Potomac Climate Miracle

I know what you are thinking – the miracle of our congress suddenly seeing the light and acting with a comprehensive program to address climate change in a realistic way.  Unfortunately, this is about a somewhat different kind of miracle.  One based on the geology of the river and not on the public spirit of politicians who regularly gather on its shores.  This miracle addresses the lack of North American data to calibrate the function of our long-term climate in a way to let us prepare for what global warming is likely to bring.  In my US Geological Survey career, I worked alongside researchers methodically using the quantified growing-season tolerances of forest trees to produce accurate values for climatic data constructed from fossil pollen.  The need for such data is obvious.  For example, regulators based the legal partition of water from the Colorado River on some 70 years of recorded discharge.  That sounds like a good sample until you realize that rainfall patterns run in decade-long cycles.  So, this was a sample of only seven data points.  Now, with more decades of discharge measurement and backward extrapolation of flows from tree ring data we know that long-term Colorado River outflow was severely over-estimated at that time.  The long-term climate extrapolation with pollen cores suffers from a similar lack of data for our region extending back beyond 20,000 years.  The basic climate cycle to which earth climate responds has about a 20,000-year frequency so that climate models are left with less than a single cycle with which to calibrate climate parameters.  And the last half of that data set from before 10,000 years ago is very limited.

     The miracle we are talking about here is the discovery of over 100,000 years of pollen data from a site adjacent to George Washington’s home at Mt Vernon.  For yet unexplained reasons, a kind of shallow slough adjacent to the Potomac has been slowly subsiding at a rate that allowed it to collect the local pollen rain for that length of time.  Gardeners know that this location lies in the same plant zone 6 to 7 transition for our area that we see on the back pages of most seed catalogs.  The site falls under the same general classification of deciduous oak-hickory forest biome that we have here in northwest Arkansas.  For those of us interested in forests of the past this is a chance to see the changing forest biome over an incredibly long slice of time – extending far back before humans ever stepped onto the landscape.  The show starts at the very end of the previous glacial episode some 130,000 years before present, into the climate optimum known as the previous interglacial, and through the ratcheting build-up of the very last continental glaciers of the Wisconsin period.   Capturing the nature of that ratcheting is important since it tells us exactly how the global reflection from a building polar ice sheet provides the feedback mechanism that climate deniers often disparage as so much hoopla.   The multiple cycles of climate response to the well-documented orbital forcing of solar insolation provide a critical opportunity for calibrating models that will be used to predict the response of climate to a kind of forcing it has not experienced in eons, and even back then under a widely differing distribution of continental solar receptors.

     The pollen data show the abrupt end of a former glacial climate (130,000 years ago) leading into an oak-hickory biome much like that of today, but without any human influence at all.  The first cool cycle was a strictly polar affair.  Only in the second cooling phase did the record show a biome like we would see today in the Adirondacks – northern hardwoods, hemlock, red spruce, and basswood.  Then another oak-hickory phase, followed at 70,000 years by a cold, dry, and fire-prone ecosystem.  That environment lasted with minor oscillations right up to the end of the datable sequence at about 20,000 years ago.  All of this is an interesting chronicle of the past.  The big surprise, however, is in the DO (Dansgaard-Oeschger) cycles expressed along the way.  These are abrupt warmings seen in the Greenland ice core records – changes with an apparent onset over less than a century.  Here seemed to be evidence that climate can tip abruptly over a threshold.  Arguments could be made that polar regions are especially sensitive to small changes, so this could just be a polar effect irrelevant to temperate latitudes.  When I first looked at the Potomac pollen story, I thought there was a misprint – two sequentially numbered figures displayed diagrams with the exactly the same data.  Then I realized that one showed the relative oak pollen percentage (indicating warmth), while the other reprised the published DO oscillations in oxygen isotope data for the ice core.   The similarity was that obvious, while demonstrating that abrupt DO shifts in climate were present in this temperate zone data – changes that could have occurred within a human generation or two.

     Of course, I would have to check out the drill site myself when the opportunity arose.  That was during an autumn 2013 visit to a Fairfax County Park located just off I-95 in Alexandria, Virginia.  Attendants at the visitor’s center barely remembered something about a drill rig once located along the far edge of the park’s central marsh.  I followed a trail through old secondary woods of mostly sweetgum and over-mature pine, across a boardwalk, and into delightful old-growth oak and hickory.   From my vantage on the boardwalk alongside logs covered with basking turtles and a single stoic heron, I looked across the marsh towards the drill site at the edge of distant woods.  Standing at this exact spot at earlier times I could have seen: mature oak-hickory forest with mastodon tracks in the pond-side mud; the autumn colors of northern hardwoods interspersed with evergreen hemlock along the water’s edge while an extinct moose-elk fed on water plants; or wooly camels browsing on subarctic vegetation in a forest-tundra mosaic.   The pollen evidence in the core makes it a little easier to believe that small oscillations in the earth’s orbit can produce such momentous changes in the landscape.  That also reminds us that species have evolved to withstand such changes, but only when changes are gradual.  We need to remember that the nature of the solar radiation forcing function created by human-induced changes in our atmospheric greenhouse will be applied in a very different way and at a very different rate from the past orbital oscillations – with potential changes that our ecosystem has never confronted before.  Most important here is the demonstration that the “feedback amplification” and “tipping point” arguments famously cited by Al Gore and derided by climate deniers are actually based on solid scientific fact.