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 Climate
Change Indicators: Ragweed Pollen Season
This indicator depicts changes in the length of ragweed
pollen season in the United States and Canada.
Key Points
Since 1995, ragweed pollen season has grown longer at 10 of
the 11 locations studied (see Figure 1).
The increase in ragweed season length generally becomes more
pronounced from south to north. Ragweed season increased by
25 days in Winnipeg, Manitoba; 24 days in Saskatoon,
Saskatchewan; 21 days in Fargo, North Dakota; and 18 days in
Minneapolis, Minnesota (see Figure 1). This trend is
consistent with many other observations showing that climate
is changing more rapidly at higher latitudes.
The trends in Figure 1 are strongly related to changes in
the length of the frost-free season and the timing of the
first fall frost. Northern areas have seen fall frosts
happening later than they used to, with the delay in first
frost closely matching the increase in pollen season.
Meanwhile, some southern stations have experienced only a
modest change in frost-free season length since 1995. |
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Background
Allergies are a major public health concern, with hay fever
(congestion, runny nose, itchy eyes) accounting for more
than 13 million visits to physicians’ offices and other
medical facilities every year. One of the most common
environmental allergens is ragweed, which can cause hay
fever and trigger asthma attacks, especially in children and
the elderly. An estimated 15.5 percent of all Americans are
sensitive to ragweed.
Ragweed plants mature in mid-summer and produce small
flowers that generate pollen. Ragweed pollen season usually
peaks in late summer and early fall, but these plants often
continue to produce pollen until the first frost. A single
ragweed plant can produce up to a billion pollen grains in
one season, and these grains can be carried long distances
by the wind.
Climate change can affect pollen allergies in several ways.
Warmer spring temperatures cause some plants to start
producing pollen earlier (see the Leaf and Bloom Dates
indicator), while warmer fall temperatures extend the
growing season for other plants, such as ragweed (see the
Length of Growing Season indicator). Warmer temperatures and
increased carbon dioxide concentrations also enable ragweed
and other plants to produce more allergenic pollen, in
larger quantities. This means that many locations could
experience longer allergy seasons and higher pollen counts
as a result of climate change. |
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About the Indicator
This indicator shows changes in the length of the ragweed
pollen season in 11 cities in the central United States and
Canada. These locations were selected as part of a study
that looked at trends in pollen season at sites similar in
elevation, but across a range of latitudes from south to
north. At each location, air samples have been collected and
examined since at least the 1990s as part of a national
allergy monitoring network. Pollen spores are counted and
identified using microscopes.
Pollen counts from each station have been analyzed to
determine the start and end dates of each year’s ragweed
pollen season. Because the length of ragweed season
naturally varies from year to year, statistical techniques
have been used to determine the average rate of change over
time. This indicator shows the total change in season length
from 1995 to 2015, which was determined by multiplying the
average annual rate of change by the number of years in the
period.
Indicator Notes
This indicator is based on data from a limited number of
cities in the central states and provinces. These cities
cover a broad range from north to south, however, which
allows researchers to establish a clear connection between
pollen season changes and latitude.
Many factors can influence year-to-year changes in pollen
season, including typical local and regional variations in
temperature and precipitation, extreme events such as floods
and droughts, and changes in plant diversity. In addition,
seasonal pollen production may differ between stations
located in densely urbanized areas and stations in outlying
areas. Adding more years of data would provide a better
picture of long-term trends, but widespread data were not
available prior to 1995.
This indicator does not show how the intensity of ragweed
pollen season (pollen counts) might also be changing.
Data Sources
Data for this indicator come from the National Allergy
Bureau, which is part of the American Academy of Allergy,
Asthma, and Immunology’s Aeroallergen Network. Data were
compiled and analyzed by a team of researchers that
published a more detailed version of this analysis in a
scientific journal with data through 2009.
Technical Documentation
Download related technical information PDF |
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 Figure
1. Change in Ragweed Pollen Season, 1995–2015This
figure shows how the length of ragweed pollen season changed
at 11 locations in the central United States and Canada
between 1995 and 2015. Red circles represent a longer pollen
season; the blue circle represents a shorter season. Larger
circles indicate larger changes.
Data source: Ziska et al., 20168 |
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EPA Page |
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This is the
EPA page for this topic. To see if the Trump
administration has changed the EPA page, simply click the
link and compare the information with this page. If you
notice changes were made to the EPA page, please post a
comment. Thanks. |
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Additional Climate Change Information |
Climate Change and Carbon Dioxide
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change
and carbon dioxide.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice. |
Carbon Dioxide and Climate Change
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of carbon dioxide
and climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice. |
Environmental Group Warns Earth's Health at Risk
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
A report by the World Wildlife Fund looked at thousands of animal populations
and found they have dropped significantly in 40 years. |
Sea Levels Rising at Fastest Rate in 3,000 years
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
A group of scientists say sea levels are rising at record rates. Another group
found that January temperatures in the Arctic reached a record high. |
Capturing CO2 Gas Is Not Easy
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
Most scientists agree that carbon-dioxide gas is partly to blame for climate
change: rising global temperatures. But capturing the CO2 gas released by power
stations is costly and difficult. |
Growth, Climate Change Threaten African Plants and
Animals
(Beginner - Listening,
reading)
A video lesson to
help with your understanding of climate change.
The English is
spoken at 75% of normal speed.
Great English listening and reading practice.
Researchers believe Africa may lose as much as 30 percent of its animal and
plant species by the end of this century. |
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