ExoTerra - Oceans

Climate Change Indicators: Ocean Heat

Much attention has been focused on the effects of climate change on forests, farms, freshwater sources and the economy. But what about the ocean?

This indicator describes trends in the amount of heat stored in the world’s oceans.

  • Line graph showing three different estimates of how the amount of heat in the ocean changed from 1955 to 2015.
     
     

    This figure shows changes in ocean heat content between 1955 and 2016. Ocean heat content is measured in joules, a unit of energy, and compared against the 1971–2000 average, which is set at zero for reference. Choosing a different baseline period would not change the shape of the data over time. The lines were independently calculated using different methods by government agencies in three countries: the National Oceanic and Atmospheric Administration (NOAA), Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO), and Japan Meteorological Agency’s Meteorological Research Institute (MRI/JMA). For reference, an increase of 1 unit on this graph (1 x 1022 joules) is equal to approximately 18 times the total amount of energy used by all the people on Earth in a year.

    When sunlight reaches the Earth’s surface, the world’s oceans absorb some of this energy and store it as heat. This heat is initially absorbed at the surface, but some of it eventually spreads to deeper waters. Currents also move this heat around the world. Water has a much higher heat capacity than air, meaning the oceans can absorb larger amounts of heat energy with only a slight increase in temperature.

    The total amount of heat stored by the oceans is called “ocean heat content,” and measurements of water temperature reflect the amount of heat in the water at a particular time and location. Ocean temperature plays an important role in the Earth’s climate system—particularly sea surface temperature (see the Sea Surface Temperature indicator)—because heat from ocean surface waters provides energy for storms and thereby influences weather patterns.

    Increasing greenhouse gas concentrations are trapping more energy from the sun. Because changes in ocean systems occur over centuries, the oceans have not yet warmed as much as the atmosphere, even though they have absorbed more than 90 percent of the Earth’s extra heat since 1955. If not for the large heat-storage capacity provided by the oceans, the atmosphere would warm more rapidly.Increased heat absorption also changes ocean currents because many currents are driven by differences in temperature, which cause differences in density. These currents influence climate patterns and sustain ecosystems that depend on certain temperature ranges.

    Because water expands slightly as it gets warmer, an increase in ocean heat content will also increase the volume of water in the ocean, which is one cause of the observed increases in sea level. 

    This indicator describes global trends in sea surface temperature.

    • Line graph showing changes in average global sea surface temperature from 1880 to 2015.
       
       

      This graph shows how the average surface temperature of the world’s oceans has changed since 1880. This graph uses the 1971 to 2000 average as a baseline for depicting change. Choosing a different baseline period would not change the shape of the data over time. The shaded band shows the range of uncertainty in the data, based on the number of measurements collected and the precision of the methods used.

      Data source: NOAA, 20166
      Web update: August 2016

      Oceans are at the brink

      For decades, the ocean has been absorbing carbon dioxide dumped into the atmosphere by burning fossil fuels. It has also absorbed a lot of the extra heat produced by elevated atmospheric carbon dioxide levels.

      But even the ocean has limits, and we are bumping up against them, with damaging consequences for the whole world. Even with its vast capacity to absorb heat and carbon dioxide, the physical impacts of climate change on the ocean are now very clear and dramatic. According to a 2016 report, temperatures in the shallowest waters rose by more than 0.1 degree Celsius (0.18 degree Fahrenheit) each decade between 1970 and 2010. 

      Here are five ways these warmer temperatures are affecting our oceans:

      1. Coral bleaching

      As early as 1990, coral reef expert Tom Goreau and I pointed out that mass coral bleaching events observed during the 1980’s were probably due to anomalously warm temperatures related to climate change.

      Mass coral bleaching results in the starvation, shrinkage and death of the corals that support the thousands of species that live on coral reefs.

      2. Fish migration

      In addition, many fish species have moved toward the poles in response to ocean warming, disrupting fisheries around the world.

      3. Drowning wetlands

      Rising sea levels, partly the result of heat absorbed by the ocean, is also “drowning” wetlands. Wetlands normally grow vertically fast enough to keep up with sea level rise, but recently the sea has been rising too fast for wetlands to keep their blades above water.

      Coral reefs and sea grass meadows are also in danger of “drowning” since they can only photosynthesize in relatively shallow water.

      4. Ocean acidification

      The ocean has absorbed about 30 percent of the carbon dioxide humans have sent into the atmosphere since the start of the Industrial Revolution – some 150 billion tons.

      However, this great service, which has substantially slowed global warming, has been accomplished at great cost: The trend in ocean acidification is about 30 times greater than natural variation, and average surface ocean pH, the standard measure of acidity, has dropped by 0.1 unit - a highly significant increase in acidity.

      This is damaging many ocean species that use calcium carbonate to form their skeletons and shells. Studies have shown that calcium carbonate formation is disrupted if water becomes too acidic.

      Ocean acidification also appears to be affecting whole ecosystems, such as coral reefs, which depend on the formation of calcium carbonate to build reef structure, which in turn provides homes for reef organisms.

      5. A disastrous positive feedback loop

      Finally, acidification also appears to be reducing the amount of sulfur flowing out of the ocean into the atmosphere. This reduces reflection of solar radiation back into space, resulting in even more warming. This is the kind of positive feedback loop that could result in run-away climate change – and of course, even more disastrous effects on the ocean.

       

      References

      1. For example, see: Ostrander, G.K., K.M. Armstrong, E.T. Knobbe, D. Gerace, and E.P. Scully. 2000. Rapid transition in the structure of a coral reef community: The effects of coral bleaching and physical disturbance. P. Natl. Acad. Sci. USA. 97(10):5297–5302.

      2. Pratchett, M.S., S.K. Wilson, M.L. Berumen, and M.I. McCormick. 2004. Sublethal effects of coral bleaching on an obligate coral feeding butterflyfish. Coral Reefs 23(3):352–356.

      3. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press.www.ipcc.ch/report/ar5/wg1.

      4. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press.www.ipcc.ch/report/ar5/wg1.

      5. Trtanj, J., L. Jantarasami, J. Brunkard, T. Collier, J. Jacobs, E. Lipp, S. McLellan, S. Moore, H. Paerl, J. Ravenscroft, M. Sengco, and J. Thurston. 2016. Chapter 6: Climate impacts on water-related illness. The impacts of climate change on human health in the United States: A scientific assessment. U.S. Global Change Research Program. https://health2016.globalchange.gov.

      6. NOAA (National Oceanic and Atmospheric Administration). 2016. Extended reconstructed sea surface temperature (ERSST.v4). National Centers for Environmental Information. Accessed March 2016. www.ncdc.noaa.gov/data-access/marineocean-data/extended-reconstructed-sea-surface-temperature-ersst.

      7. IPCC (Intergovernmental Panel on Climate Change). 2013. Climate change 2013: The physical science basis. Working Group I contribution to the IPCC Fifth Assessment Report. Cambridge, United Kingdom: Cambridge University Press.www.ipcc.ch/report/ar5/wg1.

      8. NOAA (National Oceanic and Atmospheric Administration). 2016. NOAA Merged Land Ocean Global Surface Temperature Analysis (NOAAGlobalTemp): Global gridded 5° x 5° data. National Centers for Environmental Information. Accessed June 2016.www.ncdc.noaa.gov/data-access/marineocean-data/noaa-global-surface-temperature-noaaglobaltemp.

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