This is largely due to the corrections in sea surface temperature measurements. (pacificclimate.org)
The figure shows that the impact of the adjustment to remove the cold bias from bucket sea surface temperature measurements warms the historical data, decreasing the amount of global warming the data indicate. (ncdc.noaa.gov)
There is definitely more to learn about how climate behaves and there are now data sets for ocean warming and carbon dioxide distribution that could benefit from better surface temperature measurements. (realclimate.org)