10 July 2010
A Story About Science – (How rising greenhouse gases are increasing the risk of the ISS hitting space junk.)
Posted by Dan Satterfield
I saw an interesting press release about a paper this past week. The headline was along the lines of “Climate Change Increasing Space Junk”.
Say what? I can predict the comments now that this will get. “You blame everything on global warming!”
Well, let’s look at what the paper really says and why they reached the conclusions they did.
I know one of the authors of this paper. Judith Lean is a solar physicist at the U.S. Naval Observatory. Dr. Lean is one of the scientists who has shown very conclusively that the sun is NOT responsible for more than a small part of the warming temps. over the last 50 years (in spite of numerous political web sites online that claim otherwise).
She quickly responded and the story behind this is really fascinating.
First some basics.
The atmosphere has 5 main layers.
Most weather happens in the bottom 10km called the troposphere. From around 10km to 50km is the Stratosphere, and above that is the mesosphere and thermosphere.
The thermosphere extends several hundred miles above the surface into what most people think of as “space”.Temperatures in this layer of the atmosphere are several thousand degrees, but there is so little air that if you were to stick a hand out of a spacecraft, it would feel very cold. (Just before the blood in your hand boiled).
The Stratosphere is getting colder.
It’s common knowledge that the planet’s temperature is rising. The troposphere is warming. What is not commonly known is that the stratosphere is cooling for the same reason. Greenhouse gases warm the troposphere and cool the stratosphere.
The reasons for cooling aloft are rather complex but John Cook of Skeptical Science has an understandable explanation of it here. Gavin Schmidt of NASA GISS has a more detailed and more complicated explanation here.
The Solar Cycle.
The big player here is the sun. It goes through an 11 year cycle of sunspots. When the sun is producing a lot of spots it puts our quite a bit more energy in the extreme ultraviolet portion of the electromagnetic spectrum. This causes the thermosphere to heat up and increases the density at any given altitude. When the sun is at the quiet end of the cycle, just the opposite occurs and satellites in orbit run into fewer molecules (mainly oxygen).
Since we just had a very long lasting solar minimum, you would expect the thermosphere to have cooled and so it did!
So what you ask?
Satellites and space junk are running into fewer molecules, so they will stay in orbit longer. In most cases this is bad, since there is a HUGE amount of junk up there. If you are on the ISS, it is good news. You do not need a boost back to your preferred orbital altitude by the space shuttle as often.
What the authors of the paper “Record‐low thermospheric density during the 2008 solar minimum
J. T. Emmert,1 J. L. Lean,1 and J. M. Picone2
(Geophysical Research Letters- published 19 June 2010)
discovered was this. The thermosphere density hit a record low during the recent solar minimum. Besides that, the drop in extreme ultraviolet radiation (EUV) does not seem to be enough to account for all of the drop.
The big discrepancy.
This last solar cycle saw a 29% drop in thermosphere density. Only about 10% of this drop can be blamed on less EUV radiation from a quiet sun. It’s estimated that about 3-6% can be added for extra co2 from humans in the atmosphere. (remember that co2 causes warming near the surface but cooling in the upper atmosphere.)
So what about the 13-16% of the density drop that is not accounted for.
Here is what the authors say about it:
Internal MLT(mesosphere and lower thermosphere) processes
possibly in combination with longer‐term anthropogenic
changes are therefore appealing candidates for explaining
the density changes. In addition to enhanced radiative
cooling by CO2 and CH4 (CH4 is Methane, another greenhouse gas, that is increasing), changes in two other MLT minor
species, O3 (O3 is Ozone) and H2O, also influence thermospheric densities, but trends in these latter species were
small during cycle 23 . Prior work has established that greenhouse‐gas
cooling of the thermosphere is enhanced during solar
minima, a relationship that the prolonged (by more than a
year) 2008 minimum may have amplified. If changes in the
radiative properties of the MLT are responsible for the
temperature and composition changes of the upper thermosphere,
then the density anomalies may signify that an as yet
unidentified climatological tipping point, involving energy
balance and chemistry feedbacks, has been reached.
So the short of it all is this- Climate change AND a quiet sun are causing space junk to stay in orbit longer. The data also indicates that some fundamental changes might be underway in the atmosphere.
You might wonder how the density of the thermosphere is estimated. Simply by how quickly satellite orbits decay. How is the sun’s EUV radiation measured? Directly since 2002 but before that the noise of the sun at a frequency of 10.7 cm is used. The data indicates that this proxy itself may be starting to change for unknown reasons.
I wanted to tell this story because it’s a nice example of how scientist use data to discover new things. You would not think at first that there would be any connection to how fast satellites fall out of orbit to climate change, but now you know there very likely is.
Remember in your junior high science book where it says that scientists make observations and then formulate a hypothesis. This is true, but it rarely works in the exact order or as neatly as your book implied!