When Fire and Water Collide: Looking to Lakes to Understand Fire’s Deep Past
Reference: Shi, Y., B. Pan, M. Wei, X. Li, M. Cai, J. Wang, X. Xu, J. Hu, and W. Shi. 2020. Wildfire evolution and response to climate change in the Yinchuan Basin during the past 1.5 Ma based on the charcoal records of the PL02 core. Quaternary Science Reviews. 241:106393. https://doi.org/10.1016/j.quascirev.2020.106393
Is the uptick in wildfires normal?
Each year that passes by seems to bring with it news of increasingly large, unpredictable wildfires around the world. 2020 has been no exception, as major wildfires rage from California to the east coast, releasing massive billows of hazardous smoke that have made it all the way to Europe. But, do we actually know that wildfires have gotten worse in recent years? A comprehensive review from the University of East Anglia for ScienceBrief.org confirms that recorded observations of wildfire behavior show that fire seasons are lengthening and affecting more of the planet than was typical in previous decades. Other studies have determined that the development of megafires in the Rocky Mountains has increased by 50% and that 61% of western fires since 1950 have occurred in the past 20 years. Most of these studies cite climate change as the primary reason for increased wildfire activity and severity, which has prompted push back and debate as to whether climate or issues with forest management are really to blame. In order to truly understand how climate change is related to fire cycles, we must look to the past to understand what a “normal” pattern of wildfire looks like and how Earth’s climatic shifts have impacted wildfire activity historically.
What we can learn from fire regimes
While wildfires can be severely detrimental to human livelihoods and therefore tend to be viewed as negative events, it is important to recognize that fires are also a natural and necessary part of nature’s balance. Each ecosystem has its own “fire regime,” a term that refers to the typical frequency and intensity of fires that occur in a particular region over long periods of time. These patterns are determined by the type of vegetation in a region (all the way down to the species level), amount of dry plant matter available to “fuel” fires, and local climate. The health of an ecosystem depends on these fire cycles and can be altered significantly in either negative or positive ways if fire patterns change. Identifying past wildfire occurrences in an area can help us to figure out whether or not fire frequency is increasing from the norm and to predict when future wildfires will happen. It is up to us to learn how we can live in better harmony with fire and to use science to understand how climate change could alter the way that fire affects the planet.
Scientists use lake sediments to reconstruct fire histories
In the case of ecosystems with very long time-gaps between fire cycles, it is not possible to use observations or consult recorded history to determine the “typical” fire cycle of an area. Instead, scientists have to get a little creative and look to lake sediments for clues. As ash, leaves, and pollen fall on a lake, they sink to the bottom and “lock in” information about past wildfires and vegetation in layers of sediment. When charcoal is released during forest fires, it gets trapped in these layers in different amounts depending on how severe the fire was. Scientists can date these layers to determine when these past fires occurred in the same way that archaeologists date artifacts. Additionally, plant fossils and pollen can be found within lake sediments and can sometimes be identified to the species so well that changes in forest composition over time can be tracked. Examining what regrows post-fire and what species exist right before fires occur can provide important information about what conditions trigger fire and how an ecosystem will recover.
A new study by Yulan Shi and colleagues ambitiously mapped out 1.5 million years of fire history in the Yinchuan Basin of China to understand the relationship between fire cycles and major climatic changes. This determination of how changes in climatic factors such as temperature and humidity affect fire cycles will inform predictions of how modern human-driven climate change might impact wildfire patterns. The study found that major climatic shifts did strongly impact fire frequency, with the most intense fire seasons occurring during major regional transitions from moist climates to very dry (arid) conditions and during periods of warming. This is an important finding for modern times as studies find that global warming could shift 25% of the planet to become arid. While fire frequency was mainly climate driven, the overall amount of charcoal found in lake sediments (corresponding to the plant material burned) was based largely on the vegetation present before fire as determined by pollen found in the sediment layers. Interestingly, the researchers also found that the driving force of pre-human era climatic changes and related shifts in wildfire regimes were due to changes in Earth’s orbit, called Milankovich cycles. This research validates that climate is in fact responsible for changes in fire cycles, particularly for fire frequency, and will help us to anticipate how wildfires will behave in the future. This time around, instead of Milankovich cycles, we are the ones provoking the climate to change and it is up to us to reverse these impacts before fire patterns become too altered for our health and the health of our planet.