How could an asteroid have the dinosaurs become extinct 65 million years ago?

The asteroid was about 6 miles across. When it crashed into what is now the Yucatan Peninsula, it threw so much dust into the upper atmosphere, that sunlight was blocked for months. Plants died without sunlight, and the herbivorous dinosaurs died without plants to eat. The carnivores soon died out, too, because their prey was gone.

There is also evidence that the Chixculub impact preceded the dinosaur extinction by hundreds of thousands of years, until another, possibly larger asteroid struck eliminated the non avian dinosaurs once and for all.

Even without the asteroids, dinosaurs were already dealing with the negative affects of climate change.