Are the Models Untestable?

Some global warming deniers assert that the global climate models (GCMs) used to analyze and predict climate change can be ignored because they are "untestable" or "have no predictive ability." In the words of "Mug Wump" on the amazon.com Science Discussion Forum "Global Warming is Nothing but a Hoax and a Scare Tactic:"

If you had any scientific evidence that human CO2 causes global warming, you would have produced it by now. Ergo, reality eludes you. Don't believe or argue with me. There are many, many scientists who once rubbed shoulders with the AGW idiots like brothers that can't take enough showers now to wash off the stink--they've come clean and have acknowledged that global warming alarmism is nothing but a hoax. Amaze us all and admit that simple fact. While you're at it, why not admit that GCMs have been shown to have zero predictive ability? Oh wait, you can't: that requires an understanding of math. It's a lot easier for you to stick to the a Leftist-libs' kind of 'truth' and continue sticking science and the search for objective truth in the back.
-Posted Tuesday, September 22nd, 2009, at 5:28 PM PDT.

Are the models, in fact, untestable? Are they unable to make valid predictions? Let's review the record. Global Climate Models have successfully predicted:

1. That the globe would warm, and about how fast, and about how much.
2. That the troposphere would warm and the stratosphere would cool.
3. That nighttime temperatures would increase more than daytime temperatures.
4. That winter temperatures would increase more than summer temperatures.
5. Polar amplification (greater temperature increase as you move toward the poles).
6. That the Arctic would warm faster than the Antarctic.
7. The magnitude (0.3 K) and duration (two years) of the cooling from the Mt. Pinatubo eruption.
8. They made a retrodiction for Last Glacial Maximum sea surface temperatures which was inconsistent with the paleo evidence, and better paleo evidence showed the models were right.
9. They predicted a trend significantly different and differently signed from UAH satellite temperatures, and then a bug was found in the satellite data.
10. The amount of water vapor feedback due to ENSO.
11. The response of southern ocean winds to the ozone hole.
12. The expansion of the Hadley cells.
13. The poleward movement of storm tracks.
14. The rising of the tropopause and the effective radiating altitude.
15. The clear sky super greenhouse effect from increased water vapor in the tropics.
16. The near constancy of relative humidity on global average.
17. The expanded range of hurricanes and cyclones--a year before Cyclone Catarina showed up off the coast of Brazil, something which had never happened before.

Looks like a pretty good track record to me.

Are there problems with the models, and areas where they haven't gotten it right yet? Sure there are. The double Inter-Tropical Convergence Zone which shows up in some coupled models, ENSO variability, insufficiently sensitive sea ice, diurnal cycles of moist convection, and the exact response of climate to clouds are all areas of ongoing research. But the models are still the best thing we have for climate prediction under different scenarios, and there is no reason at all to think they're getting the overall picture wrong.