Explained | Earth has just ended a 13-month streak of record heat, what to expect next?

By Christopher Merchant for The Conversation

Reading: A 13-month streak of record-breaking global warmth has ended.

From June 2023 until June 2024, air and ocean surface water temperatures averaged a quarter of a degree Celsius higher than records set only a few years previously. Air temperatures in July 2024 were slightly cooler than the previous July (0.04 degree Celsius, the narrowest of margins) according to the EU’s Copernicus Climate Change Service.

July 2023 was in turn 0.28 degree Celsius warmer than the previous record-hot July in 2019, so the remarkable jump in temperature during the past year has yet to ease off completely. The warmest global air temperature recorded was in December 2023, at 1.78 degree Celsius above the pre-industrial average temperature for December – and 0.31 degree Celsius warmer than the previous record.

Global warming has consistently toppled records for warm global average temperatures in recent decades but breaking them by as much as a quarter of a degree for several months is not common. The end of this streak does not diminish the mounting threat of climate change.

So what caused these record temperatures? Several factors came together, but the biggest and most important is climate change, largely caused by burning fossil fuels.

What caused the heat streak Temperatures typical of Earth 150 years ago are used for comparison to measure modern global warming. The reference period, 1850–1900, was before most greenhouse gases associated with global industrialisation – which increase the heat present in Earth’s ocean and atmosphere – had been emitted.

July 2024 was 1.48 degree Celsius warmer than a typical pre-industrial July, of which about 1.3 degree Celsius is attributable to the general trend of global warming over the intervening decades. This trend will continue to raise temperatures until humanity stabilises the climate by keeping fossil fuels in the ground where they belong.

But global warming doesn’t happen in a smooth progression. Like UK house prices, the general trend is up, but there are ups and downs along the way.

Behind much of the ups and downs is the El Niño phenomenon. An El Niño event is a reorganisation of the water across the vast reaches of the Pacific Ocean.

El Niño is so important to the workings of worldwide weather as it increases the temperature of the air on average across all of Earth’s surface, not only over the Pacific. Between El Niño events, conditions may be neutral or in an opposite state called La Niña that tends to cool global temperatures. The oscillation between these extremes is irregular, and El Niño conditions tend to recur after three to seven years.

The warm El Niño phase of this cycle began to kick in a year ago, reached its peak around the end of 2023 and is now trending neutral, which is why the record-breaking streak has ended.

The 2023/2024 El Niño was strong, but it wasn’t super-strong. It doesn’t fully explain the remarkable degree to which the past year broke temperature records. The exact influence of other factors has yet to be fully untangled.

We know there is a small positive contribution from the Sun, which is in a phase of its 11-year sunspot cycle in which it radiates fractionally more energy to the Earth.

Methane (also a byproduct of the fossil fuel industry, alongside cattle and wetlands) is another important greenhouse gas and its concentration in the air has risen more rapidly in the past decade than over the previous decade.

Scientists are also assessing how much measures to clean up air pollution might be adding to warming, since certain particulate air pollutants can reflect sunlight and influence the formation of clouds.

A temperature ratchet Across the global ocean, 2023 was a devastating summer for coral reefs and surrounding ecosystems in the Caribbean and beyond. This was followed by heavy bleaching across the Great Barrier Reef off Australia during the southern hemisphere summer.

While it is El Niño years that tend to see mass mortality events on reefs around the world, it is the underlying climate change trend that is the long-term threat, as corals are struggling to adapt to rising temperature extremes.

As the Pacific Ocean is now likely to revert towards La Niña conditions, global temperatures will continue to ease back, but probably not to the levels seen prior to 2023/24.

El Niño acts a bit like a ratchet on global warming. A big El Niño event breaks new records and establishes a new, higher norm for global temperatures. That new normal reflects the underlying global warming trend.

A plausible scenario is that global temperatures will fluctuate near the 1.4 degree Celsius level for several years, until the next big El Niño event pushes the world above 1.5 degree Celsius of warming, perhaps in the early 2030s.

The Paris agreement on climate change committed the world to make every effort to limit global warming to 1.5°C, because the impacts of climate change are expected to accelerate beyond that level.

The good news is that the shift away from fossil fuels has started in sectors such as electricity generation, where renewable energy meets a growing share of rising demand. But the transition is not happening fast enough, by a large margin. Meeting climate targets is not compatible with fully exploiting existing fossil-fuel infrastructure, yet new investment in oil rigs and gas fields continues.

Headlines about record breaking global temperatures will probably return. But they need not do so forever. There are many options for accelerating the transition to a decarbonised economy, and it is increasingly urgent that these are pursued.

(Christopher Merchant is a Professor of Ocean and Earth Observation, University of Reading.)