Aircraft emissions could influence climate change through cloud formation

September 28, 2009
What's Up with Those Ice Clouds?
Airplane exhaust could affect the ice crystals that form cirrus clouds, thus affecting radiant heat and contributing to climate change.

( -- Aircraft emissions can affect the properties of cirrus clouds, contributing to climate change. This was a key finding from PNNL scientist Dr. Xiaohong Liu and his colleagues from a recent study. The team concluded that black carbon and/or metallic material from airplane exhaust could affect radiant heat and climate by acting as efficient sources for making ice crystals, thus affecting the creation of cirrus clouds.

The team also found that when particles from fossil fuel and burning are emitted from the Earth's surface, they can travel at least 5 miles higher. At that altitude, these tiny bits of sulfate and black carbon create more ice-crystal-forming cirrus clouds.

The effect aerosol particles have on clouds remains the largest uncertainty in assessing future . can influence the Earth's radiant heat balance by scattering incoming radiation back to space or by affecting the properties of clouds. In addition, how ice particles are formed from ice nucleation on aerosols and how they are distributed in clouds can influence precipitation, which affects the distribution of heat.

This study was one of the first to model the effect of human-caused aerosols on cirrus clouds and to quantify the amount of energy that cirrus clouds reflect into space and absorb. Many previous studies have shown how aerosols change warm clouds—those with temperatures greater than 0°C—but few on cirrus clouds, which are more difficult to measure because they float at least 8 km, or 5 miles, from the Earth's surface. Cirrus clouds cover about 30 percent of the Earth's surface.

Some of the improvements to date about ice microphysics parameterization have now been incorporated into the National Center for Atmospheric Research Community Atmospheric Model Version 3 (CAM3) climate model that will be released for future users.

The team used a computer model to examine the effects on cirrus clouds of two sources of human-caused aerosols—tiny particles of sulfate and black carbon from fossil fuel and burning biomass.

Scientists used the CAM3. They used an ice microphysics treatment that predicts ice water and the number of ice crystals, coupled to a global aerosol model known as the Lawrence Livermore National Laboratory Chemical Transport Model. They found that the relative humidity in the upper troposphere plays a key role in determining how ice crystals are formed. Researchers modified the CAM large-scale cloud condensation and evaporation parameters to allow ice supersaturation in the upper troposphere. They then combined the CAM3 with ice crystal number prediction together with an aerosol model to study the impacts of human-caused aerosols on climate through changing ice cloud properties, such as cloud radiation and microphysics. Model simulations were conducted with both present-day and pre-industrial emissions.

Researchers analyzed the changes in ice water content, ice crystal number concentration, temperature, humidity, and cloud cover between the experiments for present-day and pre-industrial aerosols to identify the effects of human-caused ice formation particles (sulfate or black carbon) on the cirrus clouds and hydrological cycle. In addition, researchers calculated the changes in the shortwave and longwave and net radiation between the pre-industrial simulation and the present-day simulations to estimate the radiative forcing resulting from the aerosol effects on cirrus clouds. Then they analyzed the different present-day simulations to compare the relative importance of heterogeneous ice crystal formation versus homogeneous ice freezing to identify the contribution from aircraft-emitted soot as compared to that from the Earth's surface sources.

Future studies will focus on improving microphysics formulations in the climate model and evaluating the model simulations using data from Atmospheric Radiation Measurement (ARM) Climate Research Facility sites and field campaigns. Researchers will use both old and new ARM data. For example, they will use data from the ARM field campaign SPARTICUS (Small Particles in Cirrus) between November 2009 and March 2010, which will focus specifically on measuring properties of cirrus clouds.

More information:

Liu, X., J. E. Penner, and M. Wang. 2009. "Influence of Anthropogenic Sulfate and on Upper Tropospheric Clouds Using CAM3 Coupled with an Aerosol Model." Journal of Geophysical Research, 114, D03204, doi:10.1029/2008JD010492

Penner J. E., Y. Chen, M. Wang, and X. Liu. 2009. "Possible Influence of Anthropogenic Aerosols on Cirrus Clouds and Anthropogenic Forcing." Atmospheric Chemistry and Physics 9(3):879-896.

Provided by PNNL

Explore further: Cloud formation affected by human activity, study says

Related Stories

Cloud formation affected by human activity, study says

September 12, 2006

University of Toronto researchers and their collaborators have discovered that solid ammonium sulphate aerosol – an airborne particle more prevalent in continental areas - can act as a catalyst to the formation of ice clouds, ...

New observations on properties of water

December 13, 2006

Experimental studies conducted by Ph.D. Anatoli Bogdan at the University of Helsinki, Finland, have received broad interest in the scientific world, as the results might have applications even in the cryopreservation of cells ...

Science paper examines role of aerosols in climate change

September 5, 2008

A group of scientists affiliated with the International Geosphere-Biosphere Programme (IGBP) have proposed a new framework to account more accurately for the effects of aerosols on precipitation in climate models. Their work ...

The insides of clouds may be the key to climate change

February 17, 2007

As climate change scientists develop ever more sophisticated climate models to project an expected path of temperature change, it is becoming increasingly important to include the effects of aerosols on clouds, according ...

Recommended for you

Mysterious deep-Earth seismic signature explained

November 22, 2017

New research on oxygen and iron chemistry under the extreme conditions found deep inside the Earth could explain a longstanding seismic mystery called ultralow velocity zones. Published in Nature, the findings could have ...

Scientists dispute missing dryland forests

November 21, 2017

Scientists are disputing the possibility that a significant portion of the world's forests have been missed in an earlier accounting of ecological diversity.


Adjust slider to filter visible comments by rank

Display comments: newest first

Sep 28, 2009
This comment has been removed by a moderator.
5 / 5 (1) Sep 29, 2009
Another branch in the vast catalog of things we don't know... so much more research needed... humility triumphs over hype and fear in the long run.
1 / 5 (1) Sep 29, 2009
Old news - known since WWII. Europe experienced colder temps after huge number of bomber flights.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.