New research suggests that the correct answer is (a), twice as long as previously believed. Back in 1872-1876 the HMS Challenger sailed 69,000 miles across the Atlantic, Indian and Pacific Oceans, taking temperature measurements at several depths at each of 300 sites. Today’s researchers from the University of California, San Diego, compared Challenger’s temperature readings with data from the Argo project, which uses 3,500 floats to measure the temperature and salinity of the oceans every 10 days. The team found a 1.1 degree Fahrenheit (0.59-degree Celsius) increase in the surface temperature of the oceans over the last 135 years. Comparison of subsurface temperature differences between Challenger and Argo had to be corrected for several errors in the Challenger data. For example, the vessel’s scientists didn’t measure the depth of their thermometers directly, but had to rely on the length of the lines lowering the thermometers into the water. They had no way to ensure that the lines were vertical, so the actual depth was a little less than the length of the line, and because temperatures at shallower depths are usually warmer than temperatures at deeper depths, the Challenger readings were therefore a little warmer than they should have been. After correcting for this error and others, the research team found that global ocean temperatures increased by an average of 0.59 degrees F (0.33 degrees C) in the upper ocean down to about 2,300 feet (700 meters). This temperature change is twice that observed over the past 50 years, which suggests that the oceans have been warming for many decades.