Climate Change Daily Earth Science Hydrosphere Repost

Environmental change creeps into our deep oceans

Temperature and salinity shift beyond natural variations

Far below the waves, a dark and quiet world provides one of the most stable environments on our planet. When change does arrive, it comes slowly. It’s a cold world where temperatures remain constant and crushing pressures are the norm—and now, an environment where scientists look for subtle signs of global climate change. Because natural variations in deep-sea environmental conditions are minimal, researchers can filter them out to understand the far-reaching effects of anthropogenic (human-induced) change. The deep sea is an active laboratory for monitoring the planet earth.

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, but temperatures in the abyss don’t budge.

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 unchanging deepwater environments. Thus, making “the deep” an ideal place to monitor subtle changes resulting from our human endeavors far above, on the surface of the planet.

Temperature and salt

Two critical factors defining local environmental conditions in the deep oceans are temperature and salinity. These variables also connect the deep ocean to the surface through thermohaline circulation.  

Somewhere in the North Atlantic, cold salty water is sinking into the abyss. Perhaps this action occurs in the Labrador Sea, or maybe the descent takes place in the waters between Greenland and Scotland. But the water is disappearing from the surface, sinking to the seafloor and feeding a deep ocean current known as the “global conveyor belt.” This water movement from shallow to deep is the driver of the Atlantic Meridional Overturning Circulation (AMOC).The Gulf Stream owes its existence to the AMOC. 

Overturning circulation refers to the transfer of water from the ocean’s surface to the seafloor and back again. This process is also called thermohaline circulation. “Thermo” since cold water is denser than warm water, and “haline” because saltwater is denser than freshwater. When cold saline water becomes denser than the surrounding water, it sinks into the depths. This process starts a journey lasting up to 1600 years before the water surfaces again in the North-Central Pacific Ocean.

The temperature and salinity of deep ocean water are initially derived from surface conditions. So, changes at the surface eventually make their way into the deep oceans.

Monitoring the deep

Measuring conditions in the deep oceans is challenging, and surface data dominate historical ocean temperature measurements. 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. Prior to Argo stations, measurements from the deep were sparser and more sporadic.

An August 17, 2020 paper in Nature Climate Change (Silvy et al.) used Argo data and older data to investigate multiple climate models. The research focused on investigating the effects of anthropogenic climate change on the deep oceans. These scientists located temperature and salinity changes exceeding natural background variations and used the data to determine both when and how climate change affects this hidden realm. The first signs of climate change emerged in the Southern Ocean between 1980 and 1990. However, meaningful changes didn’t appear in the Northern Hemisphere’s oceans until the most recent decade.

The modeling indicates that up to 50% of the deep oceans today show the effects of climate change, and by 2080 up to 80% of the planet’s deep ocean environments will be affected.

The impact

Temperature and salinity are shifting beyond natural background variations, but why does it matter? Oceans drive global weather patterns, provide sustenance to billions of people, and serve as the primary reservoir for excess heat from global warming. Approximately 90% of this excess heat has been absorbed into the world’s oceans. Heat is important because a major component of environmentally healthy oceans is thermohaline driven circulation. The overturn of water from the surface to the bottom continually refreshes the deep oceans, keeping them oxygenated. Without overturn circulation systems, the deep ocean ecosystems would become anoxic, killing off life, and created an abyssal dead zone.  

The deep oceans are changing as temperature and salinity values stray past natural variations due to the influence of climate change and global warming. Deepwater ecosystems are some of the most environmentally stable systems on the planet. How they will respond to these changes is unknown. Likewise, how these changes affect thermohaline circulation, a critical component of deep-sea life, is also unknown. But the first stage in any endeavor is understanding the scope of the problem. The work to date is informative and spotlights emerging issues, but it is only a start.


Riding the global conveyor belt (Source: ArcheanWeb) –  Also:

Thermohaline circulation (Source: ArcheanWeb) –  Also:


More than half of the world’s oceans already impacted by climate change (Source: Lawrence Livermore National Laboratory) –  Also:

Human-induced changes to the global ocean water masses and their time of emergence (By Yona Silvy, Eric Guilyardi, Jean-Baptiste Sallée, & Paul J. Durack; Nature Climate Change) –  Also:

Deep Ocean Ecosystem (Modified) – By NOAA Office of Ocean Exploration and Research, Deep-Sea Symphony: Exploring the Musicians Seamounts –, Public Domain,  

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.