An exceptionally strong El Niño event could propel atmospheric carbon dioxide (CO2) levels to record highs in 2027, the Met Office has warned. El Niño events are known to amplify human sources of emissions while simultaneously reducing the planet's capacity to absorb these greenhouse gases.
Forecasters predict that the concentration of CO2, as measured at the Mauna Loa Observatory in Hawaii, could rise by approximately 3.51 parts per million (ppm) next year. This projected increase would be the largest year-on-year rise ever forecast by the Met Office, rivaling the record set in 2024 due to significant wildfires. If this forecast holds true, the total atmospheric concentration could reach a historic 432.2 ppm.
This development could pose significant challenges to global efforts to keep average global warming below the 1.5°C threshold outlined in the Paris Agreement, with potentially severe consequences. The El Niño phenomenon is part of the El Niño Southern Oscillation (ENSO) cycle, which alternates between warm (El Niño) and cold (La Niña) phases every two to seven years.
During an El Niño phase, Pacific trade winds that typically move warm water westward slow or reverse. This leads to a buildup of warm waters in the Equatorial Pacific, contributing to widespread weather disruptions and above-average global temperatures. Experts have also warned of potential consequences for the UK, including severe storms and winter chaos.
Met Office scientist Emily Wright explained that while human emissions are the primary driver of the long-term CO2 increase, natural sinks like plants, soils, and oceans typically absorb a portion of these emissions. "El Niño events reduce how much they absorb overall, leaving more CO2 in the atmosphere," Wright stated. This reduction in absorption is partly due to El Niño typically causing hot, dry weather in tropical regions, which can hinder the carbon-sequestering capabilities of vegetation and increase the risk of wildfires.
Additionally, hotter weather increases demand for energy, such as for air conditioning, leading to higher emissions. Despite El Niño's exacerbating effects, experts emphasize that human activity remains the most significant driver of rising emissions. Professor Richard Betts, Head of Climate Impacts Research at the Met Office Hadley Centre, noted that while natural variability like El Niño can alter annual CO2 rise rates, "the long-term trend from human-caused emissions" is the underlying factor. He added that the current rate of CO2 increase remains too high to align with limiting warming to 1.5°C.
While this increased CO2 concentration may not have immediate effects on next year's weather, it contributes to ongoing long-term warming. The implications include continued risks of extreme weather events, such as droughts, floods, and sea-level rise, in the coming years and decades.
Recent data indicates that global greenhouse gas emissions have reached an all-time high, with CO2 growth rates tripling since 1960. Between 2011 and 2020, CO2 growth accelerated from 0.8 ppm per year to 2.4 ppm per year. From 2024 to 2025, the global average CO2 concentration increased by 2.8 ppm, bringing the total to 426.8 ppm by the end of 2025. This sustained increase has effectively "locked in" further warming for years to come, even with potential future reductions in emissions. Experts warn that this trend increases the climate risks for the future. The El Niño Heat Mortality Tracker also indicates that an additional 370,000 lives could be at risk globally over the next six months due to heat, with Nigeria, India, and Brazil expected to be disproportionately affected. The UK has also seen an estimated 820 deaths attributed to El Niño's heat during the recent summer. The Paris Agreement, signed in 2015, aims to limit the global average temperature increase to well below 2°C above pre-industrial levels, with an ambitious goal to pursue efforts to limit it to 1.5°C. The agreement calls for global emissions to peak as soon as possible and then undergo rapid reductions in line with the best available science.