Why 2023 was the warmest year ever, and what happens now

The announcement was confirmation of what scientists have known for a few months. The World Meteorological Organisation (WMO) had said in November last year said that 2023 was set to emerge as the warmest year on record, based on data until October. The gap with 2016 was so large that data from November and December were unlikely to change this situation.

Global surface air temperature increase relative to the average for 1850-1900, the designated pre-industrial reference period, based on several global temperature datasets shown as 5-year averages since 1850 (left) and as annual averages since 1967 (right). Credit: C3S/ECMWF.

Rising temperatures contributed to a large number of extreme weather events around the world in 2023, including heatwaves, floods, droughts, and wildfires. Canada had its most destructive wildfire season (May to September) on record, with more than 45 million acres burned.

Tumbling records

Climate records fell like dominoes in the exceptional heat spell witnessed last year.

2023 marked the first time on record that every day within a year exceeded 1 degree Celsius above the 1850-1900 pre-industrial level.

About 50% of days were more than 1.5 degree Celsius warmer than the 1850-1900 level — and two days in November were, for the first time, more than 2 degree Celsius warmer.

Daily global surface air temperature increase relative to the average for 1850–1900, the designated pre-industrial reference period, for 2023. The plot highlights temperature increases within three ranges: 1–1.5°C (orange), 1.5–2°C (red), and above 2°C (crimson). Source: ERA5. Credit: C3S/ECMWF

This, however, does not mean that the planet has breached the 1.5 degree and 2 degree Celsius thresholds set by the 2015 Paris Agreement. Those thresholds refer to long-term warming — which means global temperatures over a period of 20-30 years, on average, must not exceed 1.5 degree or 2 degree Celsius.

Global daily average sea surface temperature (SST) also went off the charts. Since mid-March 2023, the daily average SST has been the highest ever, according to Climate Reanalyzer, a website produced by the Climate Change Institute at the University of Maine, which provides visualisations of publicly available datasets and models.

It led to marine heatwaves in the Mediterranean, the Gulf of Mexico and the Caribbean, the Indian Ocean and the North Pacific, and much of the North Atlantic.

Sea ice extent in Antarctica plummeted to a new low. In September, it reached an annual maximum 16.96 million sq km, which was 1.03 million sq km less than the previous record low set in 1986.

Why was it so hot?

The main driver behind the extreme warming is the increasing greenhouse gas concentrations in the atmosphere. Greenhouse gases like carbon dioxide and methane, and water vapour trap the Sun’s energy in the Earth’s system before it escapes to space, causing warming.

Since the industrial revolution, human activities like burning fossil fuels such as coal, oil, and gases have released unprecedented levels of such gases. As a result, the planet has warmed rapidly, especially in recent decades.

In 2023, greenhouse gas concentrations reached the highest levels ever recorded in the atmosphere, according to C3S and the Copernicus Atmosphere Monitoring Service (CAMS). Carbon dioxide concentrations in 2023 were 2.4 parts per million (ppm) higher than in 2022; methane concentrations increased by 11 parts per billion (ppb).

Monthly global mean atmospheric CO2 (left) and CH4 (right) column-averaged concentration from satellites for 2003-2023 (grey curve) and 12-month average (red curve). Credit: C3S/CAMS/ECMWF/University of Bremen/SRON.

The onset of El Niño last year, after seven years, played a role. El Niño refers to an abnormal warming of surface waters in the equatorial Pacific Ocean. It increased the likelihood that temperature records would be broken, and there would be more extreme heat in many parts of the world and the ocean.

What could happen now?

Scientists have suggested that 2024 could be warmer than even 2023. Typically, in the past few decades, very hot years have been ones that began in an El Niño state. Last year, El Niño didn’t set in until around July, which means that it wasn’t the primary reason behind the abnormal heat at that point, Emily J Becker, a climate scientist at the University of Miami, told The New York Times.

Therefore, 2024 could be hotter than last year. “It’s very, very likely to be top three, if not the record,” Becker said, referring to 2024.

It is also possible that the next year may surpass the 1.5 degree Celsius warming threshold across the entire calendar year for the first time. The WMO in its 2023 State of Global Climate report said there was a 66% chance that at least one of the years between 2023 and 2027 would cross the threshold.

A long-term breach of the 1.5 degree limit would unleash far more severe climate change impacts, including more frequent and severe droughts, heatwaves, and rainfall. To prevent this, the world needs to urgently implement certain steps, including greenhouse gas emission cuts, which it has failed to do so far.