| 英文摘要: | Steel produced using coal generates 7% of global anthropogenic CO2 emissions annually1. Opportunities exist to substitute this coal with carbon-neutral charcoal sourced from plantation forests to mitigate project-scale emissions2 and obtain certified emission reduction credits under the Kyoto Protocol’s Clean Development Mechanism3. This mitigation strategy has been implemented in Brazil4, 5 and is one mechanism among many used globally to reduce anthropogenic CO2 emissions6; however, its potential adverse impacts have been overlooked to date. Here, we report that total CO2 emitted from Brazilian steel production doubled (91 to 182 MtCO2) and specific emissions increased (3.3 to 5.2 MtCO2 per Mt steel) between 2000 and 2007, even though the proportion of coal used declined. Infrastructure upgrades and a national plantation shortage increased industry reliance on charcoal sourced from native forests, which emits up to nine times more CO2 per tonne of steel than coal. Preventing use of native forest charcoal could have avoided 79% of the CO2 emitted from steel production between 2000 and 2007; however, doing so by increasing plantation charcoal supply is limited by socio-economic costs and risks further indirect deforestation pressures and emissions. Effective climate change mitigation in Brazil’s steel industry must therefore minimize all direct and indirect carbon emissions generated from steel manufacture. Growing global demand for steel, along with requirements to mitigate anthropogenic climate change, have increased the importance of reducing CO2 emissions from steel production7, 8. One mitigation strategy is to substitute the coal used as a reducing agent in steel production with biomass charcoal9. When this charcoal is produced from plantation forests grown on non-forested land (herein, plantation charcoal), it can be considered net carbon neutral under the UNFCCC Kyoto Protocol’s Clean Development Mechanism (CDM) because carbon flux to the atmosphere during charcoal production and use is offset by carbon sequestration from plantation tree growth10. Substituting coal with plantation charcoal therefore mitigates CO2 emissions from steel production at the project scale2 and, when registered, can be used to offset emissions in Annex B countries, provided that plantation charcoal production is additional and does not generate indirect emissions from deforestation elsewhere11, 12. More than half of Brazil’s steel is produced using charcoal13. Historically, this charcoal was mainly sourced from native forests (herein, native charcoal)13, 14, generating carbon emissions to the atmosphere from wood harvest, carbonization and charcoal use15, 16. However, the CDM provides policy and financial incentives in the form of Certified Emission Reduction (CER) credits to substitute the coal used in steel production with carbon-neutral plantation charcoal3, 10. In 2000, the first CDM project of this type established tree plantations for charcoal production on cleared and degraded land5; later projects used this plantation charcoal in place of coal to produce steel and mitigate CO2 emissions4. Despite approval of these projects as CER credits for utilization by Annex B countries, extensive charcoal production has also occurred outside the CDM framework to impact on Brazil’s aggregate emissions as a non-Annex B country. The size of this impact is unknown. In this study, we analysed annual steel production trajectories in Brazil between the years 2000 and 2007. We determined the quantity of each carbon source used in steel production (that is, coal, native charcoal and plantation charcoal; Supplementary Table 1) and quantified associated CO2 emissions (Supplementary Table 3). We assumed all plantation charcoal qualified as carbon neutral under the CDM, whether or not it was produced by CDM-funded projects (<8% of plantation charcoal used in steel production4). In doing so, we assumed all plantations were planted on already cleared land (see Methods) and did not cause carbon leakage. We analysed results at the national and state level to investigate the spatial impacts of charcoal production and use in Brazil. Specifically, the state-level analysis focused on Minas Gerais, Brazil’s most productive and industrialized steel and plantation charcoal producer14, 17 (Supplementary Fig. 1). We found that annual steel production in Brazil increased between 2000 and 2007 (from 28 Mt to 35 Mt; Supplementary Table 1)17 and relative coal use declined (from 50 to 46%; Fig. 1); yet annual CO2 emissions from steel production doubled (from 91 ± 10 MtCO2 to 182 ± 21 MtCO2; Fig. 2 and Supplementary Table 3). Emissions increased owing to growing industry use of native charcoal outside of CDM-funded projects (Fig. 1 and Supplementary Table 1).
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