Collaborative Research: P2C2--Late Holocene Climate: Natural or Anthropogenic?

这个合作项目的主要目的是解决一个关于全新世气候的基本问题，这个问题是：人类何时成为全球气候系统的“驾驶员”？它是与传统上认为的工业时代的到来同时发生的，还是在几千年前随着农业的出现而发生的？早期农业活动释放的碳排放量是否大到足以解释全新世晚期大气温室气体（GHG）浓度的空前上升，而这些气候驱动因素在最近之前的间冰期有所下降，科学界对此仍存在分歧。这项研究将通过识别过去6000年人类对全球气候影响的“指纹”，来验证“早期人类假说”，该假说认为，人类活动导致的全球气候变化是由全新世中期的农业引发的。该项目将利用最先进的地球系统模型来区分全新世晚期的实际气候演变和假设的“自然”轨迹，后者将在没有人类开垦土地和温室气体过度排放的情况下遵循。为了实现这一目标，将使用社区地球系统模型（CESM）进行一对6000年的瞬态模拟。在一个实验中，CESM将受到轨道参数、温室气体浓度和区域土地清除率的历史变化的驱动，而在对应的实验中，后两种强迫将根据假设的二氧化碳和甲烷的减少以及与“自然”气候演变相容的相关全球植被模式而有所不同。更广泛的影响涉及对人类对全球气候影响的科学认识可能发生独特和潜在的变革性转变。

This collaborative project generally aims to address a fundamental and yet unresolved question about Holocene climate which is: When did humans assume the "driver's seat" of the global climate system? Did it coincide with the advent of the Industrial Era, as traditionally believed, or did it instead occur many millennia ago with the emergence of agriculture? The scientific community remains divided on whether carbon emissions released by early farming activities were large enough to explain the unprecedented rise in atmospheric greenhouse gas (GHG) concentrations during the late Holocene compared with declines in these climatic drivers during recent previous inter-glacial periods. This research will test the "Early Anthropogenic Hypothesis," which posits that anthropogenic global climate change was initiated by agriculture in the middle Holocene, by identifying the "fingerprint" of human influence on global climate during the past 6,000 years.The project will capitalize on state-of-the-art earth system models to distinguish between the actual climatic evolution of the late Holocene and the hypothesized "natural" trajectory that would have been followed in the absence of human land clearance and greenhouse-gas overprints. To achieve this goal, a pair of 6,000-year long transient simulations will be run with the Community Earth System Model (CESM). In one experiment CESM will be driven by historical variations in orbital parameters, GHG concentrations, and regional land clearance, whereas the latter two forcings will differ in the counterpart experiment according to the hypothesized declines in carbon dioxide and methane and the associated global vegetation patterns compatible with "natural" climate evolution.The Broader Impacts involve the potential for a unique and potentially transformative shift in the scientific understanding of human influence on global climate.