The Santa Ana winds that help drive fall and winter wildfires in California have died down, providing welcome relief for residents. But other ecological factors contribute to fires in ways that scientists are still discovering.
I study how human actions affect fire regimes – the patterns through which fires occur in a particular place over a specific time period. People alter these patterns by adding ignition sources, such as campfires or sparking power lines; suppressing fires when they develop; and introducing nonnative invasive plants.
My research suggests that nonnative invasive grasses may be fueling wildfires across the United States. Some fires are occurring in areas that rarely burn, like the Sonoran Desert and the semiarid shrublands of the Great Basin, which covers most of Nevada and parts of five surrounding states. In the coming months, some of the grasses that help feed these blazes will germinate, producing tinder for future fires.
In a recent study, I worked with colleagues at the University of Massachusetts and the University of Colorado to investigate how 12 nonnative invasive grass species may be affecting regional fire regimes across the U.S. We found that eight species could be increasing fire in ecosystems across the country.
A fire regime is a way to describe fire over space and time or to characterize fire patterns. Understanding fire regimes can help make clear that fire is a natural and integral component of many ecosystems. Knowing historical fire patterns also enables scientists to begin to understand when new or different patterns emerge.
The link between invasive grass and fire is well established. Invasive grasses are novel fuels that can act as kindling in an ecosystem where readily flammable material might not otherwise be present. They can catch a spark that might otherwise have been inconsequential.
For example, in August 2019 the Mercer Fire burned 25 acres in Arizona, scorching native desert plants, including iconic saguaro cacti. A much larger event, the 435,000-acre Martin Fire, destroyed native sagebrush ecosystems in Nevada in July 2018. Invasive grasses helped fuel both fires.
Cheatgrass, which fueled the Martin Fire, is a well-studied invasive grass known to promote fire. But many other invasive grass species have similar potential, and their roles in promoting fire have not been assessed at large scales.
Researchers describe fire regimes in many ways. Our study focused on fire occurrence (whether or not fire occurred), frequency (how many times fires occurred) and size (the largest fire associated with a place) in 29 ecological regions across the U.S. For each location we tested whether invasive grasses were associated with differences in fire occurrence, frequency or size.
A nonnative invasive species typically comes from another continent, has become established, is spreading and has negative impacts. We used an online Invasive Plant Atlas of the United States as a starting point to determine which invasive grass species to investigate.
Next, we searched the scientific literature and the U.S. Forest Service’s Fire Effects Information System to see whether there was reason to believe that any of the invasive grass species promoted fire. This process helped narrow our scope from 176 species to 12 that were suitable for our analysis.
Who are these “dirty dozen,” and how did they get here? Buffelgrass is native to Africa and was intentionally introduced to Arizona in the 1930s, probably for erosion control and forage. Japanese stiltgrass and cogongrass are native to much of Asia and were introduced to the southeastern U.S. in the early 1900s, in some instances as packing material. Medusahead, which comes from Eurasia, was introduced to the western U.S. in the late 1800s, probably by accident as a contaminant in seed shipments.
The remaining eight species – giant reed, common reed, silk reed, red brome, cheatgrass, Chinese silvergrass, Arabian schismus and common Mediterranean grass – have similar stories. People introduced them, sometimes accidentally and at other times intentionally, without an understanding of how they could impact their new settings.
Understanding how multiple species influence fire over many years at a national scale requires using big data. One person could not collect information on this scale working alone.
We relied on composite data sets that provided thousands of records of invasive grass occurrence and abundance across the country. Combining these records with agency and satellite fire records helped us determine whether fire occurrence, frequency or size were different in places with and without grass invasions.
We also used statistical models to assess whether human activities and ecological features could be driving observed differences between invaded and uninvaded areas. For example, it was possible that grass invasions were happening near roads, which are also linked with fire ignitions. By including roads with grass invasion in our statistical models, we can be more confident in the role invasive grasses could play in altering fire regimes.
Our results show that eight of the species we studied are associated with increases in fire occurrence. Six of these species are also linked to increases in fire frequency. Invasions seem to be affecting a variety of ecosystems, ranging from buffelgrass in the Sonoran Desert to Japanese stiltgrass in eastern U.S. forests to cogongrass in southeastern pine systems.
