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Causes: Water vapour and clouds (H2O)

Lake Pukaki – image: Cody Whitelaw

Home > Climate wiki > What causes Earth’s climate to change? > Water vapour & clouds

Summary

Water vapour is the strongest greenhouse gas, accounting for 60% of warming. Volcanoes release some water vapour into stratosphere. The effects are minimal compared to human activities that have raised global temperatures, causing far more water vapour to enter the atmosphere. This in turn amplifies the effect of other greenhouse gases. This feedback effect leads to even more warming, more evaporation, and so on.

Water vapour does this because heat radiated from Earth’s surface is absorbed by water vapour molecules in the lower atmosphere. The water vapour molecules, in turn, radiate heat in all directions.

Clouds form when water molecules condense onto a surface that’s warmer than the air: dust, soot, salt crystals etc. The type of cloud, how reflective they are, how high they are, whether it’s day or night, and the percentage of ice crystals compared to water molecules in them all help determine whether they contribute to warming or cooling. 

Clouds are the biggest uncertainty in climate models as they both shade and cool the Earth and also trap heat. Recent research suggests that the existing models may be underestimating the positive feedback (warming):

The positive opacity component arises from the disproportionate reduction in the area of thick, climate-cooling clouds relative to thin, climate-warming clouds. This suggests that thick cloud area is tightly coupled to the rate of convective overturning—which is expected to slow with warming—whereas thin cloud area is influenced by other, less certain processes. The positive feedback differs markedly from previous estimates and leads to a +0.3 °C shift in the median estimate of equilibrium climate sensitivity relative to a previous community assessment. – Sokol et al 2024

Recent Earth energy budget observations show an increase in the sunlight absorbed by the Earth of 0.45 W/m2 per decade, caused primarily by a decrease in cloud reflection. – Tselioudis et al  2025

What’s in this page:

Terms: evaporation, condensation, transpiration, precipitation

 

Evaporation occurs more frequently at higher temperatures because the water molecules are moving more quickly. 

Condensation is the opposite. Water molecules bring heat energy with them, so the surface of the dust warms slightly while the temperature of the surrounding air cools slightly, allowing the droplets to condense. This is due to a fundamental law of thermodynamics. The Clausius-Clapeyron Equation describes a discontinuous phase transition between the different states (gas, liquid, solid) of water. This equation comes from experiments done 200 years ago, using a sealed bell-jar in a lab. 
 

In media articles about unprecedented flooding, you’ll often come across the statement that for every 1°C of warming, the atmosphere can hold about 7% more moisture. This figure comes from research undertaken by the French engineer Sadi Carnot and published 200 years ago this year. We now know there’s more to the story. Yes, a hotter atmosphere has the capacity to hold more moisture. But the condensation of water vapour to make rain droplets releases heat. This, in turn, can fuel stronger convection in thunderstorms, which can then dump substantially more rain. This means that the intensity of extreme rainfall could increase by much more than 7% per degree of warming. 

What we’re seeing is that thunderstorms can likely dump about double or triple that rate – around 14–21% more rain for each degree of warming. – Dowdy et al  2024

No, a law of thermodynamics hasn’t been broken. Simply put, temperatures and the volume of water inside a tiny bell jar can be strictly controlled, with no external forces allowed inside. The global atmosphere is not a sealed bell jar.  Air masses with very different temperatures are continuously moving vertically and horizontally over oceans and deserts, mountains and rainforests. They move in part due to the Coriolis force (Earth’s rotation) and are influenced by massive ocean currents and huge cyclic events such as ENSO, and the temperature of the land, ice caps, and ocean that the flow over. These and many other factors including transpiration play an enormous role in how much water is actually evaporated, condensed, and precipitates.

Transpiration is the process by which plants ‘exhale’ water vapour through their stomata. Plants lose more than 90% of their water through transpiration. However in the last 150 years as CO2 has been increasing, the density of stomata in some plants has dropped 34%. This is restricting the amount of water vapour the plants release. This has implications for the water cycle, especially in tropical rainforests, which by definition create rain largely through transpiration. 
 

Plants get more water-efficient and leak less underground soil moisture out through their pores in a carbon-rich atmosphere. Add this up over billions of leaves in very sunlit, leafy places, especially the tropics, and it means there is a bunch more soil moisture stored underground, so much so that climate models predict rainfall events will saturate the ground and more rain will run off into rivers. – Ass. Professor Mike Pritchard, UCI

Precipitation is any liquid or frozen water (rain, hail, graupel, snow) that forms in the atmosphere and falls back to Earth’s surface.

Water vapour is turbocharging the hydrology cycle

This is because higher temperatures leads to more evaporation in areas that are already dry, and increased precipitation in regions that already receive high rainfall. Higher sea surface temperatures means there’s more water vapour over the oceans. As New Zealand is surrounded by the ocean, this increases the risk of heavy rain and snow fall, and more intense and more frequent  tropical storms reaching further south. 

This brings ever increasing risks for extreme flooding. Yet with higher evaporation and transpiration, areas that are now prone to drought are likely to see longer and more intense droughts.

This century, climate change will alter New Zealand’s natural water cycle significantly. It will change how much rain and snow we receive, and at what time of year. It will change how much water is stored in the soil, snow, glaciers and aquifers. It will change how much water evaporates back to the atmosphere and how much flows through streams and rivers to the coast. And it will change the severity of droughts, floods and power shortages. – National Science Challenges

More than 60% of people living in Aotearo live on flood plains. Every aspect of our lives will continue to feel increasingly damaging impacts, including atmospheric rivers dumping large quantities of rain in very short periods (days vs months).

