These measurements come from a variety of instruments deployed from ships and airplanes and, more recently, underwater robots. Organizations around the world have calculated changes in ocean heat content based on measurements of ocean temperatures at different depths. The indicator measures ocean heat content in joules, which are units of energy. This indicator also shows changes representative of the top 700 meters (nearly 2,300 feet) of the world’s oceans, where much of the observed warming has taken place. Measurement data are available for the top 2,000 meters (nearly 6,600 feet) of the ocean, which accounts for nearly half of the total volume of water in the world’s oceans. This indicator shows trends in global ocean heat content from 1955 to 2020. For all these reasons, ocean heat content is one of the most important indicators tracking the causes and responses of a changing climate. These currents influence climate patterns and sustain ecosystems that depend on certain temperature ranges.īecause 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 of the major causes of the observed increases in sea level (see the Sea Level indicator). 4 Increased heat absorption also changes ocean currents because many currents are driven by differences in temperature, which cause differences in density. ![]() 3 If not for the large heat-storage capacity provided by the oceans, the atmosphere would warm more rapidly. 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, 1, 2 and even as the rate of ocean heat uptake has doubled since 1993. Increasing greenhouse gas concentrations are trapping more energy from the sun. 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. 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. 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. Currents also move this heat around the world. ![]() This heat is initially absorbed at the surface, but some of it eventually spreads to deeper waters. When sunlight reaches the Earth’s surface, the world’s oceans absorb some of this energy and store it as heat.
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