Peatlands and the global carbon cycle during the past millennium: a global assessment using observations and models

未来的气候变化是人类面临的最具挑战性的问题之一，一项巨大的研究努力旨在试图根据对温室气体排放的基本假设，构建对21世纪气候的现实预测。气候模型现在包括了地球系统的许多组成部分，这些组成部分在一系列时间尺度上影响气候。了解和量化地球系统过程对于预测未来气候变化至关重要，因为许多过程对气候变化提供“反馈”，要么加强温室气体浓度和温度的上升趋势(正反馈)，要么有时抑制它们(负反馈)。一个关键的反馈循环是由全球碳循环形成的，其中一部分是陆地碳循环。随着二氧化碳浓度和温度的上升，植物的碳固存增加，但同时，温度的上升会导致土壤中死亡植物材料的腐烂加剧。碳循环模型表明，这两种影响之间的平衡将导致强有力的正反馈，但这一发现存在非常大的不确定性，这一过程是未来气候变化预测中最大的未知数之一。为了减少这些不确定性，需要根据过去千年的记录等数据对模型进行验证。此外，极其重要的是要确保这些模型能够真实地反映全球碳循环，并包含其所有主要组成部分。目前的模型排除了对泥炭地对气候变化的反应的任何考虑，尽管这些生态系统的碳含量几乎与全球大气一样多，并可能对气候变化敏感。一方面，气温升高可能会增加泥炭的呼吸和腐烂，另一方面，在高纬度泥炭地，即使是非常微小的生产力增长也可能弥补甚至超过这一点。更复杂的是，泥炭地排放了相当大量的甲烷，这是另一种强大的温室气体。我们提议的项目旨在通过将气候数据和气候模型输出与模拟全球范围内泥炭地分布和生长的模型联系起来，评估过去1000年泥炭地对全球碳循环的贡献。这些模型还将估计泥炭地甲烷排放的变化。特别是，我们将检验这样的假设，即气候变暖会导致较低的碳积累率，这意味着在全球范围内，泥炭地未来吸收的碳将比现在更少。我们还将测试未来气候变化是否会对泥炭地甲烷排放产生积极或消极的反馈。为了确定我们的模型能多好地模拟泥炭地-气候联系，我们将测试上一个千年的模型输出与泥炭生长速度和水文变化(与甲烷排放有关)的化石数据。为此，我们将汇集一个大型数据库，其中包括已公布的信息以及与世界各地其他研究组织的合作伙伴合作获得的新数据，这些组织参与收集信息和样本，一旦我们进行一些额外的测年和分析，就可以利用这些信息和样本。一旦根据上一个千年的数据对该模型进行了评估，我们将对未来气候变化可能导致的全球碳循环的未来变化进行预测。这将为决定是否需要将泥炭地动力学纳入下一代气候模型提供强有力的基础。归根结底，我们预计这将减少未来气候变化预测的不确定性。

Future climate change is one of the most challenging issues facing humankind and an enormous research effort is directed at attempting to construct realistic projections of 21st century climate based on underlying assumptions about greenhouse gas emissions. Climate models now include many of the components of the earth system that influence climate over a range of timescales. Understanding and quantifying earth system processes is vital to projections of future climate change because many processes provide 'feedbacks' to climate change, either reinforcing upward trends in greenhouse gas concentrations and temperature (positive feedbacks) or sometimes damping them (negative feedbacks). One key feedback loop is formed by the global carbon cycle, part of which is the terrestrial carbon cycle. As carbon dioxide concentrations and temperatures rise, carbon sequestration by plants increases but at the same time, increasing temperatures lead to increased decay of dead plant material in soils. Carbon cycle models suggest that the balance between these two effects will lead to a strong positive feedback, but there is a very large uncertainty associated with this finding and this process represents one of the biggest unknowns in future climate change projections. In order to reduce these uncertainties, models need to be validated against data such as records for the past millennium. Furthermore, it is extremely important to make sure that the models are providing a realistic representation of the global carbon cycle and include all its major component parts. Current models exclude any consideration of the reaction of peatlands to climate change, even though these ecosystems contain almost as much carbon as the global atmosphere and are potentially sensitive to climate variability. On the one hand, increased warmth may increase respiration and decay of peat and on the other hand, even quite small increases in productivity may compensate for this or even exceed it in high latitude peatlands. A further complication is that peatlands emit quite large quantities of methane, another powerful greenhouse gas. Our proposed project aims to assess the contribution of peatlands to the global carbon cycle over the past 1000 years by linking together climate data and climate model output with models that simulate the distribution and growth of peatlands on a global scale. The models will also estimate changes in methane emissions from peatlands. In particular, we will test the hypotheses that warmth leads to lower rates of carbon accumulation and that this means that globally, peatlands will sequester less carbon in future than they do now. We will also test whether future climate changes lead to a positive or negative feedback from peatland methane emissions. To determine how well our models can simulate the peatland-climate links, we will test the model output for the last millennium against fossil data of peat growth rates and hydrological changes (related to methane emissions). To do this, we will assemble a large database of published information but also new data acquired in collaboration with partners from other research organisations around the world who are involved in collecting information and samples that we can make use of once we undertake some additional dating and analyses. Once the model has been evaluated against the last millennium data, we will make projections of the future changes in the global carbon cycle that may occur as a result of future climate change. This will provide a strong basis for making a decision on the need to incorporate peatland dynamics into the next generation of climate models. Ultimately we expect this to reduce uncertainty in future climate change predictions.