2014 marked the first time more than half of global ocean surface recorded temperatures considered extreme compared to a historical average
at the US National Oceanic and Atmospheric Administration have now found that 2014 marked the first time that more than half of the global ocean surface recorded temperatures were considered extreme compared with a historical baseline. Just three years later, such temperatures covered a high of 60 per cent of the oceans. The figure was less than a fifth in the early 1900s.
“Our new index of extreme marine heat shows the global ocean crossed a critical barrier in 2014 and it’s now normal. It’s arrived, it’s here,” says Van Houtan.While the oceans as a whole crossed the threshold of 50 per cent only recently, some oceans hit it much earlier. The South Atlantic passed the milestone in 1998. “That was a long time ago. I think that’s really jarring,” says Van Houtan.
Definitions of a marine heatwave can vary. Van Houtan and Tanaka looked at two sets of global sea surface temperature data from 1870 to 2019, using the first 50 years to establish a historical baseline of what extreme marine heat looked like then for every pixel of ocean, for every month of the year. The hottest 2 per cent of temperatures were deemed extreme. The pair then used that as a yardstick to map the prevalence of those extremes up to 2019.at the University of Tasmania, Australia.