Global temperatures leapt in February, lifting warming from pre-industrial levels to beyond 1.5 degrees, and stoking concerns about a "climate emergency".
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Unusual warmth in waters off northern Australia also prompted an alert by the Great Barrier Reef Marine Authority about the risk of widespread coral bleaching.
According to NASA analysis, average temperatures last month were 1.35 degrees above the norm for the 1951-1980 period.
They smashed the previous biggest departure from the average - set only in the previous month - by 0.21 degrees.
"This is really quite stunning ... it's completely unprecedented," said Stefan Rahmstorf, from Germany's Potsdam Institute of Climate Impact Research and a visiting professorial fellow at the University of NSW, noting the NASA data as reported by the Wunderground blog.
The blog's authors, Jeff Masters and Bob Henson, described February's spike as "a true shocker, and yet another reminder of the incessant long-term rise in global temperature resulting from human-produced greenhouse gases".
The monster El Nino event had contributed to the current record run of global temperatures by increasing the area of abnormally warm water in the central and eastern Pacific.
Compared with the rival record giant El Nino of 1997-98, global temperatures are running about 0.5 degrees hotter.
"That shows how much much global warming we have had since then," Professor Rahmstorf said.
The first half of March is at least as warm, he added, and it means temperatures "are clearly more than 1.5 degrees above pre-industrial levels".
"We are in a kind of climate emergency now," Professor Rahmstorf said, noting that global carbon dioxide levels last year rose by a record rate of more than 3 parts per million.
"Governments have promised to act [to curb greenhouse gas emissions] and they need to do better than what they promised in Paris" at the global climate summit last December, he said.
Australia has not dodged the heat, either, with record national temperatures falling at the start of March, the Bureau of Meteorology said in a special climate statement.
The heat surge also comes as the future of climate science hangs in the balance in Australia , with the CSIRO planning to slash monitoring and modelling research.
The most northerly latitudes of the planet were the most abnormally hot regions in February, with large areas reporting temperatures 12 degrees or warmer than average, the NASA data shows.
The unusual heat in the far north means the Arctic sea ice will be thinner and more vulnerable to melting as the region heads into the warmer months, Professor Rahmstorf said.
Arctic sea ice is already at its smallest extent for this time of year on record. The relatively warm seas are contributing to a warmer atmosphere, reinforcing the long-term trend.
As the Wunderground blog noted, the impacts of the unusual global heat have been felt far and wide, including in severe droughts in Vietnam and Zimbabwe.
Fiji, meanwhile, continues work to recover from Cyclone Winston, the most powerful storm recorded in the southern hemisphere.
"[This warming] is not harmless," Professor Rahmstorf said. "It has quite a negative impact on society and the biosphere."
While February's global heat spike is unlikely to be sustained as the El Nino winds down, the latest indicators "are all symptoms of the general warming trend", he said.
Australia may also see some of that impact in coming weeks, with a large region of the Great Barrier Reef under threat from coral bleaching, according to the latest data from the US National Ocean and Atmospheric Administration.
The Great Barrier Marine Park Authority said on Monday that it detected "highly variable but widespread coral bleaching" across the park.
The area around Lizard Island, situated 250 km north of Cairns, and sites further north, had fared the worst, Russell Reichelt, the authority's chair, said in a statement.
"This is the result of sea-surface temperatures climbing as high as 33 degrees during February," Dr Reichelt said. "In the far north, the surveys found severe bleaching on inshore reefs, along with moderate bleaching on mid-shelf reefs."
Cloud cover had helped to moderate bleaching on southern reefs.
"The events on the Great Barrier Reef are part of the global pressure on coral reefs during a strong El Nino weather system which also affected reefs in Hawaii and the Caribbean," Dr Reichelt said, adding that about 5 per cent of shallow reefs had died as a result of bleaching events in 1998 and 2002.
"Bleaching is a vivid reminder of the need for all of us to continue building the resilience of coral reefs to give them the best chance of dealing with increasing climate change impacts," he said.
The Japan Meteorological Agency has confirmed NASA's reading of a surge in global temperatures in February.
According to the JMA, the global average surface temperature in February was 0.62 degrees above the 1981-2010 average and 1.04 degrees above the 20th century average, and was the warmest since records began in 1891.
Last month's anomaly far exceeded the previous largest deviation for the month - set in 1998 - which came in at 0.43 degrees.