Posted on Categories Discover Magazine
As the world’s oceans have spiked a record-setting fever in recent weeks, scientists are trying to work out the precise causes.
You can see the dramatic rise in global sea surface temperature in this unsettling graph:
Since early March, the global average sea surface temperature has increased to a record high, jumping above those seen during the strong El Niño year of 2016. Also shown in this graph are SSTs from the warm El Niño years of 2002/2003, 2009/2019 and 2015, along with 2022, a relatively cool La Niña year, shown in orange. For comparison, the dashed line shows the long-term mean for 1982-2011 as well. (Credit: University of Maine Climate Reanalyzer, with annotations by Tom Yulsman)
Some experts think the heating spike is the vanguard of a looming El Niño event, which is characterized by unusual warmth of the sea surface across a vast swath of the equatorial Pacific.
A huge, subsurface blob of warm water is growing larger in the equatorial Pacific as it moves eastward and toward the surface. Sea surface temperatures off Ecuador have already risen dramatically, probably heralding what could be a strong El Niño event. (Credit: NOAA Climate.gov animation, based on data from NOAA’s Climate Prediction Center, via the ENSO Blog)
El Niño is part of a see-sawing pattern called the El Niño-Southern Oscillation, or ENSO. For the past three years, the other part of the oscillation, La Niña, has exerted a cooling influence on global surface temperatures. But it has faded. Now, a gargantuan blob of warm ocean water is rising from the depths in the Pacific and moving eastward along the equator, as seen in the animation above.
This seems to be causing sea surface temperatures to rise significantly off the coast of Ecuador. And that, in turn, is being built on top of the long-term background warming of the oceans — which have absorbed most of the heat that has built up in the climate system due to human activities.
The evolution of sea surface temperature anomalies over 90 days, starting on Jan. 30 and ending on April 29, 2023. Note how relatively cool surface waters in the equatorial Pacific give way to warmer water, particularly right off the coast of Ecuador. Strong warming signals are seen elsewhere as well, including in the North Pacific and off the coasts of Africa and Spain. (Credit: NOAA)
Other experts believe there’s even more to the current oceanic heating spike than an El Niño emerging on top of continued long-term warming. They point to concerning ocean heat waves elsewhere. These include a persistent one in the North Pacific, and another in the Atlantic off the coasts of Africa and Spain, both visible in the animation above.
Regardless, should El Niño does develop as expected, we may well see record high global warming.
“ENSO has a strong relationship with the global average temperature: in general, the warmest year of any decade will be an El Niño year, and the coolest a La Niña one,” writes University of Miami scientist Emily Becker in NOAA’s invaluable ENSO Blog. “If El Niño develops this year, it increases the odds of record-warm global temperature.”
University of Pennsylvania climate scientist Michael Mann cautions that we should focus on the longer-term outlook, not just the impact of El Niños and La Niñas, which come and go.
“Let’s not get too distracted by the fleeting impacts of El Nino and La Nina on surface temperature,” Mann said on Twitter recently. “It’s the steady overall ocean heating we should be most concerned about. It will continue until carbon emissions reach zero.”
Mann and 23 other scientists recently published a study on the overall heat content of the oceans (as distinct from just the temperature at the surface). The research shows that in 2022, the world’s oceans “were again the hottest in the historical record and exceeded the previous 2021 record maximum.” This continues a very worrisome trend of unremitting ocean warming caused by humankind’s continuing emissions of greenhouse gases.
“The global long-term warming trend is so steady and
robust that annual records continue to be set with each new
year,” Mann and his colleagues write. “The warming has accelerated in recent decades, with a
faster rate of warming evident since roughly 1990.”
All the heat absorbed by the oceans doesn’t just stay there. Like water spilling from a giant reservoir, some of the heat flows back out into the atmosphere, where it has a massive effect on the climate system. By warming the atmosphere, this energy alters the patterns of precipitation
and temperature globally. Among the impacts are heavier downpours, but also hotter, long-lasint heat waves, more intense droughts, and more vicious wildfires.