Christine Rogers, a research associate at the Harvard School of Public Health, measures particulates – pollen grains and fungal spores – in outdoor air and correlates levels with asthma events. She also examines how those particulate levels might change over time because of global warming. “One of the most predictable effects of global warming is that CO2 is going to increase,” she says. “But also, seasonality is going to change. Springs will come earlier, lengthening our growing seasons. Both of these trends affect plants’ biomass, making them larger at maturity and, logically, able to produce more pollen.” To measure the effects of global warming on pollen production, Rogers and her colleagues forced ragweed plants to germinate two and four weeks earlier than they normally would, simulating the early springs that are likely to become more frequent. Half of each plant group was exposed to the normal, ambient CO2 level of 350 parts per million, and the other half to double that amount. Rogers found that total pollen production under ambient CO2 was higher in the plants grown in early spring than in those grown later. At high CO2 levels, however, plants grown later had higher total pollen production than those at ambient CO2.