The oceans that cover 70% of the planet have been absorbing 90% of the extra heat that is accumulating as a result of human-caused changes to the atmosphere, the International Union for Conservation of Nature (IUCN) reports in a survey of the science. And the heat is beginning to show in everything from animal behaviour to threats to human health.
In a particularly telling illustration, the Guardian reports, the work by 80 scientists in a dozen countries determined that if the heat absorbed by the oceans over the last century had instead warmed the atmosphere, Earth’s surface would now be 36ºC warmer than it was in 1900, instead of about 1ºC warmer.
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But the heat that has been absorbed into the top two kilometres of the oceans has been having an effect nonetheless, warming the upper few metres more than 1.4ºC in that period.
“The warming is having its greatest impact upon the building blocks of life in the seas, such as phytoplankton, zooplankton, and krill,” the Guardian notes. “Changes in abundance and reproduction are, in turn, feeding their way up the food chain, with some fish pushed out of their preferred range and others diminished by invasive arrivals.”
More than 500 marine species are regarded as threatened—a number likely to rise as seas continue to warm.
“Due to a domino effect,” IUCN warns, “key human sectors are at threat, especially fisheries, aquaculture, coastal risk management, health, and coastal tourism.” According to the research, 4.3 billion people who currently rely on marine fisheries face food insecurity from combinations of overfishing and climate-driven species declines.
Warmer oceans also harbour more potent human disease vectors like the vibrio bacteria, which can cause cholera. Ocean warming also supports “harmful algal bloom species that can cause food poisoning,” and spawns “more severe, if not more numerous, hurricanes due to the extra energy in the oceans and atmosphere.”
And those are not the worst of the effects humanity may be unleashing as our emissions slowly heat up the oceans. As the Guardian describes the far-from-remote nightmare scenario: “Warming waters could unlock billions of tonnes of frozen methane, a powerful greenhouse gas, from the seabed, and cook the surface of the planet.”