Most people are unaware that water vapour is the atmosphere’s most abundant greenhouse gas (GHG). Since it is not directly linked to the burning of fuels, it is rarely associated with human activity—unlike carbon dioxide (CO₂) and methane (CH₄), the better-known GHGs. The combustion of fossil fuels primarily releases CO₂, while methane largely comes from livestock farming, landfills, rice cultivation, and leaks during the extraction and transport of gas and oil.

However, water vapour enters the atmosphere through evaporation, mainly from the oceans, due to heat. It is thus part of one of the essential phases of the planet’s water cycle: evaporation, condensation, and precipitation.

In the atmosphere, water vapour acts like other greenhouse gases, such as CO₂ and CH₄, by trapping heat that would otherwise escape into space. This creates a positive feedback loop: the more heat and evaporation; the more vapour, the greater the atmosphere’s capacity to retain heat. In other words, warming induces more humidity, and that humidity amplifies the warming.

The ocean holds 96.5% of all water on the planet—around 1,338 million km³. By contrast, the water present in the atmosphere totals just 12,900 km³, a mere 0.001% of the total. It’s a tiny proportion, but one with an outsized role: beyond determining rainfall patterns, water vapour, together with air temperature, regulates the energy balance of the atmosphere.

At a planetary scale, the water cycle may seem minimal. But at the scale of the biosphere—the thin atmospheric layer where life thrives—the axis sustains it. This is why any change in water vapour distribution can disrupt the cycle and deeply affect climate phenomena: droughts, floods, hurricanes, typhoons, or polar ice melt.

Warmer seas mean more water vapour, and oceans are steadily heating up. A key effect of global warming is that most of the added energy in the climate system accumulates in the sea. It’s estimated that oceans absorb around 89% of the excess heat in the atmosphere, raising water temperatures and causing thermal expansion—already evident in rising sea levels.

In 2024, the average sea surface temperature reached a historic high: 20.87 °C, which is 0.51 °C above the 1991–2020 average. This warming caused a significant increase in atmospheric water vapour and an uneven redistribution of moisture on a global scale.

According to the Copernicus Climate Change Service, 2024 was the hottest year on record and was accompanied by 5% more water vapour in the atmosphere compared to the reference period. This isn’t just a data point—it’s an accelerator of extreme events, and 2024 was devastating.

Extreme rainfall intensified. A DANA storm hit Valencia, Spain, causing 228 deaths. In Kenya’s capital, Nairobi, catastrophic floods in the informal settlements of Kibera, Mathare, and Mukuru left at least 188 dead and nearly 200,000 displaced.

Elsewhere, unprecedented heatwaves struck across the globe. India and Pakistan recorded 50 °C in Shaheed Benazirabad, 8.5 °C above average. In New Delhi, temperatures reached 49 °C—one of the longest heatwaves in its history. In April, Thailand, Vietnam, and the Philippines saw highs of 38.8 °C. In Europe, Greece and Turkey exceeded 43 °C and 44 °C starting in June. During the Hajj pilgrimage, Mecca reached 49 °C, contributing to the deaths of over 1,300 pilgrims. Even Antarctica, in July, experienced an unprecedented winter heatwave, with temperatures up to 28 °C above seasonal norms.

This warming is no fluke. In addition to ocean heating, it’s linked to broader patterns such as El Niño and ice loss. Scientists expected global cooling with the arrival of La Niña, but it hasn’t occurred.

More vapour in the atmosphere means more energy available to fuel intense rainfall, destructive storms, and extreme weather events. The good news is that understanding water vapour’s role helps improve extreme weather prediction and design more accurate early warning systems.

Organisations such as the European Centre for Medium-Range Weather Forecasts (ECMWF), which manages the Copernicus Climate Change Service, and the US National Oceanic and Atmospheric Administration (NOAA), which runs the GFS model, lead global monitoring of key atmospheric variables like water vapour.

Copernicus provides an open-access portal where anyone, from scientists and journalists to citizens, can consult and download climate data, satellite imagery, maps, projections, and historical analysis.

The atmosphere is responding to a new climatic balance, where every drop of humidity matters and every fraction of a degree transforms its behaviour. In the global climate crisis context, free and rigorous access to scientific data is more important than ever. We must understand atmospheric variables and measure them using reliable scientific tools developed by agencies with decades of experience. Climate science saves lives, and scientists are our most reliable witnesses. Trusting and supporting them is the responsibility of governments, the media, and each of us.