Climate change is not the only threat to humanity that the United Nations has been called upon to deal with. Another risk that bears consideration is that of the inevitable collisions that will occur between our planet and other big rocks in space. A group called The Association of Space Explorers has raised the issue recently, arguing that the near pass of Asteroid 99942 Apophis in 2036 should prompt some coordinated global thinking on appropriate responses to possible impacts. Their report refers the the Tunguska event of 1908, a three to five megatonne explosion which started fires large enough to engulf New York City. Apophis has a 1-in-45,000 chance of striking the Earth, but would generate a 500 megatonne blast if it did so. On an astronomical timeline, it is inevitable that an object of this magnitude will eventually strike the Earth.
The group has released a fifty page report entitled Asteroid Threats: A Call for Global Response (PDF). It covers both some technological options for deflecting incoming asteroids and decision-making processes through which such plans could be put into action. It makes a strong case that the probability of successful deflection is much higher if action is taken early. An extension of that is the need to take decisions before it is certain a collision will occur: a situation that significantly increases the probability that such a decision will need to be made within the foreseeable future.