We - the human species - have arrived at a moment of decision. It is unprecedented and even laughable for us to imagine that we could actually make a conscious choice as a species, but that is nevertheless the challenge that is before us.
Our home - Earth -is in danger. What is at risk of being destroyed is not the planet itself, but the conditions that have made it hospitable for human beings.
Without realizing the consequences of our actions, we have begun to put so much carbon dioxide into the thin shell of air surrounding our world that we have literally changed the heat balance between Earth and the Sun. If we don't stop doing this pretty quickly, the average temperature will increase to levels humans have never known and put an end to the favorable climate balance on which our civilization depends.
In the last 150 years, in an accelerating frenzy, we have been removing increasing quantities of carbon from the ground - mainly in the form of coal and oil - and burning it in ways that dump 70 million tons of CO2 every 24 hours into the Earth's atmosphere.
The concentrations of CO2 - having never risen above 300 parts per million for at least a million years - have been driven from 280 parts per million at the beginning of the coal boom to 383 parts per million this year.
As a direct result, many scientists are now warning that we are moving closer to several "tipping points" that could - within 10 years - make it impossible for us to avoid irretrievable damage to the planet's habitability for human civilization.
Just in the last few months, new studies have shown that the north polar ice cap - which helps the planet cool itself - is melting nearly three times faster than the most pessimistic computer models predicted. Unless we take action, summer ice could be completely gone in as little as 35 years. Similarly, at the other end of the planet, near the South Pole, scientists have found new evidence of snow melting in West Antarctica across an area as large as California.
This is not a political issue. This is a moral issue, one that affects the survival of human civilization. It is not a question of left versus right; it is a question of right versus wrong. Put simply, it is wrong to destroy the habitability of our planet and ruin the prospects of every generation that follows ours.
On Sept. 21, 1987, President Ronald Reagan said, "In our obsession with antagonisms of the moment, we often forget how much unites all the members of humanity. Perhaps we need some outside, universal threat to recognize this common bond. I occasionally think how quickly our differences would vanish if we were facing an alien threat from outside this world."
We - all of us - now face a universal threat. Though it is not from outside this world, it is nevertheless cosmic in scale.
Consider this tale of two planets. Earth and Venus are almost exactly the same size, and have almost exactly the same amount of carbon. The difference is that most of the carbon on Earth is in the ground - having been deposited there by various forms of life over the last 600 million years - and most of the carbon on Venus is in the atmosphere.
As a result, while the average temperature on Earth is a pleasant 59 degrees, the average temperature on Venus is 867 degrees. True, Venus is closer to the Sun than we are, but the fault is not in our star; Venus is three times hotter on average than Mercury, which is right next to the Sun. It's the carbon dioxide.
This threat also requires us, in Reagan's phrase, to unite in recognition of our common bond.
Next Saturday, on all seven continents, the Live Earth concert will ask for the attention of humankind to begin a three-year campaign to make everyone on our planet aware of how we can solve the climate crisis in time to avoid catastrophe. Individuals must be a part of the solution. In the words of Buckminster Fuller, "If the success or failure of this planet, and of human beings, depended on how I am and what I do, how would I be? What would I do?"
Live Earth will offer an answer to this question by asking everyone who attends or listens to the concerts to sign a personal pledge to take specific steps to combat climate change. (More details about the pledge are available at algore.com.)
But individual action will also have to shape and drive government action. Here Americans have a special responsibility. Throughout most of our short history, the United States and the American people have provided moral leadership for the world. Establishing the Bill of Rights, framing democracy in the Constitution, defeating fascism in World War II, toppling Communism and landing on the moon — all were the result of American leadership.
Once again, Americans must come together and direct our government to take on a global challenge. American leadership is a precondition for success.
To this end, we should demand that the United States join an international treaty within the next two years that cuts global warming pollution by 90 percent in developed countries and by more than half worldwide in time for the next generation to inherit a healthy Earth.
This treaty would mark a new effort. I am proud of my role during the Clinton administration in negotiating the Kyoto protocol. But I believe that the protocol has been so demonized in the United States that it probably cannot be ratified here - much in the way the Carter administration was prevented from winning ratification of an expanded strategic arms limitation treaty in 1979. Moreover, the negotiations will soon begin on a tougher climate treaty.
Therefore, just as President Reagan renamed and modified the SALT agreement (calling it Start), after belatedly recognizing the need for it, our next president must immediately focus on quickly concluding a new and even tougher climate change pact. We should aim to complete this global treaty by the end of 2009 - and not wait until 2012 as currently planned.
If by the beginning of 2009, the United States already has in place a domestic regime to reduce global warming pollution, I have no doubt that when we give industry a goal and the tools and flexibility to sharply reduce carbon emissions, we can complete and ratify a new treaty quickly. It is, after all, a planetary emergency.
A new treaty will still have differentiated commitments, of course; countries will be asked to meet different requirements based upon their historical share or contribution to the problem and their relative ability to carry the burden of change. This precedent is well established in international law, and there is no other way to do it.
There are some who will try to pervert this precedent and use xenophobia or nativist arguments to say that every country should be held to the same standard. But should countries with one-fifth our gross domestic product - countries that contributed almost nothing in the past to the creation of this crisis - really carry the same load as the United States? Are we so scared of this challenge that we cannot lead?
Our children have a right to hold us to a higher standard when their future - indeed, the future of all human civilization - is hanging in the balance. They deserve better than a government that censors the best scientific evidence and harasses honest scientists who try to warn us about looming catastrophe. They deserve better than politicians who sit on their hands and do nothing to confront the greatest challenge that humankind has ever faced - even as the danger bears down on us.
We should focus instead on the opportunities that are part of this challenge. Certainly, there will be new jobs and new profits as corporations move aggressively to capture the enormous economic opportunities offered by a clean energy future.
But there's something even more precious to be gained if we do the right thing. The climate crisis offers us the chance to experience what few generations in history have had the privilege of experiencing: a generational mission; a compelling moral purpose; a shared cause; and the thrill of being forced by circumstances to put aside the pettiness and conflict of politics and to embrace a genuine moral and spiritual challenge.
Al Gore, vice president from 1993 to 2001, is the chairman of the Alliance for Climate Protection. He is the author, most recently, of "The Assault on Reason."