An international team of researchers has found that ocean temperature and associated sea level increases between 1961 and 2003 were 50% higher than estimated in the 2007 Intergovernmental Panel on Climate Change report. The team involved researchers from the Centre for Australian Weather and Climate Research (CSIRO), the Antarctic Climate and Ecosystems Cooperative Research Centre and Lawrence Livermore National Laboratory (LLNL).
The results are reported in the 19 June edition of the journal Nature. The team compared climate models with improved observations that show sea levels rose by 1.5 millimeters per year in the period from 1961-2003. That equates to an approximately 2½-inch increase in ocean levels in a 42-year span.
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 rise. 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.
The ocean warming and thermal expansion trends for 1961 to 2003 are about 50% larger than earlier estimates but about 40% smaller for 1993 to 2003, consistent with the recognition that previously estimated rates for the 1990s were biased by instrumental errors.
The research corrected for small but systematic biases recently discovered in the global ocean observing system, and uses statistical techniques that “infill” information in data-sparse regions. The results increase scientists’ confidence in ocean observations and further demonstrate that climate models simulate ocean temperature variability more realistically than previously thought.
This is important for the climate modeling community because it demonstrates that the climate models used for assessing sea-level rise and ocean warming tie in closely with the observed results.—Peter Gleckler, LLNL
Climate model data were analyzed from 13 different modeling groups. All model data were obtained from the WCRP CMIP3 multi-model dataset archived at the LLNL’s Program for Climate Model Diagnosis and Intercomparison (PCMDI).
Although observations and models confirm that recent warming is greatest in the upper ocean, there are widespread observations of warming deeper than 700 meters.
Results were compared with recent estimates of other contributions to sea-level rise including glaciers, ice caps, Greenland and Antarctic ice sheets, and thermal expansion changes in the deep ocean. When these independent lines of evidence are examined collectively, the story is more consistent than found in earlier studies.
The oceans store more than 90% of the heat in the Earth’s climate system and act as a temporary buffer against the effects of climate change. The ocean warming and thermal expansion rates are 50% larger than previous estimates for the upper 700 meters of oceans, and greater than that for the upper 300 meters.
This is just the tip of the iceberg, so to speak. Our ability to quantify structural uncertainties in observationally based estimates is critically important. This study represents important progress.—Peter Gleckler
Catia M. Domingues, John A. Church, Neil J. White, Peter J. Gleckler, Susan E. Wijffels, Paul M. Barker & Jeff R. Dunn (2008) Improved estimates of upper-ocean warming and multi-decadal sea-level rise, Nature, doi:10.1038/nature07080