A variety of proxies have been developed to reconstruct paleo‐CO₂ from fossil leaves. These proxies rely on some combination of stomatal morphology, leaf δ¹³C, and leaf gas exchange. A common conceptual framework for evaluating these proxies is lacking, which has hampered efforts for inter‐comparison. Here we develop such a framework, based on the underlying physics and biochemistry. From this conceptual framework, we find that the more extensively parameterised proxies, such as the optimisation model, are likely to be the most robust. The simpler proxies, such as the stomatal ratio model, tend to under‐predict CO₂, especially in warm (>15^°C) and moist (>50% humidity) environments. This identification of a structural under‐prediction may help to explain the common observation that the simpler proxies often produce estimates of paleo‐CO₂ that are lower than those from the more complex proxies and other, non‐leaf‐based CO₂ proxies. The use of extensively parameterised models is not always possible, depending on the preservation state of the fossils and the state of knowledge about the fossil's nearest living relative. With this caveat in mind, our analysis highlights the value of using the most complex leaf‐based model as possible.

中文翻译：

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叶基古二氧化碳代理的定量评论：其可靠性和适用性的后果

已经开发了多种代理来从化石叶中重建古CO ₂。这些代理依靠气孔形态的一些组合，叶δ ¹³ C，和叶的气体交换。缺乏用于评估这些代理的通用概念框架，这阻碍了进行比对的努力。在这里，我们基于基础的物理和生物化学开发了这样的框架。从这个概念框架中，我们发现参数更广泛的代理（例如优化模型）可能是最可靠的。诸如气孔比率模型之类的简单代理往往会低估CO ₂，特别是在温暖（> 15 ^° C）和潮湿（> 50％湿度）环境。这种对结构性预测不足的识别可能有助于解释以下常见的观察结果：较简单的代理通常产生的古CO ₂估计值低于较复杂的代理和其他非叶CO ₂代理的估计值。取决于化石的保存状态和有关化石最近的近亲的知识状态，不一定总是可以使用广泛参数化的模型。考虑到这一警告，我们的分析强调了使用最复杂的基于叶子的模型的价值。