Chemtrails, Contrails, & Geoengineering

These three concepts are often conflated by the public, which has led to a raft of conspiracy theories.  Many chemtrail advocates maintain that the persistent condensation trails (CONTRAILS) which appear behind commercial jets are a form of geoengineering intended to cool the climate.  The overspreading trails are said to be composed of chemicals, concocted by the government, whose composition may be harmful to humans and the environment.  

Though the facts tell a much different story, there are good reasons to be concerned about the unpredictable and potentially dangerous effects of large-scale climate engineering.  Those concerns are shared by many scientists, environmentalists, and politicians.  You don’t have to wear a tin foil hat to conclude that reengineering the atmosphere could have disastrous consequences.

Now the science.  Geoengineering proposals to seed the atmosphere with particles that reflect sunlight in order to cool the earth, have little in common with aircraft CONTRAILS.  The particles, usually Sulfur dioxide, have to be dispersed in the stratosphere at an altitude of 20 km or higher (the higher the better).  This is about twice the height of commercial aircraft flights and well beyond the service ceiling of such aircraft.  Dispersing the particles below 19 km - 20 km (i.e., in the troposphere) would be far less effective, resulting in a much shorter residence time for particles due to turbulence, mixing, and descending air currents which would carry them to earth.  

With a heavy payload of chemicals, there are very few, if any, high-altitude aircraft that can loft that much weight to such heights.  Consequently, the task would require specially modified vehicles, custom designed vehicles, or other technologies such as rockets and balloons (above).  A number of journal articles have been written evaluating different aerial platforms for performing “stratospheric aerosol injection” (SAI).  The preferred solution appears to be an entirely new aircraft, the Stratospheric Aerosol Injection Lofter (SAIL).  To date, this aircraft is only a concept (right).  

So what about aircraft CONTRAILS?  They form when the water vapor, generated by combustion inside the jet engine, turns to ice crystals (water vapor is a natural by-product of burning fossil fuels).  Their persistence is a function of temperature and humidity, which can be forecasted with a reasonable degree of accuracy from daily, upper air observations.  

But more to the point, CONTRAILS have exactly the opposite effect sought by geoengineering:  They tend to warm the climate instead of cooling it, even though they block a slight amount of sunlight (left).  The thin ice crystal clouds (cirrus) allow most shortwave solar radiation to strike the earth. This warms the surface.  Heat from the surface radiates skyward in the form of longwave radiation, which is reflected back (reradiated) by the CONTRAILS.  The net effect is a slight increase in atmospheric temperature.  Consequently, scientists have sought to limit CONTRAIL effects by proposing different technologies, fuels, and flight paths that would mitigate their formation.

The upshot is that there is no connection between persistent aircraft CONTRAILS (so-called “chemtrails”) and attempts to cool the atmosphere through geoengineering.  If some climate activists had their way, contrails would disappear tomorrow.  In the meantime, large scale attempts to inject particles into the stratosphere should be prohibited, and the science behind those efforts should be met with deep skepticism.  Our knowledge of the atmosphere is still rudimentary in many ways.  If we damage it, there is no backup system.

Right:  An infrared satellite image showing dozens of contrails over the southeastern United States during a single morning in January 2004. Yale Environment 360.   NASA

Sources:

Effects of Different Stratospheric SO2 Injection Altitudes on Stratospheric Chemistry and Dynamics

Stratospheric aerosol injection tactics and costs in the first 15 years of deployment

Review of possible very high-altitude platforms for stratospheric aerosol injection

Optimizing Injection Locations Relaxes Altitude-Lifetime Trade-Off for Stratospheric Aerosol Injection

How Airplane Contrails Are Helping Make the Planet Warmer

We Could Refreeze Earth’s Melting Poles With Aerosol-Spraying Planes

Solar Geoengineering: Should we artificially cool the planet?