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，在当前气候变化以及过去的自然气候变化(如冰期周期)中都发挥着关键作用。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，应用于改善未来的气候预测。在上一个冰河时代(大约2万年前)最冷的时候，大气中的二氧化碳浓度比工业化前的水平低约百万分之90。大多数研究人员认为，在冰河时代，这些缺失的碳被储存在深海中。大约18,000年前，大气中的二氧化碳浓度开始上升，标志着上一个冰河时代的结束。二氧化碳上升的原因还没有得到很好的理解，也没有引起激烈的争论。一种流行的假设是，南半球西风向南移动，由于南大洋海洋上升流的增加，导致海洋释放二氧化碳。然而，对于这一机制是否真的有效，气候模型存在分歧。最近对南极冰芯的研究提供了证据，证明在上一个冰河时期，这些西风的位置反复变化，以应对所谓的丹斯加德-奥施格事件。该项目将使用冰芯样本，以非常高的分辨率测量南半球西风带这些自然变化期间大气二氧化碳的过去变化。这将允许直接测试二氧化碳上升的上升流假说。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。这些最新的趋势为更好地理解二氧化碳与西风之间的关系提供了巨大的推动力。这项项目的工作旨在产生一个基准二氧化碳记录，将被气候研究人员广泛使用。该项目通过培训一名博士后研究员来促进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每年将访问大约20个中学教室，谈论极地和气候研究。该项目将在十年时间分辨率上测量关键气候事件(海因里希事件2-5和丹斯加德-奥施格事件3-8和12)期间WAIS冰芯中大气二氧化碳的演变，其中冰芯标志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，在这些时间里有微小的变化，但它们缺乏解决这个问题的定论。该项目包括对二氧化碳分析装置的改进，以提高样品吞吐量和分析精度。建议的样本数量为720个(240个独特深度，一式三份)，时间间隔为27,000至40,000年，以补充现有的低分辨率二氧化碳记录。冰芯分析与建模和数据分析相结合，以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，利用同位素启用的一般环流模型和再分析数据的现有产出。特别是，这项拟议的工作调查了南极洲周围大西洋地区发生与海因里希事件有关的大型风力驱动的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，不需要南极实地工作。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.