Feedbacks QUEST: Quantifying biogeochemical feedbacks on climate change

气候系统非常复杂。它不仅包括物理大气和海洋，还包括栖息在陆地表面、海洋和沉积物上的生物。这些影响气候；例如，它们是水蒸气、二氧化碳（CO2）、氧化亚氮（N2O）和甲烷（CH4）等温室气体的源和汇。它们还会产生大气颗粒/云滴，以及由挥发性有机碳、氮和硫化合物形成的气溶胶，它们可能会直接散射光线或改变云的性质。气候的变化可能反过来反馈，影响陆地或海洋上生命的数量和类型。这些都是生物地球化学对气候的反馈，它们必须被添加到（通常更容易理解的）物理反馈（冰，云等）中，以预测整个气候系统的性质。更好地确定和量化最重要的生物地球化学对气候变化的反馈的尝试是早就应该进行的。20多年前就提出了量化气候反馈（包括一些植被反馈）的框架，在开创性的工作中，拉什夫首次尝试综合和量化相关的生物地球化学（包括生物地球物理）反馈。随后，人们对个体反馈或群体反馈进行了大量研究，但对所有相关生物地球化学反馈进行概述和评估的尝试很少。在现有的文献中，处理倾向于定性而非定量，并且大多数关注的是二氧化碳循环中的反馈，而对其他生物地球化学循环的关注较少。从现有的评估中可以清楚地看出，纯粹的物理反馈（特别是那些涉及水蒸气、冰和雪的反馈）主要是积极的，并使地球系统成为气候变化短期驱动因素的一个相当强大的放大器。在这样一个系统中，小的附加正反馈可以产生很大的影响。据Lashof估计，生物地球化学反馈提供了一个重要的额外正反馈，但由于不确定性，总体气候敏感性可能在很大的范围内。作为第一步，这项研究将更新Lashof的半定量分析，从而更好地理解当今最重要的生物地球化学反馈是什么。在QUEST项目中还没有研究这些机制的地方，我们将使用并修改地球系统模型的QUEST系列，以便在可能的情况下对这些反馈进行更多的定量估计。（QUEST系列包括GENIE“中等复杂性”模型框架，用于研究百年到百万年的时间尺度，以及QUEST地球系统模型，现在由许多更高分辨率的模块构建，可用于长达几个世纪的运行长度。）我们期望集中关注的反馈（因为它们目前在QUEST的其他地方没有得到强调）将是：在GENIE中，涉及温室气体一氧化二氮和甲烷的长期反馈，以及对其汇的大气化学反馈的参数化，在QESM中，对陆地和海洋气溶胶前体源的影响的估计。我们还将利用这些模式对整个时间尺度范围内大气二氧化碳的不同反馈强度进行综合评估。最终的结果将是对现有知识的综合，包括一些研究较少的生物地球化学反馈，以及建模工具，以便在QUEST模型中进行进一步的探索。

The climate system is immensely complex. It consists not only of the physical atmosphere and ocean, but of the living things that inhabit the land surface, seas and sediments. These influence the climate; for example they are sources and sinks of greenhouse gases such as water vapour, carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). They also produce atmospheric particles / cloud droplets, and aerosols formed from volatile organic carbon, nitrogen and sulphur compounds, that may scatter light directly or alter the properties of clouds. Changes in the climate may in turn feed back, influencing the amount and type of life on the land or in the ocean. These are the biogeochemical feedbacks on climate, and they must be added to the (generally rather better understood) physical feedbacks (ice, clouds etc) to predict the properties of the full the climate system An attempt to better determine and quantify the most important biogeochemical feedbacks on climate change is long overdue. A framework for quantifying climate feedbacks (including some vegetation feedbacks) was outlined over 20 years ago, and in pioneering work Lashof made a first attempt to synthesise and quantify pertinent biogeochemical (including biogeophysical) feedbacks. Subsequently, there has been much work on individual feedbacks or groups of feedbacks, but there have been few attempts to take an overview and assessment of all pertinent biogeochemical feedbacks. In existing reviews, the treatment tends to be qualitative rather than quantitative and most attention is devoted to feedbacks in the carbon dioxide cycle with other biogeochemical cycles getting less attention. What is clear from existing assessments is that purely physical feedbacks (notably those involving water vapour, ice and snow) are predominantly positive and make the Earth system a fairly strong amplifier of short-term drivers of climate change. In such a system, small additional positive feedbacks can have large effects. Biogeochemical feedbacks were estimated by Lashof to provide a significant additional positive feedback, but the uncertainty is such that the overall climate sensitivity could lie within a large range. This research will, as a first step, update the semi-quantitative analysis of Lashof, resulting in a better understanding of what the most important biogeochemical feedbacks are today. Where these mechanisms are not already being studied in QUEST projects, we will then use and modify the QUEST family of Earth system models to make more quantitative estimates where possible of these feedbacks. (The QUEST family consists of the GENIE 'intermediate complexity' model framework, useful for studies on centennial to million year time scales, and the Quest Earth System model, now being built from a number of higher resolution modules and useful for run lengths of up to a few centuries.) Feedbacks that we expect to concentrate on (because they are not presently being emphasised elsewhere within QUEST) will be: in GENIE, long-term feedbacks involving the greenhouse gases nitrous oxide and methane, and a parameterization of atmospheric chemistry feedbacks on their sinks, in QESM, estimates of effect of both terrestrial and marine sources of aerosol precursors. We will also use the models to make a comprehensive assessment of the strength of different feedbacks on atmospheric carbon dioxide across the full range of timescales. The final output will be a synthesis of current knowledge, including some of the less well studied biogeochemical feedbacks, and modelling tools to allow their further exploration in the QUEST models.