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Ask a scientist: Hot oceans

From bleaching corals to weakening currents, Stanford scientists help readers navigate the effects of warmer oceans.

A paddler crosses over a coral reef off the coast of Belize in the Caribbean Sea. The corals pictured here, known as sea fans, are part of the second largest reef system in the world. (Image credit: Velvetfish/Getty)

The world’s oceans are heating up, and along with them, efforts to understand what a record rise in temperature means for the future of marine life and coastal communities.

Last summer, the surface of Earth’s ocean was hotter than during any period in more than 200 years of recordkeeping. The waters off Florida’s coast spiked to a feverish temperature of 101 degrees Fahrenheit in July. Corals expelled their life-giving algae and bleached white. Days later, scientists published research flagging early signs of major Atlantic Ocean currents grinding to a halt by 2100, a change that would increase temperatures globally and fuel stronger storms, among other impacts.

The planet is now on course to have record-high sea-surface temperatures in 2024. The National Oceanic and Atmospheric Administration recently warned that a major coral bleaching event – the fourth in recorded history, and the second in the past decade – will sweep the globe this summer. It’s predicted to be the most extensive event to date, affecting reefs in the Atlantic, Indian, and Pacific oceans.

We gathered oceanographers and coral scientists to bring context to the headlines and predictions. Below, they share insights from decades of research and recent field observations.

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Earth system science assistant professor Earle Wilson leads the Polar Ocean Dynamics Lab at the Stanford Doerr School of Sustainability. He is also a an assistant professor, by courtesy, of oceans and of geophysics, and a center fellow, by courtesy, at the Woods Institute for the Environment.

Our experts include Earle Wilson, a physical oceanographer and assistant professor of Earth system science at the Stanford Doerr School of Sustainability who studies how polar ocean processes influence global climate. Wilson’s research involves understanding sea ice formation in the Southern Ocean, where temperatures dip low enough to freeze seawater. These drifting surface formations help drive ocean circulation and reflect sunlight so the poles remain relatively cool. 

“Antarctica’s sea ice defied our expectations for decades,” Wilson said. Unlike Arctic sea ice, which has declined since the late 1970s, Antarctic sea ice began declining only five years ago. “People are grappling with what we’re seeing as a long overdue expression of anthropogenic climate change or natural variability.”

Stephen Palumbi
Steve Palumbi is the Jane and Marshall Steel Jr. Professor of Marine Sciences, a professor of oceans in the Stanford Doerr School of Sustainability, a professor of biology in the School of Humanities and Sciences, and a senior fellow at the Woods Institute for the Environment. Learn more about the Palumbi Lab.

Additional expertise comes from marine ecologist Steve Palumbi, a professor in the Stanford Doerr School of Sustainability and in the School of Humanities and Sciences who has devoted his career to coral conservation. Members of the Palumbi Lab including marine biologist DeVant’e Dawson, coral scientist Courtney Klepac, and field operations director Marilla Lippert also weighed in from the sandy shores of Turneffe Atoll in Belize, along with project collaborator Nicole Craig, a reef resilience specialist at The Nature Conservancy.

The team is stationed in Belize – where last summer temperatures rose two degrees Fahrenheit higher than usual – in anticipation of another extreme heat wave. Part of a collaboration known as Super Reefs, the team seeks to identify which corals may endure the heat two summers in a row. The work could help local management agencies prioritize conservation of the hardiest, most heat-resistant species.

Researchers Courtney Klepac, Nicole Craig, DeVant'e Dawson, and Marilla Lippert (pictured left to right) joined a virtual discussion in late April 2024 to share observations of coral bleaching from Turneffe Atoll in Belize. (Image credit: Katie Jewett)

“As a Belizean, being a part of this conservation project means a lot to me personally,” said Craig. “I’m hopeful that what we find in this study can help us chart a path forward for local reefs.”

How does the ocean regulate Earth's climate?

Wilson: The ocean is a critical heat sink for our global climate. Over 90% of the excess warming associated with anthropogenic climate change is taken up by the ocean. The ocean also exchanges carbon with the atmosphere. It’s the single largest reservoir of carbon in the carbon cycle. Thirty percent of the carbon dioxide that’s emitted by humans ends up in the atmosphere, and another 25% ends up dissolved in the ocean. 

What role do ocean currents play in distributing heat and absorbing carbon dioxide? 

