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With the longer-term record of El Niño events that we now had from our proxy temperature
reconstructions (we could reliably reconstruct the history of El Niño back through the early
seventeenth century), we could see whether the relationship held up over a time period that was both
longer than, and completely independent of, the modern period Handler had analyzed. What's more,
Amy Clement and Mark Cane of Columbia's Lamont Doherty Earth Observatory had provided an
important link in establishing a plausible physical mechanism for the purported relationship. In a
provocative 1997 article, they used the so-called Cane-Zebiak model of the El Niño phenomenon to
demonstrate that the very same mechanisms responsible for El Niño held some counterintuitive
implications for how the tropical Pacific Ocean and atmosphere system might respond to an external
heating.
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The mechanism they identified, known as the “tropical Pacific Ocean thermostat,” implied
that the eastern and central tropical Pacific might actually cool down! The pattern of cooling
resembled the opposite of El Niño—La Niña—and it resulted from subtle ways that wind patterns
and ocean currents influence each other in the equatorial Pacific. Clement, Cane, and collaborators
argued that this mechanism could paradoxically cause the climate to look more like the cold La Niña
state even as global warming proceeds.
A pseudo-skeptic who denies the reality of climate change might like this result simply because
it provides an argument for why fossil fuel burning could lead to less global warming than the models
predict. But as true skeptics, we liked the result for an entirely different reason; it questioned the
simplistic notion that the globe should simply warm or cool in unison in response to external
influences (be they natural or human and nature), and it challenged us to think about the subtleties of
how the climate system responds to such influences—the anomalies.
If the thermostat mechanism caused the climate to look more like La Niña in response to heating
(by either increased greenhouse gas concentrations or an upturn in solar output), it ought—my
colleagues and I reasoned—to exhibit an El Niño-like pattern in response to the cooling influence of
an explosive volcanic eruption. Not any old volcanic eruption would do; the volcanic aerosols would
have to block out sunlight from reaching the surface of the tropical Pacific Ocean, something that—
because of large-scale wind patterns—only a tropical eruption will do.
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It made perfect sense to us,
now, why Handler had seen a clear relationship only with tropical volcanic eruptions.
We confirmed Handler's original findings in our own independent analysis of the relationships
skeptical of the original Handler work, such as climate researcher Alan Robock of Rutgers
University, appeared to view our findings as credible. We had established the same relationship as
Handler using a completely independent and longer time interval. Moreover, a reasonably compelling
physical mechanism could now be articulated for why tropical volcanic eruptions would lead to an El
Niño-like response of the climate system. We weren't claiming that volcanoes were the trigger for El
Niño events. Our results, instead, simply suggested that tropical volcanic eruptions made the
environment somewhat more favorable, roughly doubling the probability of an El Niño event
occurring. I would subsequently collaborate with Cane and coworkers to show that the Cane-Zebiak
Handler had passed away in 1998 and hadn't lived to see the vindication of his original hypothesis.
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In the years since the original 1998 hockey stick article, my collaborators and I continued to
investigate such puzzles of past climate change in search of possible insights about the present and
