Nov. 22, 2021
Let's talk about the 2021 grass pollen season - our mid-grass pollen season update.
We’re now at the midpoint of Canberra’s grass pollen season, a season that typically peaks in November and declines as the end of the year approaches.
In early October, we forecast an above average grass pollen season for 2021 based on a wet winter, high soil moisture and predictions of above average spring rains – little did we know that it was going to be a record year that exceeded all expectations.
The Bureau of Meteorology’s forecast of an extended La Nina event during spring and into summer has resulted in a very wet spring and the month of November is on track to be one of the highest rainfall months on record. The high rainfall has led to increased grass growth and more abundant flowering, resulting in the most exceptional grass pollen season on record.
The shape of the daily pollen count for this grass season is depicted in Figure 1, which shows the daily pollen count and level reported each day from September through to mid-November. EXTREME days are when the concentration is 100 or more grass pollen grains and these days are the worst for hay fever and asthma sufferers. So far this season there have been 13 EXTREME grass pollen days – we normally only see ~2 EXTREME days in a season – and this is twice as many as we recorded in our previous record year of 2020 when we saw 7 EXTREME days across the season.
Figure 1. Daily grass pollen counts for Canberra, September to mid November 2021. The histogram shows daily counts colour coded to match the accepted pollen levels reported by CanberraPollen.com.au.
The number of grass pollen grains recorded each day have also been extraordinary with an all-time record day for grass pollen recorded on the 17th November with 510 grass pollen grains/m3. This was preceded by 4 consecutive days with EXTREME levels exceeding 200 grass pollen grains/m3.
We can also check how the season’s going by plotting the cumulative grass pollen count since October 1. That’s what’s shown in Figure 2. The graph depicts the cumulative grass pollen count average from all records, the previous highest grass pollen year (2020) and the lowest recorded grass pollen year (2017). The current season is the red line in the graph and shows the stepwise nature of the curve due to major rain events washing out airborne pollen on the days when it rained heavy enough to suppress grass pollen production.
The 2014 season is also shown as it closely follows the beginning of the 2021 season when very low grass pollen was recorded during the first 2 weeks of October and then a significant rise is recorded as the grass season takes off in the later part of October. Cooler temperatures during these low grass pollen weeks may have delayed the flowering response in both 2014 and 2021. While the 2021 grass pollen season was tracking the 2014 curve during the month of October, since November 1 the 2021 curve diverged and is on a steep trajectory to be the biggest grass pollen season on record.
Figure 2. Cumulative grass pollen count from 1st October through to 31st December for the period 2007-2009 and 2014-2021. The data summarizes the grass pollen count from the lowest record season (2017) to the highest record season in 2020. The average cumulative pollen count is shown as the black dashed line (including counts from 2007-2009 and 2014-2020). The cumulative grass pollen count for 2021 up until 21st November 2021 is shown by the red line.
Is the record grass pollen season related to climate change?
The Canberra Pollen Monitoring Program has been recording daily pollen counts for over 10 years, and while this isn’t enough to clearly show that climate change is having an impact on the length, strength and nature of the pollen seasons, there are trends in the data that are of concern.
Studies in the Northern Hemisphere have shown that with greater levels of carbon dioxide in the air there is the potential for greater production of pollen – particularly in the grasses (Albertine et al. 2014, Ziska 2021). A recent study in Brisbane has led to similar conclusions on a short-term data set – with over 3x greater abundance of grass pollen recorded in the last 15 years of monitoring (Addison-Smith et al. 2021).
The recent record levels of grass pollen (and allergenic tree pollen) in the Canberra daily pollen count in 2021 and an increase in the number of people suffering from some form of allergic rhinitis in Canberra over the last few years are trends that are in line with expected effect of climate change on pollen production and human health.
This is one of the main reasons that it is important for us to continue to monitor the daily pollen count in Canberra so we can better understand the impacts of climate change on the air we breathe and the pollen count - helping those who suffer from hay fever and asthma to better adapt to future climate change impacts on respiratory health.
We’ll update you again in our end-of-year season wrap up or as any new information comes to light. In the meantime, keep checking the Canberra Pollen app and website for the latest information.