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Marine heatwaves have become longer and more frequent over the last century, with rippling environmental effects on oceans worldwide, according to a new study published by researchers from Canada, Australia and the United Kingdom.

The total yearly days of heatwaves has increased by 54 percent from 1925 to 2016, write the scientists, in a study published in the journal Nature Communications.

The temperature trends were calculated using a mix of data: satellite data since the 1980s; daily checks at a half-dozen sites stretching back to the early 20th century; and monthly data collected on sea surface temperatures, the scientists show.

“We needed ocean temperature measurements resolved daily,” said Eric Oliver, of Dalhousie University in Nova Scotia. “On a global scale, that’s only been available since the early 1980s when satellites first began observing the ocean surface. But a few sites around the world have daily measurements going back to the early 20th century, and we also have monthly measurements of sea surface temperatures globally going back over a similar period.”

The satellite data covered from 1982 to the end of the study period in 2016, using the National Oceanic and Atmospheric Administration’s Optimum Interpolation Sear Surface Temperature data set.

The daily data stretching back to the first decades of the 20th century was available as six sites: three in the United States (at Pacific Grove, Scripps Pier, and Newport Beach; and one each in Norway (Arendal), the United Kingdom (Port Erin), and Canada (Race Rocks)).

The monthly sea surface temperature data was averaged and gridded from other sites over the entire time period to develop proxies, they added.

The standard definition of a marine heatwave was applied: they occur when sea surface temperatures exceed, for five days, a seasonally varying threshold that is itself determined as the 90th percentile of temperatures on a 30-year climate period.

Results indicated that the waters are getting hotter in several respects. The frequency of marine heatwaves increased by 34 percent over the century timeframe. At the same time, the duration of the heatwaves lengthened by 17 percent.

Taken together, the frequency and duration pushed the increase of total number of marine heatwave days to 54 percent, write the scientists.

The marine-heatwave study follows a high-profile paper in the journal Scientific Reports last September. That study concluded that heatwaves are getting longer and more frequent – and are starting to appear in some regions during even the spring and autumn.

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