I have always enjoyed poetry. Back in the 60s a poet named Rod McKuen was popular, and I enjoyed some of his poems – to the point of memorizing some. I still remember one called “Clouds,” which begins:
Clouds are not the cheeks of angels, you know.
They’re only clouds. Friendly sometimes, but you can never be sure.
Western Oregon is a great place for cloud lovers like me. Most of our clouds are natural, but we get a lot of artificial ones: condensation trails, or contrails, from aircraft.
I’ve written about these before. There are even some conspiracy theories that suggest that these are created intentionally, or that certain chemicals in the exhaust cause them to last longer than they used to – hence the name “chemtrails” used by some people (you can hear this term a lot on late-night radio. But contrails have long been suspected of affecting climate as well, although it’s not clear just how much.
But recently NASA issued a press release suggesting that cirrus clouds formed by
contrails may be responsible for the warming trend in the United States that occurred between 1975 and 1994. Patrick Minnis, senior research scientist at NASA’s Langley Research Center in Hampton, Va., published a paper in the Journal of Climate suggesting that the observed one percent per decade increase in cirrus cloud cover over the United States is likely due to contrails, and since high clouds are known to cause temperature increases, the study “demonstrates that human activity has a visible and significant impact on cloud cover and, therefore, on climate. It indicates that contrails should be included in climate change scenarios,” according to Minnis.
I’m a cloud lover, but not a cloud expert. But I know several people who are. I asked two of them, Drs. Jim Coakley and Bill Tahnk of OSU’s College of Oceanic and Atmospheric Sciences, to comment on Minnis’ study. Both generously offered their comments.
According to Jim Coakley, Minnis assumed that the observed increase in cirrus is due entirely to effects of contrails, but cirrus may be changing as a result of changing climate conditions, not the other way around. Also, Minnis used a “GCM” (climate model) which may have overestimated the temperature effects because it assumed that contrails are more or less constant (which of course they are not).
Bill Tahnk said, “Clouds are tricky. On the one hand, they reflect visible radiation back to space before it reaches the ground – a cooling effect. But they also trap terrestrial radiation – a warming effect. The net result of these two processes depends on the cloud altitude. High clouds are thought to trap more that they reflect (thus warming effect) and low clouds are thought to reflect more than they trap hence overall cooling.”
Since contrails are high clouds, their effect is largely to warm the atmosphere. But Jim and Bill have studied other activities – such as exhaust from large ships – which produce low clouds whose effect would be to cool the atmosphere.
Bill asks, “Does [increased cloudiness] account for all the warming? Hard to say, since the greenhouse gases all were on the increase during the same period.” Since clouds can be almost perfect absorbers AND reflectors of radiation, it is necessary to know a lot of details to understand their effects: cloud amounts, cloud location, cloud thickness, and cloud height. “That is why the treatment of clouds continues to be the greatest source of uncertainty in even the most sophisticated climate models.”
Wow. I can just imagine how Rod McKuen’s poem would sound if he’d known more about the physics of clouds:
Clouds are not just perfect absorbers, you know.
They’re perfect reflectors. They cause warming sometimes, but you can never be sure.
Better yet, here are Joni Mitchell’s words in “Both Sides Now”:
“It’s cloud illusions I recall; I really don’t know clouds at all.”