Terrestrial biosphere: The burning issue
2
© 2008 Macmillan Publishers Limited. All rights reserved. © 2008 Macmillan Publishers Limited. All rights reserved. NEWS & VIEWS nature geoscience | VOL 1 | OCTOBER 2008 | www.nature.com/naturegeoscience 643 Andrew C. Scott is in the Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK. e-mail: [email protected] F ires have scorched the Earth’s surface for over 400 million years, and have been an integral part of the Earth system for the past 350 million years 1,2 . But since humans have begun to use and control fire, its significance in many ecosystems has changed 3 . is connection between humans and fire has oſten masked our understanding of natural fire, especially the influence of climate change on fire frequency and extent — a topic that is likely to become increasingly important over the coming decades. Today we can try to map fires and their emissions with satellite images 4,5 , which may help to unravel the relationship between fire and agricultural development. However, the complexity of human use and suppression of fire 3 makes it difficult to disentangle any influence of climate on fire from the impacts of humans. On page 697 of this issue, Marlon and colleagues 6 compile hundreds of globally distributed charcoal records spanning the past 2,000 years in order to shed light on humans’ changing influence on fire. Our understanding of ancient wildfire comes mainly from the occurrence and distribution of charcoal 1 . Charcoal is formed through biomass burning and then carried by water and wind, before being preserved in lakes and other sedimentary basins. e amount of charcoal in sediment cores has been correlated to the amount of biomass burned at the time of deposition. Such sediment records suggest that wildfires have been sparked naturally, for example through lightning, since the Earth’s atmospheric oxygen reached the levels required to sustain fire 1,2 . Hundreds of millions of years later, humans grew more advanced and learned to use fire to hunt and to alter landscapes 3 . e development of large-scale agriculture also altered patterns of biomass burning. Adding to the complexity of fire interpretation, these changes to the natural fire regime have made it difficult to distinguish the response of wildfires to individual influences, for example from climate or agriculture. In an attempt to provide a basis for investigating fire dynamics, a global charcoal database for the past ten thousand years has been developed 7 . Marlon and colleagues have improved the resolution of the reconstruction for the past two millennia, compiling decadal-scale records that are taken mainly from terrestrial lakes and peats. eir analysis shows that biomass burning declined from ad 1 to 1750, rose sharply between 1750 and 1870, and then declined abruptly thereaſter. Comparing their record with information from climate records and models, Marlon and colleagues suggest that the long initial fall in biomass burning — which also coincides with a growing global population — parallels a cooling trend. Biomass burning was constrained, they argue, more by climate than human activity during this period. Marlon and colleagues interpret the rapid increase in biomass burning aſter 1750 as a result of the complex interactions between fire, climate and atmospheric chemistry. With the dawn of heavy industry and fossil fuel use, atmospheric CO 2 levels began to rise: increased CO 2 concentrations essentially act as ‘fertilizer’, enabling more plant growth, which is then available for burning. Increasing CO 2 also leads to global warming and increases in aridity. is in turn can lead to increased burning. Changes in vegetation patterns in response to changing climate also affect the fire regime 8 . Land use changes during this time, such as the use of slash and burn farming techniques, would have also increased the total biomass burned during this interval. Wildfires have been a natural part of the Earth system for millions of years. A new charcoal database for the past two millennia shows that human activity increased biomass burning after AD 1750 and suppressed it after AD 1870. TERRESTRIAL BIOSPHERE The burning issue nature geoscience | VOL 1 | OCTOBER 2008 | www.nature.com/naturegeoscience 643 Figure 1 Letting fires burn. Although fires are a natural part of the Earth system, humans tend to strive to put them out. There is increasing tension between those who believe wildfires should be allowed to burn, and those who want to control them. The work by Marlon and colleagues 6 reveals early patterns of fire and may help to make more informed decisions about fire management. JON KEELEY
Transcript of Terrestrial biosphere: The burning issue
© 2008 Macmillan Publishers Limited. All rights reserved.
© 2008 Macmillan Publishers Limited. All rights reserved.
