Carbon lies at the heart of the planet’s major biogeochemical balances and constitutes an essential thread for understanding climate change, ecosystem degradation, and contemporary ecological transition strategies. Far from being a uniform entity, carbon; manifests itself in different forms and dynamics, often described through color codes black, brown, blue, green, red, and grey, which help to better grasp its origin, behavior in the environment, and its economic, social, and climatic implications. Although simplified, this typology has become established in scientific and policy debates as a pedagogical and analytical tool that facilitates the design of public policies, financial mechanisms, and sustainable management strategies [1].
Black Carbon
Black carbon is mainly associated with fine particles resulting from the incomplete combustion of fossil fuels, biomass, and biofuels. It is a major component of atmospheric aerosols and is characterized by a strong capacity to absorb solar radiation, making it a powerful short-term climate warming agent [2]. Unlike carbon dioxide, whose atmospheric lifetime spans decades or even centuries, black carbon persists for only a few days or weeks, but its radiative impact is particularly intense. It contributes not only to global warming but also to the accelerated melting of glaciers and snow when it is deposited on light-colored surfaces, reducing their albedo [3].
Moreover, black carbon poses a major public health challenge, being closely linked to respiratory and cardiovascular diseases, especially in urban areas and regions that rely on biomass for domestic cooking [4]. Consequently, reducing black carbon emissions is often regarded as a measure with immediate co-benefits for both climate and health.
Brown Carbon
Brown carbon, which is sometimes less highlighted, refers to a specific fraction of particulate organic matter mainly originating from biomass combustion and natural decomposition processes. It preferentially absorbs radiation in the ultraviolet and visible ranges, with optical properties distinct from those of black carbon [5].
Brown carbon plays a complex role in the climate system, as its radiative effects can vary depending on its chemical composition, atmospheric aging, and environmental conditions. It is often associated with wildfires, agricultural burning, and emissions from peatlands, linking it directly to land-use dynamics and landscape management practices [6]. In a context of climate change marked by increasing frequency and intensity of fires, brown carbon is becoming a key indicator of climate feedbacks that are still insufficiently integrated into global models.
Blue Carbon
Blue carbon refers to carbon stored in coastal and marine ecosystems, notably mangroves, seagrass meadows, and salt marshes. Although these ecosystems cover a relatively limited surface area globally, they possess an exceptionally high carbon sequestration capacity, often exceeding that of terrestrial forests on a per-area basis [7].
Blue carbon is mainly stored in sediments, where it can remain trapped for millennia, provided the ecosystem remains intact. The degradation or destruction of these environments leads not only to the loss of essential ecosystem services coastal protection, biodiversity nurseries, nutrient filtration but also to the massive release of previously stored carbon [8].
Recognition of blue carbon has led to its gradual integration into climate policies, particularly within nationally determined contributions (NDCs) and carbon finance mechanisms, although methodological challenges remain regarding its measurement, monitoring, and verification [9].
Green Carbon
Green carbon corresponds to carbon captured and stored by terrestrial ecosystems, particularly forests, grasslands, agricultural soils, and inland wetlands. It is the most familiar form of carbon in nature-based climate change mitigation strategies. Photosynthesis is the main driver of this dynamic, transforming atmospheric carbon dioxide into plant biomass and soil organic matter [10].
Green carbon is, however, characterized by a certain vulnerability, as stocks can be rapidly released back into the atmosphere following deforestation, soil degradation, fires, or land-use change [11]. This reversibility raises critical questions about the permanence of biological carbon sinks and their integration into carbon markets. Nevertheless, green carbon remains a fundamental pillar of carbon neutrality approaches due to its co-benefits for biodiversity, food security, and rural livelihoods [12].
Red Carbon
Red carbon is a more recent and less standardized concept, generally used to denote carbon associated with activities that are highly destructive to ecosystems or that generate social conflicts, such as illegal deforestation, unsustainable mining, or certain carbon-intensive infrastructure projects [13]. It emphasizes the ethical and socio-environmental dimension of carbon flows, highlighting that not all forms of carbon storage or emissions are equivalent from a sustainable development perspective.
Red carbon thus reflects a critical reading of climate policies that focus solely on carbon volumes without considering impacts on human rights, natural resource governance, and the resilience of local communities [14]. This notion is particularly mobilized by civil society organizations and certain academic currents seeking to rebalance the climate debate in favor of environmental justice.
Grey Carbon
Grey carbon, finally, refers to carbon embedded in industrial goods and services, notably through emissions linked to raw material extraction, manufacturing, transport, use, and end-of-life of products. It is closely related to the concept of carbon footprint and life-cycle assessment [15].
Grey carbon is often invisible to the final consumer, as it is emitted upstream in the value chain, sometimes in regions far removed from the place of consumption. In the context of globalized trade, it raises the issue of shared responsibility between producers and consumers, as well as that of carbon border adjustment mechanisms [16]. Reducing grey carbon involves improving energy efficiency, eco-design, circular economy approaches, and the decarbonization of industrial processes, particularly in sectors such as steel, cement, and chemicals [17].
Conclusion
Taken together, these different types of carbon illustrate the complexity of interactions between human activities, ecosystems, and climate. They show that combating climate change cannot be limited to a one-dimensional approach centered on carbon dioxide alone, but requires a nuanced understanding of the multiple forms carbon takes within the Earth system. This differentiated approach opens the way to more integrated policies capable of maximizing environmental and social co-benefits while reducing the risks of counterproductive solutions.
As international frameworks such as the Paris Agreement and the Kunming–Montreal Global Biodiversity Framework increasingly recognize the links between climate, biodiversity, and development, the typology of colored carbons could play a growing role in guiding investments, designing economic instruments, and assessing progress toward sustainability [18].
References
[1] IPCC, Climate Change 2021: The Physical Science Basis, Cambridge University Press, Cambridge, 2021.
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[3] Flanner M.G., Zender C.S., Randerson J.T., Rasch P.J., Present-day climate forcing and response from black carbon in snow, J. Geophys. Res., 112 (2007) D11202. https://doi.org/10.1029/2006JD008003.
[4] WHO, Health Effects of Black Carbon, World Health Organization, Copenhagen, 2012.
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[11] IPBES, Global Assessment Report on Biodiversity and Ecosystem Services, IPBES Secretariat, Bonn, 2019.
[12] Griscom B.W., Adams J., Ellis P.W., et al., Natural climate solutions, Proc. Natl. Acad. Sci. USA, 114 (2017) 11645–11650.
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[14] Newell P., Mulvaney D., The political economy of the ‘just transition’, Geogr. J., 179 (2013) 132–140.
[15] ISO, ISO 14040: Life Cycle Assessment – Principles and Framework, International Organization for Standardization, Geneva, 2006.
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[17] IEA, Industrial Decarbonisation, International Energy Agency, Paris, 2022.
[18] UNFCCC, Paris Agreement, United Nations Framework Convention on Climate Change, Bonn, 2015.
