Our guest blogger, Sheryl Canter, is an Online Writer and Editorial Manager at Environmental Defense.

If you have a garden, you know the climate is warming. In temperate zones, the last frost in spring comes earlier, and the first frost in fall comes later. The longer growing season may allow you to grow vegetables you never could grow before. But you also may have noticed your weeds are more aggressive, insect pests are more of a problem, and pollen plagues you all summer long. You're not imagining things!

For over 40 years, gardeners have relied on the USDA Plant Hardiness Zone Map as a guide to what they can grow in their area. But the USDA zone map hasn't been updated since 1990, and gardeners have seen detectable shifts since that time.

In 2003, the American Horticultural Society (AHS) updated the zone map with a grant from the USDA, and published a draft of the new map [PDF] in The American Gardener. Based on temperature information from July 1986 to March 2002, the map showed widespread warming, with zones edging northward.

The USDA rejected the new map without explaining why, and said they would update it themselves. Four years have passed and still they have not released a new map. But the National Arbor Day Foundation has just released one, current for 2006. Like the 1990 and 2003 maps, the Arbor Day map is based on 15 years of data. The changes between 1990 and 2006 are dramatic; the U.S. is clearly getting warmer.

Global warming is caused by elevated concentrations of greenhouse gases in the atmosphere - notably carbon dioxide (CO2). Plants use sunlight, water, and CO2 to synthesize the glucose they need to grow - a process called photosynthesis. Thus when CO2 concentrations in the atmosphere increase, it acts as a fertilizer, accelerating plant growth. This may sound good at first, but there's more to the story.

CO2 fertilization affects different plants to different degrees. As Duke University biologists discovered, one plant that loves additional CO2 is poison ivy [PDF]. With increased CO2, poison ivy grows 2.5 times faster, and produces a more potent version of the rash-causing chemical urushiol. Other types of woody vines also grow much faster with higher levels of CO2 - fast enough to strangle and topple trees.

Accelerated plant growth has some other bad side effects. One is increased pollen production, creating misery for asthma and allergy sufferers. A Harvard study [PDF] showed that elevated CO2 concentrations caused up to a 55 percent increase in ragweed pollen production.

Another consequence is that high levels of CO2, while increasing crop yields, decrease the plants' nutritional value. Obviously this is bad for humans eating the plants, but it's also bad for humans growing them. Insects eat dramatically more plant matter when the plants are less nutritious (and ironically, can still starve to death from poor nutrition). Farmers using more pesticides to control infestations will increase pollution in rivers and streams.

So the next time you hear people arguing that global warming will be good for gardeners and farmers, set them straight!

Cross posted from Climate411.org, a blog by Environmental Defense's chief scientist, Bill Chameides.

Tons of CO2 pollution. We are always hearing about how many tons of CO2 pollution we emit. The average American car emits about seven tons of CO2 in a year; the average American family, about 24 tons; the United States as a whole, over seven billion tons; and worldwide, almost 30 billion tons. The Virgin Earth Challenge offers $25 million to whoever can economically remove one billion tons of CO2 from the atmosphere each year.

But what is a ton of CO2?

People keep saying to me, I thought CO2 is a gas. How can a gas have weight? I explain that CO2 is made up of atoms, and atoms have mass, and with gravity mass has weight. As often as not, my explanation is met with a blank stare. So let me try a different tack.

Picture a football field, and then imagine a round balloon with one end lined up on the goal line and the other on the 10 yards line – that is, a balloon with a diameter of 10 yards. If that balloon were filled with CO2, it would weigh about 1 ton; it would be a 1-ton CO2 balloon.

In 2006 an American family emitted the equivalent of 24 CO2 balloons by powering their home, driving their cars and flying. If you lined those balloons from end-to-end they would go from goal line to goal line almost two-and-a-half times – 240 yards!

Within a year or two, a little more than half of those CO2 balloons will be absorbed from the atmosphere into the ocean or trees. Unfortunately, the rest of the balloons will hang around for a very long time. One hundred years from now there will still be almost half a football field of CO2 balloons left to overheat the earth. And that's just the CO2 from one family in 2006. There will also be CO2 balloons from 2007, 2008, and so on.

In total, Americans were responsible for more than seven billion CO2 balloons in 2006. Lined up end to end they would circle the earth 1,600 times, and the number grows each year. Isn't it about time we start taking the CO2 out of some of those balloons?

Do you have ways of your own to imagine what a ton of CO2 looks like? If so, post them here!