NEWS & VIEWS
nature geoscience | VOL 1 | OCTOBER 2008 | www.nature.com/naturegeoscience 643
Andrew C. Scottis in the Department of Earth Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK.
e-mail: [email protected]
F ires have scorched the Earth’s surface for over 400 million years, and have been an integral part of the Earth
system for the past 350 million years1,2. But since humans have begun to use and control fi re, its signifi cance in many ecosystems has changed3. Th is connection between humans and fi re has oft en masked our understanding of natural fi re, especially the infl uence of climate change on fi re frequency and extent — a topic that is likely to become increasingly important over the coming decades. Today we can try to map fi res and their emissions with satellite images4,5, which may help to unravel the relationship between fi re and agricultural development. However, the complexity of human use and suppression of fi re3 makes it diffi cult to disentangle any infl uence of climate on fi re from the impacts of humans. On page 697 of this issue, Marlon and colleagues6 compile hundreds of globally distributed charcoal records spanning the past 2,000 years in order to shed light on humans’ changing infl uence on fi re.
Our understanding of ancient wildfi re comes mainly from the occurrence and distribution of charcoal1. Charcoal is formed through biomass burning and then carried by water and wind, before being preserved in lakes and other sedimentary basins. Th e amount of charcoal in sediment cores has been correlated to the amount of biomass burned at the time of deposition. Such sediment records suggest that wildfi res have been sparked naturally, for example through lightning, since the Earth’s atmospheric oxygen reached the levels required to sustain fi re1,2. Hundreds of millions of years later, humans grew more advanced and learned to use fi re to hunt and to alter landscapes3. Th e development of large-scale agriculture also altered patterns of biomass burning. Adding to the complexity of fi re interpretation, these changes to the natural fi re regime have
made it diffi cult to distinguish the response of wildfi res to individual infl uences, for example from climate or agriculture. In an attempt to provide a basis for investigating fi re dynamics, a global charcoal database for the past ten thousand years has been developed7.
Marlon and colleagues have improved the resolution of the reconstruction for the past two millennia, compiling decadal-scale records that are taken mainly from terrestrial lakes and peats. Th eir analysis shows that biomass burning declined from ad 1 to 1750, rose sharply between 1750 and 1870, and then declined abruptly thereaft er. Comparing their record with information from climate records and models, Marlon and colleagues suggest that the long initial fall in biomass burning — which also coincides with a growing global population — parallels a cooling trend. Biomass burning was
constrained, they argue, more by climate than human activity during this period.
Marlon and colleagues interpret the rapid increase in biomass burning aft er 1750 as a result of the complex interactions between fi re, climate and atmospheric chemistry. With the dawn of heavy industry and fossil fuel use, atmospheric CO2 levels began to rise: increased CO2 concentrations essentially act as ‘fertilizer’, enabling more plant growth, which is then available for burning. Increasing CO2 also leads to global warming and increases in aridity. Th is in turn can lead to increased burning. Changes in vegetation patterns in response to changing climate also aff ect the fi re regime8. Land use changes during this time, such as the use of slash and burn farming techniques, would have also increased the total biomass burned during this interval.
Wildfi res have been a natural part of the Earth system for millions of years. A new charcoal database for the past two millennia shows that human activity increased biomass burning after AD 1750 and suppressed it after AD 1870.
TERRESTRIAL BIOSPHERE
The burning issue
nature geoscience | VOL 1 | OCTOBER 2008 | www.nature.com/naturegeoscience 643
Figure 1 Letting fi res burn. Although fi res are a natural part of the Earth system, humans tend to strive to put them out. There is increasing tension between those who believe wildfi res should be allowed to burn, and those who want to control them. The work by Marlon and colleagues6 reveals early patterns of fi re and may help to make more informed decisions about fi re management.
JON
KEEL
EY
© 2008 Macmillan Publishers Limited. All rights reserved.
© 2008 Macmillan Publishers Limited. All rights reserved.
NEWS & VIEWS
644 nature geoscience | VOL 1 | OCTOBER 2008 | www.nature.com/naturegeoscience
However, further changes in agricultural land use, such as the introduction of non-native crops and the development of widespread and intensive livestock grazing eventually reduced the build-up of biomass. Marlon and colleagues suggest that this reduction in fuel, along with the implementation of fi re suppression policies in some regions, caused the reduction in biomass burning in the period aft er 1870, despite a warming climate. Ice core records of δ13CH4 and CO (ref. 8), which have been suggested to be proxies of biomass burning, provide additional support for their fi re reconstruction.
Th ere are geographic gaps in the data set, but the authors suggest that they have a good coverage of climatic zones, except for grassland and dry shrubland. Although reconstructions of fi res in these regions are diffi cult to construct owing to minimal charcoal production during burning, such fi res are important for global biomass burning estimates. Grassland fi res9 produce large amounts of emissions, including greenhouse gases and particulates4. Rare records of charcoals from lake sediments and reconstructions from black carbon and carbon isotopic data from deep-sea cores may provide a way to fi ll this gap.