Our statistical models suggest that grass invasion, along with human activities, are likely affecting fire patterns in these ecosystems.
Surprisingly, none of the invasive grass species analyzed appeared to influence fire size. We interpret this result to mean that the areas we studied are seeing more of the same types of fires that already occur there, at least in terms of size.
With an understanding of interactions between invasive grasses and fire, agencies that handle either fire or invasive species may find opportunities to work together to control invasions that can lead to more frequent burns. Our research can also strengthen predictions of future fire risk by incorporating the presence of invasive grasses into fire risk models.
Although it sometimes may feel as though the world is on fire, this information can provide potential for remediation, and may help communities prepare more effectively for future wildfires.
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Record fires are raging in Brazil’s Amazon rainforest, with more than 2,500 fires currently burning. They are collectively emitting huge amounts of carbon, with smoke plumes visible thousands of kilometres away.
Fires in Brazil increased by 85% in 2019, with more than half in the Amazon region, according to Brazil’s space agency.
This sudden increase is likely down to land degradation: land clearing and farming reduces the availability of water, warms the soil and intensifies drought, combining to make fires more frequent and more fierce.
The growing number of fires are the result of illegal forest clearning to create land for farming. Fires are set deliberately and spread easily in the dry season.
Ironically, farmers may not need to clear new land to graze cattle. Research has found a significant number of currently degraded and unproductive pastures that could offer new opportunities for livestock.
New technical developments also offer the possibility of transforming extensive cattle ranches into more compact and productive farms – offering the same results while consuming less natural resources.
The devastating loss of biodiversity does not just affect Brazil. The loss of Amazonian vegetation directly reduces rain across South America and other regions of the world.
The planet is losing an important carbon sink, and the fires are directly injecting carbon into the atmosphere. If we can’t stop deforestation in the Amazon, and the associated fires, it raises real questions about our ability to reach the Paris Agreement to slow climate change.
The Brazilian government has set an ambitious target to stop illegal deforestation and restore 4.8 million hectares of degraded Amazonian land by 2030. If these goals are not carefully addressed now, it may not be possible to meaningfully mitigate climate change.
Since 2014, the rate at which Brazil has lost Amazonian forest has expanded by 60%. This is the result of economic crises and the dismantling of Brazilian environmental regulation and ministerial authority since the election of President Jair Bolsonaro in 2018.
Bolsonaro’s political program includes controversial programs that critics claim will threaten both human rights and the environment. One of his first acts as president was to pass ministerial reforms that greatly weakened the Ministry of the Environment
Regulations and programs for conservation and traditional communities’ rights have been threatened by economic lobbying.
Over the last months, Brazil’s government has announced the reduction and extinction of environmental agencies and commissions, including the body responsible for combating deforestation and fires.
Although Brazil’s national and state governments are obviously on the front line of Amazon protection, international actors have a key role to play.
International debates and funding, alongside local interventions and responses, have reshaped the way land is used in the tropics. This means any government attempts to further dismantle climate and conservation policies in the Amazon may have significant diplomatic and economic consequences.
For example, trade between the European Union and South American trading blocs that include Brazil is increasingly infused with an environmental agenda. Any commercial barriers to Brazil’s commodities will certainly attract attention: agribusiness is responsible for more than 20% of the country’s GDP.
Brazil’s continued inability to stop deforestation has also reduced international funding for conservation. Norway and Germany, by far the largest donors to the Amazon Fund, have suspended their financial support.
These international commitments and organisations are likely to exert considerable influence over Brazil to maintain existing commitments and agreements, including restoration targets.
Brazil has already developed a pioneering political framework to stop illegal deforestation in the Amazon. Deforestation peaked in 2004, but dramatically reduced following environmental governance, and supply change interventions aiming to end illegal deforestation.
Moreover, private global agreements like the Amazon Beef and Soy Moratorium, where companies agree not to buy soy or cattle linked to illegal deforestation, have also significantly dropped clearing rates.
