The buildup of greenhouse gases in the atmosphere has resulted in global climate change that is having a significant effect on many allergenic plants through increases in plant productivity and pollen allergenicity and shifts in plant phenology. Based on experimental studies, increased atmospheric levels of carbon dioxide have directly increased plant productivity. This has affected the total amount of pollen produced in some species. Research has also shown increased levels of birch allergen at warmer temperatures. Warmer temperatures have resulted in earlier flowering for many spring-flowering species in many countries, recorded through visual observations of flowering and by airborne pollen. Increases in the cumulative season totals of various pollen types also have been recorded; some of these increases may be explained by changes in plant distribution.