Last month was the world's warmest January on record and raised further questions about the pace of climate change, scientists say.

January 2025 had been expected to be slightly cooler than January 2024 because of a shift away from a natural weather pattern in the Pacific known as El Niño.

But instead, last month broke the January 2024 record by nearly 0.1C, according to the European Copernicus climate service.

The world's warming is due to emissions of planet-heating gases from human activities - mainly the burning of fossil fuels - but scientists say they can't fully explain why last month was particularly hot.

It continues a series of surprisingly large temperature records since mid-2023, with temperatures around 0.2C above what had been expected.

"The basic reason we're having records being broken, and we've had this decades-long warming trend, is because we're increasing the amount of greenhouse gases in the atmosphere," Gavin Schmidt, director of Nasa's Goddard Institute for Space Studies, told BBC News.

"The specifics of exactly why 2023, and 2024, and [the start of] 2025, were so warm, there are other elements involved there. We're trying to pin those down."

January 2025 ended up 1.75C warmer than January temperatures of the late 19th Century, before humans started significantly warming the climate.

Early last year, global temperatures were being boosted by the natural El Niño weather pattern, where unusually warm surface waters spread across the eastern tropical Pacific. This releases extra heat into the atmosphere, raising global temperatures.

This year, La Niña conditions are developing instead, according to US science group Noaa, which should have the opposite effect.

While La Niña is currently weak - and sometimes take a couple of months to have its full effect on temperatures – it was expected to lead to a cooler January.

"If you'd asked me a few months ago what January 2025 would look like relative to January 2024, my best shot would have been it would be cooler," Adam Scaife, head of monthly to decadal predictions at the UK Met Office, said.

"We now know it isn't, and we don't really know why that is."

A number of theories have been put forward for why the last couple of years have been warmer than anticipated.

One idea involves a prolonged response of the oceans to the 2023-24 El Niño.

While it was not especially strong, it followed an unusually lengthy La Niña phase from 2020-23.

The El Niño event might therefore have "lifted the lid" on warming, allowing ocean heat that had been accumulating to escape into the atmosphere.

But it's unclear how this would still be directly affecting global temperatures nearly a year after El Niño ended.

"Based on historical data, that effect is likely to have waned by now, so I think if the current record continues, that explanation becomes less and less likely," says Prof Scaife.