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Met Office to revise global warming data upwards

26.11.2010 14:58 Age: 6 yrs
Category: Oceans
By: Leon Clifford

Global warming has been underestimated due to a bias in measurements made by ocean buoys and ships, the UK Meteorological Office said on 25 November 2010.

This could mean that warming may have been as much as 0.03C per decade larger than previously thought, according to Met Office scientist Vicky Pope.

The increase in average global surface temperatures has been around 0.16C per decade between 1970 and 2000 but has ranged between 0.05C and 0.13C in the last ten years.  The trend figures for the period between 2000-2009 are based on data from three different source; one from NASA (0.13C), one from the US National Oceanic and Atmospheric Administration (0.07C) and one from HadCRUT3 (0.05C) - the dataset managed by the Met Office and by the University of East Anglia's Climate Research Unit.

However, the increasing number of measurements of sea surface temperature (SST) from ocean buoys and the decreasing proportion of measurements from ships in the years since 1980 may have had the effect of depressing the recorded increase in temperatures. As a result, Met Office scientists have reviewed the whole sea surface temperature data set between 1850 and 2006 to take account of this bias. A paper has been submitted to the Journal of Geophysical Research which looks in more detail at all the biases in sea-surface temperature measurements from ships and buoys between 1850 and 2006 and reassesses the sea surface temperature records and their uncertainties in the light of this.  And since sea surface temperatures feed into the bigger picture this could lead to a reassessment of the global average surface temperature.

Satellites indicated there was a bias

Satellite data has reported a bigger increase in sea surface temperatures than in situ data from buoys and ships, according to Met Office scientist John Kennedy. “We suspect that there has been this difference for quite a while. And when we make a correction for the data from buoys we find that the trend from in situ data is much closer to the trend observed by the satellites,” explained Kennedy. “This is what makes us think, all other things being equal, that the increase in the number of buoys leads to a cooling bias in the global average sea surface temperature."

In particular, Met Office scientists found that sea surface temperature observations from the Along-Track Scanning Radiometer (ATSR) satellite instrument and observations from the same area made by in situ platforms were different. Kennedy and colleagues from the Met Office have submitted a paper on this issue that has been accepted for publication in the journal Remote Sensing of Environment (RSE) which is called “Using AATSR data to assess the quality of in situ sea-surface temperature observations for climate studies”.

The RSE paper states that “The trend in global-average SST between 1991 and 2007 calculated from in situ data was compared to its counterpart calculated from the ATSR instruments. The in situ record warms more slowly than the ATSR record, probably due to a decrease in the fraction of relatively warm-biased ship observations contributing to the global-average SST over the period and a corresponding increase in the fraction of relatively unbiased drifting buoy observations.”

Warm ships and cold buoys

The problem seems to be related to the way that some ships measure the temperature of the water which leads to the average temperature measured by ships being higher than the average sea surface temperature measured by a thermometer on a buoy. “On average the difference ranges between 0.13C and 0.18C,” Kennedy told Reporting Climate Science .Com. The scale of this difference across the globe and over the years is sufficient to add a warming of 0.03C per decade to the HadCRUT surface temperature record.

But Kennedy cautions that the 0.03C figure is an upper limit on the shortfall in warming due to the buoy-ship switch over during the period between 1998-2009. It assumes that the bias in global average sea surface temperature is as large as the RSE paper suggests and that the sea surface temperature contributes  around 70 per cent of the average global surface temperature.

As the number of buoys has increased, so the proportion of ship measurements as a fraction of all sea surface measurements in the HadCRUT data has fallen. This has meant that over time the weighting given to the warmer ship measurements has fallen with the result that any underlying increase in actual sea surface temperatures has been at least partly offset.

Furthermore, there have been changes in the ships themselves, with more temperatures being recorded directly from sea surface waters rather than from water taken in through coolant intakes or processed in some other way, according to Kennedy. This may also have led to an increase in the amount of more realistic temperature measurements which would have had the effect of reducing any underlying increase in sea surface temperatures.

The proportion of the data from buoys has risen over time. The earliest records in the data go back to 1850 and contain relatively sparse measurements from ships. From around 1979 the number of measurements from buoys increased dramatically and they now account for around 60 per cent of the contribution to sea surface temperature in the HadCRUT data.

In fact, since 2005 buoys have accounted for around 90 per cent of all measurements. They make many hundreds of measurements but they tend to be confined to a smaller area. Ships generally take fewer observations, but because ships travel in a more purposeful way, the observations from each ship tend to cover a much larger area.  This is why buoys account for much less than 90 per cent of the input into the sea surface temperature data.

“The buoys are not the only issue. There are a great many factors. Ships vary from year to year. Early ships used buckets. Some ships today use sensors on the hull. Instruments are not standardised. So we are looking at just one aspect of all these variations,” said Kennedy.

For front page of Reporting Climate Science .Com see here.

For Remote Sensing of Environment see here.