Heatwaves across Europe and south Asia have dominated the news recently. But these events are really a surface expression of more fundamental changes that are affecting our planet: the Earth itself is accumulating heat faster than ever before.
We lead a large international team of scientists who come together every year to provide an update on the state of the climate system. This year, we find that Earth’s energy imbalance – the difference between the amount of energy entering and leaving the planet – has doubled in recent decades, and is now at record levels.
This extra heat is a key indicator of the pace and scale of human-caused climate change. In a climate unaffected by human greenhouse gas emissions, the Earth’s energy imbalance would be zero. But, since the 1970s, the Earth has become increasingly out of balance. This rate of increase is faster than expected, and work is underway to understand exactly why this is happening.
Where the extra heat goes
Most of this excess heat doesn’t stay in the atmosphere. Around 90% of it is being absorbed by the oceans, which act as a vast heat sink, but the consequences are playing out across the entire Earth system. The oceans are warming, ice sheets and glaciers are melting, and permafrost is thawing.
Sea levels are rising too, driven by thermal expansion of the warming ocean (a physical increase in the volume of seawater as it absorbs heat) and melting of land-based ice. Since 1901, the global sea level has increased by about 23cm. That might not sound dramatic, but even relatively small increases in sea levels can make storm surges more dangerous, increase coastal flooding and damage ecosystems and infrastructure.
The rate of sea level rise is also speeding up. Between 1901 and 2018, it rose by around 1.7mm per year. In the past decade (2006-25), this rate has more than doubled to over 3.6mm per year.
Arthur Villator / shutterstock
The oceans themselves, warmed by all that stored energy, are also increasing the odds of more extreme events. Marine heatwaves – prolonged periods of unusually high ocean temperatures – are now around three times more frequent than they were in the early 1990s. In 2025, an average area in the ocean would have experienced about 65 marine heatwave days – nearly one day in five – although some regions saw far more and others far fewer.
This is substantially higher than in previous decades and without human intervention, we would only expect to see around three to four days of heatwaves a year. Marine heatwaves can devastate marine life, disrupt fisheries, and weaken the natural coastal protections that many communities rely on.
The impacts of the energy imbalance also don’t stay at sea. Warmer oceans lead to stronger evaporation and a moister atmosphere, making weather extremes such as continental heatwaves, intense rainfall and droughts more likely too.
A warming world meets El Niño
On top of the changes caused by human activity, the energy imbalance combines with natural climate patterns such as El Niño – a periodic warming of the tropical Pacific Ocean that shifts weather patterns across the globe. While El Niño events themselves are natural, today’s are happening in a world that already has a warmer baseline.
Scientists are currently watching for the development of a particularly strong “Godzilla” El Niño, driven by exceptionally warm ocean conditions. As UN Secretary-General António Guterres put it, El Niño can “pour fuel on the fire of a warming world”.
And that fuel shows up in many ways: not just higher temperatures, but also shifts in rainfall patterns. Some regions, such as Australia, may experience drought, while others will see heavier downpours and flooding. No matter what the changes are, they reflect a system holding more energy to drive extreme events and deliver potentially catastrophic impacts.
Whatever our experiences of the upcoming El Niño, the underlying driver of longer-term climate change remains the same. Both the Earth’s energy imbalance and global temperatures are increasing due to record greenhouse gas emissions.
In 2025, human-caused global warming reached about 1.37°C above pre-industrial levels, edging closer to the 1.5°C threshold associated with a dramatic escalation in climate risks and impacts. At the current pace of global warming, that limit could be crossed in around four years.
The prospect of a strong El Niño in the very near future is a stark reminder of how natural variations in the climate system and human-caused warming can combine to produce particularly intense impacts. But it is the steady, continuous build-up of heat in the Earth system that will shape the decades ahead.
The key point is that as long as more heat enters the Earth system than leaves it, temperatures will continue to rise. Until we bring the planet back into balance – by drastically reducing greenhouse gas emissions and ultimately reaching net zero – that heat will keep building. And as it does, the extremes we are experiencing now will only become more frequent, more intense and harder to manage.
