The following statement has expired and is here for historical purposes and does not represent statements of the AMS that are “in force” at this time.

An Information Statement of the American Meteorological Society 
(Adopted by the AMS Council on 2 November 2010)

                                                                                                                     
This statement highlights the causes and possible effects of inadvertent weather modification1 at local and regional scales due to aerosol2 and gas emissions3 and to changes in land use.  The known effects can have unanticipated and often undesirable socioeconomic consequences.  This statement assesses the impacts of inadvertent weather modification and suggests potential respective actions.

The climatic effects of greenhouse gases (GHGs) have been summarized by the AMS Information Statement on Climate Change.  This policy statement, however, highlights the key understanding of anthropogenic effects on weather in order to support effective decision making for emission controls, alternate water resources, severe-storm preparedness, and climate-change mitigation and adaptation strategies.  Further, understanding anthropogenic effects on weather is important to improve short-range and longer term weather predictions.

1. Status of inadvertent weather modification

This section summarizes the current knowledge of the physical processes affecting weather modification as a result of changes in land use, aerosol, and gas emissions.

a. Aerosol radiative effects

By partially blocking solar radiation from heating the surface, air pollutants lower surface heating and evaporation rates.  This slows vertical air motions, and hence causes slower dispersal rates of air pollutants, and suppresses formation of convective clouds and precipitation.  Reduced surface evaporation has major implications for the global hydrological cycle and how it responds to the combined forcing of GHGs, land use change, and aerosol pollution.  In addition, surface deposition of dark aerosols accelerates ice-melt rates, hence affecting water resources.  While these conclusions are based on sound physical meteorology, many of these effects are yet to be quantified.

b. Cloud-mediated effects of aerosol

Aerosols act mostly as cloud-drop condensation nuclei (CCN), and some of them as ice nuclei (IN), both of which change cloud radiative and precipitation properties in complex ways.  Over oceans, emissions from fossil-fuel-burning ships produce tracks, observed to dramatically influence the extent and persistence of local shallow cloud cover, reducing the amount of solar radiation received at the surface and enhancing the amount reflected back to space.  Aerosols also suppress precipitation from shallow or short-lived clouds (e.g., orographic cap clouds). Their impacts on deep convective clouds are much less certain, but are of potentially great importance.  Recent research suggests that, depending on meteorological conditions, aerosols can either increase or decrease rainfall from such clouds.  In warm moist atmospheres, aerosols often invigorate deep convective clouds, usually resulting in greater electrical activity, stronger damaging winds, and a greater likelihood of flash floods.  Studies indicate that aerosols might also modulate the intensity of tornadoes and hurricanes.

c. Changes in land use

One example of significant land use change is the rapid global increase in urbanization and its associated changes in land surface properties and topography that create "urban heat islands" and urban barrier effects that perturb regional air flows, which thus redistributes precipitation, runoff, and flood risk over and around cites. Land-use changes alter surface albedos, as well as surface fluxes of heat, water vapor, and momentum to the atmosphere, and thus modify local and regional atmospheric circulations, which in turn can modify weather. For example, when a forest is removed and replaced by an agricultural field, it can result in a significantly different albedo, especially after a snow storm.  Artificial lakes, and wind and solar farms also change the surface fluxes and albedo. Such changes also occur indirectly through increases in nitrogen deposition and atmospheric CO2, which alter leaf area amounts and thus the portioning of latent and sensible heat fluxes.  Poor agricultural practices that favor wind erosion, such as from summer fallow, overgrazing, and deforestation, as well as from tillage, can produce large quantities of dust that absorb and reflect solar radiation thereby modifying clouds and precipitation processes.

d. Integrated effects

The cumulative changes in surface and atmospheric heat and moisture profiles modify atmospheric circulation and weather patterns on all scales, including synoptic storm tracks, in ways that are just beginning to be explored.  In the aggregate, these changes can affect air quality, ecosystems, and water resources.  The cumulative impacts of inadvertent weather modification may thus result in local or regional-scale climatic alterations superimposed on, and interacting with, natural and GHG-induced climate variability and change. Understanding of inadvertent weather modification, still in its infancy, is thus necessary for understanding the sources, triggers, and response mechanisms of climate change.

2. Mitigation

Mitigation or avoidance, of these unintended impacts requires:

• Application of new knowledge to curtail pollutant emissions and adverse land-use changes and to mitigate their impacts.
• Advancement of scientific and engineering understanding to elucidate the causes of atmospheric changes and to lay the foundation of knowledge for countering their adverse impacts.

Achieving these objectives requires:

• Documentation of anthropogenic weather forcings.
• Process studies (both observations and simulations) of how such forcings affect meteorological conditions.
• Simulations of the extent to which such local and regional forcings influence hemispheric-scale systems, such as the subtropical and polar jet streams.

3. Adaptation

Adaptation is necessary when impacts cannot be fully mitigated.  Adaptation to the unavoidable components of unintended weather modification requires:

• Consideration of environmental impacts of inadvertent weather modification as part of development and planning processes (e.g., crop adaptation, management practices, and water utilization).
• Implementation of strategies to enhance depleted water resources in response to reduced precipitation (e.g., through desalination).
• Evaluation and planning of public response to risks from inadvertent weather modification that can influence severe weather events.

4. Recommendations

High-priority research and new technological capabilities are required to improve understanding of the impacts of inadvertent weather modification.  These might include:

• Further use of satellite remote sensing of land, trace gas, aerosol, cloud, and precipitation properties.
• Enhanced documentation of emissions of aerosols and their precursors; their chemical evolution; radiative properties; CCN and IN activity; and their transport and deposition.
• Expanded in situ measurements of aerosol–atmosphere and land–atmosphere interactions over a range of cloud regimes, from fair weather to severe convective storms and to hurricanes.
• Detailed simulations of these processes at a hierarchy of scales, up to global.

These research efforts on unintended weather modification should be recognized as addressing parts of the broader question of climate variability and change, which crosses geopolitical boundaries.  As was the case with acid rain and stratospheric ozone depletion, national and international frameworks should be developed for addressing the related environmental and ethical issues for inadvertent weather modification.

[This statement is considered in force until November 2013 unless superseded by a new statement issued by the AMS Council before this date.]

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