dr. chiariello: the prediction for california for the next century is that rainfall should increase.he option that we chose was to supplement each major rainfall by about 50%. narrator: over the growing season, the researchers allow the plants to grow to maturity. then they carefully harvest 10-centimeter squares of plant material. dr. iariel: the first cut at understanding whether or not plants are responding to elevated co2 or to any of these other factors is to see how much plant biomass there is at the end of a growing season. the plant biomass is partlovground. it's what we see in terms of leaves and flowers and stems. and part of it is below ground in the form of roots and chemicals that have leaked intohe soil around the roots. narrator: once the plots are harvested, the samples are dried and then weighed to calculate the amount of carbon the plants removed from the atmosphere each year. this key measure, known as the net primary production, is used in computer models that predict climate change effects on other ecosystems. yeah. after eight years of conducting this experiment