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Climate Change is Making You Sneeze: Why Allergy Seasons Are Longer, Stronger, And Worse Than Ever Before

In just the past few years, seasonal allergies have begun to worsen at tremendous rates – and it’s due to climate change. This commonly overlooked side effect of climate change is causing public health issues and mass discomfort all over the world. Scientists have found overwhelming evidence that climate change is lengthening and intensifying allergy seasons. Two of climate change’s most well known characteristics are causing this inconspicuous consequence to occur: Rising global temperatures and increased concentrations of carbon dioxide in the atmosphere. While at first glance these occurrences seem intuitively to be unrelated, they are in fact the driving forces behind this growing trend.

During certain stages of their reproductive cycles, plants naturally release pollen^[¹^, ²^]. Traditionally, this process occurs in sync with seasonal weather patterns. For instance, trees typically release pollen in early spring; In late spring or summer grasses will pollinate, and weeds follow around the autumn^[³^]. However, as climate change adjusts average global temperatures, the seasonal cues we have historically associated with allergies are falling out of sync as allergy season length and pollen levels increase^[⁴^, ⁵^]. Warmer winters are causing plants to emerge from dormancy much earlier than they did just a few decades ago. At the same time, warmer autumns are delaying the first frost, allowing plants to continue producing pollen beyond their historical growing season. This is causing pollen seasons to begin earlier, end later, and thereby expose allergic individuals for much longer periods^[⁴^].

In a study published in Proceedings of the National Academy of Sciences (PNAS) in 2021, researchers analyzed nearly three decades of pollen measurements from 60 monitoring stations across North America, covering the years 1990 to 2018. Their findings were incredible – and disturbing. They concluded that the North American pollen season starts 20 days earlier than it did in 1990 – less than forty years ago. They showed that Airborne pollen concentrations have increased by over 21%, meaning the density of pollen in the air has increased dramatically in a very short period of time. By analyzing the last few decades, they showed that human-caused climate change accounted for over half of the observed increase in pollen season length, identifying rising temperatures as the primary driver. The study proved that pollen seasons hadn’t just lengthened. The seasons had intensified as well – and it was largely due to human actions^[⁷^].

Map of long-term worsening of pollen seasons in North America. Source: PMC

However, temperature isn’t the only factor worsening pollen seasons. Carbon dioxide in the atmosphere is one of the primary greenhouse gases responsible for global warming – but it is also essential for photosynthesis^[⁸^, ³^]. Plants use CO₂ to convert sunlight into energy. This means that higher atmospheric CO₂ levels stimulate plant growth, a process known as the CO₂ fertilization effect^[⁹^, ¹⁰^]. For most plant species, more CO₂ causes faster growth, larger plants, more flowers, and therefore more pollen production. While larger plants may initially seem like a positive effect, in reality this is particularly concerning^[¹¹^, ¹⁰^]. If allergenic plants such as common ragweed (Ambrosia Artemisiifolia) produce more pollen, they can cause more widespread allergic reactions. Ragweed is also one of the leading causes of hay fever worldwide, meaning that allergies are not the only negative effects from higher levels of pollen in the air^[³^, ¹²^]. Laboratory experiments have repeatedly shown that ragweed grown under elevated CO₂ conditions produces significantly more pollen than plants grown under normal atmospheric conditions.

Worse, recent research suggests that the problem may be even worse than previously thought. A study published in 2021 found that ragweed exposed to elevated CO₂ not only produced more pollen, but also found that the pollen triggered stronger allergic inflammation. It showed that climate change didn’t only increase the quantity of pollen released, but the potency of allergenic pollen as well. Future allergy seasons will not simply last longer – they could also become much more severe^[¹³^, ¹⁴^].

Climate change is also allowing allergenic plant species previously unable to survive in ecosystems take over^[³^]. As temperatures rise, many plant species are expanding into regions that were previously too cold. For instance, ragweed has recently spread northward across parts of Europe and North America into areas it could normally not survive in. Regions which have traditionally experienced mild allergy seasons are now encountering entirely new pollen sources^[¹⁵^, ³^]. Simultaneously, rainfall pattern shifts, increased droughts and changing land usage is influencing which plants can dominate local ecosystems. Some drought-tolerant weeds which produce highly allergenic pollen thrive in disturbed environments, potentially increasing pollen exposure in urban and suburban areas^[³^]. Climate change is reshaping entire plant communities, and these negative ecological shifts are predicted to continue throughout the following century^[¹⁶^].

Climate change is not only affecting pollen negatively, however. Air pollution is amplifying climate change’s effects on pollen to higher and higher degrees^[¹⁷^, ³^]. Many cities simultaneously experience higher temperatures, increased ozone pollution and more airborne particulate matter^[³^, ¹¹^]. These pollutants interact and amplify pollen in several major ways. For instance, research has shown that pollutants can damage the outer coating of pollen grains, causing them to release allergenic proteins more easily after inhalation. Fine particulate pollution can also carry these allergens farther into the lungs, increasing asthma attacks and severe allergic reactions^[³^, ⁵^].

Wildfire smoke additionally amplifies these effects. As climate change causes larger, more frequent wildfires in many regions, smoke particles can inflame airways already irritated by pollen^[⁹^, ¹²^]. For individuals with asthma or allergic rhinitis, this combination can significantly worsen respiratory symptoms.

The impacts of seasonal allergies extend beyond a simple inconvenience. Individuals with allergic rhinitis can experience sleep disturbances, persistent fatigue, worse academic performance and decreased work productivity while creating a large economic burden worldwide^[¹⁸^, ¹⁹^, ²⁰^, ²¹^]. More severe cases also contribute to asthma exacerbations^[²²^, ²³^]. Researchers are increasingly describing allergies as one of the clearest examples of climate change’s effects on everyday life^[²⁴^]. Unlike other aspects of climate change, such as melting Arctic ice or rising sea levels which seem psychologically and geographically distant, longer allergy seasons are a major consequence that millions of people are feeling already^[⁴^, ²⁵^]. People who only needed allergy medication for a few weeks each spring twenty years ago may now be experiencing symptoms from late winter all the way into the summer.