Terrestrial Carbon Cycle Dynamics in CMIP5 Last Glacial Maximum and mid-Holocene climate simulations

1. 最新的最先进的全球气候模型目前正在运行下一代未来气候变化预测，这些预测将被纳入政府间气候变化专门委员会（IPCC）评估报告的第五版。这些模型比以前更复杂，并且大多数首次包括广泛的生物地球化学过程（例如相互作用植被，碳循环动力学，大气尘埃）。这些模式模拟为促进对地球系统反馈的科学理解和对人为温室气体排放的未来影响提供了更可靠的预测提供了很大的机会。为了发展这种认识，模式间比较对于阐明导致模式差异的过程以及它们如何影响气候变化的预测至关重要。在模拟当前气候时显示出广泛一致性的气候模式在对未来变化的估计上可能存在很大差异。模拟过去的气候，如末次冰期极大期（LGM； 21000年前），提供了机会来测试这些相同的模型重建与今天非常不同的气候的能力，从而提供了更大的潜力来评估重要的地球系统反馈过程，以及它们如何对未来的预测做出贡献，而不是单独使用现代验证。特别是通过碳循环等生物地球化学过程产生的气候反馈效应，已被CMIP5 （Coupled Model Intercomparison Project）和IPCC AR4强调为一个关键的科学问题。为了理解碳循环的反馈，过去的气候对于研究不受人类排放干扰的地球系统至关重要。2. 目的和目标本研究的主要目的是利用过去两个特定时期的模拟来测试用于未来气候预测的最先进的气候模式的能力，以重现非常不同的气候。第一个时期是全新世中期（6000年前），当时北半球的夏季日照比今天要大，导致季风系统的加剧。水文循环和陆地生物圈之间的反馈导致撒哈拉沙漠出现了大规模的植被，被称为“绿色撒哈拉”。关于全新世中期撒哈拉植被的范围有很好的证据，它为气候模式模拟的测试提供了一个强有力的特征。第二个时期是LGM，当时大冰原覆盖了北半球的大部分地区，大气中的二氧化碳含量较低（180 ppm），全球气温降低了几度。这一时期的一个主要问题是陆地碳库和气候-碳反馈的变化是如何导致大气中二氧化碳水平较低的。我们的目标是量化全球陆地碳储量的变化，通过量化和评估这些自然过程，这将有助于在未来的气候变化预测中限制气候-碳循环反馈。潜在的应用和效益本研究将直接解决评估现实的当前气候模型，并解决自然碳-气候反馈如何影响气候变化的问题。因此，它们将提高我们对未来气候预测的重要因素的理解。我们的分析结果将用于IPCC的第五次评估报告。最终，它们将为全球和英国的可允许碳排放政策提供信息。此外，来自广泛研究领域的科学家可能会对这个项目的结果感兴趣。这些学科包括古气候和古环境研究、地球系统建模、古生物学、生物地球化学和未来气候变化研究。

1. ContextNew state-of-the-art global climate models are currently running the next generation of future climate change predictions, which will be incorporated into the fifth release of the Intergovernmental Panel on Climate Change (IPCC) assessment report. These models are more sophisticated than before and most include a wide range of biogeochemical processes for the first time (e.g. interactive vegetation, carbon cycle dynamics, atmospheric dust). These model simulations provide great opportunities to advance scientific understanding of Earth system feedbacks and provide more robust projections for future impacts of anthropogenic greenhouse gas emissions. In order to develop this understanding inter-model comparison is essential to elucidate the processes that cause model discrepancies and how they influence predictions of climate change. Climate models which show broad agreement when modelling present day climate can differ considerably in their estimates of future changes. Modelling past climates such as the Last Glacial Maximum (LGM; 21,000 years ago) provide the opportunity to test the ability of those same models to recreate climates very different to today, and thereby offer greater potential to evaluate important Earth system feedback processes and how they contribute to future projections than by using modern-day validation alone. In particular, the effect of climate feedbacks through biogeochemical processes, such as the carbon cycle, has been highlighted by CMIP5 (Coupled Model Intercomparison Project) and the IPCC AR4 as a key scientific issue. For understanding feedbacks through the carbon cycle, past climates again are vital to enable to study the Earth system in a state that is unperturbed by human emissions. 2. Aims and objectivesThe main aim of this study is to use the simulations of two particular past time periods to test the ability of state-of-the-art climate models that being used for future climate projections to reproduce very different climates. The first time period is the mid-Holocene (6,000 years ago), when summer insolation in the northern hemisphere was larger than today, causing intensification of monsoon systems. Feedbacks between the hydrological cycle and the terrestrial biosphere resulted in large-scale vegetation in the Sahara, known as the 'Green Sahara'. There is good evidence about the extent of Sahara vegetation during the mid-Holocene, and it provides a strong feature to test in the climate model simulations. The second time period is the LGM, when large ice-sheets covered much of the northern hemisphere, carbon dioxide levels in the atmosphere were lower (180 ppm) and global temperatures were reduced by several degrees. One of the major questions from this time period is how changes in terrestrial carbon reservoirs and climate-carbon feedbacks contributed to the low atmospheric carbon dioxide levels. We aim to quantify changes in global terrestrial carbon storage and by quantifying and evaluating these natural processes this will help to constrain climate-carbon cycle feedbacks in future climate change projections.3. Potential applications and benefitsThis study will directly address evaluate realistic current climate models are and address the question of how natural carbon-climate feedbacks influence changing climates. They will thereby improve our understanding of the factors which are important for future projections of climate. The results of our analyses will contribute to the fifth assessment report of the IPCC. Ultimately, they will inform global and UK policy on allowable carbon emissions. In addition, scientists from a wide spectrum of research fields are likely to be interested in the results of this project. These disciplines include palaeoclimate and palaeoenvironmental research, Earth system modelling, palaeontology, biogeochemistry, and future climate change research.