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Because water has a vastly larger capacity for heat storage than the atmosphere, eventually between 80 to 90 percent of the heat trapped by the greenhouse effects ends up being stored in the oceans. Thus, the trend in ocean temperatures is an excellent indicator of long-term trends in global GHG effects.
Using data from the upper 700 meters of the oceans, researchers found that between 1993 and 2008 (the time period that has the most data available) the oceans on average warmed 0.18 degree C. Because the oceans are so large and its heat-storing capacity is so substantial, that seemly modest increase represents a lot of heat. One researcher analogized it to the heat released by 2 billion Hiroshima-size atomic bombs.
The amount of heat in the oceans varies from year-to-year. From 1993 to 1998, the warming trend was slow; the trend rose sharply from 1998 to 2003. From 2003 to 2008 the water temperatures held fairly steady. Ocean temperatures are measured by a variety of instruments. Starting in 2003 there was a large-scale deployment of robotic temperature probes known as Argo [see http://www.argo.ucsd.edu/]. The steady period could thus reflect the nature of the Argo instrument probes. However, the researchers are inclined to believe that it more likely reflected a temporary lull in heat storage due to natural processes, such as ocean circulation patterns. The steady state may reflect the partial diffusion from surface layers and partially due to currents carrying warm waters to levels below those measured. The earlier steady state (1993 to 1998), in contrast, may be a reflection of the cooling brought about by large volcanic eruptions, whose cooling effects often last for many years. [See, for example, http://vulcan.wr.usgs.gov/Glossary/VolcWeather/description_volcanoes_and_weather.html.] In contrast, since 2003, satellite data shows Earth's heat absorption to be increasing.
It is because these "grand views" of data trends may conflict one with the other that I find more interesting the micro-pictures presented by studies of trends in animal and biota data (e.g., increases in tree destruction by beetles, changes in migratory timing and patterns). When one puts these together, there would appear to be little doubt that there is a long-term trend upward. The rate may be open to debate, but the direction is not.
Reports on ocean temperature trends can be found at http://www.nature.com/nature/journal/v465/n7296/full/nature09043.html, http://www.nature.com/nature/journal/v465/n7296/full/465304a.html, and http://www.cgd.ucar.edu/cas/Trenberth/trenberth.papers/NatureNV10.pdf.
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