Concerns about the possible effects of global climate change are now widespread.  Demands for efficient policies to address this perceived threat have also spread.  Geoengineering may offer a partial response.  Geoengineering, or climate engineering, is defined by the U.S. Environmental Protection Agency, as "the intentional modification of Earth's environment to promote habitability." [1]

Geoengineering encompasses a family of concepts that might offer practical means of offsetting some of the effects of climate change.  These concepts are all designed to slightly raise the amount of sunlight that Earth reflects back into space.  Collectively, they are sometimes called solar radiation management (SRM).  The evident difficulties that efforts to cut greenhouse gas emissions continue to meet have recently brought about heightened interest in SRM, and many prominent scientists and economists now support increased research into this concept.

SRM is not a single technology; rather, it is a family of possible technologies.  Below are examples of prominent SRM concepts, but it is important to note that other proposals do exist, and many experts believe that we have barely begun to explore the full range of possible methods of global cooling through SRM:

Atmospheric Aerosol Injection: Strategy entails the injection of small volumes of matter (generally sulfur-based aerosol particles) into the stratosphere so as to scatter back into space a fraction of the solar radiation that otherwise would have reached Earth, thereby decreasing global mean temperatures. 

Cloud Whitening: Strategy entails the spraying of seawater droplets into low-level marine clouds so as to increase cloud albedo and longevity, thereby creating a cooling effect.

Space-Based Solar Radiation Management: Strategy entails the creation of solar shields or reflective mirrors to be situated in space so as to deflect a portion of the solar radiation otherwise directed towards Earth, thereby cooling the planet.

In addition to SRM, another family of concepts is also sometimes referred to as 'geoengineering'.  These technologies work by capturing carbon dioxide from the atmosphere and sequestering it in a location where it will, at least for some period of time, not contribute to global warming.  At the simplest level, merely planting very large numbers of trees might qualify (although in that case, the period during which the carbon dioxide is no longer in the atmosphere is relatively short).  Below are some prominent examples of this family of concepts:

Air Capture of Carbon: Strategy entails the construction of devices (in effect, artificial trees) to remove carbon dioxide from the atmosphere and store it for very long periods in a safe, secure manner, thereby decreasing carbon dioxide concentrations in the Earth's atmosphere.

Oceanic Iron Fertilization: Strategy entails a form of air capture based on the injection of iron into oceans so as to stimulate phytoplankton growth, which would serve to increase carbon dioxide uptake and thereby decrease the atmospheric concentration of carbon dioxide.

[1] U.S. EPA, "Economics of Climate Change" http://yosemite.epa.gov/ee/epa/eed.nsf/Webpages/ClimateEconomics.html