But human borders mean nothing to air, water, windblown soil or seeds or migrating fish, birds or mammals.

If America wants to retain its position as a global power, its president must listen to the people and show strong leadership at this turning point in human history.

It is not too late to take another path.

Some argue we should get coal, oil and gas out of the ground as quickly as possible, build more pipelines and make as much money as we can selling it here and abroad. Their priorities are the economy and meeting short-term energy needs so we can live the lives to which we've become accustomed.

The increasing frequency of extreme weather events, droughts and floods is in line with what climate scientists have been predicting for decades - and evidence is mounting that what's happening is more severe than predicted, and will get far worse still if we fail to act.

And that, quite simply, is the issue. We live in a finite world with finite resources. Although it may sometimes seem quite big, earth is really very small - a tiny blue and green oasis of life in a cold universe.

Human beings are often at their best when responding to immediate crises - car accidents, house fires, hurricanes. We are less effective in the face of enormous but slow-moving crises such as the loss of biodiversity or climate change.