What is the biggest impact of climate change on your life right now?  Many of the worst effects of climate change are just beginning to be felt, but there is one impact that is already with us in a big way: an increase in seasonal allergies due to pollen.  If you are an allergy sufferer, you probably already know this: in almost every urban area in the Northern Hemisphere, allergy season is getting longer and more intense. Pollen in the air is expected to double by 2040, but it is already much worse than it was just a few years ago.  Let’s take a look at why pollen causes allergies, and then look at why pollen is getting more abundant and obnoxious. We will focus on trees, although grasses and some herbaceous plants like ragweed are also important. In the spring in most regions, trees are the dominant source of allergenic pollen.

Pollen and allergies

bur oak male flowers

Bur oak male flowers dropping pollen

Trees make pollen to reproduce, but their challenge is to get the pollen to a receptive female flower.  A simple solution would be for pollen from a flower to fertilize the same flower, but that rarely happens. Trees are self-infertile – they cannot produce viable seeds from their own pollen. Like people (but unlike many crop plants), trees carry a heavy load of deleterious genes which makes self-reproduction risky. Its why we don’t marry our siblings, and trees don’t mate with themselves.

black locust flower

Black locust flowers are animal pollinated

The solution is to produce pollen that gets carried to a receptive female flower on another tree of the same species. Many trees take advantage of the sexual surrogacy offered by bees, beetles, birds and other animals. Trees induce pollinators to visit by offering rewards – pollen, nectar, scent – to animals that will visit them and then may visit a nearby tree of the same species. Trees that are pollinated by animals are usually showy, with bright colors and large petals or bracts.

Many people think they are allergic to the showy flowers of spring, such as black locust. They are not. These trees do not throw any appreciable amount of pollen into the wind. What is more likely is that black locust, oak and mulberry flower at about the same time. People come to associate their allergy symptoms with the showy flowers, not noticing the more muted oak and mulberry flowers.

Oak pollen

Oak pollen on a camera bag

Many trees take a simpler approach – they just cast pollen out into the wind. These are the ones you have to worry about.  You can imagine that the chance of a pollen grain tossed into the wind by an oak flower landing on a receptive flower of the same kind of oak is pretty slim. The solution is to produce ridiculous amounts of pollen. A medium-size oak tree may produce a kilogram of pollen during a 7-10 day period of flowering. That  is equivalent to 33 billion little grains trying to get into your nose from just one tree.

Why is that a problem to you? Pollen is rich in protein, and these proteins serve a lot of functions from pollen tube growth to kin recognition. Pollen protein is what causes allergies – your body has learned to recognize some pollen as a foreign invader and marshalls and immune response. This immune response causes blood vessels in your nose and sinuses to swell. This entrance of the invader into your body, but it is also what makes you sneeze and get a snuffy nose and itching eyes.

Male strobili of eastern white pine

Male strobili of eastern white pine

You may be noticing right about now, depending on where you live, that pollen coats car windows, floats on water and generally makes itself known. It may surprise you to find out that the pollen you can see rarely causes allergies. In Kentucky, where I live, pine trees are producing prodigious amounts of pollen and it is everywhere and obvious.  Yet pine pollen rarely causes allergies. Why? It’s too big. The average grain of pine pollen is about 60-90 micrometers in diameter. This is too big to get very far up your nose.  Oak pollen is much less visible. Even though there is about as much oak pollen in the air in my neighborhood as pine pollen, you don’t see the oak pollen unless you shake it on to a dark surface. Oak pollen is tiny, 24-38 micromters, or less than half the size of pine pollen. You may not notice oak pollen, but your nose does.

Pollen and Climate Change

There are several ways that climate change could make your allergies worse. Climate change is altering the range of many plants, so you may encounter trees that have moved into the area where you live. Climate change is also changing seasons, which may mean earlier or longer periods of pollen exposure. And climate change may increase the amount of pollen that a plant can produce in a given season. It turns out that all of these things are already happening.

birch flowers

Birch male flowers releasing pollen

Alix Rasmussen of the Danish Meteorological Institute has been looking at changes in birch pollen in Copenhagen over a 23 year period (1977-2000). He found that pollen release started 14 days earlier on average by 2000, the peak pollen release was 17 days earlier and the pollen season lasted 5 days longer.  The total amount of pollen released was larger, the peak days had more pollen and there were more days with pollen. Rasmussen found similar results at a more rural location. This is one of the clearest studies showing that climate change is already making allergy season worse. There is a growing body of research confirming what most allergy sufferers already know: climate change is upon us and it is making allergy season worse.

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