May 22, 2008

A Sea Surface Story

Filed under: Temperature History

Sometimes we wonder if authors of papers are not outright campaigning for coverage in World Climate Report. Chose a title like “Ocean surface warming: The North Atlantic remains within the envelope of previous recorded conditions” and you will be guaranteed coverage by our skeptical scientists!

The latest article, with the title above, appeared recently in Deep-Sea Research which if you don’t know is published by Elsevier (one of the largest publishers of academic journals in the world). The research of interest here was conducted by a team of oceanographers from the United Kingdom’s Swansea University, and Australia’s University of Darwin, the University of Queensland, and CSIRO. Victoria Hobson and her three associates begin the article noting “There is increasing evidence that warming global temperatures will have profound effects on the Earth’s ecosystems, with the global mean air surface temperature rising by around 0.6°C during the 20th century, with 11 of the last 12 years (1995–2006) ranking amongst the 12 warmest years in the instrumental record of global surface temperature since 1850.” They could win an award from Al Gore with that mouthful! Turning attention to the ocean, they write “Oceans may play a crucial role in regulating the climate, and since the 1950s the heat content of the world’s oceans has increased by ~2 x 10^23 J, equivalent to a mean volume warming of 0.06°C. While this increase is an order of magnitude less than that observed for terrestrial systems it may be even more important as water heats at a much slower rate than air because of its heat capacity.”

They continued in their introduction stating “The North Atlantic has undergone a net warming since 1976, and this recent warming parallels similar warming from 1910 to 1945. Between these two recent warming eras the North Atlantic cooled by 1°C. Whether these changes form part of a natural cycle or part of a long-term trend of progressive ocean warming is hotly debated.” OK – this team seems to be getting off the reservation by suggesting that an important climate change issue is “hotly debated.” The global warming alarmists insist that the “debate is over,” but yet in another article in the scientific literature, we read that the debate is alive and well – you decide.

Hobson et al. used a sea surface temperature dataset that has been widely used in the climate change community, and nicely described some of the uncertainties in the measurements and corrections used to address the shortcomings. The note that the SST data consist of monthly means calculated from in-situ measurements aboard ships, drifters and moored buoys, and since 1981, satellite data. There are both spatial and temporal biases in the data; the opening of the Panama and Suez Canals affected ship passages, and the World Wars reduced shipping traffic and restricted areas travelled. Thus, the dataset is more complete, with the least uncertainty in the post-World War II era.” Furthermore, “Inconsistencies arise, for example, from the choice of bucket used to collect surface water, or the method of sample collection (bucket versus engine intake), the bias in SST due to different observational practices may amount to 0.3°C on average.
Corrections have been applied to the data to account for the changing construction and use of buckets used to collect the water samples on ships.” The dataset is not perfect, but it is arguably the best dataset of its type.

Rather than simply analyze the raw readings, they introduced a novel approach that would be based on the position of isotherms (lines of equal temperature). The state “The mean summer position of the 12, 15 and 18°C isotherms in the North Atlantic for August and September from 1854 to 2005 were determined at 2° longitudinal intervals. These three isotherms were chosen as indicators of the summer SST behaviour across the North Atlantic as they lie in the region of most marked seasonal warming and cooling, and so are sensitive indicators of inter-annual thermal variability of the North Atlantic.”

The figure below (Figure 1) shows the mean latitudinal position of the isotherms over varying time intervals based on data availability thresholds. Ocean warming would cause a northward (upward on the plot) trend, and the global warming crusade would quickly point to the upward trends in the past decade. But when we examine the entire record, no overall northward trend appears whatsoever. Hobson et al. used multivariate statistical procedures to develop an “Index of Northerly Extent” that captures temporal and spatial variance in all three isotherms.


Figure 1. The mean latitudinal position north for the 12°C (black, from 1911 to 2005), 15°C (blue, from 1856 to 2005) and 18°C (red, from 1876 to 2005) isotherms. The length of timescale varies because of variations in data availability.

Guess what year had the most northerly extent of the isotherms – 1932!!! The second highest ranking year was …. 1898! The remaining years ranked third to tenth were 2005, 1952, 1998, 2001, 1939, 1899, 1936, and 2003. These results, along with others, led the team to conclude “We have shown that the current ‘warm era conditions’ do not eclipse prior ‘warm’ conditions during the instrumental record.” Furthermore, they write “In short, our analyses suggest in recent years the position of summer isotherms in the North Atlantic has not moved markedly beyond the window of previous values.”

We couldn’t say it any better – once again, the real world is not cooperating with expectations from predictions from numerical climate models for a world of never-seen-before environmental conditions resulting from ever-increasing atmospheric greenhouse gas concentrations.

Reference:

Hobson, V.J., C.R. McMahon, A. Richardson, G.C. Hays. 2008. Ocean surface warming: The North Atlantic remains within the envelope of previous recorded conditions. Deep-Sea Research, 55, 155–162.




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