Pollen and the climate

Better growing conditions result in more pollen. Climate change creates better conditions for certain pollen-bearing plants and consequently a longer pollen season. Warmer summers and milder winters may extend the season when allergenic pollens such as hazel, alder, elm, birch, grass and wormwood are released. Moreover, the changed climate can lead to better growing conditions for pollen-bearing plants which previously have not been widely distributed in Denmark, for example common ragweed (ambrosia artemisiifolia).

Citizens and municipalities
The changed climate may mean that people with a pollen allergy have more or fewer symptoms, and that the symptoms will last for longer periods. It may also mean that more people develop allergies to pollen. For the Danish municipalities, this may result in more days off work due to sickness or more days with reduced working capacity and reduced quality of life.

CO2 increases pollen production
The content of CO2 is the atmosphere has already increased by 29% since before the Industrial Revolution. Researchers believe that the atmospheric content of CO2 is likely to double by 2100. Even now, it is possible to register a doubling in the amount of CO2 in the air in large towns and cities, so they can in effect be regarded as small micro scenarios for global warming.

Many herbs and grasses are expected to react by growing more vigorously and with more inflorescences and thereby more pollen, for example Timothy-grass (Phleum pratense) and orchard grass (Dactylis glomerata). Several studies show that doubling the CO2 content in the air increases pollen production for common ragweed by 61%.

For trees, the effect of increased CO2 levels is more complex. According to model calculations for oak, beech and birch, the effect of increased CO2 levels is less the higher the temperature. This is because of increased respiration in relation to the level of photosynthesis. The catkins and leaves also compete for resources. However, we are already seeing more intense pollen seasons for trees. The total amount of pollen in a single season has increased markedly, and the number of days with high pollen counts is also up.

Longer pollen season and new pollen types
The warmer climate is already having major consequences in terms of how plants are spreading and the pollen season. A warmer climate is expected to extend the growing season for plants by two months. The pollen season will also be longer because hazel will start to flower earlier. Today, the pollen season for trees starts three weeks earlier than 25 years ago when the first pollen counts were recorded. For alder and hazel, the season now starts as much as five to six weeks earlier. In addition, the amount of pollen has increased significantly for e.g. birch and grass pollen, and there is a trend of more days with very high pollen counts. At the other end of the pollen season, certain grasses and the invasive species common ragweed will flower in October. The summer period is expected to become warmer and drier, while the winter months will be milder and damper. This will have serious consequences for the vegetation. Some species will grow and produce more while others will find it hard to adapt. For example, researchers believe that grey alder (alnus incana) will become more widespread in Denmark, and that more invasive species will become established. The pollen from common ragweed poses a particular threat.