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Between 90-95 per cent of fires are caused by lightning strikes and until recently it was assumed the long-term effects contributed to global warming.
First time research involving ANSTO scientist Scott Chambers, explained a surprising result which reveals that during forest regrowth, there is in fact a cooling effect on climate, counteracting any warming that occurred during the burn.
When forests go up, what's the global warming effect? During the hot summer months in the northern hemisphere, remote forest fires can rage out of control, unchecked or fought due to little or no human presence. Large areas are burned to the ground sending carbon into the atmosphere. Between 90-95 per cent of fires are caused by lightning strikes and until recently it was assumed the long-term effects contributed to global warming.
First time research involving ANSTO scientist Scott Chambers and featured in a new book How We Know What We Know About Our Changing Climate, explained a surprising result which reveals that during forest regrowth, there is in fact a cooling effect on climate, counteracting any warming that occurred during the burn.
"Forest fires are a part of the natural life cycle of black and white spruce and larch trees that make up most of the forests of Siberia, Alaska and Canada," said Scott. "Both the spruce and larch are from the pine tree family yet the larch is deciduous, unlike the spruce which is an evergreen, but both take time to grow.
"Unlike the Australian bush, which can regenerate a few years after a fire, high-latitude pine forests take from 60 to 80 years to regrow, but only recently has research been conducted to understand climate change effects during each regrowth phase."
Scott explained that there are many stages of post-fire 'succession' between a charred black landscape and its return to a black spruce pine forest.
"Each of these stages, which can last for years and cover vast areas, emits and absorbs energy, water vapour and carbon dioxide very differently to the original forest," he said.
"In Siberia there are also further complications because the frozen ground underneath the burned forest melts and releases methane. In extreme cases, swamps will develop after the ice-melt and forest will not return to the area at all."
Over three summers in Alaska and Siberia, and completed in 2005, Scott set up paired measurement sites over burned and unburned forests to investigate forests in different stages of growth (from 0 to 20 years post burn).
The purpose of Scott's initial research and on-going research, in collaboration with Professor Jim Randerson from the University of California, was to estimate the net effect of the change in land surface characteristics, energy and greenhouse gas exchanges on regional and global climate.
"After fires or other types of disturbances, the entire forest zone can be a patchwork of forest in different stages of growth, so it has a variety of signals," Scott explained. "It is also well documented that northern latitudes are responding more rapidly to changing global climate than other parts of the world, so this was expected to lead to a change in fire frequency and fire severity.
"These factors have the potential to drastically alter the stages of post-fire growth and the intermediate land surface characteristics which it was understood could ultimately feedback to climate," he said.
The results, however, were extremely unexpected. "We found that the results challenged the current model because it implied that future increases in forest fires may not accelerate climate warming as previously thought," said Scott, adding that the reasons for this were to do with the stages of regrowth.
"The first things that begin to grow are small grasses, shrubs and berries followed by deciduous trees like Birch and Aspen," he explained. "These are brighter trees with wide, shiny leaves which reflect the sunlight or energy, cooling the climate, compared to pines which absorb energy and warm it. As they grow faster than pines, they dominate the forest first so at this stage there is a negative effect to climate warming.
"The cooling process is also down to the fact that the deciduous Birch and Aspen trees transpire readily - due to their rapid growth and assisted by their larger leaves - cooling the air in much the same way as an evaporative air conditioner.
The slow growing high-latitude pines, on the other hand, are accustomed to periods of extreme water stress, and with their smaller, needle-like leaves do not have high levels of transpiration, so once these dominate the forest the climate begins to warm."
Although Scott no longer works on this project he still keeps in touch with colleagues who do.
"A science project is never really over as you always have an interest in how the research is continuing, so I keep in touch with the scientists who are still working on this subject," he said.
Scott's research at ANSTO is focussed on radon - a radioactive gas emitted from the earth as a result of uranium breaking down in the ground. This gas can help scientists trace pollution in the atmosphere and establish where it came from by measuring the radon levels in the air.
Understanding and measuring radon is also important for developing more accurate climate models for scientists to work from.
Published: 01/03/2009