damn right they're legit...
taken from
http://www.defra.gov.uk/environment/climatechange/research/pdf/geo-engineering-0409.pdf :
'...using airborne sub-microscopic particles such as sulfate, metals, dielectrics, resonant scatterers or dust [A12]. These aerosol particles would be created by releasing aerosol precursors into the stratosphere. This could be done by: releasing precursors at the Earths surface and allowing them to be carried into the stratosphere; firing them into the stratosphere from the Earths surface; or delivering them into the stratosphere using high-altitude balloons or aircraft [B2] (possibly by
addition to aviation fuel, which could reduce the cost of delivery [Q15]).
Injection could either take place in the tropics (with the aim of obtaining
global coverage) or in the Arctic (with the aim of reducing warming in this region, which is particularly vulnerable to anthropogenic climate change).
There are a number of uncertainties about the potential impacts of these schemes on the environment21,22. In particular, the effects of stratospheric aerosols on the climate system are not fully understood [AD4] - although they are
known to affect circulation patterns, stratospheric ozone concentrations (which affect climate) [AD2] and upper tropospheric cloud formation (a particular concern is that these schemes could increase the cover of high cirrus clouds in the tropics, which could increase warming).
Changes observed after volcanic eruptions (which can inject aerosols into the stratosphere) suggest that the climatic response to stratospheric aerosol forcing is
regionally variable [AD3]. In particular, they indicate that there may be significant decreases in precipitation over land23 (which could lead to drought) and changes in the North Atlantic Oscillation (which could lead to warmer winters over Eurasia) [B6].
The potential impact of the schemes on ecosystems also remains uncertain, but aerosols can affect photosynthesis by increasing the amount of diffuse solar radiation and decreasing the amount of direct solar radiation [A14] and can
cause environmental pollution.
Sulfate-aerosols - The most widely-discussed proposal in this category involves the injection of sulfate aerosols into the stratosphere24. It has been estimated that this scheme would require ~1.5 to 3 teragrams of sulfur to be added to the stratosphere each year to counter the effects of a doubling of CO2 levels25, although another study suggested that ~5 teragrams of sulfur per year might be needed to mitigate future warming26 [cf. B3, F4].
The aerosols could be produced: either by injecting sulfur dioxide into the stratosphere, where it would be converted into
sulfuric acid droplets; or by releasing long-lived sulfur compounds such as carbonyl sulfide (OCS) at the surface [AD1]. Unlike in the troposphere, sulfate aerosols in the stratosphere do not get washed out within a few weeks, but have a
residence time of ~1 to 2 years24.
Consequently, they are transported further in the stratosphere than in the troposphere and thus have greater
coverage of the globe, particularly if they are injected at the tropics [C27]. It might, however, be difficult to produce a spatially-uniform change in the radiative properties of the stratosphere using the methods of aerosol-precursor delivery that have been proposed27.
Under this option, if greenhouse-gas concentrations continued to rise, increasing quantities of sulfur would need to be
injected continuously into the stratosphere to mitigate temperature change, which may not be sustainable in the long term. Also as noted above if failure occurred, rapid climate change could result28.
The
climatic impacts of the scheme also remain uncertain. A study that simulated the injection of sulfate-aerosol precursors into the stratosphere using a General Circulation Model found that injection at the tropics produced sustained cooling over most of the world, but also
disrupted the Asian and African summer monsoons, with
detrimental effects on food supply28.
The scheme could also lead to significant reductions in stratospheric ozone concentration (particularly in the Arctic)10. An additional risk is that aerosols would be washed out of the atmosphere, causing
acid rain [AD5]. The effect of fallout over a few decades is likely to be small compared to the impacts of acid rain in the recent past [C32], but
the magnitude of this effect still needs to be quantified [AD5].
Threats:

Uncertain climate system impacts

Uncertain ecological impacts

Fallout may contribute to acid rain (sulfate aerosols) [AD5]
 Uncertain effects on stratospheric ozone

Failure to maintain could lead to rapid temperature rise/climate change

Ocean acidification (via increased CO2)'
there could be many reasons for chemtrails.