Every major global climate record was broken last year, the World Meteorological Organization (WMO) has said, with its chief voicing particular concern about ocean heat and shrinking sea ice.
The UN weather agency said in its annual State of the Global Climate report that average temperatures hit the highest level in 174 years of record-keeping by a clear margin, reaching 1.45 degrees Celsius above pre-industrial levels.
Ocean temperatures also reached the warmest in 65 years of data with over 90% of the seas having experienced heatwave conditions during the year, the WMO said, harming food systems.
"The WMO community is sounding the red alert to the world," said WMO Secretary-General Celeste Saulo, who took over the job in January.
"What we witnessed in 2023, especially with the unprecedented ocean warmth, glacier retreat and Antarctic sea ice loss, is cause for particular concern."
Ms Saulo later told reporters that ocean heat was particularly concerning because it was "almost irreversible", possibly taking millennia to reverse.
"The trend is really very worrying and that is because of the characteristics of water that keep heat content for longer than the atmosphere," she said.
Climate change, driven by the burning of fossil fuels, coupled with the emergence of the natural El Nino climate pattern, pushed the world into record territory in 2023.
The WMO's head of climate monitoring, Omar Baddour, told reporters there was a "high probability" that 2024 would set new heat records, saying that the year after an El Nino was typically warmer still.
Today's report showed a big plunge in Antarctic sea ice, with the peak level measured at one million kilometres squared below the previous record - an area roughly equivalent to the size of Egypt.
That trend, combined with ocean warming which causes water to expand, has contributed to a more than doubling of the rate of sea-level rise over the past decade compared with the 1993-2002 period, it said.
Ocean heat was concentrated in the North Atlantic with temperatures an average three degrees Celsius above average in late 2023, the report said. Warmer ocean temperatures affect delicate marine ecosystems and many fish species have fled north from this area seeking cooler temperatures.
Ms Saulo, a meteorologist from Argentina who has promised to strengthen global warning systems for climate disasters, said she hoped the report would raise awareness of the "vital need to scale up the urgency and ambition of climate action".
"That's why we spoke about the Red Alert because we must care for the people and how they will suffer from these more frequent, more extreme events," she said.
"If we do nothing, things will become worse and that will be our responsibility."
Lead author of the WMO report John Kennedy said that while global temperatures were expected to increase, 2023 was warmer than expectations.
"People made predictions for what the global temperature would be in 2023 and those were quite far below what actually happened," he said.
"So 2023 was a bit of a surprise even though we expected warming. It was a very warm year, globally."
Speaking on RTÉ's News at One, Mr Kennedy said many of the changes being seen in the environment are "irreversible".
"In terms of the global temperatures, as long as we keep putting greenhouse gases into the atmosphere, temperatures will keep increasing. The same goes for ocean warming and sea level rises," he said.
"The choice is when we want to stop that rise and we need to reduce greenhouse gas emissions into the atmosphere to do that."
Mr Kennedy said the report summarises all the changes being seen as a result of global warming.
"Across the earth we're seeing signs of that warming," he said.
"So the oceans are warming, because of that the sea level is rising. Sea level is also rising because ice on the land is melting, glaciers and ice sheets in Greenland and Antarctica."
Mr Kennedy added that many but not all events can be clearly linked to global warming.
"For some of the extreme events like heavy hail, there's not such a clear link," he added.
"So we have to look at these on a case-by-case basis in some cases, but for heatwaves there's a very clear signal."
Speaking on RTÉ's Drivetime, Sadhbh O'Neill, senior climate advisor at Friends of the Earth Ireland, said she is concerned that people could accept climate records "as the new normal" without realising the extent of the climate crisis.
She said policy makers and politicians need to act like it is an emergency and action needs to happen fast.
"I think the public can grasp an emergency," Ms O'Neill said.
"We saw it during Covid, with high levels of understanding and compliance. We need to tap into that."
She said major corporations and industry also need to heed the calls to take action as climate change "is actually accelerating".
"We need a whole new way of thinking, something much stronger than the Paris agreement that is going to bind countries to phase out fossil fuels," Ms O'Neill said.
