The growing season for ragweed, the main cause of allergies in the U.S., has expanded by an average of 11 days since the 1970s, an analysis shows.

As the northern hemisphere warms, the last spring frost of the year is occurring ever earlier, and the first fall frost ever later. As a result of this shift, trees, grasses, and weeds have more time to grow, flower, and release pollen that triggers allergy attacks. The longer growing season is extending pollen production by all 17 types of ragweed that grow in the U.S. in the fall, according to Climate Central.

Rising carbon dioxide levels are also at play. A single ragweed plant can release 1 billion grains of wind-dispersed pollen, and high levels of carbon dioxide can stimulate plant growth and intensify that production. Researchers have found that the concentration of pollen in the air has, between 1990 and 2018, increased by 21 percent.

In a report released earlier this year, Climate Central noted, “Cutting greenhouse gas emissions is ultimately the most meaningful action to slow the rate of warming, curb the expanding allergy season, and limit CO2 influence on allergen production.”

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