Abrupt CO2 Change and the Southern Hemisphere Westerlies: Testing the Upwelling Hypothesis

二氧化碳（CO2）是主要的大气温室气体，在当前的气候变化以及过去的自然气候变化（如冰期周期）中都起着关键作用。了解二氧化碳变化的自然原因是气候研究的一个重要目标，可用于改善未来的气候预测。在最后一个冰河时代（大约2万年前）最冷的时候，大气中二氧化碳的浓度大约是百万分之90 (ppm)，低于工业化前的水平。大多数研究人员认为，在冰河时期，这些缺失的碳储存在深海中。大约18000年前，大气中的二氧化碳浓度开始上升，标志着最后一个冰河时代的结束。二氧化碳上升的原因还没有被很好地理解，并引起了激烈的争论。一种流行的假设是，南半球的西风向南移动，由于南大洋的海洋上升流增加，导致海洋释放出二氧化碳。然而，气候模型对这种机制是否真的有效存在分歧。最近对南极冰芯的研究提供了证据，证明在最后一个冰河时期，这些西风的位置在所谓的Dansgaard-Oeschger事件中反复变化。这个项目将使用冰芯样本，以非常高的分辨率测量过去在南半球西风带自然变化期间大气中二氧化碳的变化。这将允许对二氧化碳上升的上升流假说进行直接测试。目前，由于平流层臭氧消耗和全球变暖，南半球西风带再次向南移动。这可能会减少世界海洋的碳吸收，加剧人为排放造成的变暖。这些最近的趋势为更好地理解二氧化碳和西风之间的关系提供了巨大的推动力。这个项目的目的是产生一个基准的二氧化碳记录，将被气候研究人员广泛使用。该项目通过培养一名博士后研究员，促进STEM（科学、技术、工程和数学）劳动力的发展。一个正在进行的科学推广项目将每年访问大约20个中学教室，讨论极地和气候研究。该项目将在关键气候事件（Heinrich事件2-5和Dansgaard-Oeschger事件3-8和12）期间，以年代际时间分辨率测量WAIS分水岭冰芯中大气二氧化碳的演变，在这些事件中，冰芯标记表明南半球西风带发生了变化。现有的冰芯二氧化碳记录表明，在这段时间里存在着微小的变化，但它们缺乏对这个问题的定论的分辨率。该项目包括改进二氧化碳分析装置，以提高样品吞吐量和分析精度。在距今27000年至40000年的时间间隔内，建议的样品数量为720个（240个不同的深度，一式三份），以补充现有的低分辨率CO2记录。冰芯分析与建模和数据分析相结合，利用基于同位素的环流模型和再分析数据的现有输出，更好地了解南半球西风变化的南极水同位素证据及其与二氧化碳变化的联系。特别是，拟议的工作调查了南极洲周围大西洋地区与海因里希事件相关的大型风力上升流事件的可能性-这是一种以前未被认识到的气候变化模式。该项目使用现有的冰样本，不需要在南极进行实地考察。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Carbon dioxide (CO2) is the primary atmospheric greenhouse gas that plays a key role both in current climate change as well as in past natural climate variations such as the ice-age cycle. Understanding the natural causes of CO2 change is an important goal of climate research, with applications in improving climate projections for the future. At the coldest point of the last ice age (around 20,000 years ago) the atmospheric concentration of CO2 was around 90 parts per million (ppm) below the pre-industrial level. Most researchers think that during the ice age this missing carbon was stored in the deep ocean. Around 18,000 years ago, the CO2 concentration in the atmosphere started rising, marking the end of the last ice age. The cause for the CO2 rise is not well understood and heavily debated. A popular hypothesis is that the Southern Hemisphere westerly winds shifted southwards, causing CO2 release from the ocean due to increased ocean upwelling in the Southern Ocean. However, climate models disagree on whether this mechanism actually works. Recent research on Antarctic ice cores has provided evidence for repeated shifts in the position of these westerly winds during the last ice age in response to so-called Dansgaard-Oeschger events. This project will use ice-core samples to measure past changes in atmospheric CO2 at very high resolution during these natural shifts in the Southern Hemisphere westerlies. This will allow a direct test of the upwelling hypothesis for the rise in CO2. Currently, the Southern Hemisphere westerlies are again shifting southwards due to stratospheric ozone depletion and global warming. This may reduce carbon uptake by the world's oceans, exacerbating warming from anthropogenic emissions. These recent trends provide great impetus to better understand the relationship between CO2 and the westerly winds. This project work aims to produce a benchmark CO2 record that will be widely used by climate researchers. The project contributes to development of the STEM (Science, Technology, Engineering and Mathematics) workforce by training a postdoctoral researcher. An ongoing science outreach project will visit around 20 middle school classrooms annually to talk about polar and climate research. The project will measure in decadal temporal resolution the evolution of atmospheric CO2 in the WAIS Divide ice core during key climatic events (Heinrich events 2-5 and Dansgaard-Oeschger events 3-8 and 12) where ice-core markers suggest shifts occurred in the Southern Hemisphere westerlies. Existing ice-core CO2 records imply minor variations during these times, but they lack resolution to be conclusive on this issue. The project includes improvements to the CO2 analytical setup that will enhance sample throughput and analytical precision. The proposed number of samples is 720 (240 unique depths in triplicate) in the interval 27,000 to 40,000 years before present to complement the existing low-resolution CO2 record. The ice-core analyses are integrated with modeling and data-analysis to better understand the Antarctic water isotopic evidence for shifts in the Southern Hemisphere westerly winds and their link to CO2 variations, using existing output from isotope-enabled general circulation models and reanalysis data. In particular, the proposed work investigates the possibility of large wind-driven upwelling events in the Atlantic sector around Antarctica associated with Heinrich events--a previously unrecognized mode of climate variability. This project uses existing ice samples and does not require Antarctic field work.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.