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Poison Ivy in a Warming World

If you’re one of the tens of millions of people who love being outdoors in the summer, you’ve probably had at least one nasty encounter with poison ivy, that noxious weed that causes blisters and an awful itch that can drive you absolutely crazy.

Rising carbon dioxide levels are likely to make the nuisance even more, well, annoying. Most plants use CO2 as a raw material to drive their growth. As human emissions of this heat-trapping gas continue to increase, plants have more material to work with. Some plants can use the extra CO2 more effectively than others — and poison ivy, unfortunately, is one of them.

According to a 2007 study by Lewis Ziska, a plant physiologist at the U.S. Department of Agriculture, poison ivy plants respond to higher levels of CO2 by growing bigger and producing more urushiol, the toxic oil that causes that terrible rash. There’s some evidence that the urushiol gets more concentrated, with more in every square inch of leaf, but the problem is mostly just that the plants produce more leaves.

The 2007 study was based on controlled experiments that exposed plants to CO2 levels to the 1950s compared to levels more comparable with today. The results showed that poison ivy is most likely worse now than it was when Eisenhower was president. It also projected forward to the end of this century based on the major climate study released by the IPCC back in 2007. The data behind our graphic, however, have been recalculated based on projections from the new IPCC report issued just a few months ago.

Future emissions depend on decisions that have yet to be made by politicians. To cover its bases, the IPCC makes its projections based on different emissions scenarios, and for the purposes of the poison ivy studies, Ziska and his co-authors used a high-emissions scenario, consistent with the rate of emissions we’re producing now.

Ziska also told Climate Central that his calculations are based on concentrations of CO2 as documented at NOAA’s high-altitude Mauna Loa research station in Hawaii. Concentrations on the ground, where poison ivy actually grows, are usually about 10 percent higher — and in urban areas, they can be up to 30 percent higher. So however big and bad poison ivy grew in Ziska’s experiments, it could be even worse in the gardens, woods and parks where people run into the awful stuff.

There are plenty of reasons to worry about the impacts of climate change (just check out the new U.S. National Climate Assessment for some of them). This is just one more you can add to the mix.