Fresh Science

During the past four decades, the oceans have been soaking up heat, expanding and rising at a rate about 50 per cent faster than previously estimated by the IPCC, a team of Australian and US oceanographers has found.

The team’s research published in Nature today, corrects errors in ocean temperature data that had led to conflict between observed and simulated changes. The effect of major volcanic eruptions on ocean temperature can even be clearly seen in the data.

The results will give policy makers more confidence in the models and in predictions of future sea level rise due to ocean warming.

The Nature report contributes to a body of work supporting the climate models used for the IPCC projections. The Nature report and a suite of international observations indicated that the rate of sea level rise is tracking near the upper limit of IPCC projections. If that continued global mean sea level would rise by about 800 mm by 2100.

“The modellers and the observers had been debating for years. Modellers felt that the trouble was in the observations; and observers thought the models were wrong,” says Catia Domingues, from the CSIRO Wealth from Oceans Flagship, the lead author on the paper and a CSIRO post-doctoral Fellow with the Centre for Australian Weather and Climate Research.

“We have been able to account for biases in about 70% of the ocean temperature data and that, combined with more sophisticated analysis techniques, gives us far more reliable and accurate rates of ocean warming,” says Catia.

“Our findings reveal that a reported significant 15-year change in global ocean temperature between 1970 and 1985, which climate models could not simulate, was not natural, but rather due to errors in the observations”.

The errors in the ocean temperature data also concealed how fast sea level has been rising due to ocean warming and subsequent thermal expansion (think of the ocean as like the mercury in a thermometer).

“Uncovering a significantly larger thermal expansion contribution for the top 700 m of the oceans helps to explain, for the first time, the rate at which global sea level has risen from 1961 to 2003. “And this outcome sheds light on one of the key uncertainties of the 2007 IPCC AR4 report”.

The detective work of CSIRO’s Dr Susan Wijffels and NASA’s Dr Josh Willis and others was also critical to arriving at the more accurate estimates of ocean warming described in Nature.

“Their team scrutinized millions of ocean observations, tracked down small but systematic errors, and provided us with corrections,” says Catia.

“More accurate ocean observations and increased confidence in the models they underpin will help us to be better prepared for changes in our living conditions, no matter where we reside, near or far from the oceans.”

Catia Domingues is one of 16 early-career scientists chosen for Fresh Science, a national program sponsored by the Federal and Victorian governments. She is presenting her research to the public for the first time at the Melbourne Museum.

Background information

Catia Domingues is the lead author of the paper. She is from the CSIRO Wealth from Oceans Flagship, and an Office of the Chief Executive CSIRO post-doctoral Fellow with the Centre for Australian Weather and Climate Research

The science team included researchers from the Centre for Australian Weather and Climate Research, the Antarctic Climate and Ecosystem Cooperative Research Centre and the Lawrence Livermore National Laboratory, California.  Co-authors were John Church, Neil White, Peter Gleckler; Susan Wijffels, Paul Barker and Jeff Dunn.

Abstract

Improved estimates of upper-ocean warming and multi-decadal sea-level rise
Catia M. Domingues, John A. Church, Neil J. White, Peter J. Gleckler, Susan E. Wijffels, Paul M. Barker & Jeff R. Dunn
Nature 453, 1090-1093 (19 June 2008)
Accepted 3 May 2008
http://www.nature.com/nature/journal/v453/n7198/full/nature07080.html | Changes in the climate system’s energy budget are predominantly revealed in ocean temperatures and the associated thermal expansion contribution to sea-level rise. Climate models, however, do not reproduce the large decadal variability in globally averaged ocean heat content inferred from the sparse observational database, even when volcanic and other variable climate forcings are included. The sum of the observed contributions has also not adequately explained the overall multi-decadal rise2. Here we report improved estimates of near-global ocean heat content and thermal expansion for the upper 300 m and 700 m of the ocean for 1950-2003, using statistical techniques that allow for sparse data coverage and applying recent corrections to reduce systematic biases in the most common ocean temperature observations. Our ocean warming and thermal expansion trends for 1961-2003 are about 50 per cent larger than earlier estimates but about 40 per cent smaller for 1993-2003, which is consistent with the recognition that previously estimated rates for the 1990s had a positive bias as a result of instrumental errors.. On average, the decadal variability of the climate models with volcanic forcing now agrees approximately with the observations, but the modelled multi-decadal trends are smaller than observed. We add our observational estimate of upper-ocean thermal expansion to other contributions to sea-level rise and find that the sum of contributions from 1961 to 2003 is about 1.5 0.4 mm yr-) of 1.6 0.2 mm yr-1. doi:10.1038/nature07080; Received 27 December 2007;

RAPID ROUNDUP: Warming and rising of oceans (Nature) – experts react.

Professor Gary Meyers is Director of the Integrated Marine Observing System at the University of Tasmania.

“This paper is a landmark in our understanding of change in the global oceans over the last several decades. Using all of the historical data and different data-types and correcting their inherent problems was a huge statistical challenge. The authors found a solution that is both pragmatic and elegant.”

Professor Nathan Bindoff is a physical oceanographer and Director of the Tasmanian Partnership for Advanced Computing (TPAC). Partners include the University of Tasmania, CSIRO Marine & Atmospheric Research and the Antarctic Climate and Ecosystems CRC. He was a Coordinating Lead Author of the IPCC AR4 Working Group 1 chapter on oceanic climate change and sea level observations (Chapter 5). 

“This paper resolves a long standing problem in fully understanding all of the contributions to sea-level rise.  The contributions to sea-level rise just simply didn’t add up from the 1950 to the 1990’s. This has been a problem in the IPCC intergovernmental reports in 2001 and 2007. It could have been either a measurement problem of the contributions from the ice sheets, glaciers, or from the oceans, or the sea-level measurements themselves. This paper provides an explanation for this problem, and uncovers the fact the oceans have been absorbing more heat than we had previously understood.  The absorption of heat by the oceans is still the biggest contributor to sea-level, and what is shown so beautifully in this paper is that we now have excellent agreement between the effect of natural forcing on sea-level by volcanic eruptions such as Pinatubo and changes in the ocean contribution to sea-level.  I find it remarkable, that the observations can now resolve the effects of volcanic eruptions and just increases our confidence in using these corrected data for understanding the climate change and its acceleration in the oceans.  For example, this work feeds into issues around the rate of sea-level rise by 2100 and whether there will be decreases in the oceans capacity to absorb carbon dioxide.”