What was done
The authors used International Comprehensive Ocean-Atmosphere Data Set sea-surface temperature data to calculate, in annual time steps, the mean August-September positions of the 12, 15 and 18°C isotherms in the North Atlantic Ocean from 1854 to 2005 at 2-degree longitudinal intervals.

What was learned
This effort revealed, in the words of Hobson et al., that (1) the three isotherms "have tended to move northwards during two distinct periods: in the 1930s-1940s and then again at the end of the 20th century," that (2) "the chances of this occurring randomly are negligible," that (3) the 15°C isotherm "reached a maximum latitude of 52.0°N in 1932, and a latitude of 51.7°N in 2005, a difference of approximately 33 km," and that (4) "of the 10 most extreme years, 4 have occurred in the 1992-2005 warm era and 3 have occurred in the 1926-1939 era."

What it means
Considering the totality of their findings, the UK and Australian researchers concluded, in the broadest of terms, that "current 'warm era conditions' do not eclipse prior 'warm' conditions during the instrumental record," which sounds an awful lot like what we have concluded, over and over, in our U.S. Temperature Record of the Week feature, i.e., that during the period of most significant greenhouse gas buildup over the past century (1930 and onward) there has been little to no net increase in mean near-surface air temperature throughout the United States. Now the same appears to also have been true throughout a large sector of the North Atlantic Ocean.