Historical sea voyage sends manmade warming signal

The HMS Challenger sailed 69,000 miles, taking around 360 temperature soundings on the first global marine expedition from 1873-1876. This painting, by William Frederick Mitchell, is from its earlier life as a warship, in 1858.

Data from a 19^th century scientific mission, the first global marine research expedition, have provided strong evidence that humans have influenced climate throughout the entire 20th century. From 1873–1876 the HMS Challenger, a corvette of the British Royal Navy, sailed 69,000 miles and took hundreds of ocean temperature soundings. Last year, scientists used its measurements to show the top 700 metres of the ocean has warmed around 0.33°C since Challenger’s voyage. Understandably, seeing a global effect in the limited Challenger data is fraught with difficulty. Nevertheless, Will Hobbs from the University of Tasmania, Australia, and Joshua Willis at NASA’s Jet Propulsion Laboratory in Pasadena, California, have now checked if this warming is linked to humans. “Even accounting for all uncertainties and limitations, the temperature change could not be realistically explained by natural variability alone, implying a long-term human signal,” Will told me.

Having swapped cannons for labs, among HMS Challenger’s projects was a series of around 360 soundings for temperature. At each sounding, its scientists dropped pressure-protected thermometers into the ocean attached to a rope every 100 fathoms (182 m) down to 1000 fathoms depth. “The scientists kept detailed records of how each measurement was taken, problems encountered, and how accurate and precise their measurements were,” Will said. “All instruments were calibrated in a lab before and after the expedition. So we have a lot of information about what level of accuracy we can expect, in some ways more than from modern automated observing systems, which are usually left to their own devices after deployment.”

Under the sea

To look at ocean warming over the whole 20th century and earlier scientists compared data from the Challenger and the modern Argo float network, which transmits ocean data via satellite. Credit: Scripps Institution of Oceanography

All the measurements were published after the voyage by the Royal Society, and so are therefore freely available today. Last year, scientists at the University of Southampton, UK, and the Scripps Institution of Oceanography at the University of California, San Diego, digitised this data. They compared it against modern temperature data from the worldwide network of robotic Argo floats, finding twice the temperature rise previously measured since the 1950s. That suggests the oceans have been warming at a similar rate since the start of the 20^th century or before.

Will and Joshua wanted to test if this temperature change could be ‘reliably attributed to humans’. In a paper published in Geophysical Research Letters last month, they do this by first calculating all the sources of uncertainty. Thanks in part to the information the Challenger scientists collected, they could select only the most reliable soundings, reducing the number of measurements to 150-250, depending on the depth. But some uncertainty still arises as this isn’t much to base an estimate of all the world’s oceans on. They could also refer to the 19^th century records for instrument precision uncertainties.

The hardest uncertainty to deal with is ‘sounding line bias’, arising because the depth of each temperature measurement the Challenger made was taken from the length of rope used. “If there was a strong current the line would be dragged away from the ship so that the thermometer would be in shallower and therefore warmer water than the rope length implied,” Will said. This means that 19^th century oceans were probably cooler than Challenger’s measurements, and temperatures have therefore risen by more than the Scripps and Southampton scientists estimated. “This underestimate is around 10-17% of the total calculated warming,” Will added.

Signal gets through

University of Tasmania’s Will Hobbs will now use the HMS Challenger data in understanding how sea temperature influences climate predictions. Credit: University of Tasmania.

Finally, for uncertainty arising from natural month-to-month and year-to-year variability of the ocean, Will and Joshua turned to simulations. “We don’t have observations of natural variability in the sub-surface ocean on timescales of longer than a few decades,” Will said. “We rely on models to tell us what the natural variability on timescales of centuries or beyond might be.” The scientists then combined all these other potential explanations for the warming over the 135 year period between the Challenger voyage and the modern Argo data. But the warming was still much higher than natural variability and observational uncertainty combined, leaving human CO2 emissions as the likely culprit.

Will and Joshua also used the Challenger data to work out values for changes that previously only climate models could give estimates for. That includes warming from 1873-1955, which is also highly unlikely to have happened by chance. The researchers also looked at sea level rise, and the balance of heat entering and leaving the atmosphere. They found that two-fifths of sea level rise from 1873 to 1955 came because sea water expands and takes up more space at higher temperatures. The rest likely came from melting ice sheets and glaciers.

“We were able to give the first observation-only estimate of ocean thermal expansion prior to the 1950s with ranges of uncertainty,” Will said. “We think this will be useful for scientists trying to understand current and past changes in sea level.” And the Challenger mission that started 140 years ago will continue to help Will as he now examines how ocean warming affects future predictions of surface climate change. “As a unique long term record the Challenger data will be a very useful baseline for model assessment,” he said.

Temperature changes for the upper 750m of the ocean. On all three maps soundings taken by the HMS Challenger are shown by circles. The circle colours indicate the difference between the Challenger soundings and modern temperatures recorded by Argo floats, red for warmer, blue for colder, according to the key shown on the right of the maps. In the top map, these are compared against average values for warming over the same period from several climate simulations. In the lower two maps they are compared against change since the mid-20th century calculated from measurements, with the exact dates shown above each map. The temperature changes across the whole maps over the periods they cover are also shown by colour according to the scale on the right. Image used with permission from Wiley/Geophysical Research Letters, see reference below.

Journal references:

Roemmich, D., John Gould, W., & Gilson, J. (2012). 135 years of global ocean warming between the Challenger expedition and the Argo Programme Nature Climate Change, 2 (6), 425-428 DOI: 10.1038/nclimate1461
Hobbs, W., & Willis, J. (2013). Detection of an observed 135 year ocean temperature change from limited data Geophysical Research Letters DOI: 10.1002/grl.50370