>In advance of the UN negotiations next week in Cancun, the press and blogs today have included numerous elaborations on the UK Met Office press release Scientific evidence is Met Office focus at Cancun. The Australian article “Global temperature rises may be underestimated due to errors, Met Office study says” by Ben Webster includes the following statement, “The long-term rate of global warming was about 0.16C a decade in the 1970s, 1980s and 1990s but it slowed in the past 10 years to between 0.05C and 0.13C, depending on which of three major temperature records is used. The Met Office said that changes in the way ocean temperatures were measured had resulted in an under-estimate of about 0.03C in recent years.”
But what the Met Office fails to mention is that the dataset being discussed in the press release, their HADSST2 data, which is the sea surface temperature dataset used in their HADCRUT3 and HADCRUT3v global temperature products, is biased upwards by almost 0.12 deg C after 1998 due to a change in source data in 1998. I’ve illustrated and discussed this bias in two previous posts: Met Office Prediction: “Climate could warm to record levels in 2010” and The Step Change in HADSST Data After the 1997/98 El Nino.
The new source Sea Surface Temperature data was not fully consistent with the source dataset the Hadley Centre used prior to 1998. So when they merged the two datasets, the Hadley Centre failed to account for the inconsistency and created an upward bias in their HADSST2 data. This bias is easily seen when the other Hadley Centre sea surface temperature dataset, HADISST, is subtracted from the HADSST2 data, Figure 1. Note that the HADISST has relied primarily on satellite-based measurements since 1982, but the HADSST2 data is based on buoy and ship readings. The upward step is approximately 0.12 deg C. The bias created by the change in measurement methods over the past decade that was reported on in The Australian would only offset a portion of that shift.
Hopefully, when the Hadley Centre finally releases its updated Sea Surface Temperature dataset (HADSST3) they will eliminate the upward step. And for those interested, here’s a link to a Met Office Scientific Advisory Committee (MOSAC) publication, “Climate monitoring and attribution,” that provides an overview of the upcoming HADSST3 and HADISST2 datasets. Refer to page 3 under the heading of “3. Progress in development of marine datasets.”
The HADSST2 and HADISST data used in this post are available through the KNMI Climate Explorer: