Winkler Alexander J, Myneni Ranga, Reimers Christian, Reichstein Markus, Brovkin Victor
Max-Planck-Institute for Biogeochemistry, Jena, Germany.
Guest at Max-Planck-Institute for Meteorology, Hamburg, Germany.
PLoS One. 2024 Aug 1;19(8):e0306128. doi: 10.1371/journal.pone.0306128. eCollection 2024.
Current strategies to hold surface warming below a certain level, e. g., 1.5 or 2°C, advocate limiting total anthropogenic cumulative carbon emissions to ∼0.9 or ∼1.25 Eg C (1018 grams carbon), respectively. These allowable emission budgets are based on a near-linear relationship between cumulative emissions and warming identified in various modeling efforts. The IPCC assesses this near-linear relationship with high confidence in its Summary for Policymakers (§D1.1 and Figure SPM.10). Here we test this proportionality in specially designed simulations with a latest-generation Earth system model (ESM) that includes an interactive carbon cycle with updated terrestrial ecosystem processes, and a suite of CMIP simulations (ZecMIP, ScenarioMIP). We find that atmospheric CO2 concentrations can differ by ∼100 ppmv and surface warming by ∼0.31°C (0.46°C over land) for the same cumulated emissions (≈1.2 Eg C, approximate carbon budget for 2°C target). CO2 concentration and warming per 1 Eg of emitted carbon (Transient Climate Response to Cumulative Carbon Emissions; TCRE) depend not just on total emissions, but also on the timing of emissions, which heretofore have been mainly overlooked. A decomposition of TCRE reveals that oceanic heat uptake is compensating for some, but not all, of the pathway dependence induced by the carbon cycle response. The time dependency clearly arises due to lagged carbon sequestration processes in the oceans and specifically on land, viz., ecological succession, land-cover, and demographic changes, etc., which are still poorly represented in most ESMs. This implies a temporally evolving state of the carbon system, but one which surprisingly apportions carbon into land and ocean sinks in a manner that is independent of the emission pathway. Therefore, even though TCRE differs for different pathways with the same total emissions, it is roughly constant when related to the state of the carbon system, i. e., the amount of carbon stored in surface sinks. While this study does not fundamentally invalidate the established TCRE concept, it does uncover additional uncertainties tied to the carbon system state. Thus, efforts to better understand this state dependency with observations and refined models are needed to accurately project the impact of future emissions.
当前将地表变暖控制在一定水平（如1.5℃或2℃）以下的策略主张，分别将人为累计碳排放总量限制在约0.9或约1.25万亿吨碳（10¹⁸克碳）。这些允许的排放预算基于各种模型研究中确定的累计排放与变暖之间的近似线性关系。政府间气候变化专门委员会（IPCC）在其《决策者摘要》（§D(1.1)和图SPM.10）中对这种近似线性关系给予了高度置信度评估。在此，我们在特别设计的模拟中，使用包含具有更新陆地生态系统过程的交互式碳循环以及一组CMIP模拟（ZecMIP、ScenarioMIP）的最新一代地球系统模型（ESM）来检验这种比例关系。我们发现，对于相同的累计排放量（≈1.2万亿吨碳，2℃目标的近似碳预算），大气二氧化碳浓度可能相差约100 ppmv，地表变暖相差约0.31℃（陆地为0.46℃）。每排放1万亿吨碳的二氧化碳浓度和变暖程度（对累计碳排放的瞬态气候响应；TCRE）不仅取决于总排放量，还取决于排放时间，而排放时间此前一直主要被忽视。对TCRE的分解表明，海洋热量吸收正在补偿碳循环响应引起的部分而非全部路径依赖性。时间依赖性显然是由于海洋中尤其是陆地上滞后的碳固存过程导致的，即生态演替、土地覆盖和人口变化等，而这些在大多数ESM中仍未得到很好的体现。这意味着碳系统处于随时间演变的状态，但令人惊讶的是，它以一种与排放路径无关的方式将碳分配到陆地和海洋汇中。因此，即使对于相同总排放量的不同路径，TCRE有所不同，但与碳系统状态相关时它大致是恒定的，即地表汇中储存的碳量。虽然这项研究并没有从根本上否定已确立的TCRE概念，但它确实揭示了与碳系统状态相关的额外不确定性。因此，需要通过观测和改进模型来更好地理解这种状态依赖性，以便准确预测未来排放的影响。
