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| Enlarge ImageJeff Donnelly and colleagues have studied lakes and ponds overlying a large aquifer (yellow) in southeastern Massachusetts to reconstruct the history of past water table and pond levels. The research provides clues to past climate conditions and the potential for future climate change in the region. The brown box within the aquifer represents the area in the illustration below. (Courtesy of Jeff Donnelly, Woods Hole Oceanographic Institution) |
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| Enlarge ImageAn enlarged view of the brown box in the map above, showing several ponds studied by WHOI scientists. At top, a cross-section (from points A' to A) of the area, showing water table levels today and two scenarios for water table levels 3,000 and 5,000 years ago during the Holocene Epoch when drought conditions prevailed in the New England region. (Courtesy of Jeff Donnelly, Woods Hole Oceanographic Institution) |
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| Enlarge ImageAn image from a ground-penetrating radar (GPR) is juxtaposed below a photo of the lake surface. The GPR image reveals a sandy ancient shoreline below layers of organic silt that subsequently accumulated on top of it when the lake level rose. The buried shoreline was created when the climate was drier and lake level was lower. (Jeff Donnelly, Woods Hole Oceanographic Institution) |
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| Enlarge ImageDonnelly and colleagues sink metal coring tubes into lake bottoms to collect plugs of sediment that are analyzed and dated to reconstruct a record of past climate conditions. (Tom Kleindinst, Woods Hole Oceanographic Institution) |
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| Enlarge ImageDonnelly retrieves a core taken from the bottom of Oyster Pond in Woods Hole, Mass. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution) |
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Jeff Donnelly, Associate Scientist
Geology and Geophysics Department
Woods Hole Oceanographic Institution
Between 5,400 and 3,000 years ago, something happened to New England’s
climate. The region became drier. Water levels in lakes dropped.
Several droughts persisted for hundreds of years, changing the makeup
of local tree and plant populations. Animal and insect populations probably shifted accordingly.
Climate change is not just about temperature change. We don’t know what
caused it, but New England’s hydrology—the distribution and circulation
of its water—fundamentally changed in the not-so-distant past, and the
humid conditions we have come to expect in the northeastern United
States were different. Records of past climate conditions give us
reason to believe such a change could happen again.
Changes in precipitation
Researchers have looked at pollen grains buried in sediments to seek
clues about past vegetation and climate patterns. The types and
abundances of different pollens can reveal quite a bit about the
average temperatures during a certain period, because some plants thrive
and others fade as the Earth warms and cools.
But pollen alone does not necessarily tell us the entire story. My
research group is working to demonstrate how the sedimentary patterns
within small lakes and ponds can be used to track shifts in moisture
over time.
As climate changes—whether on a global or local scale—groundwater
tables rise and fall. Some lakes and ponds offer good records of such
changes because they are hydrologically “closed”; that is, they are not
filled or drained by streams or inlets, but rather their basins are
filled with groundwater from below and precipitation from above. As the
broad, regional water table fluctuates, these closed water basins rise
and fall to different levels in unison.
For instance, during more humid times, a lake might be 20 feet deep and
cover a large area. In hotter, drier times—such as a well-documented
drought in 1964 in southern New England—the water table lowers; the
amount of water in lakes diminishes, their banks lower, and their
surface area shrinks. To the rhythm of climate conditions shifting
between wet and dry, the sandy shorelines of such lakes migrate in and
out, moving closer and farther from the center of the lake.
Warmer winters, lower lakes
My colleagues and I are reconstructing the water-level histories of
lakes from Cape Cod to New York over the past 11,000 years, also known
as the Holocene Epoch, when the world emerged from the last ice age.
From rowboats and other small vessels, we carry ground-penetrating
radar to image lake-bottom sediments. We search for the reflections of
the radar pulses made by ancient sandy shorelines lurking beneath
organic-rich silts, which subsequently accumulated over the shorelines
in more humid times. Once we have found those sand layers, we sink
metal coring tubes into the lake bottom to collect plugs of sediment
that we analyze and date.
Our studies so far have revealed a drier New England in the past. In
the sediments drawn from ponds in Massachusetts and New Hampshire, we
have found a common “low stand” from 5,000 to 3,000 years ago, when
water levels were much lower and the lakes covered much less area.
Working with Bryan Shuman of the University of Minnesota, we created a
simple model of conditions in New England. We suspect that warmer
winters (something most New Englanders might idly wish for) may have
changed precipitation patterns and ice coverage in the area. With less
snow melt to fill lake and pond basins, less ice preventing winter
evaporation, and longer growing seasons for plants, the lakes and ponds
would have had a much smaller supply of water.
The forecast for droughts
In many ways, the time period we focused on—5,000 to 3,000 years
ago—may be one of the best analogs of future conditions in the
northeastern U.S. in an era of global warming. The recent National Assessment of the Potential Consequences of Climate Variability and Change
states that, in New England, “winter minimum temperatures show the
greatest change, with projected increases ranging from 2° to 3º C (4°
to 5º F).”
Such warming would decrease snow levels and related groundwater, lake,
reservoir, and river recharge. Our reconstructions of the history of
hydrologic variability in key temperate regions such as New England
offers essential clues for understanding the causes and potential
impacts of climate change.
Water availability has a wide range of ecological, economic, and social
impacts. It affects vegetation and wildfires. The Massachusetts aquifer
that underlies one of our research sites is the second largest in the
state and an important water supply for large-scale cranberry
production. The lakes we study are about 50 kilometers from the major
urban centers of Boston and Providence, which rely on regional
reservoirs for water. Water and land managers, farmers, businesses, and
civic leaders will need to make informed decisions on how to manage
fresh water supplies and mitigate drought.
If drought is coming to the Northeast, we need to look back at Earth’s
history and appreciate that a changing global climate could mean
drought that lasts not years, but centuries.
The research was supported by the
National Science Foundation, and the Ocean and Climate Insitute and the
Coastal Ocean Institute at WHOI.
Posted: December 7, 2006 [top] |
