(CN) — To understand where you’re going, you have to know where you’ve come from. That’s the idea researchers were pursuing with the recent geochemical analysis of a single stalagmite from a cave in Grand Canyon National Park.
More specifically, they measured the ratio of stable isotopes in calcite deposits within the stalagmite to help predict how the volume of groundwater aquifers may be affected by a warming climate. The calcite deposits date to the early Holocene period, an epoch ranging between approximately 11,700 and 8,500 years ago that was marked by higher-than-average global temperatures.
The results, published Monday in the journal Nature Geoscience, may help planners better manage water resources in the burgeoning Colorado Plateau in the Four Corners region of New Mexico, Arizona, Utah and Colorado, where population and agricultural pressures increasingly affect the rates with which aquifers are recharged by summer monsoon rains.
The frequency and intensity of those monsoon rains are expected to change with a warming climate, although existing research varies widely as to whether resulting groundwater will increase or decrease. By taking a glimpse into the stalagmite, researchers determined that “water infiltrating into the cave increased in line with regionally warming temperatures” during the Holocene period, when the average global temperature was 1.8 to 3.6 degrees Fahrenheit (1 to 2 degrees Celsius) higher than it is today.
The monsoon rains are crucial to sustaining development in the region, which is home to more than 1 million people but only receives an annual average of 8 inches of precipitation. For millennia, the summer rains have been fueled by moist air circulating off the Gulf of California, while winter occasionally delivers rain and snow from the mid- to high-latitude Pacific Ocean storms.
The researchers concluded “there was a gradual intensification and incursion of the [monsoon rains] over the Grand Canyon in the Early Holocene,” which can most likely be attributed to warming atmospheric temperatures, a reduced temperature gradient and “a decrease in snow and ice cover on the Colorado Plateau.” They suggest the conditions resulted in rain and snow events “sufficiently intense and frequent to infiltrate into the Grand Canyon cave.”
They further suggest that “future warming, which could cause temperatures to rise above those of the early Holocene, may also lead to an expansion of the North American monsoon and possibly greater groundwater recharge rates of summer rainfall on the high-elevation Colorado Plateau.”
But that may only occur if both the frequency and intensity of rains increase. The researchers noted a past increase in intensity of precipitation “may not have led to greater mean summer or mean annual precipitation because the frequency of such events may have decreased.”
“While the Early Holocene is not a direct analogue for future climate, our data do suggest that warm-season groundwater infiltration may vary as a function of precipitation intensity, at least for the high-altitude Colorado Plateau region,” the researchers concluded.
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