56 | April 2019
Take ragweeds, which are the third-most common aller-
gen in the United States. Roughly one in four Americans
are sensitive to their pollen.
Scientists have conducted laboratory experiments to de-
termine how ragweed responds to various concentrations of
CO2. One study showed that, as CO2 concentrations rise from
280 to 370 parts per million—which is what actually occurred
in the global atmosphere between about 1900 and 2000—rag-
weed pollen production more than doubles. Under a CO2 con-
centration of 600 parts per million, ragweed pollen production
doubles again. Today, atmospheric CO2 stands around 410
parts per million; if emissions grow unchecked, we could reach
600 parts per million in about 40 years. Similar patterns hold
for timothy grass pollen, another common allergenic plant that
sends out pollen early in the summer.
And that's not all. In 2005, a group of researchers led
by Lewis Ziska of the U.S. Department of Agriculture found
that higher CO2 levels also increased the potency of rag-
weed pollen for allergic people. If CO2 concentrations reach
600 parts per million, ragweed pollen could become not just
more common, but also about 1.7 times more allergenic
than it was in 2000.
Methodology: The first and last day below 32 degrees
were retrieved from the Applied Climate Information Sys-
tem. The length of the freeze-free season is the number
of days between these two points in time. Locations were
only included in the analysis if they had a freeze season
of at least 90 days.
ern United States, becoming about 19 days longer in the
Southwest and 16 days longer in the Northwest. And as the
frost-free season has lengthened, so has the pollen season
of ragweed, one of the most commonly allergenic plants
in the United States. In parts of the upper Midwest, for in-
stance, the ragweed pollen season lengthened by about
three weeks between 1995 and 2011.
For Americans with pollen allergies, it's not just the length
of the growing season that matters, but when the growing
season starts. Managing allergies effectively can require
that treatment begins several weeks before the allergens
proliferate in the air. Across the country, the growing sea-
son is starting earlier than it did in 1970—a week earlier, on
average. (Some locations that did not see longer growing
seasons overall may still have experienced earlier spring
freezes or earlier fall frosts.)
If warming emissions continue to climb unchecked, by
the end of the century, the growing season will lengthen by
at least an additional month in most of the United States, rel-
ative to late-twentieth century averages. And as the climate
warms, plants can move into new areas, exposing people
to allergenic pollen that they previously did not encounter.
More CO2 = More Pollen
By trapping more heat in the atmosphere, humanity's car-
bon emissions have made the growing season longer. But
carbon emissions also directly spur pollen production in
some allergenic plants, independent of changes in tem-
perature.
