Just over a year ago, a forest fire caused considerable damage at the University of Cape Town and surrounding areas. Irreplaceable African collections were destroyed. the last active windmill in Africa was badly damaged. Many were surprised by the fire and the widespread damage it caused.
What have we learned about the risk of a repeat event?
The risk of fire is the consequence interrelated factors and is complex. As part of my doctoral thesis, I examined the relationship between wind and precipitation variability and specifically to 2015-2017 drought in Cape Town.
In this article, I examine some of these factors in relation to the 2021 fire and their potential influence on future events.
Hot, windy weather during increasingly drier and warmer autumns after the summer dry season poses a particular risk. The fires around the university are necessary and inevitablebut dense, flammable, and often overgrown brush around the campus perimeter and some of the trees around buildings increase the risk.
But there are many things we don’t know that we urgently need do more research.
SANParks, the agency responsible for managing South Africa’s national parks, has commissioned a study investigate the cause of – and the response to – the fire. The investigation found it was deliberately launched before 9 a.m. on April 18, 2021. The suspects were driving on a highway along Devil’s Peak, the university’s dramatic backdrop jutting from Table Mountain .
At first the fire spread slowly in calm conditions. But from 10 a.m., conditions suddenly deteriorated with the arrival of dry, gusty winds from the northwest. The blaze has intensified and destroyed buildings in the area where natural areas turn into built-up spaces – the so-called the wild lands urban interface.
The fire causes
A recent article introduces the concept of ignition pick-ups – basically all the places from which a fire could start to reach you. The catchment area of the university campus is much smaller than it was in pre-colonial times when fires could freely cross the Cape Flats.
During the fire season (approximately from December to April), the winds come mainly from south or southeast. Southeast of the university are suburbs unlikely to experience large fires. To the south, the native Afro-temperate forest of Newlands would probably act as a “natural firebreak”.
Only the fire starts in the small grassy area between the forests and the university, overgrown with flammable trees, present a significant risk from the south.
Fires descending from the west would tend to spread more slowly. But in certain conditions, the risk of downhill spotting – fires ignited by blazing embers thrown ahead of the main line of fire –can be bigger.
Spotting was responsible for the fire that tore through buildings in the middle of campus. He went through a four lane highway in several places, because fires were started 350 m in front the main fire front.
Northwest winds also pose a considerable weather and fire hazard to the university. In a recent post we examined the relationship between wind direction and precipitation in the region. Usually northwesterly winds bring cool, wet and cloudy weather after breaking through the cold Benguela Current along the west coast of southern Africa. Sometimes in the fall, however, like the day of the fire, they are hot, dry, and gusty.
The wind pattern around Table Mountain is extremely complex. Nearby wind data is difficult to obtain, but we know that at airport and portstations, northwesterly winds were gusting to more than 40 km/h around 2 p.m.
This led to a very high predicted fire danger rating. Near the university, the South African Meteorological Service recorded maximum temperatures of 35.8°C.
Relative humidity is the other critical meteorological determinant of fire behavior. At 11 a.m. it was just 10% in the city. It’s drier than the desert air usually is. Such low humidity is known to cause extreme fire behavior.
Warmer and drier autumns
A recent study found a long-term pattern of autumnal drying, which is expected to worsen with climate change. The dry autumns were associated with a wave 4 pattern in the southern hemisphere (four pairs of highs and lows around the hemisphere). It is clearly visible in April 2021 Temperature and pressure anomalies (deviations from normal).
Long-term continuous temperature data for individual locations in South Africa is very difficult to gather. In grid Berkeley Land maximum temperature data for the region around Cape Town since 1910, the three hottest April months were 2014, 2017 and 2021. Other temperature data sets give similar results. So do close station data available since 2000.
Given the findings of the recent drought attribution studies for the region, it is likely that autumnal heat and dryness have become more common recently due to climate change. However, many other variables which are more difficult to analyze also influence the sensitivity of plants to fire.
After the March 2009 fire, the University of Cape Town removed exotic trees to reduce fuel loads. He has now eliminated most of the remaining stands of pines and other exotic trees along the northwest perimeter of the upper campus.
Over the past two decades, SANParks has also made significant progress in clean up stands of invasive trees. This despite intense opposition, mainly from local recreational users who frequently support the preservation of the “forest” with arguments based on ecological and climatic considerations. misconceptions.
However, at the time of the 2021 fire, dense stands of flammable, mostly exotic shrubs and low trees dominated certain bordering areas the campus, especially in the north where the fire was coming from. Despite extensive clearing operations since the fire, a dense thicket of invasive exotic shrubs and trees is emerging on both sides of the university’s perimeter. It will need to be monitored as a future fire hazard.
Consequently, the rapid spread of fire near the University of Cape Town is a constant risk once fuel loads have been restored.
A longer version of the article can be found here.