(CBS) They're miserable in Chicago, hurting in Atlanta and suffering in New York.

That's because it's the allergy season — but as CBS News correspondent Mark Phillips reports, what we used to call the "allergy season" is becoming the "allergy year."

What allergy sufferers have suspected is true: Allergy season is starting earlier, lasting longer and becoming more intense. The stuff that produces the pollen — the grasses, weeds and trees — is the villain you can see. But there's another villain you can feel — global warming. It's good for the pollen makers, but bad for us.

At the British National Pollen Research Center, they've been tracking airborne pollens in Britain around the Northern Hemisphere. Researchers have found that as average temperatures have risen, so have pollen levels. Not only that, the nasty stuff is showing up earlier each year.

"On average, the pollen season is starting earlier by about five days per decade," says Jean Emberlin of the research center. "That means people are starting to have hay fever much earlier in the year than they were two or three decades ago."

Pollen season, which in the 1970s used to start sometime in May, now begins in early March.

Want more evidence? Huge clouds of pollen can now be tracked by satellite as they move across continents.

At the School of Public Health at Harvard, they've found an environmental double whammy. Not only is climate change speeding up the growth of allergy-causing plants, but carbon dioxide — one of the greenhouse gases that causes global warming by trapping the sun's heat — also works on its own to increase pollen output.

"An earlier spring increases pollen production in late-flowering plants such as ragweed," says Christine Rogers of the Harvard School of Public Health. "High (carbon dioxide) also increases plant productivity and results in greater pollen production."

The longer-term consequences of global warming may still be debated. But one consequence — the effect on public health — is already here.

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