Wilson: The ocean plays an essential role in the uptake and sequestration of heat and carbon from our atmosphere. Much of this uptake occurs in cold, high-latitude regions, such as the North Atlantic, where dense, deep waters form and sink into the ocean interior. The polar oceans also provide a pathway for these carbon-rich deep waters to return to the surface. For example, deep waters formed in the North Atlantic eventually upwell in the Southern Ocean centuries later, where they can release dissolved carbon dioxide back into the atmosphere. Thus, the polar ocean provides an essential gateway to the otherwise sequestered ocean interior. 

What is causing recent increases in ocean temperatures?

Palumbi: This year, we’ve observed record-high ocean temperatures that are linked to the El Niño Southern Oscillation. This cyclical phenomenon happens when warm water builds up in the Eastern Pacific and sweeps across the world. The effects can be seen in places like Belize, where there was an unprecedented marine heat wave last summer that impacted the coral reefs near where our team is now. 

Wilson: The record-breaking surface temperatures in recent years are also a continuation of a warming trend that began with the Industrial Revolution over 150 years ago. It is often difficult to disentangle what led to warming anomalies in a specific year or region. Climate variability, due to processes like the El Niño, will always play a role. However, if we step back and examine the global ocean warming over the past few decades, it is a fact that these longer time-scale trends are due to rising atmospheric carbon dioxide levels. 

What are the social and environmental consequences of warmer oceans?

Arctic sea ice, like the floes pictured here, has been in decline since the late 1970s. This image was taken in Savissivik, Greenland in January 1970. (Image credit: Vittoriano Rastelli/Getty)

Wilson: There are too many to list here, but one of the most important effects of ocean warming relates to how marine ecosystems will adapt to a warmer, more acidic ocean. I’m most familiar with the Southern Ocean, where we’re seeing huge krill loss near the Antarctic Peninsula. Krill are extremely sensitive to temperature and depend on sea ice for food and part of their life cycle. As sea ice retreats and the ocean warms, those populations will be severely impacted. At the same time, species that prefer warmer water may expand their presence in these regions. These trends will affect any society that relies on the ocean for food and economic revenue.

Palumbi: The top part of the ocean is full of marine life that many people depend on. Warm water can cause big dead zones – warmer water means lower dissolved oxygen. You get a stinking mess of dead fish. In other places, corals might turn white, bleach, and die, which means they’re no longer a good place for lots of fish that people eat as their main source of protein.

Have ocean temperatures reached a tipping point?

Wilson: A tipping point is when a system undergoes a change that isn’t easily reversed. There are many types of tipping points in the climate system, each related to different processes. For example, there is a concern that if the surface waters in the North Atlantic become too warm or too fresh, they may not be able to sink deep enough to fuel the overturn in circulation, which plays a major role in oceanic heat and carbon uptake. 

While models and theory suggest that this tipping point is possible and potentially imminent, we have only had direct continuous observations of the Atlantic Meridional Overturning Circulation since 2004. Also known as AMOC, this powerful ocean current circulates water throughout the Atlantic. Observations of AMOC show variability in strength over time but no obvious signs of a slowdown.

There is also a fear that warming ocean temperatures may trigger rapid ice loss and sea level rise that may not be easily reversed in our lifetimes.

Craig: In Belize, we’ve tipped. We are at Calabash Caye Field Station where I’ve spent time swimming in the reefs just offshore. In mid-April, we visited that exact site. It made me sad to see how the reef has declined from healthy and vibrant to only 5% coral cover. The decline wasn’t gradual at all. People who were here last year didn’t see what we saw in April. It’s alarming. 

Palumbi: Corals have inborn resilience. They can survive, persist, and slowly recover from disturbances and damage. At some point, though, the coral cover becomes so diminished that the reef collapses. People in the Caribbean have been worried about this tipping point for a long time. As Nicole Craig voiced, the tipping point in Belize just happened. Last summer a heat wave swept around the world, and we anticipate another one later this summer.

What can marine resource managers and scientists do to prepare for a future of warmer oceans?

Palumbi: The chief thing is to stop putting carbon dioxide into the atmosphere. That trend has to reverse. We have to buy enough time to keep coral reefs alive until we can grapple with climate change. 

Klepac: It’s important to raise awareness about how events are increasing in severity and frequency. Coastal development can also wreak havoc on a coral reef. Improving wastewater management and curbing agricultural runoff will improve the water quality for corals that live near human populations. 

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