The UnAustralian

Friday, August 01, 2003
 
Chicken Little in Reverse (or Climate Models: Do They Work?)

Much is made about the scientific consensus behind global warming, and so it's popular for global warming skeptics to emulate their ideological bedfellows, the creationists, and publish long lists of scientists (with the odd TV repairman and popstar thrown in to bulk out the numbers) who don't believe in human induced global warming.

However, scientific consensus isn't determined by a popularity contest. Rather, scientific battles are fought out in the various science journals, and eventually one theory (or sometimes both) is unable to explain known physical phenomena and predict unknown phenomena. About the closest one gets to scientific consensus is when one competing theory gives up the good fight, and migrates to crackpot websites, which have considerably lower standards than the scientific literature.

But enough of this waffle, and onto the science. It's easy to write off the giant computer programs that model the Earth's atmosphere as some kind of black magic. However, what's important, is how well their results correlate with reality. And a recent paper in Science gives us the chance to test this.

The Earth's atmosphere isn't uniform, rather it is made up of several different layers with names like the stratosphere, troposphere and thermosphere (this NASA website gives more details). From a global warming perspective, the troposphere (the part of the atmosphere closest to the earth) and the stratosphere (the layer about the troposphere - which also contains the ozone layer) are the most relevant. Both of these layers are defined by their temperature profiles (the way the temperature changes with altitude) and the boundary between the two is known as the tropopause.

Even since humanity has been putting weather balloons into the sky, we've known where the tropopause is. And from this data, we also know that the sky is falling... up. Or rather, the tropopause has been slowly but steadily rising.

And now, how to explain this. There are two probable causes of the rise. One is that the troposphere is heating up, and the other is that the stratosphere is cooling.

So what could cause this heating and cooling? Well, here are some ideas: solar variability and volcanos are both known to cause this. Likewise changes in the amount of sulphate particles, ozone and greenhouse house gases could also do this. Luckily for us, scientists have been measuring how all of these factors change for a long period of time.

The authors of the study mentioned above look at all of these, and plugged the numbers into a climate model known as the Department of Energy Parallel Climate Model (PCM). And loe and behold, what comes out, but the model predicts that the tropopause will increase in height, by an amount similar to what has been observed (it overestimates the increase in height by a bit, but more on that later).

Then, they look at each contribution separately (ie. they ran the model, and only allowed one of these variables to change). This gave some interesting results. During the 20th century, changes in the solar intensities, cause a very slight rise in height, meanwhile the sulphates and the volcanos both cause large decreases in the height (-32 and -39% of the observed change respectively). However, the two super-giants are the increase of greenhouse gases, and the decrease of ozone. The both cause a rise in the height of the tropopause of 77 and 88% of the observed change respectively. Surprise surprise, the two bullies are both caused by human activity. Also interestingly, if we just model solar effects, and volcanos, their ability to influence the change in the tropopause height drops with time (ie. other effects become larger, reducing the relative contribution of the natural effects).

Now, there are some uncertainties in this analysis. For one, it is readily apparent that the model overestimates the effects of volcanos (volcanic eruptions have a huge effect - the troposphere cools, whereas the stratosphere warms with a massive bang, both literally and on the charts of the climatologists). The other variables are also not modeled perfectly, but the model does a pretty good effort. Also, there are problems with the accuracy of the experimental dataset. Two different datasets, give similar but not identical numbers. These two effects, are probably responsible for the difference between the observed and modeled change in tropopause height.

So from this, we see that our climate model does a pretty job at modeling the change in tropopause height. But to simply reproduce experimental data isn't enough. In order to give the global warming skeptics something else to think about, we should also predict trends in the data.

The scientists involved in the work, poured over the model outputs looking for trends. The immediate trend that springs to mind, is that the rise in the troposphere's height is heterogeneous. Rather it depends on the latitude (or maybe the longitude - I always get the two confused, but the one which runs from the North Pole to the South Pole). The climate model predicts a certain spatial distribution (this is mostly from the extra loss of ozone in polar regions), and when the experimental data is examined, we see the same pattern.

Now, if the global warming skeptics want to change their current status as roadkill on the scientific highway, this is the sort of stuff that they should be doing. Generate realistic explanations for observations. Explain how the tropopause can change in height - without a large greenhouse gas contribution. And use numbers, not BS hand-waving. Publish the results in scientific journals (real ones - Energy & Environment doesn't count). Make predictions - especially ones which proponents of global warming don't. Stop the strawman arguments. Stop misrepresenting scientific papers.

You get the picture.


Source: Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes. By B. Santer, M. Wehner, T. Wigley, R. Sausen, G. Meehl, K. Taylor, C. Ammann, J. Arblaster, W. Washington, J. Boyle and W. Bruggemann. Science Volume 301 25 July 2003 Page 479.
| 12:11 AM