It was tediously written, ponderous in length, had 43 co-authors, and published in a journal that rarely issues a press release. Our thanks to a science writer at Electricity Daily who boiled the whole thing down to its essence, and to lead author James Hansen, who reviewed the news report and let it stand without comment. However, climate can vary chaotically, i.e., in the absence of any forcing. The slightest alteration of initial or boundary conditions changes the developing patterns, and thus next year's weather in inherently unpredictable. This behavior results from the nonlinear fundamental equations governing the dynamics of such a system.
Average temperature is chosen because it is believed to be the least sensitive to chaos.
If it does, they posit, that shows forcings have an effect independent of chaos. While the experiment worked to some degree for the stratosphere and the sea surface, where things are fairly simple, it failed utterly for the troposphere, where climate exists. For the most detailed modeling, the unforced case yields a correlation with observation of 26 percent, or basically no correlation. In fact, the only model to exceed a 50 percent correlation for the troposphere is one in the unforced mode that hits 59 percent. When forcing is added, its performance dips to 19 percent.