As global average temperatures rise, it is widely believed the frequency of wildfires will increase. However, this may not be the case. According to analysis of sediment from lake beds in Northern Alaska, the frequency of wildfires didn’t relate to changes in temperature variation over the last few thousand years. This is strange, surely a warmer climate will dry out vegetation faster, creating more fuel for fires to ignite and spread? Apparently not, there appears to be a far more potent controlling factor at play…
In Southern California, the temperatures easily hit 95°F (35°C) today and I noticed the entire neighbourhood pumping a small reservoir’s-worth of water into their manicured lawns (creating an impressive river down the street). Our garden looks a little dry in comparison, I refuse to turn the sprinklers on until we really need it (for now, the hose will do). Summer appears to have arrived early, making me slightly nervous; the wildfires that blighted this region over the the last few years are sure to return. To make matters worse, we had a surprisingly wet winter, helping the spring growth of vegetation. It may be nice and green now, but all I see is surplus firewood.
However, as the last few thousand years have shown us, no matter how hot it gets, the frequency of wildfires may actually decrease.
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Using samples from sediment cores at the bottom of Alaskan lakes, climatologist Philip Higuera of Montana State University has discovered it could be the type of vegetation that grows in response to temperature increases that affects the frequency of subsequent wildfires. There is little indication to suggest the frequency of wildfires increased as global average temperatures increased over the past 15,000 years. This might be counter-intuitive, but it would appear nature has an automatic fire-retardation mechanism.
“Climate is only one control of fire regimes, and if you only considered climate when predicting fire under climate-change scenarios, you would have a good chance of being wrong,” Higuera says. “You wouldn’t be wrong if vegetation didn’t change, but the greater the probability that vegetation will change, the more important it becomes when predicting future fire regimes.”
Using radiocarbon dating techniques, Higuera’s team were able to accurately date the different layers in the metre-long sediment samples. From there, they analysed the charcoal deposits, therefore deriving the wildfire frequency in North Alaska woodlands. In addition, they analysed pollen content to understand what species of plant were predominant over the past 15,000 years. Then, using known climate data for the same period, the researchers were able to correlate the fire frequency with plant species and then relate the whole lot with trends in climate change. The results are very interesting.
One of the key discoveries was that climate change a was less important factor than vegetation changes when related to frequency of wildfires. According to sediment samples over the millennia, despite very dry periods in climate history, wildfire frequency decreases sharply. It appears that during periods of temperature increases, vegetation species change from flammable shrubs to fire-resistant deciduous trees.
“Climate affects vegetation, vegetation affects fire, and both fire and vegetation respond to climate change,” Higuera adds. “Most importantly, our work emphasizes the need to consider the multiple drivers of fire regimes when anticipating their response to climate change.”
Although we may not escape the clutches of wildfires in Southern California this year, the last 15,000 years have shown us that this may gradually change as vegetation adapts to hotter conditions, becoming more fire-resistant…