Temperatures in the world’s oceans have broken fresh records, testing new highs for more than a month in an “unprecedented” run that has led to scientists stating the Earth has reached “uncharted territory” in the climate crisis.

The rapid acceleration of ocean temperatures in the last month is an anomaly that scientists have yet to explain. Data collated by the US National Oceanic and Atmospheric Administration (NOAA), known as the Optimum Interpolation Sea Surface Temperature (OISST) series, gathered by satellites and buoys, has shown temperatures higher than in any previous year, in a series stretching back to 1981, continuously over the past 42 days.

The world is thought to be on the brink of an El Niño weather event this year – a cyclical weather system in the Pacific, that has a warming impact globally. But the El Niño system is yet to develop, so this oscillation cannot explain the recent rapid heating, at a time of year when ocean temperatures are normally declining from their annual March and April peaks.

Prof Mike Meredith of the British Antarctic Survey said: “This has got scientists scratching their heads. The fact that it is warming as much as it has been is a real surprise, and very concerning. It could be a short-lived extreme high, or it could be the start of something much more serious.”

Warming oceans are a concern for many reasons. Seawater takes up more space at higher temperatures, accelerating sea level rise, and warmer water at the poles accelerates the melting of the ice caps. Hotter temperatures can also be dire for marine ecosystems, as it can be difficult or impossible for species to adapt. Corals in particular can suffer devastating bleaching.

Some scientists fear that the rapid warming could be a sign of the climate crisis progressing at a faster rate than predicted. The oceans have acted as a kind of global buffer to the climate crisis over recent decades, both by absorbing vast amounts of the carbon dioxide that we have poured into the atmosphere, and by storing about 90% of the excess energy and heat this has created, dampening some of the impacts of global heating on land. Some scientists fear we could be reaching the limit of the oceans’ capacity to absorb these excesses.

Meredith said it was still too soon to tell. “The rate [of temperature rise] is stronger than climate models would predict,” he said. “The cause for concern is that if it carries on, this will be well ahead of the climate curve [predicted] for the ocean. But we don’t know yet if that is going to happen.”

The alarm was first raised several weeks ago. Climate scientists warned the Guardian early this month that preliminary data from NOAA showed the average temperature at the ocean’s surface had been at 21.1C since the start of April – beating the previous high of 21C set in 2016.

“The current trajectory looks like it’s headed off the charts, smashing previous records,” Prof Matthew England, a climate scientist at the University of New South Wales, said at that time.

Since then, temperatures have varied slightly from day to day but have shown little sign of abating. For temperatures to continue so far above the average for such a long period at this time of year was “an anomaly”, said Ben Webber, lecturer in climate science at the University of East Anglia.

“What we are seeing is very unusual,” he said. “This is heading in an unprecedented direction, and could be taking us into uncharted territory.”

Simon Good, an expert on ocean observation at the UK’s Met Office, said: “For the last three years the tropical Pacific has been in the opposing phase with a La Niña bringing cooler conditions. With the strong potential for an El Niño event in the mix we can expect sea-surface temperatures to temporarily rise further, with a consequent knock-on effect for global temperature as a whole.”

Mark Maslin, professor of Earth system science at University College London, said the climate crisis was taking hold before our eyes. “Climate scientists were shocked by the extreme weather events in 2021,” he said. “Many hoped this was just an extreme year. But they continued into 2022 and now they are occurring in 2023. It seems we have moved to a warmer climate system with frequent extreme climate events and record-breaking temperatures that are the new normal. It is difficult to see how anyone can deny climate change is happening and having devastating effects around the world.”