Data from a 19th 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.” Read the rest of this entry »