Caves may reveal if global warming is causing stronger hurricanes
Scientists have shown that cave formations could help settle the contentious debate on whether hurricanes are strengthening in intensity due to global warming.
Measuring oxygen isotope variation in stalagmites in Actun Tunichil Muknal cave in central Belize, a team of researchers have identified evidence of rainfall from 11 tropical cyclones over a 23 year period (1978-2001). The research -- the study of ancient storms is called paleotempestology -- could help create a record of hurricanes that would help researchers understand hurricane frequency and intensity.
"Tropical cyclones (including hurricanes, tropical storms, typhoons, and cyclones) produce rainwater that is different from other summertime precipitation," explained Amy Benoit Frappier, an assistant professor in the Department of Geology & Geophysics at Boston College and lead author of the study published in Geology. "Tropical cyclones produce isotopically light rainwater primarily because 1) their cloud tops are very high and cold, and 2) their humid air tends to prevent lighter water molecules from evaporating back out of the raindrop as they fall."
Frappier says that this property of tropical cyclone rainfall -- first identified by Jim Lawrence and Stan Gedzelman in the mid-1990s -- was last year used by researchers to show that tree-rings can record tropical cyclones (Miller et al. PNAS, 2006) and is now revealing itself in a stalagmite in Belize.
"When a hurricane rains over a cave, suddenly the isotopic composition of the water soaking into the ground becomes much lighter than normal. As that storm water percolates down into the cave, some of the oxygen is transferred from the water to calcium carbonate, a mineral in the form of cave formations such as stalagmites," she explained. "This leaves behind a very thin layer of calcium carbonate with an unusually light O isotopic composition. We used a computer-controlled dental drill to carefully mill off layers of powder from a fast-growing stalagmite, where each sample reflects cave drip water over periods of a week to a month. Analyzing these rock powders using standard techniques, we were able to detect brief spikes from recent hurricanes and tropical storms that produced rain over the cave - even when those storms struck only weeks apart!"
"We also found that the relative size of spikes that we measured was related to the intensity of the storm, which is encouraging for the prospect of reconstructing the intensities of pre-historic landfalling storms," she continued. "Future work will explore whether this proxy can be widely applied, how reliable it is, and how far back in time it can take us. We plan to extend the historical record of landfalling storms using this technique to learn more about the links between climate and tropical cyclones at times when climate was different from the modern period."
Currently, reliable history for hurricanes only dates back a generation or so. Prior to that, the official hurricane records kept by the National Oceanic and Atmospheric Administration's Atlantic basin hurricane database (HURDAT) are controversial at best since storm data from more than 20 years ago is not nearly as accurate as current hurricane data due to improvements in tracking technology. The lack of a credible baseline makes it nearly impossible to accurately compare storm frequency and strength over the period.
"Before aircraft and satellite monitoring were available, the Atlantic hurricane data are likely woefully underestimated - except where a hurricane ran directly over a ship or coastal community and there were meteorological observations of pressures and/or winds recorded," Chris Landsea, a scientist at the NOAA National Hurricane Center, told mongabay.com by email last year. "Given that ship captains did their best to NOT sail into the eye of hurricanes, there is a very large underreporting bias in our databases during the late 19th and early 20th Centuries, except for hurricanes at landfall along populated coastlines."
"What data we do have - and there certainly are biases in HURDAT that need to be addressed storm by storm - suggest that the middle of the 20th Century was about as busy as the last active 11 years have been (1995 to 2005). Disentangling trends due to bias in the hurricane dataset and possible global warming induced changes is then very problematic," he added.
While several studies published since early 2005 have linked recent climate warming to the increasing occurrence and strength of hurricanes over the past thirty years, the research has proved controversial since they relied on potentially flawed data. The new study may help climate researchers re-analyze existing tropical cyclone databases to address these concerns.
"The statistical models presented here open the door to developing highly resolute speleothem records of prehistoric [tropical cyclone] frequency and intensity," write the authors who go on to say that the addition of further cave observation sites outside of Belize could "ultimately contribute to risk assessment and climate change impact detection and/or attribution programs."
Citation: Frappier, A.B. et al. "Stalagmite stable isotope record of recent tropical cyclone events". Geology, February 2007; v. 35; no. 2; p. 111–114; doi: 10.1130/G23145A.