Th ese gaps also extend to two potentially important regions. One is the Amazon, which has recently suff ered massive biomass burning at the hands of its inhabitants10. Local pressure on the use of its resources is in confl ict with the global need to reduce CO2 emissions and preserve the rich forests of the region as a carbon sink. Tropical peatlands such as those found in Indonesia are similarly missing from the database. Increasing burning of these vast carbon stores — started by humans and fuelled by drought — is making a signifi cant contribution to global greenhouse emissions11.
Th e work by Marlon and colleagues highlights the need for an expanded database, collected from new areas and time slices — not an easy task when current funding favours the investigation of big questions over the collection of primary data. Th e nature of the relationship between charcoal production, transport and sedimentation, and the eff ect of this relationship on our interpretation of the charcoal record, also needs our attention.
Another area where our understanding could be improved is the prehistoric use of fi re by humans, including landscape burning. How did the use of fi re by the fi rst human populations of North America
aff ect vegetation? What were the eff ects of the burning of savannas by Aboriginal populations in Australia, and how might we recognize these impacts using the fossil charcoal record? In addition, we need a better idea of natural fi re systems, before the advent of humans, in order to understand the full impact of humans on biomass burning.
An integration of fi eld data and modelling as used by Marlon and colleagues is an important contribution to our understanding of the relationship between fi re, climate and man. But only time will tell if they have the correct answers.
Published online: 21 September 2008.
References1. Scott, A. C. Palaeogeogr. Palaeocl. Palaeoecol.
164, 281–329 (2000).2. Scott, A. C. & Glasspool, I. J. Proc. Natl Acad. Sci. USA
103, 10861–10865 (2006).3. Pyne, S. J. World Fire: Th e Culture of Fire on Earth
(Henry Holt, New York, 1995).4. van der Werf, G. R. et al. Atmos. Chem. Phys.
6, 3423–3441 (2006).5. Tansey, K. et al. Geophys. Res. Lett. 35, L01401 (2008).6. Marlon, J. R. et al. Nature Geosci. 1, 697–702 (2008).7. Power, M. J. et al. Clim. Dyn. 30, 887–907 (2008).8. Ferretti, D. F. et al. Science 309, 1714–1717 (2005).9. Bond, W. J. & Keeley, J. E. Trends Ecol. Evol.
20, 387–394 (2005).10. Cochrane, M. A. Nature 421, 912–919 (2003).11. Page, S. E. et al. Nature 420, 61–65 (2002).
Th e reality of anthropogenic-induced climate change is no longer in question. Nevertheless, the amount of carbon dioxide vented into the atmosphere each year as a result of human activities shows no sign of levelling off (let alone decreasing).
marine boundary layer. Th ey propose that wind-driven vessels (see image), based on a 1920s design forgotten during the turmoil of the 1929 economic depression, could spray seawater into the atmosphere (Phil. Trans. R. Soc. A doi:10.1098/rsta.2008.0136; 2008).
Th e beauty of the proposal is its relative simplicity. Th e resources required are just seawater and wind, the vessels should not need much maintenance, and the eff ect of cloud albedo increases could be quite large compared with the eff ort needed to generate the spray.
As with all geoengineering proposals, more research needs to go into both the effi ciency of the scheme at an industrial scale and the possible (unexpected) side eff ects. Th e injection of seawater into the atmosphere could be stopped immediately should undesirable eff ects develop, such as substantial reductions in precipitation over populated land areas. But, as the authors concede, the response of the Earth system could linger much longer.
Heike Langenberg
As a result, geoengineering solutions are beginning to look more attractive, at least as a last resort. Proposals come in many fl avours of expense, impact and economic viability, with ocean fertilization, the injection of sulphate aerosols into the upper atmosphere and carbon sequestration being some of the more prominent ideas.
Writing in a special issue on “Geoscale engineering to avert dangerous climate change”, John Latham and colleagues propose a diff erent way of cooling the Earth, which involves seeding low-level maritime clouds with seawater particles (Phil. Trans. R. Soc. A doi:10.1098/rsta.2008.0137; 2008). Th e scheme aims to exploit the Twomey eff ect, in which an increase in the number of cloud condensation nuclei — seawater particles in this case — leads to more, but smaller, cloud droplets. Th e resulting brighter clouds refl ect more sunlight back into space, leading to the desired cooling eff ect.
In a companion paper, Stephen Latham and colleagues suggest a relatively low-cost engineering solution to the challenge of injecting the seawater particles into the
Cool sprayCLIMATE CHANGE
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