Where There’s Smoke There’s Fire

Recent wildfires highlight how closely coupled fire and weather extremes are. Around 400 million hectares of vegetated surface are burned every year. Sustained fires are changing fire regimes and contributing to climate change via massive CO2 emissions and regional climate effects, says David Bowman, professor of forest ecology at the University of Tasmania.

by David Bowman in New Scientist (7 October 2009):

IF WE are lucky, an international climate agreement will be forged in Copenhagen later this year and emissions targets set in a bid to limit global warming to 2 °C above pre-industrial levels. Agreement in Denmark or not, I find the breezy way so many politicians and commentators talk as if such an increase were no big deal truly amazing.

The truth is we have no idea what will happen at those places where you and I live if we raise the global thermostat 2 °C or more. Worse, there is another factor that could crank that thermostat still higher, making life quite as intolerable as the well-understood threat from rising sea levels: fire.

The fossil record shows that fires started to occur soon after vegetation was established on Earth during the Silurian, about 420 million years ago. Ancient terrestrial life’s exposure to fire gives us good reason to think fire is an important evolutionary factor and, more controversially, that life co-evolved with fire.

Fire can be seen as a physicochemical process, a “fire triangle” of oxygen, fuel and heat for ignition. Combustion can occur if the concentration of oxygen is higher than 13 per cent, and variation in oxygen levels correlates with fire activity in Earth’s history. Fluctuations in atmospheric oxygen through geological time significantly affected fire risk. For example, in the Permian, oxygen levels were substantially higher than now, and even moist giant moss forests sometimes burned.

Those Permian coal fossils flag up another key detail: the burial of decay-resistant charcoal and organic matter may have led to long-term reductions in the proportion of CO2 in the atmosphere and increased relative oxygen levels, because the carbon is geologically sequestered as coal and the oxygen left in the atmosphere (until chemical weathering draws it down). So fire in the biosphere should be considered both as a physicochemical process and as a fundamental biogeochemical process, feeding back between biosphere, geosphere and hydrosphere.

Seeing fire as biology seems odd, but landscape fires only occur because life creates fuel: so some ecologists now say fires should be seen as “biologically constructed”, drawing parallels with decomposition and herbivory.

For me, then, the Greeks had it right with their classification of fire, air, earth and water. But we have much to learn about how life and fire affect each other. Take tropical savannahs, the most flammable vegetation on Earth. Researchers think that falling atmospheric CO2 concentrations about 8 million years ago stimulated the global development of tropical savannah dominated by grasses which use the C4 photosynthetic pathway. These tropical grasses are highly productive in hot, wet climates, and under low CO2 concentrations they have a physiological advantage over woody vegetation, which uses the C3 pathway.

The Greeks had it right with their classification of fire, air, earth and water.

Savannah was stimulated because those grasses produce large quantities of fine and well-aerated fuels, greatly increasing the frequency of fire, further disadvantaging woody plants because frequent fires create a population bottleneck by killing “fire-tender” juveniles. Savannah trees had to develop rapid growth to escape the fire trap. Expanding savannahs may also have caused a climate feedback that created hotter, drier conditions, favouring yet more savannah: one of the great examples of fire-vegetation-climate feedback proposed by some ecologists.

The spread of highly flammable savannahs (the supposed home of hominins) most likely contributed to man’s eventual mastery of fire. Our ancestors appear to have used fire as far back as 400,000 years ago, with evidence of domestic use of fire dating back 50,000 to 100,000 years ago. One of the hallmarks of the transition from hunter-gatherer to sedentary economies has been to use fire to convert forest into agricultural or pastoral landscapes. This is ongoing, with tropical rainforests cleared to create farmland and cattle pasture. In the developed world, by contrast, suburban sprawl into rural and natural landscapes has resulted in homes being juxtaposed with flammable vegetation – and by increasing attempts to control fires.

Clearly, human use of fire for economic and ecological gain, even down to cooking, helped make us what we are. Yet we can’t completely control it: estimates show that around 400 million hectares of vegetated surface are burned every year but it is unclear how much is down to human activity or natural factors.

A new concern is that global climate change is driving the greater incidence of the kind of extreme weather that promotes fire. We don’t know if this year’s deadly wildfires in Victoria, Australia, the suburbs of Athens in Greece, and north of Los Angeles were ultimately a consequence of climate change, but as a biogeographer they fill me with dread. We are set to reconfigure Earth’s life zones – with potent consequences for ecology.

A key to understanding those consequences is the notion of the “fire regime”, where different vegetation has characteristic fires in terms of recurrence, intensity, seasonality and biological effects. Indeed, fire can be thought of as an emergent property of vegetation in the same way that vegetation can be thought of as an emergent property of climates. In other words, Earth has a “pyrogeography”.

Those recent wildfires highlighted how closely coupled fire and weather extremes are. A real worry is that such sustained fires are changing fire regimes and thereby changing vegetation types – by selecting for more flammable and fire-tolerant species – and contributing to climate change via massive CO2 emissions and regional climate effects.

The relationship between climate and human activity is hard to untangle, but there are things we know now which, with further research, will help improve models for the Intergovernmental Panel on Climate Change. Among these is that since the start of the industrial revolution, all types of landscape fire combined produced CO2 emissions equal to 20 per cent of those from burning fossil fuels. Fire also influences climate by releasing black-carbon aerosols which absorb heat from the sun. These may have the strongest effect on global warming after CO2 levels. Crucially, fire also influences most components of radiative forcing – changes in the difference between incoming and outgoing radiation energy in a climate system.

Managing wildfire is one of the really big challenges in adapting to climate change, up there with facing rising sea levels. To paint a precise picture of the climate warming caused by fire needs improved measurements, but the indicators are that there is a positive feedback – that fire begets fire. Fundamentally, adaptation demands we rethink our place in flammable landscapes. Like it or not, fires are going to change the way we live and where we live. We will have to recognise that in our shadow there lies a flame.

David Bowman is a professor of forest ecology at the University of Tasmania, Hobart. He is building a multidisciplinary group looking at sustainable management of biodiversity and ecosystem services in Tasmania, one of the world’s best natural labs for researching landscape fire. This essay is based on a Science paper (vol 324, p 481).

Source: www.newscientist.com