Luis Alvarez is the man behind the asteroid theory. In 1980 he published a paper showing a thin layer of Iridium in the strata dividing the Cretaceous and Tertiary periods (the "K-T boundary"). This layer was seen at many places all over the globe, and Alvarez proposed that an asteroid impact threw enormous amounts of Iridium-rich debris into the atmosphere, blotting out the sunlight and causing the mass extinction at the end of the Age of the Dinosaurs.
This theory seized the imagination of much of the scientific community, and absolutely captivated the press. By the mid-1980s, you didn't hear any alternative stories about the end of the dinosaurs.
Like Anthropogenic Global Warming, the asteroid impact extinction proposal is terribly weak. First, while there is solid evidence for an impact crater off Mexico at this time (the Chicxulub impact basin), it is by no means the largest such impact crater that's been discovered. Other, larger impacts did not cause extinction, and other mass extinctions had no asteroid impact. Mass extinction events occur all the time, most recently 10,000 years ago with the Wooly Mammoths and Sabre Tooth Tiger. The unanswered question here is what was different at the end of the Cretaceous.
Second, the finest granularity of measurement we have for the fossil record 65 million years ago is about 100,000 years; there's no way that we can tell that a "sudden" extinction took less than 100,000 years. There are lots of signs that dinosaurs were in trouble in the time leading up to the end of the Cretaceous - the rate of formation of new Genus groups (the next size up in the classification scale from species; humans are genus Homo and species Sapiens). There were fewer and fewer genus groups represented in the fossil record as the end approached, and a very small number of genera made up the vast majority of the populations (e.g. 80% of large specimens were Triceratops in the Hell Creek fossil layer - the one right below the C-T Boundary). This is not a sign of a healthy, robust population.
Third, not all large animals went extinct. Crocodiles grow to very large - we might even say dinosaurian - size, and they are very little different than the crocodillians from the Cretaceous. Large mammals from the Cretaceous survived the extinction event - for example, Repenomamus Giganticus, best described as an ancient version of R.O.U.S that fed on dinosaurs. Plant species were hit much less hard by the C-T extinction, which doesn't fit the whole "dust cut off all the sunlight" hypothesis.
Fourth - and in my opinion most significant - is the discovery of dinosaur fossils above the K-T Boundary. The first paper describing this was published in Science in 1985 (Sloan et al.). [UPDATE: Wolfwalker in the comments points out that this can happen if the ground is disturbed; it's not the strongest point in the argument.]
So what gives? Longtime readers know that I'm sort of nasty and suspicious, and I think that this is Yet Another Example of a scientific error cascade driven by the politics of the day.
The 1980s saw the heating up of the Cold War. The Soviet Union invaded Afghanistan in 1979, shocking President Carter and a State Department over-invested in Détente. Ronald Reagan promised to confront Soviet expansionism, for example the deployment of Persing nuclear missiles in Europe to counter the Russian SS20 missiles already there. The peace movement - its core backed by the Soviet Union - agitated for a "Nuclear Freeze".
Some scientists (most notably Carl Sagan) stepped into the fray on the Nuclear Freeze side, with a literal theory of a freeze. Sagan proposed that a large scale exchange of nuclear weapons would throw so much dust into the atmosphere as to trigger a global cooling similar to the one that supposedly ended the dinosaurs. This was featured prominently on the PBS TV show Cosmos:
As I wrote in my post on Sagan and the Drake Equation:
Sagan talks a lot about the variable for intelligent civilizations that blow themselves up. He even tries a couple of different values for that variable, so see how the number N changes. Go watch it again, paying attention to this emphasis. It's quite striking.So what does all this mean? It means that scientists are human. They want to be relevant. They want their work to be important, meaning they want it to apply to the issues we face today. It means that they run in herds.
It means that we should all be properly skeptical about the pronouncements from Gray Beard scientists. JayG pointed out that some scientists have gone back through the published scientific literature and pronounced the issue to be settled. Whatever.
Max Plank famously said that a new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die. What's happening with Global Warming is actually very unusual, and only occurring because the AGW hypothesis is still in its infancy. It hasn't had time to calcify into Received Wisdom. The asteroid-killed-the-dinosaurs may need a generation of paleontologists to suffer their own personal extinction event before the science is updated.