Although it may seem like world leaders are panicking for no reason, the paper acknowledges that it is not unusual for an asteroid of this size to hit Earth. But if a 500 meter diameter asteroid were to hit Earth, it would create a crater 9 kilometers across and nearly 1 kilometer deep, so you’d better be prepared. The ejecta produced by such an impact would release enormous amounts of heat into the atmosphere, which would rain down on Earth and cause firestorms.
Conversely, if an asteroid of this size were to hit the ocean, we would face the threat of a massive tsunami that could destroy cities and, with it, the governing institutions of the affected areas. That will be the case.