The Hundredth Monkey Phenomenon

Given the prospect that even a small degree of global warming could cause a massive global extinction on the scale of the dinosaurs, we should be worried about it. Also, it may even be too late to prevent such a catastrophe simply by cutting back emissions:

Everyone with a bit of sense knows that the only way to combat global warming-induced climate disruption is through aggressive action to boost energy efficiency, shift to non-carbon energy production, and change our urban systems to increase lifestyle efficiency. Problem is, because of thermal inertia, we may already be too late to avoid catastrophic levels of planetary warming. We could well face a scenario in which our best efforts to cut carbon emissions end up still not happening fast enough to avoid a “tipping point” disaster (choose your apocalypse here: methane clathrates melting; complete collapse of ecosystems; heatwaves, wildfires and droughts killing millions; all of the above).

Fortunately there is at least one way to actively cool the Earth by shading it from the Sun with hundreds of tiny umbrellas. The above quote and description of this solution are found here.