We have read the science. Global warming is real, and we are a prime cause.

We have heard the warnings. Unless we act, now, we face serious consequences. Polar ice will melt. Sea levels will rise. A third of our plant and animal species could vanish. There will be famine in Africa and Central Asia.

Largely lost in the debate is the good news: We can do something — more easily, and at far less cost, than most of us imagine.

These are the conclusions of the latest report from the Intergovernmental Panel on Climate Change, the scientific body that recently shared the Nobel Peace Prize. It is sobering reading. But let's remember its optimistic bottom line as world leaders gather in Bali this week, seeking an agreement on climate change that all nations can embrace.

We do not yet know what such an accord might look like. Should it urge governments to tax greenhouse gas emissions or endorse a global carbon-trading system? Should it provide mechanisms for preventing deforestation, accounting for 20 percent of carbon dioxide emissions, or help less developed nations adapt to the inevitable effects of global warming?

The answer is some variation on all these things — and much, much more. But at Bali, the goal is simpler and more immediate. We must set an agenda to a better future and a timeline that produces a deal by 2009.

It helps to have a vision of how the future might look if we succeed. That is not merely a cleaner, healthier, more secure world. Handled correctly, our fight against global warming could set the stage for an eco-friendly transformation of the global economy — one that spurs growth and development rather than crimps it, as many nations fear.

We have witnessed three economic transformations in the past century. First, the Industrial Revolution, then the technology revolution and now globalization. We stand at the threshold of another great change: the age of green economics.

The evidence is all about us, often in unexpected places. Visiting South America recently, I saw how Brazil has become one of the biggest players in green economics, drawing some 44 percent of its energy needs from renewable fuels. The world average is 13 percent. In Europe: 6.1 percent.

Much is made of the fact that China is poised to surpass the United States as the world's largest emitter of greenhouse gases. Less well known, however, are its more recent efforts to confront grave environmental problems. China is on track to invest $10 billion in renewable energy this year, second only to Germany. It has become a world leader in solar and wind power. At a recent summit of East Asian leaders, Premier Wen Jiabao pledged to reduce energy consumption (per unit of gross domestic product) by 20 percent over five years — not far removed, in spirit, from Europe's commitment to a 20 percent reduction in greenhouse gas emissions by 2020.

This is the way of the future. Some estimates show that growth in global energy demand could be cut in half over the next 15 years simply by deploying existing technologies yielding a return on investment of 10 percent or more. The IPCC report lays out the very practical ways, from tougher standards for air conditioners and refrigerators to improved efficiency in industry, building and transport. It estimates that overcoming serious climate change may cost as little as 0.1 percent of global GDP a year over the next three decades.

Growth need not suffer. Research by the University of California at Berkeley indicates that the United States could create 300,000 jobs if 20 percent of electricity needs were met by renewables.

A leading Munich consulting firm predicts that more people will be employed in Germany's enviro-technology industry than in the auto industry by the end of the next decade. The U.N. Environment Program estimates that global investment in zero-greenhouse energy will reach $1.9 trillion by 2020 — seed money for a wholesale reconfiguration of global industry.

Already, businesses in many parts of the world are demanding clear public policies on climate change — regulation, emissions caps, efficiency guidelines. Our job, in Bali and beyond, is to shape this nascent global transformation — to open the door to the age of green economics and green development. What's missing is a global framework within which we, the world's peoples, can coordinate our efforts to fight climate change.

Scientists have done their job. Now it's up to the politicians. Bali is a test of their leadership.

Ban Ki-moon is secretary general of the United Nations. He wrote this commentary for The Washington Post.