The American Meteorological Society has headlined the two policy statements [which I was fortunate to be able to assist on the committtee that completed them]. The two Statements are
It has taken two years to complete this process. It does represent the broadening of the AMS to consider that the role of humans in the climate system involves much more than just the effects due to the addition of CO2 and several other greenhouse gases.
Excerpts from the first AMS Statement include
“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.”
“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.”
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.”
This Statement [as I have posted on previously; i.e. see] provides wider support for the findings that we reported on in the paper
Pielke Sr., R., K. Beven, G. Brasseur, J. Calvert, M. Chahine, R. Dickerson, D. Entekhabi, E. Foufoula-Georgiou, H. Gupta, V. Gupta, W. Krajewski, E. Philip Krider, W. K.M. Lau, J. McDonnell, W. Rossow, J. Schaake, J. Smith, S. Sorooshian, and E. Wood, 2009: Climate change: The need to consider human forcings besides greenhouse gases. Eos, Vol. 90, No. 45, 10 November 2009, 413. Copyright (2009) American Geophysical Union.
The second Statements implicitly involves geoengineering (although the AMS has a separate statement for that; see my post). Excerpts from that Statement read
“Cloud seeding techniques have been developed over nearly 70 years through experimentation and trials. In this statement we focus on the policy issues that pertain to local-scale application of these techniques covering areas from a few to several hundreds of square kilometers. Larger-scale efforts to intentionally modify weather and climate using these or other techniques are discussed in a separate AMS policy statement on geoengineering.”
“From time to time methods have been proposed for modifying extreme weather phenomena, such as seeding severe thunderstorms with aerosols to diminish tornado intensity, or seeding tropical cyclones to cause changes in their dynamics and steer them away from land and/or diminish their intensity. Some experimentation has taken place in these areas, but current knowledge of these complex weather systems is limited and the physical basis by which seeding might influence their evolution is not well understood. ”
“Weather modification techniques other than cloud seeding have been used in various areas of the world for short periods of time to achieve goals similar to those of cloud seeding. Much less is known about the effects of these other techniques, and their scientific basis is even further from being demonstrated, either statistically or physically, than it is for cloud seeding. Application of weather modification methods that are not supported by statistically positive results combined with a well-understood physical chain of processes leading to these results, and that can also be replicated by numerical cloud modeling, should be discouraged.”
The scientific uncertainty (and thus dangers of deliberate intervention in the global climate system – “geoengineering”) indicate that geoengineering is a very poor idea.