Oeps ...

Global temperatures did not dip sharply in the 1940s as the conventional graph shows, scientists believe. They say an abrupt dip of 0.3° C in 1945 actually reflects a change in how temperatures were measured at sea.

Until 1945, most readings were taken by US ships; but after the war, UK vessels resumed measurements, and they took the sea's temperature differently.

In the 1940s, there was no universally accepted way of measuring sea temperature.

-- British vessels typically lowered a bucket over the side of the ship, pulled it up full of seawater, and put a thermometer in.

-- US vessels, on the other hand, usually had thermometers in place near the pipe where water was pumped in to cool the engines.

The bucket method is known to produce readings slightly lower than the actual temperature, because a fraction of the water will evaporate as it is hoisted on board.
But engine intake measurements can produce higher temperatures than are actually there.

See : http://news.bbc.co.uk/2/hi/science/nature/7423527.stm.

Statistics ! Garbage in, garbage out !