Tackling Clouds for Improved Predictions of Future Climate


Blog Action Day

Today, JPL Earth scientist Hui Su joins thousands of other bloggers in more than 130 countries around the world for the Blog Action Day ‘09 Climate Change.

Blog Action Day is an annual event that unites the world’s bloggers in posting about the same issue on the same day, with the aim of sparking discussion around an issue of global importance. The theme of this year’s event, climate change, affects us all and will be the topic of international climate negotiations taking place in Copenhagen, Denmark, this December.

As a world leader in studying Earth’s climate, NASA researchers play a vital role in shaping our understanding of global change. In today’s post, Su discusses the critical role clouds play in climate, and why learning more about them is a key to predicting how our climate will change in the future.

For more information on Blog Action Day, visit: http://www.blogactionday.org .


Hui Su
Hui Su

Clouds are among the most fascinating natural phenomena and have inspired countless works of literature and art. Their ever-changing forms make them a great challenge to atmospheric scientists working to predict how our climate will change in the future in response to increasing greenhouse gases such as carbon dioxide.

Clouds occur at many different heights in our atmosphere and take many different forms. There are three main types of clouds: stratus, cumulus and cirrus. Stratus clouds are low clouds, usually within 2 kilometers (7,000 feet) above the surface. They look like a gray blanket, extending thousands of kilometers across the sky. Cumulus clouds look like puffy cotton balls and extend vertically for large distances. The third type is wispy and feathery-looking cirrus. Cirrus clouds are usually high in the sky, about 7 kilometers (23,000 feet) above the surface. These three types of clouds have different impacts on Earth’s climate due to their unique abilities to reflect sunlight and trap heat radiated from Earth’s surface.

Artist's concept of NASA's CloudSat spacecraft
Artist’s concept of NASA’s CloudSat spacecraft, which is providing the first global survey of cloud properties to better understand their effects on both weather and climate. Image credit: NASA/JPL

cirrus cloud diagram
Su et al. (2008, Journal of Geophysical Research) suggested that cirrus clouds increase as sea surface temperature becomes warmer, further enhancing surface warming. Image credit: NASA/JPL/Caltech
› Full image

Stratus clouds can effectively block sunlight from reaching the surface; therefore, they act as an umbrella that cools Earth. Cirrus clouds are relatively transparent to sunlight but can trap terrestrial radiation, JUST AS carbon dioxide does, so they have a net warming effect on Earth. Cumulus clouds can block sunlight and also trap terrestrial radiation. Their net effect varies greatly depending on their actual heights and thicknesses.

Climate scientists have long struggled to quantify how different types of clouds change when global warming occurs. For example, an increase in stratus clouds may cool Earth’s surface, compensating for global warming; while an increase in cirrus clouds may further warm Earth’s surface, exacerbating global warming. Up to now, scientists have not been able to come to a consensus as to whether stratus, cumulus or cirrus clouds will increase or decrease as global temperatures increase.

A key advancement in cloud studies in recent years has been the availability of global satellite observations of clouds, especially the measurements of clouds at different heights provided by NASA satellites like CloudSat, managed by NASA’s Jet Propulsion Laboratory (JPL). These observations are allowing scientists to better simulate clouds in climate models, which are the primary tools climate scientists use to predict future climate change. Up till now, the dynamic nature of clouds has made them very difficult to simulate in current climate models. But by applying space data, we at JPL are working closely with modelers to improve cloud simulations and thereby improve predictions of future climate change.

To learn more about JPL’s research in this field and the CloudSat mission, visit:
http://cloudsat.atmos.colostate.edu/home .

    4 Responses to “Tackling Clouds for Improved Predictions of Future Climate”

  1. Tackling Clouds for Improved Predictions of Future Climate | Hometown Pasadena Says:
    October 15th, 2009 at 12:15 pm

    [...] the Full Story at JPL [...]

  2. RAJAPANDI Says:
    October 24th, 2009 at 1:37 am

    very good web site

  3. oussama hamed Says:
    October 29th, 2009 at 3:11 pm

    again i have to say
    what is unnatural is unnatural
    and what we are doing nowadays (The excessive use of polluting energy) nature cant accept it.
    Then all exciting actions have a certain reaction or responce
    and what we can see here its a bad responce can take this beautiful earth to the Abyss…
    so please find solution for the pollution u will got a resolution for the problem of the climate change
    thanks for listening and lets move our planet in danger…
    stop all machines working with petrol and the other polluting energies
    that is the only solution ( i guess that we can live with simple energies )

  4. Cedders Says:
    November 19th, 2009 at 3:44 am

    Nice, honest, accessible article for blog day. These seem to have been the major uncertainties in climate change research for the past twenty years, and I hope they can be resolved soon.

    A naive question: shouldn’t the inevitable increase in evaporation from enhanced greenhouse effects/forcings mean more cloud of every type? Surely, it has to condense somehow. And presumably low cloud is the main known negative feedback mechanism, as it must have been back ten/hundreds of millions of years ago, say in the Carboniferous? So equilibrium has to be reached by more stratus, and climate change will therefore mean not just higher temperatures but less sunlight, and so lower agricultural yields (as well as more severe droughts and floods)? I know these questions are speculative, but are they the kind of thing NASA is working on, and if not who is?

    Hui Su responds:

    It is reasonable to think that increasing sea surface temperatures caused by increased greenhouse gases would give rise to increasing evaporation, which provides the necessary source of moisture for clouds to form. However, formation of clouds is affected by factors beyond moisture supply. One of the important factors is vertical wind. For example, deep cumulus clouds are associated with strong updraft, in which water vapor is carried upward to a colder environment and condenses rapidly to form cloud droplets. On the other hand, marine stratiform clouds are usually embedded in prevailing downward winds. The low-cloud cover may decrease as the downward-moving winds weaken. Considering the globe as a whole, things that go up must come down somewhere.

    With that said, in a global warming scenario, we expect regions of enhanced upward motions are accompanied by regions of enhanced downward motions. A recent study published in Science (Clement et al, Science, 2009) suggested that the Northeast Pacific (including the California coast) has experienced decreased low-cloud cover in the past 50 years in response to the warming of sea surface temperature, partly due to the reduced downward-moving winds in this area. The decreasing low cloud cover with increasing sea surface temperature thus provides a positive climate feedback. Scientists at NASA and around the world are working hard to understand various physical processes that contribute to cloud changes associated with global warming.

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