1998 period. According to a such a critic, I would need to use a cumulative MEI to account for this accumulated energy, instead of the non-cumulative MEI I used above. Figure 5 below presents John Christy's cumulative MEI, in contrast to the non-cumulative MEI I presented above in figure 3:

Figure 5: Cumulative TSI (total solar irradiance) and cumulative MEI used by Christy et al. [5, page 18].

But scientists rarely (if ever) use a cumulative MEI. In fact, I know of no peer-reviewed scientific paper that uses a cumulative MEI. Instead the cumulative MEI remains relegated to non-peer-reviewed sources (or fake, predatory "journals" not listed on a reputable citation index) which are more amenable to unfounded claims than are peer-reviewed sources. Scientists instead use a non-cumulative MEI in their peer-reviewed work. Even Spencer and Christy, two critics of mainstream climate science, use a non-cumulative MEI in their peer-reviewed research. Christy even cites peer-reviewed sources that use a non-cumulative MEI, including including work from his research colleague Roy Spencer.

There are at least three reasons why scientists employ a non-cumulative MEI instead of a cumulative MEI:
  • A cumulative MEI is arbitrary, since it involves arbitrarily selecting a period over which the MEI accumulates.
  • A cumulative MEI compromises the relationship between the MEI and the temperature trends one uses the MEI to predict.
  • A cumulative MEI violates basic physics.

Christy, as a climate scientist, makes a version of the first two points in his peer-reviewed work. Yet he previously used a cumulative MEI in his non-peer-reviewed blog articles to argue that solar-induced changes in ENSO caused most of the post-1970s global warming; I discuss Christy's double standard on this in "Myth: The Sun Caused Recent Global Warming and the Tropical Stratosphere Warmed". Timothy Osborn, another climate scientist, argues for the first and third points in response to Christy's use of a cumulative MEI.

Osborn points out that a warmer Earth would radiate more energy into space, instead of all the energy just accumulating. Scientists can observe this increased radiation during a warm El Niño; the radiation increase occurs largely because El Niño increases cloud cover and these clouds then reflect the solar radiation Earth would otherwise absorb. This cloud-based mechanism compensates for less emission of radiation by clouds during El Niño. 

So increased radiation during warm El Niño events means that energy from a two-year El Niño event does not simply accumulate for 10+ years, contrary to the cumulative MEI. Thus a cumulative MEI violates basic physics, making a non-cumulative MEI more appropriate that a cumulative MEI. And a non-cumulative MEI reveals that El Niño did not cause the post-1997 warming trend, as I explained above and as Christy himself showed.




3. Posts Providing Further Information and Analysis





4. References


  1. "Global temperature evolution 1979–2010"
  2. "Tropospheric warming over the past two decades"
  3. https://www.esrl.noaa.gov/psd/enso/mei/ (accessed June 12, 2017)
  4. "Ranking the strongest ENSO events while incorporating SST uncertainty"
  5. "On the Existence of a “Tropical Hot Spot" & The Validity of EPA’s CO2 Endangerment Finding"