Rob Jackson is the Nicholas Chair of Global Environmental Change and director of Duke University's Global Change Center. He is the author of "The Earth Remains Forever" and a children's book, "Animal Mischief."

Timing is everything in life, and the time to debate global warming - and whether or not humans are a major cause of it - is over.

Greenhouse gas concentrations have risen rapidly since the start of the Industrial Revolution. Atmospheric carbon dioxide has increased by a third, and methane concentrations have doubled. How do we know this with certainty? Ice-core records from Antarctica and Greenland allow scientists to study actual bubbles of air trapped in glaciers back almost a million years. Carbon dioxide and methane concentrations are higher today than at any time in the past million years, probably tens of millions.

We also know that the Earth is warming quickly. We know this from weather stations, aircraft measurements, balloons, buoys, boreholes and satellites. Pictures help us, too. Look at images of the Arctic ice cap at the end of the summer in 1979. Look again at the end of summer in 2005, and you'll see ice that has shrunk by 20 percent. The North Pole may be ice-free in summer within 50 years.

Couldn't natural causes, such as volcanoes or sunspots, be the culprits causing this warming? No, but don't take my word for it.

Read the consensus statements from the National Academy of Sciences or the Intergovernmental Panel on Climate Change. Read the official climate change statement for 40,000 earth scientists in the American Geophysical Union: "Scientific evidence strongly indicates that natural influences cannot explain the rapid increase in global near-surface temperatures observed during the second half of the 20th century." Read the official statement of the 11,000-member American Meteorological Society: "Because human activities are contributing to climate change, we have a collective responsibility to develop and undertake carefully considered response actions." Not if human activities are contributing to climate change, but because they are.

A well-established tie

Scientists have known how greenhouse gases work for almost two centuries. In 1827, physicist Jean Baptiste Fourier first showed how carbon dioxide, methane and other greenhouse gases trapped infrared radiation to warm the Earth. By the 1890s, Nobel Prize-winning chemist Svante Arrhenius had already described how emissions given off when we burn coal, oil and other fossil fuels would warm the Earth further.

Given the link between people and global warming, I'm not going to give the usual litany of reasons for acting now: the spread of tropical diseases into temperate locales; heat waves and droughts that harm people and crops; more intense hurricanes and storms; rising sea levels that could submerge the Outer Banks this century and that will keep rising for a thousand years after we stabilize greenhouse gas concentrations. What I'd like to do instead is to link global warming to other issues we care deeply about, including money, security and health.

By reducing our need for foreign oil, we could reduce our country's trade deficit and improve our national security. Using less fossil fuel would also save lives. Conservatively, tens of thousands of Americans die every year from air pollution. Hundreds of thousands more get sick, with extra cases of asthma, bronchitis, emphysema, cancer, stroke and heart disease.

Because vehicle emissions and coal-fired power plants are our two biggest sources of air pollution, we could save lives now and reduce global warming by improving efficiency and using renewable fuels. Get mad that we haven't done this already.

In 1908, Henry Ford's first Model T rolled off the assembly line and got 20 miles to the gallon. Today the fleet average for the United States is still about 20 miles. With gas topping $3, people are snapping up smaller cars from foreign companies such as Honda and Toyota, while U.S. automakers suffer.

Other countries have been smarter about renewable energy, too. Denmark now generates one-fifth of its electricity using wind power, and Danish companies control half of the global market for wind turbines, a multibillion-dollar business that creates tens of thousands of jobs. Americans invented the silicon solar panel, but our complacency with fossil fuels let that market slip away to Asia and Europe as well.

Tackling global warming now makes environmental and economic sense. Be patriotic and conserve energy; buy green power; live close to work and enjoy spending more time with your family and friends.

Changing your light bulbs will help, too, but it won't fix the problem entirely. We need to join the world with a federal policy that prices carbon for efficiency and energy innovation. Make the politicians take notice.

The time to debate global warming is over. The time to start working is now.