The ocean covers more than 70% of the surface of Earth and makes up a staggering 97% of all the water on our planet. These waters are not only the cradle of life but also the engine driving the planet’s climate, weather, and air quality. The photic zone, a thin veneer of the ocean’s surface, teems with microscopic life that produces much of the oxygen we breathe, while the oceans themselves act as a colossal carbon sink, moderating the increasingly warming global climate.

Yet despite its huge significance for life as we know it, there are still many things we don’t know about the ocean. By one account, there may be around two million species of marine life, but over 90% is undescribed. Besides its many hidden mysteries, the ocean is also under increasing pressure from human activity. Here are just a few interesting facts and big questions about Earth’s oceans shared by some of the world’s foremost experts.

1. Up to 80% of our oceans remain unexplored and unmapped

Beneath the ocean’s surface lies a world less known than the surface of the moon. While satellite imagery offers us a glimpse of the ocean surface, the deep sea is a frontier that continues to elude even our finest instruments. The challenges are manifold, from the prohibitive costs to the technical challenges posed by the deep sea’s extreme conditions. Yet, progress is being made.

Modern technologies, including sonar and autonomous vehicles, are slowly peeling back the layers of this mystery, revealing the contours of our planet’s final unexplored territory.

“The international ARGO program, consisting of a fleet of robotic instruments that drift with the ocean currents and move up and down between the surface and the first 2000 m of the water column, has also led to a greater spatial and temporal coverage of oceanic waters. In a sense, oceans represent the great frontier for the next generation of explorers and researchers, where vast opportunities for inquiry and investigation remain,” Dr. Martine Lizotte from Université Laval wrote in a Metafact review.

2. Sea levels are rising — and they’re rising fast

Experts worldwide unanimously agree in their assessment: sea levels are indeed rising. The Intergovernmental Panel on Climate Change (IPCC), a collective of hundreds of scientists under the auspices of the United Nations and the World Meteorological Organisation, has provided compelling evidence of this trend. With “high confidence,” their reports highlight a clear increase in sea levels, corroborated by various global datasets and two independent sources of evidence: tide gauges and satellite altimeters.

Not only is sea level rise happening, but it is also accelerating. The 20th century saw an average increase of approximately 2.2 mm per year. However, recent decades have witnessed an acceleration to 3.4 mm per year. Initially, thermal expansion — where warming ocean waters increase in volume — was the primary driver. Yet, the narrative has shifted. The current rise is predominantly due to glacier ice loss, melting of the Greenland ice sheet, and mass loss from the Antarctic ice sheet.

The implications of rising sea levels are dire, especially for the 230 million people living less than one meter above sea level. These communities face increased vulnerability to flooding, storm surges, and the salinization of freshwater sources critical for drinking and agriculture. The danger is not a distant threat but a current reality, with the potential to reshape our world in profound and irreversible ways.

“There are two sources of evidence which independently concur that sea-level has been rising at about 3.4 mm per year over the past few decades. First, tide gauges across the world give us a picture of how sea-level is changing in relation to the elevation of land masses. In tectonically stable coastlines (like Australia), tide gauges show a consistent increase in sea level when analyzed over inter-decadal time scales. The second line of evidence is satellite altimeters, which continuously assess the distance between the ocean surface and the satellite using active sensors. These provide a global picture of sea-level rise consistent with (a) the tide gauge records, corrected for land subsidence and (b) the projections of models relating sea-level to changes in ocean temperature and volume,” Professor Neil Saintilan from Macquarie University wrote in a Metafact review.

3. The oceans absorb a lot of human-made carbon dioxide from the atmosphere — but it’s not as much as we used to think and the process is slow

Oceans act as a buffer against climate change by absorbing copious amounts of carbon dioxide (CO₂). However, like a sponge that is already soaking wet, there is only so much the oceans can take.

One estimate suggests that the ocean will absorb 80% to 85% of the extra carbon people spew into the atmosphere by burning fossil fuels. This may be just wishful thinking though as more recent studies suggest the ocean takes up much less carbon — and it does so exceedingly slowly. The consensus now is that oceans can take in only one third of human-produced carbon pollution annually.

Prof Katsumi Matsumoto from the University of Minnesota points out that while the ocean is the largest carbon reservoir, capable of holding 50-60 times more carbon than the atmosphere, the process is not immediate. It could take centuries for oceans to absorb a substantial portion of anthropogenic CO₂ emissions.

“Vertical mixing takes surface waters that become more saturated with anthropogenic CO₂ down into the deep ocean and brings up deep waters that are not saturated with anthropogenic CO₂. Eventually, the CO₂ that is absorbed by the oceans can get neutralized by CaCO₃ on the sea floor… On timescales of continental weathering and movement (i.e., tectonics), the huge carbon reservoir of the rocks comes into play. But they are much too long for humans,” Matsumoto wrote in a Metafact review.

4. The oceans are becoming increasingly acidic

Amidst the climate crisis, the world’s oceans are facing a trifecta of challenges: rising sea levels, increasing temperatures, and the less talked about but equally critical issue of acidification.

Ocean acidification is a process driven by the absorption of carbon dioxide from the atmosphere. As CO₂ dissolves in seawater, it forms carbonic acid, lowering the water’s pH and making the oceans more acidic. Though the current average pH of ocean surfaces stands at approximately 8.1 — still alkaline — the ongoing emission of CO₂ means this number is gradually decreasing.

Experts emphasize that while the oceans’ pH will not dip below 7 (under which it would become acidic), the trend toward lower pH levels is undeniable and will persist for decades if not centuries. This subtle shift in pH levels has profound implications for marine life, particularly species that rely on calcium carbonate for their shells and skeletons.

While there’s a consensus on the ongoing process of acidification, the extent of its impact and the future it portends for our oceans hold some uncertainty. The division among experts more often pertains to semantics and the interpretation of what constitutes “acidic” conditions. However, the underlying agreement is that the continuous emission of CO₂ is propelling the oceans toward a more acidic state, with significant ecological and economic consequences.

“The large-scale addition of CO₂ to the atmosphere is now being transferred in part to the ocean, where it contributes to ocean acidification. So, acidification will happen; it is a predictable chemical response. But it’s important to keep scale in mind. The amount of acidification will be undetectable to the average person; it is not as if the ocean is going to turn into a churning vat of acid. However, it will become acidic enough to disrupt the calcification of crustaceans and shellfish. And that by itself could cause huge problems in the marine ecosystem,” Professor Steven Campana from the University of Iceland wrote in a Metafact review.