Stratus clouds

Cirrus clouds

Cumulus clouds

Evaporation, Precipitation and Climate Change

Rising global temperatures are likely to accelerate evaporation rates worldwide. So more evaporation is likely to lead to more rainfall, in a globally averaged sense. Many scientists speak of a "spun up" water cycle as a result of global warming; meaning there would be more water cycling through the atmosphere as a result of the increased precipitation rate.

There is no guarantee, however, that these increased evaporation and precipitation rates will be distributed evenly worldwide. Some areas may experience floods, and other areas droughts, as the traditional locations of rain belts and deserts shift in response to a changing climate. Some climate models predict that coastal regions will become wetter and the middle of continents will become drier. Also, some models forecast more evaporation and rainfall over oceans, but not necessarily over land. 

Warmer temperatures and increased carbon dioxide levels may encourage plant growth in many regions. Increased transpiration, the release of water into the air by plants as a result of photosynthesis, would be likely in such a scenario. Uptake of water from the soils by plants would also need to increase if there were more plants, or if plants began to grow more rapidly.  

Clouds and Climate

Different types of clouds have different affects on the Earth's climate. While some types of clouds help to warm the Earth, others help to cool it. For example:

  • Stratus clouds: A thick, low grey blanket, stratus clouds block sunlight from reaching the Earth like an umbrella and thus have a net cooling effect.

  • Cirrus clouds: Wispy high clouds as much as 20 kilometers above the Earth's surface, cirrus clouds let most sunlight pass through but may trap heat from getting out of the Earth system. Therefore, they have a net warming effect.
  • Cumulus clouds: With sharp edges and a cotton ball appearance, cumulus clouds can block sunlight, but also trap the Earth's heat depending on their heights and thicknesses.

Currently, the combined effect of all clouds is one of net cooling, meaning that clouds are dampening the rate of global warming. But scientists are looking into whether clouds will have the same effect on climate as the Earth continues to warm. If the proportion of different cloud types changes, it could affect the rate of climate change. This is an area of ongoing research.

What if climate change causes the number of cooling clouds to increase? What if it causes the number of warming clouds, to increase?  If so, clouds may be able to temper the amount of warming in a negative feedback loop. Most scientists doubt that the net cooling effect of clouds will ever be large enough to completely offset ongoing warming, but it might slow the rate of ongoing warming.

However, what if climate change causes the number of warming clouds to increase? What if it causes the number of cooling clouds to decrease. Scientists say the current net cooling effect of clouds on the Earth's climate would decrease and the rate of warming would increase in a positive feedback loop.

For more information about clouds and climate:

  • CMMAP: Studying Clouds and Climate The mission of the Center for Multiscale Modeling of Atmospheric Processes (CMMAP) is to find better ways to understand clouds and their roles in the global climate system via climate models that better take clouds into account.
  • Clouds: The Wildcard of Climate Change This NSF Special Report highlights current research about understanding how climate and clouds are related. It includes background from the CMMAP project and a video narrated by NCAR Scientist Emerita Dr. Peggy LeMone.
  • The Effect of Clouds on Climate: A Key Mystery for Researchers This article from Yale Environment 360 (Aug 30, 2010) summarizes some of the challenges related to figuring out the connection between clouds and climate change.