Ocean heat content

Ocean heat content (OHC) refers to the amount of heat stored in the Earth’s oceans. As the sun warms the Earth, a very thin layer of ocean water is warmed. Through convection and dynamic forces of wind and waves, the heat is transported to lower depths. Most of the ocean’s heat is in the top 500 meters. Tropical oceans are heated most. Heat from the tropics is transported toward the poles through ocean currents. OHC is measured in joules and is used by scientists to analyze and predict global warming.

Importance of Ocean Heat Content
The oceans cover 70% of the Earth’s surface. (The northern hemisphere is 61% ocean and the southern hemisphere is 81% ocean.) Oceans are also deep. They are the world’s largest solar energy collector and energy storage system. Scientists believe computing ocean heat content over time may be the best way to monitor anthropogenic climate change.

Because the ocean and atmosphere are a coupled system, heat stored in the ocean can be released in the air and change surface temperatures. The release of ocean heat and its effect on weather and climate is most easily seen by changes in oceanic oscillations such as the El Nino Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and the Atlantic Multidecadal Oscillation (AMO).

The surface temperature record is less valuable as a monitor of climate change because it is two-dimensional, subject to time lags, and subject to measurement uncertainties from urban heat island (UHI) effect and microsite issues. Through recent improvements to the ocean monitoring system, ocean heat content may be the most accurate and reliable metric for analyzing and projecting climate change.

History of ocean observations
The earliest observations of ocean temperature were sea surface temperature readings. Ocean vessels would use wooden buckets to pull up sea water to measure the temperature of water about one meter deep. Later, ships would take measurements of water taken in by engine water intakes as deep as 20 meters deep. Sea surface temperatures are still being taken today by satellites. But these temperature readings do not give enough information deep into the oceans to compute ocean heat content.

Pre-XBT Era (1955-1966)
Data on ocean heat content goes back to about 1955. Observations during this period were sparse and mainly near coastlines in the northern hemisphere.

XBT Era (1967-2003)
Expendable Bathythermograph (XBT) was the dominant instrument during this era and is still used today. XBT can obtain values on the temperature structure of the oceans to a depth of 2000 meters. About 80 voluntary observing ships take about 14,000 observations each year. The XBT is capable of temperature accuracies of ±0.1°C.

Argo Network Era (2004 to present)
The Argo Network began deployment in 2000, reached near global coverage in 2004 and reached its complete size in November 2007 when its 3000th ocean float was launched. Argo grew out of World Ocean Circulation Experiment

The Argo Network measures the ocean temperature, currents and salinity. Argo buoys use Conductivity, Temperature, Depth (CTD) sensors to collect data and send it back in real time. The quality and distribution of the data is much better than the past when it was collected by ocean going ships and mainly restricted to shipping lanes. All data is freely available to everyone.

While the Argo Network is able to take measurements to a depth of 2000 meters, a portion of the ocean depths are not being monitored. The average depth of the oceans is 3,785 meters. It is possible to sample lower depths, but the lower depths are not watched as closely.

Polar Ocean Profiling System
Polar Ocean Profiling System (POPS) uses a network of Argo type buoys specially designed for use in the multiyear ice zone of the Arctic Ocean. One type, called Compact Arctic Drifters, have been collecting data since 2000. The buoys are able to collect data to a depth of 250 meters with plans to increase depth to 1000 meter. Researchers also use under-ice floats in this region.

Solar variation
Solar variations are changes in the level of solar radiation emitted by the Sun. Changes in total solar irradiance (TSI) is often related to the 11 year sun spot cycle. An increase in the number of sunspots increases the brightest of the Sun. But aperiodic changes may also play a role. While solar variation is not a high percentage of TSI (perhaps 0.1%), the climate sensitivity to these changes is still subject to debate.

Related to solar variation are changes to the influx of galactic cosmic rays (GCR). Low level clouds reduce sunlight to the Earth’s surface and so reduce surface temperature and OHC. Galactic cosmic rays have been shown to be essential to the formation of low clouds. However, most scientists believe it is impossible for a reduction in low clouds to have caused the observed 20th century warming of the oceans and land surface temperature.

The Maunder Minimum was probably caused by a combination of solar variation and volcanic eruptions.

Volcanic eruptions
A significant volcanic eruption is known to inject aerosols into the stratosphere scattering solar radiation, decreasing surface air temperatures and lowering ocean heat content.

Greenhouse gases
Mankind’s release of greenhouse gases, like carbon dioxide, into the lower atmosphere are believed to hold back some of the warmth the Earth would release into space causing the planet and oceans to warm. Some scientists believe the Earth is now receiving more energy than it is emitting back to space. During the 1990s, researchers at NOAA began calculating ocean heat content because of its possible importance in signaling the Earth was warming due to greenhouse gases. Substantial evidence to verify the hypothesis is not yet available.

Correcting biases in the OHCA record
Scientists use data about ocean heat content to create a time series of ocean heat content anomalies (OHCA). Certain changes to the observational network introduced a warm bias which artificially increased the ocean’s warming trend in the years 1987-1996. Exactly how much the trend was biased has been a subject of debate among scientists.

In the years from 2003-2005, the OHA originally showed the oceans were cooling. However, the cooling was determined to be the result of a data processing error and not actual cooling. Once the error was corrected, the oceans showed neither significant cooling nor significant warming during those years.