The symptoms of our planetary fever are becoming more obvious with each passing year. Now a place that has been locked in solid ice since our ancestors were swinging from the trees is turning to liquid, way ahead of previous scientific predictions. Robert Corell, one of America's leading climate scientists, warns: "If you want to see what will happen to the rest of the world, look to the Arctic. It happens there first."

Our heat is what is turning it into a landscape that we can no longer recognise. If humans continue emitting warming gases at the current rate, this will happen in most places - with rising oceans, dried-out and dying agricultural lands, and far more extreme weather events.

The speed with which this is happening suggests it won't just happen to the grandchildren and polar bears politicians keep evoking in speeches. It will happen to us. The world's climate scientists are warning that in my lifetime, we could be on course for five degrees of warming. That's a gap as big as that between the way we live now, and the last ice age. It will change our planet to one we don't understand, and cannot inhabit in anything like our current numbers.

This year, there is a chance - at five minutes to ecological midnight - to change course. The world's leaders will meet in Copenhagen to agree a successor to Kyoto. If they resolve to make substantial and binding cuts, we could keep the ecosystem the right side of the tipping point, beyond which it will collapse. But it has to happen now.

Anything you do in the next few months to pressure your politicians - marching and campaigning and volunteering for Greenpeace or Friends of the Earth - may be the most important thing you ever do.

But many people concerned about this catastrophic global warming are systematically misdirecting their political energies. They are taking their anxieties and ploughing them exclusively into cutting their own personal emissions.

This is, at best, of very limited value, and at worst a placebo that stops you from reaching for the real medicine. The only thing that will keep our climate within safe parameters is mass public pressure on our politicians to agree binding restrictions that apply to all of us - not just the nice 10 percent who will voluntarily cut back.

The time for that pressure is now. The Arctic was a canary in the coal mine. The canary is half-dead. It's time to shout.