When people talk about climate solutions, forests, wetlands, and renewable energy often get the spotlight. But out of sight, and all too often out of mind, the deep sea is working quietly to regulate Earth’s climate.
Covering over 70% of the planet, the ocean is Earth’s largest life-support system, and it has already shielded us from the worst impacts of the climate crisis. It has absorbed more than 90% of the excess heat generated by burning fossil fuels, and about 30% of human-generated carbon dioxide emissions since the 1980s. Without this massive buffer, global temperatures would already be far higher than they are today. In fact, if the heat taken up by the upper 2000 meters of the ocean between 1955 and 2010 had instead entered the lower atmosphere, Earth would have warmed by 36°C.
But the story doesn’t end at the surface. The deep ocean plays an especially critical role in climate regulation, carbon storage, heat transport, and many ways in which scientists are only beginning to fully understand.
The Twilight Zone Migration
One of the most powerful climate regulators in the deep ocean is the biological carbon pump. Microscopic plants (phytoplankton) at the ocean’s surface capture carbon dioxide through photosynthesis. When they die, or when animals eat them, some of this carbon sinks into deeper waters, effectively removing it from the atmosphere for decades to millennia.
A major driver of this process happens in the ocean’s ‘twilight zone’ (200–1000 meters deep), where countless zooplankton and fishes take part in the largest daily migration on Earth. Every night, they rise to the surface to feed, and by day, they return to the depths, carrying carbon with them. This process, known as diel vertical migration, actively transports carbon into the deep ocean, where it is far less likely to return to the atmosphere.
This daily migration is a powerful way the deep ocean sequesters carbon, keeping it locked away for decades to centuries.
Methane bubbles flow in small streams out of the sediment on an area of seafloor in the north-west Atlantic. Image courtesy of NOAA Okeanos Explorer Program, 2013 ROV Shakedown and Field Trials in the U.S. Atlantic CanyonsMethane Seeps
The deep seafloor is dotted with methane seeps, where gas escapes from beneath the seabed. Methane is about 80 times more potent than carbon dioxide as a greenhouse gas within the first 20 years after it was emitted. But, deep-sea microbes form a natural shield. They consume much of this methane before it reaches the surface, preventing it from entering the atmosphere.
These tiny organisms act as natural filters and gatekeepers, stopping a dangerous greenhouse gas from entering the atmosphere.
Deep-Sea Sediments
As marine snow (the constant drizzle of dead plankton, fecal pellets, and organic debris) settles on the seafloor, it builds up into thick layers of sediment. These sediments form one of Earth’s largest and longest-lasting carbon sinks, locking carbon away for thousands to millions of years.
Disturbing these sediments through destructive practices like deep-sea mining or bottom trawling risks releasing this stored carbon back into circulation, undermining a vital climate buffer.
Ocean Currents
The deep ocean doesn’t just store carbon, it also moves heat. Conveyor-belt-like currents, known as thermohaline circulation, transport warm and cold water around the planet, redistributing heat between the equator and the poles. This circulation helps stabilize Earth’s climate system, influencing weather patterns and shaping the environment that all life depends on.
Disruptions to these currents caused by ocean warming and melting ice pose a major risk to global climate. Slowing circulation would alter global weather and could also reduce the ocean’s ability to store heat and carbon.
Nature’s Climate Engineers
It’s not just physical processes that play a vital role. The biodiversity of the deep ocean is essential for climate stability. From corals and sponges that filter organic matter, to fish and invertebrates that drive nutrient cycling, deep-sea life underpins the processes that regulate carbon storage. Even microbes play a central role in recycling and locking away carbon on the seafloor.
Despite the deep sea being one of Earth’s most important climate regulators, it remains poorly understood and highly vulnerable to human activity. Destructive fishing, pollution, and the emerging threat of deep-sea mining could disrupt these delicate systems. Protecting the deep sea is not just an environmental issue, it is a climate solution.
