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Inadvertent Weather Modification
An Information Statement of the American Meteorological Society
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.
Mitigation or avoidance, of these unintended impacts requires:
Achieving these objectives requires:
Adaptation is necessary when impacts cannot be fully mitigated. Adaptation to the unavoidable components of unintended weather modification requires:
High-priority research and new technological capabilities are required to improve understanding of the impacts of inadvertent weather modification. These might include:
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.]
© American Meteorological Society, 45 Beacon Street, Boston, MA 02108-3693
2 An aerosol is a suspension of solid or liquid particles in a gas. Atmospheric aerosols can have natural or anthropogenic sources, the latter primarily through the combustion of fuels, but also from blowing dust due to degradation of land surfaces, particularly in semi-arid regions. The large difference in geographic locations between sources and sinks (in deposition areas) and the variety of physical and chemical processes affecting them, lead to a large spatial variability in aerosol concentration, size distribution, and composition. Aerosols are removed by gradual fall due to gravity (dry deposition) or precipitation (wet deposition), and almost all have a tropospheric half life from a few days to a few weeks. Evidence exists that aerosols modify weather systems, so that the aggregated changes could affect regional systems.
3 Trace gases that result in noticeable atmospheric effects, and that are considered as a group in this statement include: carbon dioxide, methane, carbon monoxide, sulfur dioxide, chlorofluorocarbons, tropospheric ozone, nitrous oxide, and nitrogen oxide. These gases have natural and/or anthropogenic sources.