So, what caused the early 20th century warming? A possible explanation is that it is the result of a major increase in the rate of formation of NADW (Wigley and Raper, 1987), an idea that is supported by the pattern of warming which is a maximum in the North Atlantic (Schlesinger and Ramankutty, 1995). We noted earlier that the fact that this warming is similar for the land and the ocean (in fact, the 1910 to 1940 trend over the ocean is greater than over the land) suggests that it is not externally forced (since this would normally lead to warming over the land that was greater than over the ocean), and that it originates in the ocean. This also helps to explain why the land/ocean warming differential that one would expect as a consequence of external forcing has only become evident over the past three decades." [It looks like Wigley wrote this paragraph, and Jones responded "Sounds fine!"]North Atlantic Deep Water - Wikipedia, the free encyclopedia
North Atlantic Deep Water (NADW) is a water mass that forms in the North Atlantic Ocean.