The Impact of Ocean Temperatures Hitting 40 Year High

Ocean temperatures have soared recently, hitting their highest point for April and March in over 40 years.

Data from the Climate Change Institute at the University of Maine has shown that since mid-March 2023, the average sea surface temperature worldwide—between the latitudes of 60 degrees North and 60 degrees South—has been higher than it has ever been since accurate satellite temperature records began in 1981.

"Ocean temperatures have been at all-time record warm levels for 42 straight days," Colin McCarthy, an extreme weather expert and content creator, said in a tweet on Tuesday, garnering over 330,000 views. "This is [completely] unprecedented in the modern history of Earth."

If these trends continue, increased ocean temperatures may have major impacts on many marine species at all levels of the food chain.

"This will impact many life forms and especially primary producers which live [on] the surface as they are dependent on sunlight for fixing CO2," Thomas Mock, a marine microbiologist at the University of East Anglia in the U.K., told Newsweek.

"Primary producers in the oceans are mainly algae from microscopic size to the size of trees. To efficiently fix atmospheric CO2, they require nutrients. However, if the ocean warms up, vertical mixing is reduced because a warm layer basically like a lid sits on top of the ocean. We call this ocean stratification. This limits the flow of nutrients to where primary producers are (surface layers). Hence, productivity is reduced potentially leading to a positive feedback loop as less CO2 is being fixed (loop: anthropogenic CO2 leads to heat, and this leads to more CO2 remaining in the atmosphere because of reduced uptake by oceanic primary producers)."

Increased ocean temperatures can also lead to coral bleaching, which is when heat-stressed corals expel the algae that live within them and photosynthesize the sunlight for them, turning the corals white.

"Heat also induces stress in marine organisms that can't regulate their body temperature," Mock said. "These marine heatwaves become more common. Thus, only those species will survive who can either cope with heat or who evolve quickly enough (e.g., microbes) to be able to cope (e.g., heat is considered to impose a hard selection). Another consequence of increasing temperatures is poleward (Arctic and Antarctic) migration of marine organisms out of heat zones (equator, subtropics) if they can. There is evidence for this already and for many organism groups in the ocean and on land."

Extreme Weather Events

Hotter oceans provide energy for storms to brew, creating more powerful hurricanes. They contribute to the melting of sea ice and glaciers, increasing sea levels across the world and destroying the habitats of thousands of species that rely on the ice, including the polar bear.

Ocean temperatures have been increasing since the 1880s, according to data from the National Oceanic and Atmospheric Administration, having increased by an average of 1.5 degrees Fahrenheit since 1901. This increase has been accelerating in recent decades, according to a paper in the journal Frontiers in Marine Science from 2021.

The precise data showing changes from year to year are based on measurements from satellites—NOAA satellites in this case—but also verified with measurements from ships and buoys.

"The ocean has been steadily warming as a result of climate change. So in a fairly steady fashion decade by decade, the ocean has been getting warmer," Brad de Young, a professor of oceanography at the Memorial University of Newfoundland, told Newsweek.

This exceptionally high year of ocean temperatures may be because of a combination of factors, including the fact that we are coming out of a La Niña period that results in cooler waters, and entering an El Nino, which can lead to increased temperatures.

"Month by month and year by year there are processes that move heat around in the Earth system that lead to relative cooling," deYoung said. "The huge swings associated with La Niña and El Nino are one of those. We have been in a La Niña pattern for the past few years but appear to be sliding into an El Nino phase. This means that there is a huge shift in heat in the tropical Pacific with warmer water beginning to appear in the eastern Pacific. So this dramatic surface warming of the ocean could be related to this shifting cycle of Pacific heating. While [the rise in temperatures is] striking, it is not unexpected."

Scientists agree that while there are natural fluctuations in ocean temperatures, the general long-term trend is that these temperatures are, on average, increasing.

"As we keep gradually turning up the temperature of the planet we can expect to see events such as these in a given year or season," deYoung said. "While there is relatively steady warming there are still many things that cause the temperature to go up or down from year to year. Another aspect of this story is that the ocean plays a big part in all this as it has been absorbing most of the excess heat held by the earth system as a result of anthropogenic warming."

Oceans are thought to absorb more than 90 percent of the heat that is trapped in the atmosphere by greenhouse gas emissions, according to NASA.

A major driver of the increased ocean temperatures is human-driven changes to the climate. Scientists have noted that despite the La Niña conditions, the oceans have been warmer than expected in recent years. One 2019 study in the journal Frontiers in Marine Science showed that marine heatwaves have increased in frequency with climate change, with several regions of the ocean currently undergoing heatwaves.

"Another note here is that the data here are from 60 N to 60 S because of problems getting data closer to the poles. This does cover most of the ocean and so is a reliable index of what is going on in the oceans. Indeed over time it probably underestimated the warming as the poles are warming faster than elsewhere. But on short time scales, a few months these data could somewhat over or underestimate the anomaly," deYoung said.

"Of course, this observation has not been fully digested as it is so 'fresh' and it is still developing. So it could change somewhat over the coming months," he said.

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