Poison Ivy's Past

kw: speculation, global warming, plant growth

The recent reports of the effect of higher levels of CO₂ on the growth or poison ivy got me thinking. In short, the present CO₂ level in the atmosphere is 370 ppm or so. In 2050 it is expected to be about 570 ppm. At an experimental forest in North Carolina, researchers found that poison ivy grew 150% faster, produced urushiol (the toxic oil) at a 150% greater concentration, and that the urushiol was a more toxic variety.

When I was a child, I frequently got small poison ivy rashes. This was the 1950s. There was no cortisone treatment. We could only use calomel or potassium permanganate poultices. The rash typically could be cleared up in about two weeks. At that time, CO₂ in the atmosphere was about 300 or 310 ppm.

So, let's figure. 570/370 = 1.55. (100%+150%)/(100%) = 2.5. There are two likely mathematical models. One is exponential: log(2.5)/log(1.55) = 2.1. This is the model exponent. For simplity's sake, we'll say this model predicts foliage growth and urushiol concentration both increase as the square of the CO₂ level.

The other is a linear relationship with a threshold. Plot points at (570, 2.5) and (370, 1), and you find that the slope is 1.5/200 = .0075. Then, 1/.0075 = 133.33, the level of CO₂ at which growth and urushiol production cease. The proportionality constant is 0.0075 per ppm.

What do these two models predict for a CO₂ level of 310 (when I was a pre-teen)?

First model: The square of 310/370 = 0.70, which is 30% less than 1.

Second model: Decrement of 0.0075*60 = 0.45, or 45% less.

My conclusion: It seems likely that the growth rate of poison ivy was 30% to 45% less than today, during the 1950s and 1960s, and that the concentration of urushiol was also lower by a similar amount. It is also likely that the urushiol then produced was less toxic.

I do know that my reaction to poison ivy has gotten much more severe over the past fifty years. Every case I've had since the mid-1980s required a three-to-four-week regimen with prednisone to clear up. And poison ivy looks bigger than it did then. Since I am much bigger now than when I was 12, it must be really bigger! Bigger and badder...