When you think of climate change research, mucking through mangrove mud in the Federated States of Micronesia probably doesn’t first come to mind. But there I was, waist deep in evidence. Welcome to the dirty side of climate science.
While much of spotlight of global warming has focused on the climate models themselves and data from polar ice cores, scientists are increasingly looking for physical evidence of recent manifestations of tropical climate change several thousand years ago to make sure the parameters in the models are grounded in reality, and that means taking sediment cores.
As sediment accumulates chronologically in a peat bog, pond bottom or mangrove swamp, what’s in it can tell us a lot about what the climate was like. The key is analyzing the ratio of different types of hydrogen and oxygen contained in the sediment. Because different ratios are uniquely associated with saltwater and freshwater looking at those ratios can give us an idea of the relative proportion of ocean water to rainfall at a given time and thus, an idea of the precipitation. In the tropics, hydrogen isotopes are the preferred inferer of precipitation, or “proxy.” As the amount of sediment in a given year can be highly variable, dates obtained from plant particles and other organic material located throughout the core can then provide us with a context of time. The only catch is finding undisturbed sediment…and that means going places where people have not been before, and places that are not currently connected to the ocean.
The tropics are of increasing interest to climate scientists because the equatorial region has more energy than anywhere else, thanks to the angle of incidence with which solar radiation strikes the upper atmosphere. Radiation at or near the equator on the Earth’s surface gets a direct hit, whereas the curvature of Earth means that radiation hits at an angle elsewhere. Consequently, the equatorial region is the source of the energy redistribution across the globe stretching all the way to the poles, which we commonly know through the trade winds, convection cells and pressure systems: nature’s way of maintaining energy equality. The low pressure zone near the equator, the primary redistribution mechanism above all others, is called the Intertropical Convergence Zone (ITCZ).
The best place to sample from the equator is where people don’t live, and regional influences like mountain chains are not present. Any of these local factors will overwhelm a smaller (but still large in aggregate) global effect. The problem is that the location that fits the above criteria best is the Pacific Ocean, and there are good reasons why there are so few people there; namely, there is no land to contain and isolate a sediment section…almost. Island chains like the East Caroline Islands, of which Kosrae is the easternmost island, are the exception.
For the past 7 years, the Sachs Lab of the University of Washington’s School of Oceanography has been conducting expeditions to some of these remote Pacific islands to get sediment core data in an attempt to reconstruct rainfall over the past several thousand years from those locations: Palau, Pohnepei and Kosrae in the Federated States of Micronesia, the Marshall Islands, Nauru, Wallace & Futuna, Washington and Christmas Island (Kiribati), the Galapagos and Clipperton Atoll.
The idea is to piece together the local climate across those locations, which span a range of latitudes and longitudes, to see if there is a broader pattern. And there is: the ITCZ is moving even without man’s influence.
But how exactly is this information gathered and processed? Taking cores undersea, of glaciers and of oil deposits, is done on a large scale using gargantuan equipment, for a return of the hundreds of thousands of years of data you can get from a single core.
Tropical coring, in contrast, is a rudimentary operation and successfully extracting sediment requires creativity, a crack team and a bit of on-the-fly innovation. The Sachs Labs’ recent expedition to Kosrae in August and September 2011, as well as a previous one in 2009, offered plenty of opportunities for such improvisation. Donning dive booties, long metal core tubes, and a (portable) lawn mower engine we trekked and traipsed around pristine mangrove mud, looking for ideal undisturbed conditions to plunge the cores in. The ground was a jagged obstacle course with inflexible mangrove roots threatening to impale our feet with one wrong step.
That lawnmower engine, new to our latest expedition, made the cores vibrate back and forth on their way down and could thus get through thick mud or small roots that got in the way. To better plug up the top of the core to prevent sediment movement inside during transport – critically important to maintaining the chronological integrity of the core – we filled space at the core top with insulating foam spray typically used for drywall insulation in the attic or basement. While there are seemingly always unforeseen events that make fieldwork difficult, mundane everyday items found at Home Depot helped address our core problem, and get us the data we need to better understand the tropical climate.
This past expedition is the latest in a long line of others, and there will be more.