Fire’s net impact on the global carbon cycle

July 18, 2024 by hjeffery | News

Fire’s net impact on the global carbon cycle

Douglas Kelley¹, Chantelle Burton², Garry Hayman¹, Douglas S. Hamilton³, and participants of the FLARE workshop

¹UK Centre for Ecology & Hydrology; ²Met Office, UK; ³North Carolina State University, USA

Outcomes from the FLARE workshop and three challenges for the future of transdisciplinary fire science. Full report available from https://futureearth.org/2024/07/10/new-paper-outlines-the-future-of-transdisciplinary-fire-science

Fire, often viewed as a destructive force of nature, is, in reality, highly entwined in the Earth System and provides a complex array of essential roles in maintaining healthy ecosystems (Pausas and Keeley, 2014; Clarke et al., 2013). Environmental processes that are shaped by fire include: the global carbon cycle, ecosystem dynamics, atmospheric composition and the surface energy balance (Bowman et al., 2009). Each of these processes also feedback and shape other elements of the natural world, highlighting the complexity of the role of fire in the Earth System.

Climate change and human activities are a new driving force for fire, changing fire regimes around the world and increasing the occurrence of extreme events (Kelley et al., 2019). The anthropogenic disruption of natural cycles has profound implications for our planet (Burton et al., 2022, submitted). In such a rapidly changing world, there is an urgent need for a deeper understanding of fire behaviour and increased research into the prediction of fire’s impacts on the global carbon cycle.

The FLARE (Fire Science Learning AcRoss the Earth System) working group aims to unite the science community to comprehensively understand the challenges of fire and its impacts in a rapidly changing world. Answering this challenge requires a clear path forward and engagement from researchers across all disciplines. To start the process FLARE hosted a workshop in September 2023. Scientists from a wide range of disciplines discussed the latest advances in fire science in their field, the tools of their trades, and implications of rising fire risk on ecosystems and society. Conversations led to a white paper that identified three unifying challenges for the science community to tackle in the next 5-10 years: (1) A comprehensive understanding of fire’s influence on the global carbon cycle; (2) Defining and studying fire and extreme events; (3) Exploring the complex interactions between fire and human societies.

A priority question requiring engagement from the entire fire science community (and beyond) is:

“What is the net impact of fire on the global carbon cycle?”

To effectively answer this question, science must draw upon: ecology, to understand fires diverse impacts on ecosystems; microbiology, to understand how soils microbiome is shaped by fire; Earth system science, to understand the ways in which fire influences the movement of energy and mass; meteorology, to understand how climate change will change fuel properties; biogeochemistry, to understand how the elements within fire emissions interact within different Earth System spheres; cryosphere science to understand how fire can alter melt rates; and social sciences, to understand how humans shape fire regimes and the policy around them.

The Complexity of Fire and the Carbon Cycle

Figure 1. The role of fire in the global carbon cycle is examined in the FLARE white paper and reproduced from Figure 5. Using a rainbow as a metaphor, the paper illustrates the interconnectedness of fire and the carbon cycle. To represent this relationship, a diagram of the Earth is depicted with boxes symbolizing the primary carbon sources and sinks. Each source has a colour band extending into the atmosphere, while each sink has a corresponding colour band extending into the Earth. The diagram also includes depictions of clouds and smoke to show how carbon sources interact within the atmosphere. Furthermore, it highlights the impact of fire on the cryosphere, such as the role of black carbon in ice sheet retreat and the resulting reduction in pink algae in frozen environments.

Fire’s impact extends far beyond the release of carbon dioxide and other greenhouse gases (GHG) into the atmosphere. In stable environments, ecosystems can balance carbon emissions during fires with subsequent carbon uptake through vegetation recovery and growth. This natural equilibrium helps regulate atmospheric carbon levels.

However, natural balances that do exist are being disrupted as our climate rapidly changes. Increasing temperatures, altered precipitation patterns, and shifts in vegetation cover are leading to more frequent and intense fires in many regions. Increased fire intensity releases more carbon into the atmosphere and affects an ecosystem’s ability to recover and store carbon over time. For example, reducing the interval between fire occurrences prevents vegetation from fully regrowing, reducing its capacity to absorb atmospheric carbon dioxide, and potentially shifting a biome from forests to grasslands. In other areas, where fire is vital for the healthy maintenance of an ecosystem, fires are becoming less frequent. This reduction in fire frequency disrupts the natural cycle of nutrient cycling and vegetation regeneration that fires typically support. Consequently, these ecosystems may experience altered carbon dynamics, with implications for both carbon storage and greenhouse gas emissions.

Pyrogenic Carbon

A notable example of a way in which fire shapes the carbon cycle beyond GHG emissions is the formation of pyrogenic carbon during a fire (Santín et al., 2016). Pyrogenic carbon is important due to its stability and thus ability to store carbon over long timescales. Studies estimate that 5-25% of the biomass burned in a fire can be converted to pyrogenic carbon, with up to half of that remaining stable in the long-term. Globally, it is estimated that approximately 200 Pg of pyrogenic carbon resides in the top 2 metres of soil, with annual production ranging from 196-340 Tg.

