Technical Abstract: In addition to being a significant contributor to global warming, carbon dioxide (CO2) is also the sole source of carbon for photosynthesis. Although the stimulation of plant growth by rising CO2 is usually viewed as positive, the rise in CO2 is indiscriminate with respect to the stimulation of plant species that are considered both of benefit and detriment to human society. Here we present laboratory and in situ data from studies that have examined the response of detrimental (weedy) plants to CO2 increases during the 20th century (i.e., from 290 to 375 parts per million by volume, ppmv), as well as that projected for the mid 21st century (500-1000 ppmv). Data from these studies indicated a number of possible indirect (e.g. increased use of herbicides) and direct effects (e.g. increased ragweed pollen) that may influence public health. Overall, these initial results regarding CO2 and/or temperature-induced changes in plant biology suggest a number of potentially unfavorable as well as some favorable consequences in human systems. However, these data are based on a small number of experiments and additional information is needed to reduce the biological and economic uncertainties associated with CO2-induced changes in plant biology and human health.