It's easy to think of the ocean as a permanent fixture, given how ancient it is. But those salty waters had to come from something, somewhere. How did we end up with this vast, shimmering source of life?
Over the past several billion years, the ocean has undergone a lot of change, just as it continues to evolve today. It plays a huge role in regulating the climate — including by balancing out some of the planetary warming driven by human activity. But that warming affects the ocean, too, with threats to life both under the sea and out of it.
World Oceans Day, June 8, celebrates the vitality and importance of the ocean, and how we can do our part to protect it. Here are some facts you may not have known about how the ocean morphs before our eyes, and what its past can tell us about our present.
Our planet didn't always have an ocean
Earth has been around for about 4.5 billion years, but there was no ocean in sight for the first billion or so. That's in part because the planet was too hot for water to accumulate in liquid form.
The planet eventually started to cool below the boiling point of water – 212 degrees Fahrenheit – allowing the early ocean to form around 3.8 billion years ago, according to the National Oceanic and Atmospheric Administration. At that time, water began to condense in the atmosphere and rain down in the basins on the Earth's landscape, said Peter Adams, a geologist at the University of Florida.
There's some debate around how water arrived on Earth in the first place. One theory is that volcanic activity expelled water vapor and other gases from the planet's interior, where it already existed. Another suggests that icy comets deposited water when they crashed into the early planet. The reality could be a combination of these theories, plus one other that involves the major collision event that is believed to have created the moon, Adams noted.
Regardless of the ultimate explanation, the Earth's specific location in the solar system happens to be the perfect place for water to exist in all three phases — ice, liquid and water vapor, Adams noted. If the planet were any closer to the sun, it would be too hot for liquid water to stick around; if it were any farther, water vapor couldn't exist.
"We're in the Goldilocks zone because we can move water around the planet in all three forms, and because it can exist in all three forms, we're in that sweet spot," he said.
Ocean currents circulate around the planet, similar to our own bloodstreams
The ocean helps make the Earth habitable. That's because it teams up with the atmosphere to efficiently circulate heat and water around the planet, Adams said. Without ocean currents, the regions surrounding the equator would be much warmer, he noted, and those in the higher latitudes would be much colder.
The Gulf Stream, a strong current that brings warm water from the Gulf of Mexico up the eastern coast of North America and across the Atlantic toward the British Isles, is a key example of this phenomenon. That steady influx of warm water helps stabilize Europe's climate, Adams said, which would otherwise be much colder and harsher based on its latitude alone.
But the currents and climate didn't always work exactly this way. Shifting tectonic plates — the broken-up pieces of the planet's crust that are constantly, slowly moving atop its mantle — have shaped and reshaped both land and sea for billions of years.
This map shows where the Gulf Stream originates in the Gulf of Mexico, and how it travels northeast across the Atlantic Ocean toward Northwest Europe. Image courtesy NOAA
"We're really happy with the way that our oceans circulate and redistribute heat right now," Adams said. "But if you reconfigure the tectonic plates, as has happened and continues to happen in Earth's history, you'll have different circulation patterns."
Climate change also has implications for ocean circulation. As the planet warms, the cold, fresh water long frozen as glaciers is melting and making its way to the salty ocean, said Suzanne OConnell, a geoscientist at Wesleyan University. Because freshwater is less dense than saltwater, it acts like a lid that could interfere with existing ocean currents, she said.
A disruption of those currents could drastically change temperatures in Europe and, potentially, elsewhere, according to NOAA.
"Think of the ocean as being like our blood. If you start moving your blood in a different part, … [that's] going to have major consequences," OConnell said.
The ocean is growing in some places, and shrinking in others
We can't understand the deepest parts of the ocean or the tallest mountains on land without tectonic plates, the 10 to 15 mobile slices of Earth's crust.
In some parts of the globe, the ocean floor is spreading while it contracts elsewhere. The North American, South American, European and African continents were connected a few hundred million years ago. Today, they sit atop separate tectonic plates, which they share with parts of the Atlantic Ocean.
Where those plates line up, deep within that ocean, is the Mid-Atlantic Ridge, an underwater mountain range that extends across the center of the Atlantic Ocean. At that juncture, magma is pulsing out from the mantle below and cooling underwater to form new ocean crust. It's spreading at a rate of two to five centimeters per year, according to the National Oceanic and Atmospheric Administration.
The Earth's crust is broken up into mobile tectonic plates that sit atop its mantle. Image courtesy United States Geological Survey
You can think of the process like pulling at the ends of a piece of Play-Doh until a thin spot forms in the middle, Adams said. But in this case, as tectonic plates move apart from each other, they allow the ocean floor to expand.
"That's where the new, low topography is [in the ocean]," he explained. "And, naturally, water is going to occupy the low points, because gravity's going to move it down into the lowlands. And you continue to pull the Play-Doh apart, and you get a wider and wider ocean."
Conversely, the basin that houses the Mediterranean Sea is contracting, getting smaller over time, Adams said. All of these changes are happening on a timescale far longer than a human lifespan. The movement of a plate is like what the imperceptible, slow growth of a fingernail feels like to a human, he noted.
The ocean does a lot for us, but we're asking too much of it
The fossil fuels we burn to power our lives and societies release greenhouse gases that crowd the atmosphere, causing the planet to heat up at a rapid rate. That's causing major changes – not on a geologic time scale like the one governing plate tectonics, but much faster.
The planet's temperature has fluctuated at other points in its long history, but those changes generally happened gradually, and organisms had time to evolve in response, OConnell noted. But human-driven climate change is closer in scale to more acute events that leave little time for adaptation. In less than two centuries, she noted, "we have completely changed our atmosphere."
Scientists can use ice cores, which date back around 800,000 years, to create a record of how much carbon dioxide has existed in the atmosphere over that period of time. For the bulk of that time, CO2 measurements ranged somewhere between 180 to 300 parts per million. Today, those levels hover around 420 parts per million, OConnell said.
Researchers use ice cores to assess carbon dioxide levels in the atmosphere over the past 800,000 years. Image courtesy NASA
"If we were going to warm the Earth and add all this CO2 over the next 20,000 years, no big deal," OConnell said. "But we aren't."
The ocean helps mitigate some human-driven warming of the climate because water is capable of absorbing heat — and a lot of it. But as it absorbs heat, it expands. Glacial melt is also driven by the warming planet, which deposits excess water in the ocean.
"So as the Earth's oceans are heated, not only is there more water in the oceans, but the water expands and raises the height of the water column," Adams said. Those two processes combine to contribute to sea level rise, which threatens coastal ecosystems and communities.
Today, the ocean absorbs around a third of the carbon dioxide that humans release into the atmosphere. That excess CO2 contributes to ecological changes like ocean acidification, which affects sea creatures with shells.
For OConnell, it all adds up to a call to action. We can't completely spare ourselves — or the ocean — from the consequences of climate change. But we do have the power to swiftly and meaningfully cut greenhouse gas emissions and reduce the likelihood that researchers' worst-case predictions will come to pass.
"We're going to blow through 1.5 degrees [of warming,] maybe even this decade. But the difference between 1.5 and 3 degrees is huge," she said. "So, let's not let it get to 2 degrees. Every fraction of a degree makes the difference."