The foundation of knowledge generated by science is constantly evolving—not just in terms of new discoveries such as finding a new species of plant or animal, but by refining the way scientists analyze and understand the planet and its resources.
Scripps Institution of Oceanography at UC San Diego biological oceanographer David Checkley and co-author Martin Lindegren reviewed a critical data set based in Scripps’s proverbial back yard: Measurements of seawater temperature taken daily since 1916 at the Ellen Browning Scripps Memorial Pier on the Scripps campus in La Jolla, Calif. In comparing temperature trends between the Scripps Pier and elsewhere, the researchers uncovered a key difference and a possible mechanism behind it.
The researchers attributed a greater increase in sea-surface temperature recorded at the pier than the global average increase over this period in part to the replacement of the institution’s original pier in 1988. That replacement contributed to an increase—what scientists call an artifact—of temperature taken thereafter. Adjusting the Scripps Pier sea-surface temperature record to account for this increase “reduces the long-term trend from +1.1°C/century to +0.6°C/century, consistent with the global rate of change of sea-surface temperature over the past century,” wrote Checkley and Lindegren in the paper recently published in the Journal of Physical Oceanography.
The backstory began two decades ago, when something Checkley noticed during his brisk lunchtime exercise swims around Scripps Pier stoked his curiosity. At various points of the swim, Checkley would traverse peculiar pockets of warm water.
What was that all about? Knowing the pier’s temperature records were used in sardine management, an area of his research, Checkley was intrigued.
Eventually he struck upon the hypothesis that temperature may have been affected when the wooden Scripps Pier built in 1916 was replaced by a modern concrete pier. Checkley checked with colleagues who study the coastal environment and related temperature trends but none were familiar with this idea.
When biologist Lindegren arrived from Sweden to join Checkley’s team as a Scripps Institution of Oceanography postdoctoral scholar, the time was right to test the hypothesis. Checkley describes Lindegren as a “math whiz,” highly skilled at developing methods and models that tease out information from large data sets (as one example, see this story about factors that control sardine and anchovy populations).
After putting various factors to the test, Checkley and Lindegren did indeed find a change beginning at the time of the pier replacement. As they describe in the paper, the change may be attributed to an increase in rip currents that transfer warm water found in the surf zone toward the open ocean and the seawater sampling site at the end of the pier. Waves and swell weakened by pier pilings, and scouring of sand from around those pilings, encourage development of rip currents under the pier.
Checkley also developed a new instrument to sample the sea-surface temperature irregularities at the end of the pier, using it over several days to document temperature fluctuations due to rip currents and daily heating and cooling.
“We hypothesize that rip currents increased under the pier after the wooden structure was replaced with the new concrete pier we have today,” said Checkley, who added that the larger structural presence of the concrete pilings that support the new pier may enhance rip current strength.
The researchers noted that the altered temperature record has influenced management of California’s sardine fishery as sea-surface temperature is a variable used in catch quota formulas. For instance, the researchers estimated that in 2001 the quota might have been reduced by two-thirds and the value of the sardine catch that year might have been reduced by $6 million had the artifact been considered.
Checkley said the finding enhances researchers’ understanding of the Scripps Pier sampling series. Like the California Cooperative Oceanic Fisheries Investigations program, which has produced valuable observations of the marine environment off the state since 1949, and the Keeling Curve, which has monitored carbon dioxide levels since 1958 and is regarded as a cornerstone of modern climate change research, the Scripps Pier sea-surface temperature readings span a nearly century-long breadth of temperature trends and fluctuations, offering a wealth of valuable information to scientists and resource managers for areas such as the management of fisheries off California’s coast.
Support for the study came from Scripps Institution of Oceanography and the National Science Foundation through a postdoctoral fellowship to Martin Lindegren. The California Department of Parks and Recreation, Division of Boating and Waterways, supports the collection, processing, archiving, and dissemination of the daily temperature and salinity measurements taken at Scripps Pier and at nine other stations along the California coast through the Scripps Shore Stations Program.