A record set before the peak

Sea surface temperatures in Nino3.4, a tell-tale El Nino monitoring region of the Pacific, climbed to a record 29.70C in September, according to the Copernicus climate service. Copernicus said the new measurement exceeded the peak temperatures of previously strong El Nino events of 1982/1983, 1997/1998 and 2015/2016.

The reading stands out because the event has not yet reached its peak. Forecasters expect the weather pattern - which raises drought and flood risks and exacerbates global warming - to become the strongest ever recorded this year. Copernicus told AFP the September values point to an already exceptionally strong El Nino, especially given that the episode has not peaked.

Why Copernicus will not call it the strongest yet

Despite the record, the service said in a statement to AFP that it cannot yet say the current El Nino is the strongest ever recorded. The hesitation rests on a methodological change: Copernicus said it had adopted a new index which accounts for the warming of tropical oceans, and under that measure the current event is not yet stronger than past ones.

Two further complications make historical comparison difficult. Not all forecasters use the same scale to interpret El Nino, and as climate change raises ocean temperatures it can make recent events appear stronger than past ones.

Japan's agency sees record intensity

A second measurement body has gone further. On Friday, Japan's weather agency said El Nino conditions had reached their strongest intensity on record, with temperatures in the phenomenon's monitoring area 4C above normal.

The pattern behind both readings works as follows:

• •El Nino warms surface temperatures in the central and eastern equatorial Pacific Ocean
• •It brings worldwide changes in winds, pressure and rainfall patterns
• •It typically takes place every two to seven years and lasts around nine to 12 months

September heat and the 1.5C limit

The Pacific record sits inside a broader month of heat. Copernicus said global average sea surface temperatures hit a record 21.05C for September, topping the previous high of 20.92C for the same month in 2023. The world also experienced its second hottest September on record, with surface air temperatures averaging 16.27C - 1.63C hotter than the pre-industrial average, a period defined as 1850-1900.

That baseline matters for the Paris Agreement, under which nations agreed at the 2015 UN climate talks to keep warming well below 2C above pre-industrial levels. The United Nations declared in September that the long-term 1.5C target would be missed in a matter of years.

Copernicus director Carlo Buontempo warned that the current run of warm months has further to go.

Buontempo added that crossing the Paris Agreement's long-term limit is only a matter of time, and how far we overshoot, and for how long, is still up to us. Scientists say it is crucial to reduce greenhouse gas emissions from fossil fuels to limit the period of overshoot above 1.5C and bring temperatures back down quickly.

Copernicus says the September values point to an already exceptionally strong El Nino, and experts have warned the event would likely push global temperatures to a new record in 2027.