We have financial, diplomatic and political tools we know will work to stop the whole-sale clearing of the Amazon, and in turn halt these devastating fires. Now it is time to use them.
Ten years ago, on February 7, 2009, the Black Saturday bushfires killed 173 people. More than 2,000 houses were destroyed in Victoria, including at Kilmore, Kinglake, Vectis (Horsham), Narbethong, Marysville, Strathewan, Beechworth, Labertouche (Bunyip), Coleraine, Weerite, Redesdale, Harkaway, Upper Ferntree Gully, Maiden Gully, Bendigo, Eaglehawk, Lynbrook, St Andrews, Flowerdale, Narre Warren, Callignee, and my home town of Churchill, where my mother and father still lived. Their home wasn’t burned, but many of their neighbours were badly affected by the worst bushfire day in Australia’s history.
A week before, my uncle and aunt had to seek refuge at Mum and Dad’s place when a fire ember landed in their front yard during the Boolarra bushfires. Mum has since passed and Dad still lives in Churchill.
The climate is changing due to human induced greenhouse gas emissions, and this means more bushfire danger days in what is already one of the most fire-prone countries in the world. Unfortunately, we have not done enough to curb climate change and the situation is getting worse.
Climate change means more days of extreme heat, longer heatwaves and more frequent droughts. Droughts now occur further south than in the past and have been increasing in Australia’s southeast, including Tasmania. The records continue to tumble, and the evidence of dangerous climate change continues to mount.
Back in 2008, John Brumby was Premier of Victoria and Kevin Rudd was Prime Minister. I was working on climate change for the Victorian government, developing projections for increased risk of bushfires. A 2005 study had already predicted an increase in fire weather risk throughout most of southeastern Australia over the coming decades, with “very high” and “extreme” fire danger ratings likely to increase in frequency by 4-25% by 2020 and 15-70% by 2050.
There has been more research in this area, although certainly not enough, given the huge stakes. A 2007 report for the Climate Institute of Australia predicted increases in annual average fire danger of up to 30% by 2050, and a potential trebling in the number of days per year where the uppermost values of the index are exceeded. The largest changes are predicted for the arid and semi-arid interior of New South Wales and northern Victoria.
The 2008 Garnaut Climate Change Review also warned that fire seasons will begin earlier, end slightly later, and generally be more intense. “This effect increases over time but should be directly observable by 2020,” it said.
In 2015, a further study by CSIRO and the Bureau of Meteorology concluded that:
Projections of warming and drying in southern and eastern Australia will lead to increases in [forest fire danger index] and a greater number of days with severe fire danger. In a business as usual scenario (worst case, driest scenario), severe fire days increase by up to 160-190% by 2090.
By combining all of this research, I created the graph below.
This shows that while there is some uncertainty as to the extent of increase in the number of bushfire danger days in southeastern Australia, the situation is undoubtedly getting worse, and it’s time for action.
In 2017, the independent Climate Council published a report on Victoria’s growing bushfire threat, which made several stark findings and recommendations:
Climate change is increasing the risk of bushfires in Victoria and lengthening fire seasons.
Victoria is the state most affected by bushfires, and is on the front line of increasing bushfire risk.
The economic cost of bushfires in Victoria is an estimated A$180 million a year, and this is predicted to more than double by 2050.
Bushfires will continue to adversely affect human and environmental health.
In the future, Victoria is very likely to experience an increased number of days with extreme fire danger. Communities and emergency services across Victoria must be prepared.
Reducing greenhouse emissions is vital for protecting Australians.
Our grandfathers and grandmothers had the wisdom to build amazing water infrastructure, protected by the “closed catchments” that give Melbourne and Victoria some of the best water in the world. Bushfires are a major risk to these water supplies – particularly in the catchments of major dams such as the Thomson.
A bushfire followed by a downpour that washes ash into the dam could potentially force the closure of the trillion-litre capacity Thomson reservoir, making it unusable for months. Firefighters have been battling exactly this kind of blaze at Mount Baw Baw in recent days and at the time of writing the situation has improved.
