There are a wide range of opinions and articles on this question, and here are some excerpts from a blog on the point Artificial Weather Revealed by Post 9-11 Flight Groundings:
In the August 2002 edition of Nature, which is ranked the world's most cited interdisciplinary journal, a report was published titled "Contrails reduce daily temperature range," where scientists discuss how "a brief interval when the skies were clear of jets unmasked an effect on climate."
Here is a link to the original Nature article), which states:
DTRs for 11–14 September 2001 measured at stations across the United States show an increase of about 1.1 C over normal 1971–2000 values (Fig. 1). This is in contrast to the adjacent three-day periods, when DTR values were near or below the mean (Fig. 1). DTR departures for the grounding period are, on average, 1.8 C greater than DTR departures for the two adjacent three-day periods.
It would therefore appear that airplane activity causes net cooling, at least for the places where the temperatures were measured (which conclusion is also restated by Christian Science Monitor, below).
However, elsewhere in the blog:
But according to Dahl, the immediate problem we face from climate change is global drying, not global warming ...
But Dahl says today's precipitation enhancement programs are worsening the problem of climate change, and even in the best-case scenarios, we will not be able to meet our growing water needs through weather modification.
"I think it's this simple," said Dahl in an interview. "Clouds affect the weather and the climate. We are making artificial clouds. So yes, we are changing the climate, and this movie shows that clouds made by jets are accelerating these changes far more than the buildup of CO2 or methane, and are in fact the primary factor in anthropogenic climate change."
Which appears to be suggesting that the primary problem of climate change is aridification, which is primarily caused by jet streams.
There is a NOVA/pbs article The Contrail Effect from 2006 which states:
These results suggest that contrails can suppress both daytime highs (by reflecting sunlight back to space) and nighttime lows (by trapping radiated heat). That is, they can be both cooling and warming clouds. But what is the net effect? Do they cool more than they warm, or vice versa? "Well, the assumption is a net warming," Travis says, "but there is a lot of argument still going on about how much of a warming effect they produce."
A more recent article, 2011, Airplane contrails worse than CO2 emissions for global warming: study states, citing a study in Nature Climate Change by Dr. Ulrike Burkhardt and Dr. Bernd Karcher from the Institute for Atmospheric Physics at the German Aerospace Centre:
They have discovered that aviation contrails play a huge role in the impact on the climate and an even greater impact than that created by the CO2 emissions produced. While the CO2 emissions from airplanes account for around three percent of the annual CO2 emissions from all fossil fuels and change the radiation by 28 milliwatts per square meter, the aviation contrails are responsible for a change of around 31 milliwatts per square meter.
Another article by The Christian Science Monitor, Airplane contrails and their effect on temperatures states:
As it turns out, [contrails] also impact temperatures at Earth's surface, although by how much and in which direction — up or down — is still being worked out.
...
But the real effect was in daytime highs, which were much higher. That would seem to indicate that, contrary to prevailing thinking, contrails might have a net cooling effect.
Which is all to say that while there seems to be a consensus that contrails have a climate change impact, the jury seems out - at least in the mainstream media articles that came up - on what that effect is. The claims, at least as I have read them from the above, would seem to include that contrails:
- reduce daily highs (by reflecting the sun's energy);
- prevent nighttime lows (as greenhouse gasses do);
- cause global aridification (which is worse than global warming);
- increase warming more than just CO2 (because of extra pollutants);
- increase warming more than CO2 at lower altitudes.
The over-arching claim appears to be that contrails result in either a net decrease in global temperature owing to the effect in #1, or a net increase owing to the balance of the claims.
Is there more consensus among the scientific community on the effect of contrails, especially from the perspective of the net effect?
It may be that I have misstated or overlooked a claim above, and please feel welcome to edit (if you have that capability) to rectify, or otherwise comment.
(As an aside, I am curious whether the contrail effect is related to the ostensible 20 year lull in rising temperatures.)
