Ongoing research from Canada and the UK has shown that the widely used HadCRUT4 global temperature data set has greatly underestimated the rate of warming over the past 15 years.
The HadCRUT4 global temperature anomaly plot since 1850.
Over recent decades, the HadCRUT4 dataset has covered about 84% of the globe, with unsampled regions over the poles "Arctic/Antarctic hole" and Africa, which according to the study can be a potential source of bias in global temperature reconstructions since the unsampled regions are not uniformly distributed over the Earth's surface.
According to the RealClimate.org story, the "Arctic hole" is the main reason for the near-surface temperature differences that we see between the NASA GISS data set and the other two data sets, HadCrut and NOAA. The NASA GISS set fills in these polar gaps by interpolation from the edges, while the other two do not fill them in at all.
If regions that are not covered (data gaps) warm up similar to the rest of the world then there is not much of an issue in regards to the accuracy of the data set. However, the Arctic has warmed much more than other parts of the world over the past 15 years, which presents a problem for the HadCRUT4.
Two alternative methods for global temperature recontruction were explored and were validated on the basis of their skill at reconstructing omitted sets of observations. The two methods showed superior results compared to those that exclude the unsampled regions.
The best of the previous alternative methods showed a two and a half time greater warming trend than the coverage-biased HadCRUT4 since 1997.
The coverage-biased HadCRUT4 causes a cool bias in recent temperatures, which increases from around 1998 to present.
For the 1997-2012 period the HadCRUT4 data set showed a slight warming trend of 0.05 C. per decade, but after filling in the gaps that trend becomes +0.10 C. after a applying a correction of ocean temperatures to the NASA data.
The study abstract makes a key point that has been brought up on many occasions......Trends starting in 1997 or 1998 are particularly biased with respect to the global trend. The issue is exacerbated by the strong El Niño event of 1997-1998, which also tends to suppress trends starting during those years.
The RealClimate.org story finishes with this comment.......
The public debate about the alleged "warming pause" was misguided from the outset, because far too much was read into a cherry-picked short-term trend. Now this debate has become completely baseless, because the trend of the last 15 or 16 years is nothing unusual - even despite the record El Niño year at the beginning of the period. It is still a quarter less than the warming trend since 1980, which is 0.16 °C per decade.
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