Major bushfires often occur in time of severe drought. Black Saturday itself happened towards the end of the 15-year Millennium Drought, when Victoria’s water supplies were already strained. I remember vividly the then chief executive of the Melbourne Water Corporation urging the government to deal with any fire in the Thomson Dam catchment immediately, given the threat to Melbourne’s water.
Fortunately, amid the devastation of Black Saturday we avoided major disruption to our water supplies. But this risk poses a huge challenge to both firefighters and policy-makers. The rule is that protection of human life is ranked above assets and infrastructure, and rightly so. But when there is a clear and present danger of towns and cities going without water, it’s also true that safeguarding water means saving human lives in the ensuing days.
The bitter lesson of the Californian fires
Any way you look at it, these are hard questions. On our current trajectory, we are heading for terrible trade-offs.
In 2050 my daughter Astrid and my son Atticus – Mum and Dad’s grandchildren – will be 45 and 43, respectively. I hope it is not too late for our leaders in Canberra, Davos and throughout the world to wake up and take urgent action to limit global warming 1.5℃. That would mean that the most fearful predictions of our bushfire future never come to pass.
The 2009 Black Saturday fires burned 437,000 hectares of Victoria, including tens of thousands of hectares of Mountain Ash forest.
As we approach the tenth anniversary of these fires, we are reminded of their legacy by the thousands of tall Mountain ash “skeletons” still standing across the landscape. Most of them are scattered amid a mosaic of regenerating forest, including areas regrowing after logging.
But while we can track the obvious visible destruction of fire and logging, we know very little about what’s happening beneath the ground.
In a new study published in Nature Geoscience, we investigated how forest soils were impacted by fire and logging. To our surprise, we found it can take up to 80 years for soils to recover.
Soils have crucial roles in forests. They are the basis for almost all terrestrial life and influence plant growth and survival, communities of beneficial fungi and bacteria, and cycles of key nutrients (including storing massive amounts of carbon).
To test the influence of severe and intensive disturbances like fire and logging, we compared key soil measures (such as the nutrients that plants need for growth) in forests with different histories. This included old forests that have been undisturbed since the 1850s, forests burned by major fires in 1939, 1983 and 2009, forests that were clearfell-logged in the 1980s or 2009-10, or salvage-logged in 2009-10 after being burned in the Black Saturday fires.
We found major impacts on forest soils, with pronounced reductions of key soil nutrients like available phosphorus and nitrate.
A shock finding was how long these impacts lasted: at least 80 years after fire, and at least 30 years after clearfell logging (which removes all vegetation in an area using heavy machinery).
However, the effects of disturbance on soils may persist for much longer than 80 years. During a fire, soil temperatures can exceed 500℃, which can result in soil nutrient loss and long-lasting structural changes to the soil.
We found the frequency of fires was also a key factor. For instance, forests that have burned twice since 1850 had significantly lower measures of organic carbon, available phosphorus, sulfur and nitrate, relative to forests that had been burned once.
Sites subject to clearfell logging also had significantly lower levels of organic carbon, nitrate and available phosphorus, relative to unlogged areas. Clearfell logging involves removing all commercially valuable trees from a site – most of which are used to make paper. The debris remaining after logging (tree heads, lateral branches, understorey trees) is then burned and the cut site is aerially sewn with Mountain Ash seed to start the process of regeneration.
The impacts of logging on forest soils differs from that of fire because of the high-intensity combination of clearing the forest with machinery and post-logging “slash” burning of debris left on the ground. This can expose the forest floor, compact the soil, deplete soil nutrients, and release large amounts of carbon dioxide into the atmosphere.
Predicted future increases in the number, frequency, intensity and extent of fires in Mountain Ash forests, coupled with ongoing logging, will likely result in further declines in soil nutrients in the long term. These kinds of effects on soils matter in Mountain Ash forests because 98.8% of the forest have already been burned or logged and are 80 years old or younger.
To maintain the vital roles that soils play in ecosystems, such as carbon storage and supporting plant growth, land managers must consider the repercussions of current and future disturbances on forest soils when planning how to use or protect land. Indeed, a critical part of long-term sustainable forest management must be to create more undisturbed areas, to conserve soil conditions.
