Oceans warming faster than climate models predicted: climate models doing well, but slightly under-predicting ocean warming

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Figure 1 (a) Upper (0–700 m) OHC, calculated using 40 Coupled Model Intercomparison Project, Phase 5 (CMIP5) models (historical run) (gray lines; black line is the ensemble mean). The CMIP5 results are compared with the observation-based estimate using the strategies presented in this study (red line) and National Oceanographic Data Center (NODC) mapping (dashed blue line). Two major volcanic eruptions are marked by the black arrows. (b) Annual global-averaged upper ocean warming rates from the CMIP5 model results (gray lines; red line is the ensemble mean) and from observations (blue line), computed from the first differences of OHC at 700 m (units: °C yr−1). (Figure plotted by IAP)   

To know how much global warming is happening, you  have to measure ocean warming. That is because more than 90% of the excess energy coming to the Earth from greenhouse gases goes into the ocean waters.  But how would you measure the ocean? How would you make consistent, long-term measurements that would allow people to compare ocean heat from decades ago to today? How would you make enough measurements throughout the ocean so that we have a true global picture?

This is one of the most challenging problems in climate science. Many measurements were made along ocean passageways as ships transported goods across the planet.As more ship travel occurred, and more measurements were made, the coverage of temperature measurements across the globe increased. So, over time, the temporal and spatial resolution increased. One particularly important measurement device called the eXpendable BathyThermograph (XBT). This device, originally designed to make crude measurements for navies, has been used for years by climate scientists. There is systematic bias in XBT data, which creates spurious “ocean warm decades” from 1970s to early 1980s as reported in the IPCC Fourth Assessment Report.

Past errors in the ocean heat content (OHC) record could be reduced by correcting systematic measurement biases, filling in gaps where no information is available, and by choosing a proper comparison climate. Lead author, Dr. Lijing Cheng (who works for the International Center for Climate and Environment Sciences in China) of a paper found here applied four separate improvements to data, focusing attention on the heating in the upper 700 meters of ocean waters because that depth has the best measurements and it also is the region where much of the global warming heat goes.

Going back to 1970, the upper 700-meter water layer temperature has increased approximately 0.3°C (approximately 0.55°F). While that may not sound like a lot,  this is a huge amount of water and consequently requires an enormous amount of energy. The world’s oceans were separated into the Atlantic, Pacific, and Indian. All three of these oceans are warming with the Atlantic warming the most. Ocean heating was calculated using 40 state-of-the-art climate models. Over the period from 1970, the climate models have under-predicted the warming by 15%. A remarkably close match that gives us a lot of confidence in the models. On the other hand, the models were not able to predict shorter-term fluctuations in ocean heating contained within the observed time period. Dr. Cheng summarized the work by saying,

Ocean heat content change is an important metric for the ongoing global warming. But ocean subsurface temperature data are very sparse in time and space, and they contain systematic biases for some instruments. Those problems make previous OHC estimates with large uncertainties, which have been referred as ‘pre-mature’ data. That’s why our group in IAP/ICCES keeps working on investigating and quantifying the error sources and providing our estimate on historical OHC change. In this study, we summarized our previous research and provided a new OHC estimate. We will make more efforts to provide more and more accurate OHC estimates in the future.

Further Information

New Study Finds Quicker Upper Ocean Warming than Previous Thought

http://english.cas.cn/newsroom/research_news/201506/t20150617_148976.shtml

Oceans warming faster than climate models predicted

http://www.theguardian.com/environment/climate-consensus-97-per-cent/2015/jul/20/oceans-warming-faster-than-climate-models-predicted

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