Monday, April 15, 2019

Climate Change, the Growing Season, and America’s Allergies

March 27, 2019

Nearly 20 million Americans suffer from pollen allergies. Analysis of local temperature data by Climate Central and recent scientific research show that climate change is prolonging their season of suffering.

Global warming is extending the freeze-free season, giving plants more time to grow, flower, and produce pollen. And as atmospheric carbon dioxide (CO2) levels rise, laboratory experiments suggest, some plants that produce allergenic pollen produce even more of it.

These trends will worsen as humanity dumps more carbon into the atmosphere. Like global warming’s effects on heat waves and vector-borne diseases, climate change’s consequences for pollen allergies reveal how greenhouse gas emissions are already damaging Americans’ health.

Over the past few decades, the prevalence of allergies among Americans has skyrocketed. In 1970, about one in ten Americans suffered from hay fever, which is caused by airborne allergens, such as pollen and mold spores; by 2000, three in ten did.

Asthma—which often occurs alongside pollen allergies—has become more common, too. The proportion of Americans who suffer from asthma rose from 3.1 percent to 8.4 percent of the population between 1980 and 2010. Rates are even higher among African-Americans, low-income households, and children. Today, some 6.2 million Americans under the age of 18 suffer from the chronic disease.

These are expensive, dangerous problems. Overall, allergies cost the United States more than $18 billion per year. Asthma brought people to U.S. emergency rooms 1.7 million times in 2015; the next year, it killed about 3,500 Americans. And even the more manageable effects of pollen allergies remain inconvenient and uncomfortable.


To understand how the growing season has changed across the United States, Climate Central assessed temperature data for 201 cities. Of those cities, 83% saw their freeze-free seasons lengthen since 1970. In the average city, the amount of time between the last and first freeze of the year grew by just over two weeks.


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, relative 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.


By trapping more heat in the atmosphere, humanity’s carbon emissions have made the growing season longer. But carbon emissions also directly spur pollen production in some allergenic plants, independent of changes in temperature.

Take ragweeds, which are the third-most common allergen in the United States. Roughly one in four Americans are sensitive to their pollen.

Scientists have conducted laboratory experiments to determine 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—ragweed pollen production more than doubles. Under a CO2 concentration 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 ragweed 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.

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