Specifically, clearfell logging should be limited wherever possible, especially in areas that have been subject to previous fire and logging.
Ecologically vital, large old trees in Mountain Ash forests may take over a century to recover from fire or logging. Our new findings indicate that forest soils may take a similar amount of time to recover.
Last year was a time of exceptional weather and record-breaking heat according to the Bureau of Meteorology’s annual climate statement, which was released last night.
The Bureau issued four Special Climate Statements relating to “extreme” and “abnormal” heat, and reported a number of broken climate records.
One of the headline stories for the year was drought across eastern Australia — centred on New South Wales, but also affecting Victoria, eastern South Australia and southern Queensland.
With the whole of NSW declared in drought during the latter half of 2018, this drought will be recorded as one of the more significant in Australia’s history, ranking alongside the Millennium, 1960s, World War Two and Federation Droughts. Of those historic droughts, only the Millennium Drought saw similar, accompanying high temperatures.
The below-average rainfall has persisted for around two years across much of NSW and adjacent regions. The drought conditions were particularly severe in the recent spring period, with low rainfall, persistently high temperatures, and record high evaporation.
This exceptionally dry period was influenced by sea surface temperatures to the west of the continent. Perhaps fortuitously, a developing El Niño in the Pacific Ocean failed to mature in the second half of the year. An El Niño would have typically exerted a further drying influence on eastern Australia.
The dry conditions in eastern states were severe enough to see Australia record its lowest September rainfall on record, and the second-lowest on record for any month — behind April 1902, during the prolonged Federation Drought. Over 2018, Australia’s annual rainfall was 11% below average, and the lowest recorded since 2005, during the Millennium Drought.
In contrast, above-average rainfall was recorded across parts of the tropical north, and most significantly in the Kimberley, consistent with recent trends of increasing rainfall in that region.
The drought conditions were exacerbated by record or near-record temperatures across many parts of the country. It was Australia’s third warmest year on record, behind 2013 and 2005. Daytime maximum temperatures were the warmest on record for NSW and Victoria, and second-warmest for South Australia, the Northern Territory and Australia as a whole.
Persistent dry conditions through winter are typically associated with low soil moisture and heatwaves in the following spring and summer, and 2018 followed this pattern — with the added contribution of a warming climate.
The year ended with some record-breaking heat events. Perhaps the most significant of these was the extreme heat along the central and northern Queensland coast in late November and early December, which saw maximum daytime temperatures of 42.6 °C in Cairns and 44.9 °C in Proserpine on the 26th of November.
These temperatures, combined with persistent dry conditions in the preceding months, saw catastrophic fire weather and bushfires along 600km of the Queensland coast, an event that fire agencies have called unprecedented for the state.
The year ended with a burst of heat over the Christmas-New Year period, with temperatures at least 10 degrees warmer than average across southern South Australia, most of Victoria and southern NSW, leading to Australia’s warmest December on record.
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Once again, the summer of 2018 in the Northern Hemisphere has brought us an epidemic of major wildfires.
These burn forests, houses and other structures, displace thousands of people and animals, and cause major disruptions in people’s lives. The huge burden of simply firefighting has become a year-round task costing billions of dollars, let alone the cost of the destruction. The smoke veil can extend hundreds or even thousands of miles, affecting air quality and visibility. To many people, it has become very clear that human-induced climate change plays a major role by greatly increasing the risk of wildfire.
Yet it seems the role of climate change is seldom mentioned in many or even most news stories about the multitude of fires and heat waves. In part this is because the issue of attribution is not usually clear. The argument is that there have always been wildfires, and how can we attribute any particular wildfire to climate change?
As a climate scientist, I can say this is the wrong framing of the problem. Global warming does not cause wildfires. The proximate cause is often human carelessness (cigarette butts, camp fires not extinguished properly, etc.), or natural, from “dry lightning” whereby a thunderstorm produces lightning but little rain. Rather, global warming exacerbates the conditions and raises the risk of wildfire.
Even so, there is huge complexity and variability from one fire to the next, and hence the attribution can become complex. Instead, the way to think about this is from the standpoint of basic science – in this case, physics.
