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Causes: Water vapour and clouds (H2O)

Lake Pukaki – image: Cody Whitelaw

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Home > Climate wiki > What causes climate change? > Greenhouse gases > Water vapour & clouds

Water vapour and clouds (H2O)

Summary


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, May 2024

  • Water vapour is the strongest greenhouse gas, accounting for 60% of warming. HOWEVER, it is not an anthropogenic forcing. It’s driven by and in turn amplifies the effect of other greenhouse gases. As temperatures increase the atmosphere contains more water vapour. 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 versus 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. The most recent research (2024) 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, April 2024.

Water vapour is turbocharging the hydrology cycle

That means more evaporation in areas that are already dry, and also increased precipitation (rain, hail, snow etc.) in regions that already receive high rainfall. Higher sea surface temperatures mean 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 also for tropical storms to reach 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.

  Causes

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Home > Climate wiki > What causes climate change? > Greenhouse gases > Water vapour & clouds

Summary

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, May 2024

  • Water vapour is the strongest greenhouse gas, accounting for 60% of warming. HOWEVER, it is not an anthropogenic forcing. It’s driven by and in turn amplifies the effect of other greenhouse gases. As temperatures increase the atmosphere contains more water vapour. 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 versus 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. The most recent research (2024) 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, April 2024.

Water vapour is turbocharging the hydrology cycle

That means more evaporation in areas that are already dry, and also increased precipitation (rain, hail, snow etc.) in regions that already receive high rainfall. Higher sea surface temperatures mean 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 also for tropical storms to reach 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.

Impacts on Aotearoa

“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. Links in the drop tab below will take you to key pages on NIWA, each of which lists several projects in different sectors under an overarching New Zealand Government mandated and funded programme. See also the pages on this website on floods and rivers.

Clouds

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. Here we report how stratocumulus decks respond to greenhouse warming in large-eddy simulations that explicitly resolve cloud dynamics in a representative subtropical region. 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

Fig. 1: Stratocumulus clouds
Fig. 1: Stratocumulus clouds
The important point here, is that once  the 1200ppm 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 this case, greenhouse gas concentrations in the atmosphere reaching 1200ppm. 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).

Other clouds act as blankets

Stratus and stratocumulus are only a few of the many types of clouds. Some 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, instead carry water. This may turn clouds that once reflected the sun’s heat into clouds that act as blankets.

“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.

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) from aircraft are known to create the kind of cloud cover that acts as a blanket, preventing heat from escaping Earth, thereby making the planet warmer.

“Experts are concerned that efforts to change aviation engine design to reduce CO2 emissions could actually create more contrails and raise daily temperatures even more.” – Fred Pierce, Yale360

Fig. 2: Contrails (image: Wikipedia)
Fig. 2: Contrails (image: Wikipedia)

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