““Our tools for assessing and predicting fires are advancing, but they don’t fully capture how fires affect ecosystems and carbon dynamics. To enhance our understanding, we need to improve how we integrate real-world data into our models and allow them to capture ecological adaptations to fire.” explains Douglas Kelley, a land surface modeller from UKCEH

Resprouters and Obligate/Serotinous Seeders

Species like serotinous and obligate seeders, which depend on specific fire conditions for seed release, may fail to regenerate if fire intensity or frequency changes. Resprouter plants, which rely on protected carbohydrate stores to regrow, may struggle if fires become too frequent or intense (Clarke et al., 2013; Pausas and Keeley, 2014; Zeppel et al., 2015). Carbon stored in trees often relies on fire for this critical part of a plant life cycle, and changes in fire regime can therefore have less immediate, often negative consequences for stored forest carbon (Kelley and Harrison, 2014; Kelley et al., 2014).

It’s essential to recognize that fire’s influence extends beyond terrestrial ecosystems. Smoke and ash contain black carbon, a potent climate-forcing agent that warms the atmosphere and, when deposited on snow and ice, reduces albedo, accelerating their melt. Additionally, fires can impact remote marine ecosystems through the transport of carbon and nutrients via atmospheric deposition and runoff, affecting ocean productivity and carbon storage.

““Wildfires can significantly affect the global carbon cycle. Fires in ecosystems that store large amounts of carbon, such as peatlands, permafrost and forests, can release vast quantities of carbon dioxide into the atmosphere. However, where that carbon ultimately ends up and its impact on future warming is hard to predict. Incorporating accurate fire-related carbon fluxes into Earth System Models is crucial for predicting climate outcomes and informing mitigation strategies, and it will require us to bring together experts from across the fire sciences.” emphasizes Chantelle Burton, a key contributor to the FLARE report.

Black Carbon and Ocean Deposition

Fires affect the carbon cycle indirectly through atmospheric processes. Black carbon aerosols can warm the atmosphere by absorbing solar radiation and cool it by seeding cloud formation. Once deposited on ice or bare ground, black carbon lowers surface albedo, accelerating ice melt (Kang et al., 2010; Evangeliou et al., 2018). Conversely, other aerosols like organic carbon scatter solar radiation, and nutrient-rich aerosols can stimulate primary production in land and ocean ecosystems, further complicating our understanding of fire-carbon dynamics at the global scale (Tang et al., 2021).

Peatlands and Permafrost

Peatlands and permafrost are significant carbon stores at risk from changing fire regimes. Fire-induced permafrost thaw and peatland burning are likely to increase, driven by climate warming, reduced precipitation, and human activities. The release of stored carbon from these ecosystems could have profound implications for the global carbon cycle. (Gibson et al., 2018)

Human activities significantly influence fire regimes and their impact on the carbon cycle. Changes in land use, such as deforestation and urban expansion, often escalate the frequency and intensity of fires, thereby releasing more carbon into the atmosphere. Effective fire management practices, including controlled burns and suppression efforts, play a crucial role in mitigating carbon emissions from fires. Initiatives like REDD+ (Reducing Emissions from Deforestation and Forest Degradation) aim to curb emissions from deforestation, contributing to global carbon reduction targets. Integrating comprehensive fire management strategies into national climate policies and commitments (termed “NDCs”) enhances carbon sequestration efforts and fortifies ecosystem resilience against the adverse impacts of climate change.

Unifying Question: What is the Net Impact of Fire on the Global Carbon Cycle?

To address this question, we must , consider ecological data, and understand emissions’ global transport and deposition patterns and impacts. We must increase our capacity to predict how fire regimes will change in the near-term and how human activities and societal reactions will alter these changes.

“Garry Hayman highlights the broader environmental implications of fire, stating, “Fire not only affects the carbon cycle but also interacts with other biogeochemical cycles e.g. the nitrogen cycle, influencing ecosystem health and resilience. Comprehensive studies are essential to capture these interdependencies.”

Building a Transdisciplinary Future

Addressing these challenges requires a new generation of transdisciplinary-minded fire scientists. Equity in research and knowledge exchange is vital. Underrepresented regions and communities must be engaged in productive and meaningful collaborations. Two-way capacity building through student exchanges and collaborative projects can help in fostering a more inclusive and comprehensive approach to understanding and managing fire’s impact on the carbon cycle.

“Douglas Hamilton concludes, “There are simply not enough scientists in this field of research currently to do the work needed. Building capacity and engaging in meaningful conversations across disciplines is a priority for fire science in the coming years.”

Conclusion

Fires play a complex and critical role in the global carbon cycle, influencing immediate carbon emissions and long-term carbon storage across a diverse range of Earth System spheres. Understanding these dynamics requires integrating diverse scientific disciplines, improving predictive models, and fostering global collaboration. As we continue to face the challenges posed by changing fire regimes, a holistic and inclusive approach will be key to mitigating their impact on our planet.