To understand the interplay between global warming and wildfires, consider what’s happening to our planet.
The composition of the atmosphere is changing from human activities: There has been over a 40 percent increase in carbon dioxide, mainly from fossil fuel burning since the 1800s, and over half of the increase is since 1985. Other heat-trapping gases (methane, nitrous oxide, etc.) are also increasing in concentration in the atmosphere from human activities. The rates are accelerating, not declining (as hoped for with the Paris agreement).
This leads to an energy imbalance for the planet.
Heat-trapping gases in the atmosphere act as a blanket and inhibit the infrared radiation – that is, heat from the Earth – from escaping back into space to offset the continual radiation coming from the sun. As these gases build up, more of this energy, mostly in the form of heat, remains in our atmosphere. The energy raises the temperature of the land, oceans and atmosphere, melts ice, thaws permafrost, and fuels the water cycle through evaporation.
Moreover, we can estimate Earth’s energy imbalance quite well: It amounts to about 1 watt per square meter, or about 500 terawatts globally.
While this factor is small compared with the natural flow of energy through the system, which is 240 watts per square meter, it is large compared with all other direct effects of human activities. For instance, the electrical power generation in the U.S. last year averaged 0.46 terawatts.
The extra heat is always the same sign and it is spread across the globe. Accordingly, where this energy accumulates matters.
Heat also accumulates in melting ice, causing melting Arctic sea ice and glacier losses in Greenland and Antarctica. This adds water to the ocean, and so the sea level rises from this as well, rising at a rate of over 3 milimeters year, or over a foot per century.
On land, the effects of the energy imbalance are complicated by water. If water is present, the heat mainly goes into evaporation and drying, and that feeds moisture into storms, which produce heavier rain. But the effects do not accumulate provided that it rains on and off.
However, in a dry spell or drought, the heat accumulates. Firstly, it dries things out, and then secondly it raises temperatures. Of course, “it never rains in southern California” according to the 1970s pop song, at least in the summer half year.
So water acts as the air conditioner of the planet. In the absence of water, the excess heat effects accumulate on land both by drying everything out and wilting plants, and by raising temperatures. In turn, this leads to heat waves and increased risk of wildfire. These factors apply in regions in the western U.S. and in regions with Mediterranean climates. Indeed many of the recent wildfires have occurred not only in the West in the United States, but also in Portugal, Spain, Greece, and other parts of the Mediterranean.
The conditions can also develop in other parts of the world when strong high pressure weather domes (anticyclones) stagnate, as can happen in part by chance, or with increased odds in some weather patterns such as those established by either La Niña or El Niño events (in different places). It is expected that these dry spots move around from year to year, but that their abundance increases over time, as is clearly happening.
How big is the energy imbalance effect over land? Well, 1 Watt per square meter over a month, if accumulated, is equivalent to 720 Watts per square meter over one hour. 720 Watts is equivalent to full power in a small microwave oven. One square meter is about 10 square feet. Hence, after one month this is equivalent to: one microwave oven at full power every square foot for six minutes. No wonder things catch on fire!
Coming back to the original question of wildfires and global warming, this explains the argument: there is extra heat available from climate change and the above indicates just how large it is.
In reality there is moisture in the soil, and plants have root systems that tap soil moisture and delay the effects before they begin to wilt, so that it typically takes over two months for the effects to be large enough to fully set the stage for wildfires. On a day to day basis, the effect is small enough to be lost in the normal weather variability. But after a dry spell of over a month, the risk is noticeably higher. And of course the global mean surface temperature is also going up.
“We can’t attribute a single event to climate change” has been a mantra of climate scientists for a long time. It has recently changed, however.
As in the wildfires example, there has been a realization that climate scientists may be able to make useful statements by assuming that the weather events themselves are relatively unaffected by climate change. This is a good assumption.
Also, climate scientists cannot say that extreme events are due to global warming, because that is a poorly posed question. However, we can say it is highly likely that they would not have had such extreme impacts without global warming. Indeed, all weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.
In particular, by focusing on Earth’s Energy Imbalance, new research is expected to advance the understanding of what is happening, and why, and what it implies for the future.