Cities as harbingers of climate change: common ragweed, urbanization, and public health.

BACKGROUND Although controlled laboratory experiments have been conducted to demonstrate the sensitivity of allergenic pollen production to future climatic change (ie, increased CO(2) and temperature), no in situ data are available. OBJECTIVE The purpose of this investigation was to assess, under realistic conditions, the impact of climatic change on pollen production of common ragweed, a ubiquitous weed occurring in disturbed sites and the principal source of pollen associated with seasonal allergenic rhinitis. METHODS We used an existing temperature/CO(2) gradient between urban and rural areas to examine the quantitative and qualitative aspects of ragweed growth and pollen production. RESULTS For 2000 and 2001, average daily (24-hour) values of CO(2) concentration and air temperature within an urban environment were 30% to 31% and 1.8 degrees to 2.0 degrees C (3.4 degrees to 3.6 degrees F) higher than those at a rural site. This result is consistent with most global change scenarios. Ragweed grew faster, flowered earlier, and produced significantly greater above-ground biomass and ragweed pollen at urban locations than at rural locations. CONCLUSIONS Here we show that 2 aspects of future global environmental change, air temperature and atmospheric CO(2), are already significantly higher in urban relative to rural areas. In general, we show that regional urbanization-induced temperature/CO(2) increases similar to those associated with projected global climatic change might already have public health consequences; we suggest that urbanization, per se, might provide a low-cost alternative to current experimental methods evaluating plant responses to climate change.