Multiple scientific projects are currently underway to assess the type, scale and cross-sector impacts, from river flows, loss of glaciers, farming, floods, and other extreme weather events, to the costs of insurance and how local councils need to plan for these changes. 

Clouds

Clouds come in all shapes and sizes, but two of the most consistent cloud swaths are formed by Earth’s large-scale airflow patterns. One band, near the equator, stretches around the planet like a belt. It forms as trade winds of the Northern and Southern hemispheres converge, forcing moist air upward to cool and condense into clouds. Another band occurs in the mid-latitudes, where jet streams usher large swirls of stormy weather around the planet. 

Storm clouds contracting allows more heating

In 2024, researchers found that equatorial cloud bands had narrowed, while the tracks of mid-latitude storms had shifted toward the poles, hemming in the region in which they can form and shrinking their coverage. Based on satellite imagery, they found that cloud coverage had decreased by about 1.5% per decade. The also found that 80% of the overall reflectivity changes in these regions resulted from shrinking clouds, rather than darker, less reflective ones, which could be caused by a drop in pollution. In 2025 researchers further refined this: 

Analysis of satellite observations shows that in the past 24 years the Earth’s storm cloud zones in the tropics and the middle latitudes have been contracting at a rate of 1.5%–3% per decade. This cloud contraction, along with cloud cover decreases at low latitudes, allows more solar radiation to reach the Earth’s surface. When the contribution of all cloud changes is calculated, the storm cloud contraction is found to be the main contributor to the observed increase of the Earth’s solar absorption during the 21st century. – Tselioudis et al June 2025

Stratocumulus clouds act as sunshades

Stratocumulus clouds cover 20% of the low-latitude oceans and are especially prevalent in the subtropics. They cool the Earth by shading large portions of its surface from sunlight. However, as their dynamical scales are too small to be resolvable in global climate models, predictions of their response to greenhouse warming have remained uncertain.

In the simulations, stratocumulus decks become unstable and break up into scattered clouds when CO2 levels rise above 1,200 ppm. In addition to the warming from rising CO2 levels, this instability triggers a surface warming of about 8 K globally and 10 K in the subtropics. Once the stratocumulus decks have broken up, they only re-form once CO2 concentrations drop substantially below the level at which the instability first occurred. Climate transitions that arise from this instability may have contributed importantly to hothouse climates and abrupt climate changes in the geological past. Such transitions to a much warmer climate may also occur in the future if CO2 levels continue to rise*. – Shneider et al, Nature 2019 

* CO2 levels increased from 410ppm in 2019, reaching 430ppm in 2026. See Video 2

A crucial point discovered in the research is that once the 1,200ppm threshold is reached, a tipping point is passed. 
 
A characteristic of tipping points is that once a system has been tipped into a new state, it can’t be returned to its original state simply by undoing the tipping event. 

In order for stratocumulus clouds to begin forming again, greenhouse gas concentrations in the atmosphere would need to drop below 300ppm.

See this NASA explanation of the research paper: Clouds, Arctic Crocodiles and a New Climate Model.
Fig. 1: Stratocumulus clouds

Other clouds act as blankets

Some types of clouds act as blankets, preventing heat from escaping into the atmosphere. This is why going outside in the evening when the sky is cloudy, is warmer than cloud-free nights. But as the climate warms, clouds that once carried ice will carry water instead. This could turn those  clouds that once reflected the sun’s heat into clouds that act as blankets, keeping the heat from escaping.
As the climate warms, cloud ice is gradually being replaced with water – a change that has an overall cooling effect. But what happens when there is no cloud ice left? Our climate model simulations suggest that we then reach a state where warming accelerates. – Carlsen et al 2020

Recently, researchers discovered that some some Arctic clouds are enhancing the melting of sea ice (Video 3).

Other types include those that form quickly and can dump larger quantities of rain, hail, and snow in short periods. Rather than acting as sun shades, these types of ‘weather bomb’ clouds may become more common. For example, contrails (Fig. 2) have quite different effects, depending on how they interact with cirrus clouds:

Results like Petzold’s [research] can also help inform the work that many in and around the aviation industry are doing to improve forecasting so that aviators can follow flight paths that limit the potential for long-lived contrails to form. – EOS, 2026

Contrails

Contrails, or cirrus homogenitus (man-made clouds) form at certain altitudes and weather conditions, when water vapour condenses onto the soot particles emitted by aircraft engines to form liquid droplets, which freeze into ice crystals. In dry air, they disappear quickly. In cold, humid conditions, they spread and can persist for hours, forming wide sheets of cloud.

The net climate effect of a contrail changes depending on the thickness of the cloud in which it forms. Thicker cirrus clouds can buffer the warming that contrails might contribute and can even lead to local cooling. But when contrails appear in thin clouds (many so thin that the eye can’t see them), the force of their warming can become even more significant than if they had formed in clear skies. – EOS, 2026

Contrails are a climate puzzle written across the sky: As airlines test new routes and researchers refine models, contrails are shifting from an afterthought of flight to a potential tool for cutting the carbon footprint of aviation. – Anthropocene magazine 2026

Fig. 2: Contrails (image: Wikipedia)

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