Studies conducted during an ocean-going research expedition 135 years ago have helped provide the latest evidence of climate warming and sea level rise.
The HMS Challenger embarked from Portsmouth, England on Dec. 21, 1872 on a journey that would take it across the Atlantic from east to west and then west to east, down the coast of northwest Africa, across to northern South America and then eastward, south of Africa and Australia, up into the Pacific, before turning south again, rounding Cape Horn and returning to England on May 24, 1876.
The expedition, undertaken to explore the deep sea and learn more about the ocean environment, is in many ways considered a founding voyage of oceanography; in all, it covered almost 69,000 miles and conducted 492 deep sea soundings, 133 bottom dredges, 151 open water trawls and 263 serial water temperature readings.
A new study by researchers from the University of Tasmania and NASA’s Jet Propulsion Laboratory has compared the ship’s measurements of ocean temperatures with modern observations from the international Argo array of ocean profiling floats in an attempt to glean information on ocean changes since the Challenger’s voyage. This is of particular significance given that 90 percent of climate warming goes into heating, not land or the atmosphere, but the ocean, including the deep sea.
Comparing the data sets was not, however, a simple apples-to-apples comparison.
Although comprehensive for its time, the Challenger expedition did not cover the whole ocean. For example, few temperature readings were taken at latitudes higher than 45 degrees north or south, where significant warming has occurred. And lowering thermometers deep into the water on ropes made from Italian hemp is not quite the same as collecting data from battery-powered autonomous drifting floats.
However, writing in the journal Geophysical Research Letters, the study’s authors, William Hobbs of the University of Tasmania and Joshua Hobbs of JPL, say that even accounting for such uncertainties, as well as for natural climatic variability, the difference between the Challenger data and contemporary temperature readings is greater than the estimated increase in sea temperature since the mid-20th century, when ongoing measurements began.
That suggests, they say, that ocean warming has been going on longer, and has been greater, than previously suspected.
“Our research revealed warming of the planet can be clearly detected since 1873 and that our oceans continue to absorb the great majority of this heat,” said Hobbs.
They then used this temperature increase to calculate that approximately 40 percent of the total sea level rise seen in tide gauges from 1873 to 1955 was caused by thermal expansion of the ocean. The remaining 60 percent was likely to have come from the melting of ice sheets and glaciers.
Drawing of the HMS Challenger survey vessel preparing to measure ocean temperatures by lowering thermometers deep into the ocean on ropes in 1872. Credit: NOAA