Remote sensing of recent changes in northern peatlands

泥炭地是富含有机物质的湿地，在潮湿的条件下，富含碳的碎屑得以堆积。尽管泥炭地覆盖了不到地球陆地表面的3%(Xu&Morris等人，2018)，但泥炭地被认为储存了全球土壤碳总量的六分之一到三分之一(Gorham，1991；Page等人，2011)。到目前为止，这些重要景观最集中的地区是北半球的中高纬度地区，特别是加拿大北部、斯堪的纳维亚半岛和西西伯利亚(Tarnocai等人，2009年)。近几十年来，这些地区经历了快速的气候变化，预计在未来一个世纪里，气候变暖的速度将比地球上几乎任何其他地方都快(Collins等人，2013年；Cohen等人，2014年)。人们担心，北部泥炭地的碳库可能容易受到气候快速变化的影响，随着这些系统开始变暖和变干，数千年积累的碳可能会回到大气中(Ise等人，2008年；Ciais等人，2013年)。尽管北部泥炭地对全球生物圈气候反馈具有如此重要的作用，但关于这些生态系统目前如何变化以应对最近的气候变化，仍存在巨大的知识差距。该项目将评估近几十年来北部泥炭地的水文、植被和地貌的变化，空间范围从大陆到个别田野地点。有可能使用一系列分类方法(例如，归一化差异湿度指数(NDWI)、基于分段的分类)来衡量泥炭地表面湿度的变化，例如，热岩溶湖泊的广泛发育；而植被的变化，在“绿化北极”中日益明显，可以使用归一化差异植被指数(NDVI)来评估。有机会利用重复光学DEM、测高或干涉测量来量化地表下降的程度，在较小的空间尺度上，可以通过分析重复的航空摄影，包括无人机测量，以及可能在阿拉斯加、苏格兰北部和其他盛产泥炭的地区获得的历史空中图像，来评估诸如泥炭地池塘的扩张和收缩等地貌变化。这一更详细的分析可能会采用监督小组在其他地方广泛采用的尖端技术，如运动结构和激光扫描(例如，Smith等人，2016年)，以及更传统的实地技术。网格化的仪器气候数据将用于解释这些分析产生的任何时间趋势，以及其他地貌和地形控制。对于学生来说，将有令人兴奋的机会访问包括北极在内的偏远野外地点，对卫星数据进行地面真相解释，并收集航空无人机摄影测量。学生还将被鼓励与在这一主题领域工作的广泛的国际专家小组建立合作。该项目将使用一系列尺度的遥感技术和仪器气候数据的强大组合，来研究北部泥炭地在一系列尺度上的最近变化。更具体地说，该项目将针对以下目标：1.利用卫星图像探测和量化北半球大片地区北部泥炭地植被和地表湿度的最新变化。2.利用无人机摄影马赛克和动态结构的时间序列，在较小的空间尺度(例如，区域尺度到特定的泥炭地地点)探测和量化近几十年来北部泥炭地的植被、地表湿度和地貌的变化。以确定最近的气候变化在推动北部泥炭地最近变化方面的作用。

Peatlands are organic-rich wetlands, where waterlogged conditions allow carbon-rich detritus to accumulate. Despite covering less than 3 % of the Earth's land surface (Xu & Morris et al., 2018), peatlands are thought to store between a sixth and a third of all global soil carbon (Gorham, 1991; Page et al., 2011). By far the largest concentrations of these important landscapes are in the mid- and high-latitudes of the northern hemisphere, particularly northern Canada, Scandinavia and Western Siberia (Tarnocai et al., 2009). These areas have experienced rapid climate change in recent decades, and are projected to continue to warm more rapidly than almost anywhere else on the planet over the coming century (Collins et al., 2013; Cohen et al., 2014). Concern exists that the northern peatland carbon store may be vulnerable to rapid changes in climate, and that thousands of years' worth of accumulated carbon may be returned to the atmosphere as these systems begin to warm and dry (Ise et al., 2008; Ciais et al., 2013). Despite this importance of northern peatlands to global biosphere-climate feedbacks, large knowledge gaps exist about how these ecosystems are currently changing in response to recent climate change. This project will assess changes in the hydrology, vegetation and geomorphology of northern peatlands during recent decades, on spatial scales ranging from the continental to individual field sites. There is potential for measuring changes in peatland surface wetness, for instance the widespread development of thermokarst lakes, using a range of classification approaches (e.g. normalised difference wetness index (NDWI), segmentation-based classification); while changes in vegetation, increasingly apparent in a "greening Arctic", may be assessed using the normalised difference vegetation index (NDVI). There is the opportunity to quantify surface lowering using either repeat optical DEMs, altimetry or interferometry, and at smaller spatial scales, geomorphological changes such as expansion and contraction of peatland pools could be assessed by analysing repeat aerial photography, including drone surveys, and possibly also historical air imagery that is available in Alaska, northern Scotland and other peat-rich regions. This more detailed analysis is likely to employ cutting-edge techniques such as Structure-from-Motion and laser scanning that have been extensively employed elsewhere by the supervisory team (e.g., Smith et al., 2016) as well as more traditional field-based techniques. Gridded instrumental climate data will be used to explain any temporal trends yielded by these analyses along with other geomorphological and topographical controls. There will be exciting opportunities for the student to visit remote field sites, including in the Arctic, to ground-truth interpretations of satellite data and to collect aerial drone photographic surveys. The student will also be encouraged to build collaborations with the broad group of international experts working in this topical area. This project will use a powerful combination of remote sensing techniques at a range of scales, and instrumental climate data, to study recent changes in northern peatlands at a range of scales. More specifically, the project will address the following objectives:1. To detect and quantify recent changes in the vegetation and surface wetness of northern peatlands across large areas of the northern hemisphere, using satellite imagery. 2. To detect and quantify changes in vegetation, surface wetness and geomorphology in northern peatlands, at smaller spatial scales (e.g., regional scales to specific peatland sites) during recent decades, using a time series of drone photography mosaics and Structure-from-motion.3. To establish the role of recent climate change in driving these recent changes in northern peatlands.