There are several types of validation. For example, you want to use validated math routines in your computations that account for things like roundoff error and the like. That's been done for decades.

Then you need to have some testing to see how well your hypothetical model actually matches reality. Many of these climate models come up short on this point. The Earth's climate is proving a more complicated process than many previously thought. There are tons of variables, with CO2 production being but one. If the model doesn't consider solar variability or clouds, as some don't, then they're too simplistic for a reasonable prediction.

Is the Earth warming? Sure looks like it, going back well over 100 years since the end of the Little Ice Age. My wife and I visited Glacier Bay National Park a couple years ago. The history of the park states:

In 1794, as the mother ship H.M.S. Discovery, Captained by George Vancouver, lay at anchor in Pt. Althorp, a survey crew under the command of Lt. Joseph Whidbey painstakingly maneuvered their longboats through the ice-choked waters of Icy Strait.

The remarkably accurate chart the survey produced shows a mere indentation in the shoreline, "terminated by solid compact mountains of ice," where Glacier Bay is today. The great glacier that filled the Bay was by then in rapid retreat, and was the source of the floating icepack that so hindered Whidbey. Any visitor who came by at the glacial maximum, a few decades earlier, would have found the glacier’s tongue extending out into Icy Strait almost to Lemesurier Island.

Today, the Bay extends over 20 miles inland as the glaciers have receeded. Most of that happened before 1900. Was human activity responsible, or did the climate warm on its own?

When people argue about global warming, the real question is "Are humans responsible?" Contrary to what some believe, there is a lot of legitimate scienfitic debate on the question. Will radically lowering CO2 production as called for by the Kyoto Protocol have any substancial effect in slowing the warming trend? Debateable. Very debateable. How do the costs of doing these reductions compared to the costs of mitigating the warming trend? That remains to be determined. Will having only the major industrial nations of today reduce CO2 production have any effect when countries like China and India (1/3rd of the Earth's population with rapidly growing economies) are exempted do any good? Doubtful, IMO.