The summertime misery of Georgia’s hot humid temperatures is almost upon residents of the state whose only refuges from the strength sapping sauna are air conditioned interiors and swimming pools. Pretty soon, we will wish for a return of the Ice Age. Comfortable summer temperatures haven’t occurred with regularity in this region for at least 11,000 years, though aberrant cool summers sporadically and rarely have. Just how cold was what’s now Georgia during the Wisconsinian Ice Age? Despite limited data, science can answer that question with some degree of certainty.
The earliest studies along this line used pollen and plant macrofossils as proxies for climate. W. A. Watts (formerly of Penn State but now retired) analyzed botanical samples from granite sag ponds in Bartow County in 1970 from sediment dating to the last Ice Age. He found lots of spruce and other plants representative of northern climates and assumed a drastic lowering of average temperatures compared to those of today must have been the norm then. Dr. Stephen Jackson of the University of Wyoming also excavated this site and did find forest trees with more northerly modern ranges including jack pine and red pine as well as Critchfield’s spruce, an extinct temperate species. There’s no doubt cooler summer temperatures allowed for an influx of boreal flora as far south as north Georgia.
However, Dr. Fred Rich of Georgia Southern has conducted numerous studies of plant macrofossils and pollen in Georgia’s coastal plain, and he’s found that plant communities in south Georgia consisted mostly of the same species that occur here today, albeit with a slight influx of such cool climate species as beech, spruce, hemlock, and basswood. The one study of Ice Age pollen and macrofossils in the piedmont at Nodoroc near Winder reveals a transitional mix with northern and southern species. This suggests a significant variation in temperatures between north and south Georgia during the Ice Age perhaps because proximity to the gulf stream off the Atlantic Coast meant warming oceanic winds created a “thermal enclave” here.
One climatic model predicts average annual temperatures were as much as 15 degrees C (~27 degrees F) lower in Georgia during the coldest phase of the Ice Age, or Last Glacial Maximum (~28,000 BP-~14,000 BP) due to cold winds blowing off the ice sheet ~270 miles to the north of the state. But this model is contradicted by a study of actual data gleaned from noble gas concentrations in ground water. I think this data is more accurate, in my opinion, than climate models which are little more than wild guesses.
Map of the Florida aquifer. The study mentioned below took a sample of water in Georgia from this aquifer. The bottom layer of water here was originally deposited during the LGM.
Scientists are able to determine past average annual temperatures by comparing the concentration of noble gases in air bubbles found in ancient groundwater with those found in today’s water. Groundwater is the well water that our pioneer ancestors used to depend upon for drinking water, and some people, living in the boondocks, still do. The top layer of groundwater is always being recharged by rainwater and seepage from the watershed above. This top layer doesn’t mix with the bottom layer of water which is much older. Scientists were able to determine the age of the bottom layer of groundwater in the Florida aquifer, which also underlies most of south Georgia. This water was originally deposited during the LGM ~20,000 years ago when the water table was very low. Since then, the water table has risen, and the recharged water stays on top.
Water can only absorb so much atmospheric gas before it becomes saturated. Once saturated, it ceases to absorb more gas, so scientists know that the concentration of noble gases in the bottom layer of ground water is in the same ratio as occurred from the atmosphere during the Ice Age. Scientists look at noble gases (helium, neon, argon, etc.) because they’re inert and don’t react with other elements–chemical reactions would throw off their calculations. To get samples of ground water, they bore a hole to the bottom of the aquifer and collect samples of water with a Niskin bottle.
A Niskin bottle. Scientists use this to collect water from the depths. It keeps water under a certain pressure to prevent alteration of data.
Scientists then run a sample through a mass spectrometer.
Chart of how a mass spectrometer works. The actual machine resembles a hospital MRI.
I could find no layman’s explanation for exactly how scientists determine the temperature based on noble gas concentrations, but I got the impression from reading their research papers that it’s based on simple gas laws. If the temperature, pressure, and volume of one sample are known, and the pressure and volume of a second sample are known, the temperature of that second sample can be calculated because all 3 variables are proportional. According to a study led by J.F. Clark, who used the technique described above, average annual temperatures in south Georgia were only 4 degrees C (7 degrees F) lower during the LGM. A similar study of ground water in Maryland found average annual temperatures were 9 degrees C (~16 degrees F) lower, showing a significant north south gradient. North Georgia’s temperatures fell somewhere in between.
These are average temperatures. A particularly cold year could see a drop in average temperatures even lower. I suppose a bitter winter during a stadial could see a year with average temperatures as much as 17 degrees F lower in south Georgia; and vice versa–a warm year probably had temperatures as warm as an average modern year.
Another study found reduced seasonality during the Ice Age. This one of foraminifera in the Atlantic Ocean (see last week’s blog entry for an explanation of this) determined that average August temperatures in Atlantic ocean tropical waters were 23 degrees C (73 degrees F) compared to 28 degrees C (82 degrees F) today; while mean winter temperatures were 19 degrees C (66 degrees F) compared to 21 degrees C (70 degrees F) today. This suggests summers were much cooler in the southeast then, but winters were only a little cooler. Combine cooler temperatures with lower CO2 (less humidity), and it would have been quite comfortable for humans to live in the region then.
During the Ice Age climate fluctuated dramatically probably leading to lots of bizarre weather events and freak storms. Warm air flowing north over the Gulf of Mexico often met cold air flowing south from the glacier that covered all of Canada. Because the atmosphere contained less moisture, many cold fronts were dry and windy, but on occasion moisture from the gulf would accumulate, causing blizzards and ice storms in winter and monster tornadoes in spring and summer. I’m sure that occasionally these tornadoes surpassed those we’ve experienced during this year’s unusually stormy spring. Giant tornadoes likely blasted forests to smithereens, leaving shredded timber to dry during prolonged droughts in the arid Ice Age climate. Eventually, a severe electical storm would break this drought and ignite all that dried out tinder, causing a massive wildfire that would convert forestland to prairie, creating habitat for mammoth, bison, horse, llama, jack rabbit, ground squirrel, upland plover, prairie chicken, bluestem grass, sunflowers, and myriads of other denizens of the open plains.
Clark, J.F. et. al.
“A Tracer Study of the Florida Aquifer in southeastern Georgia: Implications for Groundwater Flow and Paleoclimate”
Water Resources Research 1997