THE world remains on course for another record hot year with the first four months of 2015 setting a new high for temperatures, amid more signals the emerging El Nino event will be a strong one.
Combined land and sea surface temperatures in April alone were 0.74 degrees above the 20th century average, making it the fourth warmest on record, according the US National Oceanic and Atmospheric Administration (NOAA).
Sea-surface temperatures last month were the warmest in records going back to 1880, surpassing the previous high set in 1998 during a 'super El Nino' event. Most of the Pacific - where the El Nino forms - were particularly warm.
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Last month's warmth continued the series of above-average temperatures as climate change lifts the background readings. The last cooler-than-average April was in the late 1970s, NOAA said.
The first four months of 2015 were 0.8 degrees above the 20th century average for that period, eclipsing the same months in 2010 by 0.07 degrees, the US agency said.
The past 12 months also tied with the year to March as the warmest such period on record. All but one of the 10 warmest rolling 12-months periods have occurred in the past two years, NOAA said.
One of those periods was January-December last year, making it the hottest calendar year in records dating to 1880, eclipsing 2005 and 2010, NOAA and other agencies say.
The Pacific features prominently in terms of abnormal warmth for the first four months of the year, while north-eastern North America and the North Atlantic were notable patches of cold conditions.
NOAA said a weak-to-moderate El Nino is now present in the Pacific, and there is an 80 per cent chance it will last until the end of 2015.
During El Nino events, central and eastern equatorial regions of the ocean are relatively warm compared with the west as trade winds stall or reverse - as they have in recent weeks.
"El Nino conditions tend to enhance global temperatures, with stronger events having generally larger impacts," NOAA said.
El Nino signs
Australia's Bureau of Meteorology last week declared a "substantial" El Nino was possible this year, which could lead to warmer and drier than average conditions across eastern Australia.
The bureau's readings since then have confirmed if not extended that conclusion.
One gauge that the agency uses to identify an El Nino - the Southern Oscillation Index measuring pressure differences between Darwin and Tahiti - has continued to drop well below the minus 7 threshold used to categorise an El Nino.
Its latest 30-day moving index reading has dived to minus 17.1 as of Tuesday, the bureau said.
Another criterion for El Nino thresholds is the temperature anomaly in the equatorial Pacific region dubbed NINO3.4. Sustained anomalies of 0.8 degrees indicate an El Nino is underway.
The latest reading for this region was 1.1 degree warmth anomaly for the week to last Sunday, May 17, or well into El Nino territory.
More notable, perhaps is the prediction for October of a 2.4 degree above-average warmth for the region, based on a combination of the main global models (see below).
"This value of NINO3.4 has only been observed on a few occasions since 1980: during the 1982–83 and 1997–98 El Nino events," the bureau said this week.
Both of those previous events were 'super El Ninos', and are associated with significant droughts and fires in western parts of the Pacific - including the Ash Wednesday bushfires in South Australia and Victoria - and serious flooding in other parts of the world such as South America.