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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, and Marc Morano. 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 or the El Niño-Southern Oscillation) the oceans' surface and the lower atmosphere (a.k.a. the troposphere) warm. The strength of El Nino 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MEI-corrected temperatures still show post-1997 global warming, as shown in published research and unpublished analyses.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. 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"). Let's illustrate this point using the mid- to upper tropospheric warming that Spencer commented on:


Figure 2: Near global mid- to upper tropospheric temperature averaged for the UAH, NOAA, and RSS 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 offer 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 were natural. So Spencer is defending a false myth.



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"