Measuring Ocean Heat With a Thermometer
There is no free lunch, and the bill for a century of cheap energy is coming due for payment. The situation on planet earth is heating up. We feel it “coming in the air tonight” and see it when California, Australia, the Amazon, and Siberia burn. Where it is difficult seeing and feeling the heat is in the oceans. But this is precisely where the real action takes place. More than 90 percent of the extra heat absorbed by the earth over the past 60 years ended up in the oceans. About 180 sextillion joules of heat was added to the world’s oceans over this period. The 180 represents a net gain. This amount of energy is equivalent to the energy released by three billion Hiroshima sized atomic bombs, or about three bombs exploding every 2 seconds for 60 years — think about it!
The primary source of heat for warming our oceans is the sun. Remember, the oceans cover about 71 percent of the planet’s surface, so most sunlight striking the earth falls directly on ocean water. Dark ocean water is a poor reflector of sunlight but an excellent absorber of solar energy. The oceans constantly take in heat from this solar radiation. They also act as a buffering system for the atmosphere by absorbing excess heat trapped by increasing levels of greenhouse gases.
But the heating process was not evenly spread over the past 60 years. About 20% of the heating occurred in the first thirty years, and the remaining 80% occurred in the last 30 years. The heating rate is four times faster now than in the first 30 years of the cycle.
The oceans are vast. The energy equivalent of three billion atomic bombs raised the oceans’ average surface temperature by about one degree Fahrenheit. However, this seemingly small temperature rise should not be interpreted as a dismissal of the climate implications. Instead, it is an affirmation of the astonishing amount of heat energy contained in that 1 °F change.
Find me a thermometer, please
These interesting statistics on ocean heat are fascinating to read about but are we seeing the whole story. The data used for determining ocean heat absorption comes from NOAA and only represent measurements in the top 2,300 feet of the ocean. Our oceans’ average depth is about 12,100 feet, so NOAA’s measurements only sample an ocean’s upper layer. What’s happening in the deep oceans?
Measuring ocean surface temperatures is relatively easy. NOAA describes it well; “Satellite instruments measure sea surface temperature — often abbreviated as SST — by checking how much energy comes off the ocean at different wavelengths. Computer programs merge sea surface temperatures from ships and buoys with the satellite data, and incorporate information from maps of sea ice. To produce the daily maps, programs invoke mathematical filters to combine and smooth data from all three sources.”
Well, there you go, satellites, ships, and buoys provide the data. Actual ship recordings of temperature are good calibration points in the first ten meters below the surface, but more is needed for deeper measurements. Measuring conditions deeper below the surface is challenging, and surface measurements dominate historical ocean temperature records. But for the past two decades, automated profiling floats (named ARGO) have monitored the world’s oceans, providing data on temperature and salinity from the surface down to 6,500 feet. Before ARGO stations, measurements from the deep were sparser and more sporadic.
ARGO is an international effort to maintain an array of 3,000 buoys collecting data across the world’s oceans. Once a buoy deploys, it sinks to a prescribed depth and starts collecting data on temperature and salinity. After about ten days, it returns to the surface, shoots its data to a satellite, and then slips back into the cold and dark for another round of data-gathering. This process is “way-cool” for science nerds like me.
The data seem solid, particularly for the first 2,300 feet used in NOAA’s heat calculations. The system also appears to regularly collect data down to about 6,000 feet. Still, the abyssal depths of the ocean elude us.
Deep Ocean Environment
If we travel beneath the ocean’s surface, the first 600 feet comprise the photic zone, below which most sunlight can’t reach. The next 2,500 feet exist in a state of perpetual twilight, which fades to utter darkness. Temperatures also rapidly drop across this zone, known as the thermocline. When we move even deeper and pass the 12,000-foot mark, we enter the abyss, and temperatures hover at about 37 degrees Fahrenheit (3 degrees Celsius). If temperatures in the Arctic soar to over 100 degrees, the abyss stays near freezing. Cold blasts of sub-zero Arctic air may sweep across Canada and the central USA, however, temperatures in the abyss don’t budge. But slowly, over time, heat from climate change does reach the deep.
Change on the deep ocean floor is almost imperceptible. Drop an anchor from a ship, and when it hits the seafloor, it creates a small impact crater. If you return in 50 years, the crater will still look the same. Time passes slowly in these stable and slowly-changing deepwater environments. But now, researchers at Caltech believe they have uncovered a way to determine temperature changes in the deep, largely unsampled waters below 6,500 feet.
Earthquakes help reveal deep ocean temperatures
As odd as it sounds, earthquakes are at the heart of new research on ocean temperatures. A team, led by Wenbo Wu, recently investigated the use of seismic waves from earthquakes to monitor temperatures in the deep oceans.
The technology uses seismic monitoring facilities already in place and can also incorporate historic data for identifying temperature changes with time. At the heart of this new technique is a simple physical principle: The speed of seismic waves traveling in water is temperature dependent — seismic waves travel faster in warmer water.
The epicenter of an earthquake is its surface location, but the hypocenter is the actual location — the depth below the epicenter. When an undersea earthquake occurs, seismic waves radiate outward in all directions from the hypocenter. Most of these waves travel through the earth’s crust, but some propagate through the ocean water, where they travel long distances without significantly weakening.
Thousands of miles from the actual earthquake, seismic recording stations receive these waves. By knowing when the quake occurred and when the seismic waves arrive, researchers determine the wave’s velocity and calculate the water’s average temperature along the travel path.
Because geologically unstable areas repeatedly move and create small earthquakes, the researchers have historical records of seismic waves traveling along the same path. These historic recordings provide a view of how deep ocean water temperatures change with time. This historic framework shows deep ocean warming rates, allowing heat absorption calculations for the earth’s deep oceans.
The presence of multiple seismic monitoring stations worldwide lets researchers integrate all the information into a three-dimensional view of ocean temperatures and how they change with time. Between thermometers, ocean buoys, and seismic waves, there appears to be a better way to understand how climate change affects our oceans.
Data Snapshot Details: Sea Surface Temperature (SST) (Source: NOAA)
The Real-Time Data Management System for Argo Profiling Float Observations (By Claudia Schmid, Robert L. Molinari, Reyna Sabrina, Yeun-Ho Daneshzaden, Xiangdong Xia, Elizabeth Forteza, and Huiqin Yang; Journal of Atmospheric and Oceanic Technology)
Caltech’s Seismic Innovation Uses Undersea Earthquakes to Shake Up Climate Science (Source: California Institute of Technology)
Seismic ocean thermometry (By Wenbo Wu, Zhongwen Zhan, Shirui Peng, Sidao Ni, Jörn Callies; Science)
Climate change: Ocean Heat Content (By LuAnn Dahlman and Rebecca Lindsey — NOAA)
Ocean warming, explained (By Alajandra Borunda — National Geographic)
In 2019, Oceans Were Hotter Than at Any Other Point in Recorded History (By Rosie McCall — Newsweek)
Climate Change Indicators: Sea Surface Temperature (Source: EPA)