The response of the carbon cycle in prognostic Earth system models (ESMs) contributes significant uncertainty to projections of global climate change. Quantifying contributions of known drivers of interannual variability in the growth rate of atmospheric carbon dioxide (CO2) is important for improving the representation of terrestrial ecosystem processes in these ESMs. Several recent studies have identified the temperature dependence of tropical net ecosystem exchange (NEE) as a primary driver of this variability by analyzing a single, globally averaged time series of CO2 anomalies. Here we examined how the temporal evolution of CO2 in different latitude bands may be used to separate contributions from temperature stress, drought stress, and fire emissions to CO2 variability. We developed atmospheric CO2 patterns from each of these mechanisms during 1997–2011 using an atmospheric transport model. NEE responses to temperature, NEE responses to drought, and fire emissions all contributed significantly to CO2 variability in each latitude band, suggesting that no single mechanism was the dominant driver. We found that the sum of drought and fire contributions to CO2 variability exceeded direct NEE responses to temperature in both the Northern and Southern Hemispheres. Additional sensitivity tests revealed that these contributions are masked by temporal and spatial smoothing of CO2 observations. Accounting for fires, the sensitivity of tropical NEE to temperature stress decreased by 25% to 2.9 ± 0.4 Pg C yr−1 K−1. These results underscore the need for accurate attribution of the drivers of CO2 variability prior to using contemporary observations to constrain long-term ESM responses.

中文翻译：

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分离温度、干旱和火灾对大气 CO2 年际变化的影响

预测地球系统模型 (ESM) 中碳循环的响应为全球气候变化的预测带来了重大的不确定性。量化大气二氧化碳 (CO2) 增长率年际变化的已知驱动因素的贡献对于改善这些 ESM 中陆地生态系统过程的代表性非常重要。最近的几项研究通过分析单一的、全球平均的 CO2 异常时间序列，将热带净生态系统交换 (NEE) 的温度依赖性确定为这种可变性的主要驱动因素。在这里，我们研究了如何使用不同纬度带中 CO2 的时间演变来区分温度胁迫、干旱胁迫和火灾排放对 CO2 变异性的贡献。我们在 1997-2011 年期间使用大气传输模型从这些机制中的每一个开发了大气 CO2 模式。NEE 对温度的响应、NEE 对干旱的响应和火灾排放都对每个纬度带的 CO2 变化有显着影响，这表明没有单一的机制是主要驱动因素。我们发现干旱和火灾对 CO2 变化的贡献总和超过了北半球和南半球对温度的直接 NEE 响应。额外的敏感性测试表明，这些贡献被 CO2 观测的时间和空间平滑所掩盖。考虑到火灾，热带 NEE 对温度应力的敏感性降低了 25% 至 2.9 ± 0.4 Pg C yr−1 K−1。