Geoscience Reference
In-Depth Information
AMO appeared to be real, and at least partly responsible for certain phenomena, such as the
acceleration of recent warming in parts of the Arctic, that some had attributed to anthropogenic
climate change. Other phenomena that have been blamed on the AMO, such as the increase in Atlantic
hurricane activity in recent decades, arguably have nothing to do with it all.
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That hasn't stopped
climate change contrarians, however, from dragging out the AMO as a favorite catch-all explanation
for just about any observed climate trend. At times I have felt like I helped create a monster.
A Century Is Not Enough
The evidence for AMO-like oscillations in the climate system seemed reasonably strong. What
remained vexing to us, however, was that the short instrumental temperature record simply wasn't
adequate for any scientifically confident characterization of such a signal. Covering little more than a
century, instrumental global temperature observations could capture at most one complete cycle of the
oscillation we were studying—not enough to detect a persistent pattern. The apparent oscillation
could simply be an artifact of having such a short and noisy record from which to tease out a natural
climate oscillation from long-term climate trends.
It was the search for more compelling evidence of these oscillations that first led me to examine
longer-term proxy records of climate (records that, as you may recall, stand in past centuries as a
proxy for the direct measurements of climate that modern instruments make possible). Well, that, and
a bit of serendipity. My parents were attending an event in my hometown of Amherst, Massachusetts.
They happened to strike up a conversation with a University of Massachusetts professor over a bottle
of wine (
in vino veritas
?). As it turned out, he was a climate researcher. My parents said something
to the effect of “What an interesting coincidence! Our son is doing his Ph.D. in climate research.” The
end result was that the professor, Raymond Bradley, and I were set up on a sort-of scientific blind
date. We would meet to talk science the next time I was in town.
It was evident upon our first meeting that there were synergies between our respective research
interests. Ray's research specialty involved the use of climate proxy data such as that derived from
tree rings, ice cores, corals, lake and ocean sediments, and other natural archives of past climate.
These were to become central to our collaboration and to many other studies of changing climate
conditions.
Most widespread of all these climate proxies are tree ring data. Depending on the region and
environment, tree growth may largely be controlled by growing season conditions such as the warmth
of the growing season (typically summer) or the precipitation that falls during the rainy season. One
can therefore analyze annual growth rings (the width of the rings and, in some cases, the density of the
wood formed) for insights into past climate changes.
One obvious limitation with tree ring data is that they are restricted to the continental regions
where trees grow. That is a major problem for the mostly water-covered Southern Hemisphere, but
it's also problematic for the Northern Hemisphere. Neither are tree ring data available in polar
regions, where tundra and permanent ice cover prevail. And tropical tree species typically do not
have annual growth rings (look at a palm tree stump sometime if you don't believe this), limiting the
usefulness of tree ring data as tropical climate proxies. Climatically useful tree ring proxies are thus
primarily restricted to the midlatitude continental regions, leaving much of the globe un-sampled by
