本文节选自美国国家研究理事会（NRC）发布的《时域地球——美国国家科学基金会地球科学十年愿景（2020-2030）》（A Vision for NSF Earth Sciences 2020-2030: Earth in Time）第6个科学优先问题。
过去二十年，最大的进步是通过将气候、构造和侵蚀过程联系在一起，共同来揭示它们是如何改造地球表面的形态并被动态的地表地形所影响的。这个过程带来了一些关键的科学问题，包括岩石力学性质、地球内部流变学和动力学、风暴等短期气候因素在地貌演化中的作用，以及地貌与大气圈、冰雪圈、海平面和生命的协同演化。地质历史到人类时间尺度上地形测量的新技术使得我们现在可以试图解决这些关键的科学问题，并评估其对地质灾害、资源和气候等挑战的影响(NRC, 2015; Davis et al., 2016; McGuire et al., 2017; Barnhart et al., 2018; Huntington and Klepeis, 2018; NASEM, 2018)。
许多关系到地球系统不同部分之间的新认识是由地形的形成及其变化反映出来的。这些联系不仅涉及地幔动力学、地表过程、冰盖变化和海平面之间的相互作用（e.g., Flament，2014; Heller and Liu，2016; Austermann et al., 2017; Whitehouse et al., 2019），还体现了地貌与生命的协同演化（e. g., Badgley et al., 2017; Fremier et al., 2018）。这些现象表现在近地表变形的地质历史与最新成像的深部岩石圈和地幔结构之间的联系（Wu et al., 2016），同时体现了岩石强度、岩石圈应力、生物地球化学循环、气候以及物理和化学侵蚀之间的反馈（例如Riebe et al., 2017）。以上这些只是我们在多种时间尺度上，由于测量和模拟地形变化的新技术的发展，所衍生出的在前沿领域中许多有研究前景的部分实例。
同时，我们比以往都更急需理解地形和地貌变化对地质灾害、自然资源的保存和破坏以及对生态环境的影响，并认识这些变化会如何影响到人类社会的生存和发展（Davis et al., 2016; NASEM, 2018）。例如，在气候变化的情况下，有必要量化地形对生态系统和水文变化的影响。反过来，我们也需要了解土地利用、生态系统和水循环是如何改造地形地貌的。地形和地貌变化也会影响地震、滑坡、洪水、泥石流、火山活动和海啸的发生，进而会危害到人类的生命和财产。对地形变化的观测和基于过程的认识可以让我们深入了解这些危害的发生过程。
技术上的进步为理解地形变化在地球深部、表面过程、气候和生物圈相互作用系统中的因果关系奠定了突破的基础。例如，激光雷达、摄影测量法、InSAR和基于无人机的数据采集技术的飞速发展彻底革新了量化现代地形变化的能力（eg., James and Robson，2012; Roering et al.，2013; Deng et al.，2019）；见图1）。在接下来的十年中，学界的目标是将全球大部分地区的现代地形量化到亚米级分辨率（Davis et al.，2016）。重复测量可以获取地震、天气事件、火山喷发和人类活动对地形的响应。三十四年来，使用Landsat 3影像创建的可视化图像已经改变了我们看待地球的方式，尤其是使我们对地表过程有了全新的认识。河流的蜿蜒、冰川的运动、海岸线的变化、滑坡事件的出现以及其他大规模地表过程被以前所未有的方式所观察到，从而让我们形成了新的认识并涌现出了新的科学问题（Schwenk et al., 2017; Dirscherl et al., 2020; Nienhuis et al., 2020）。
目前，在地质时间尺度上测量地形变化的技术也取得了新的进展，并且有望在不久的将来得到显著提高，这为其进一步的研究突破提供了前所未有的机遇。新的热年代学方法（见图2）可以重建岩石从深部到地表的剥露过程（Huntington and Klepeis，2018），从而估算全球千年到百万年尺度的地势及其侵蚀过程的时间和速率（e.g., Champagnac et al.，2014；Harrison et al.，2015）。精细的古高程方法，包括火山灰的氢同位素和碳酸盐团簇同位素古温度计，可提供地质历史时期流域和山脉规模的地形变化数据（例如Garzione等，2017），这些数据与气候模型以及地质观测的结合，可提供0.5 km尺度的古高程估算值（Cassel et al.，2018）。这些新方法使我们能够探索具有重要意义的深时地形变化，例如高原隆升、硅酸盐和碳酸盐风化、海水化学与大气环流及其组成之间的相互关系（e.g., Farnsworth et al.，2019），以及造山带地形演化与物种丰度之间的潜在联系（Antonelli et al.，2018）。
What are the causes and consequences of topographic change?
Great progress over the past two decades has been made in linking climate, tectonics, and erosion processes to understand how they shape and are dynamically influenced by Earth’s surface topography. This progress has brought into focus key scientific questions concerning the role of rock mechanical properties, short-term actors such as storms, and the rheology and dynamics of Earth’s interior in landscape evolution, and the co-evolution of landscapes with the atmosphere, cryosphere, sea level, and life. New technology for measuring topography over geologic to human time scales now makes it possible to address these key questions, and their implications for urgent societal challenges related to geologic hazards, resources, and climate change (NRC, 2015; Davis et al., 2016; McGuire et al., 2017; Barnhart et al., 2018; Huntington and Klepeis, 2018; NASEM, 2018).
Topography is sensitive to processes that operate above, on, and below the Earth’s surface at many scales. Mantle dynamics and plate boundary evolution drive the surface morphology of continents over time scales of millions to hundreds of millions of years and spatial scales of tens to thousands of kilometers as erosion actively removes mass. Actors like earthquakes, volcanic eruptions, storms, and glaciers affect regional to local (i.e., hillslope-scale) topography on time scales of minutes to millennia. Topography itself influences these processes and their interactions by affecting global and local climate, lithospheric stresses, and erosion processes. Topography is also the fundamental feature of the landscapes on which we live. Quantifying topographic change is therefore crucial to advance many areas of geoscience—from understanding Earth-system interactions over geologic time, to predicting landslides, ecosystem gradients, and the distribution of freshwater and soil resources in the coming decades.
Many newly recognized connections among different parts of the Earth system are expressed in topographic form and change. Such connections involve phenomena as diverse as the interactions of mantle dynamics, surface processes, ice-sheet changes, and sea level (e.g., Flament, 2014; Heller and Liu, 2016; Austermann et al., 2017; Whitehouse et al., 2019); and the co-evolution of landscapes and life (e.g., Badgley et al., 2017; Fremier et al., 2018). They are manifest in links between the geologic history of near-surface deformation and newly imaged deep lithosphere and mantle structures (Wu et al., 2016); and in feedbacks among rock strength, lithospheric stresses, biogeochemical cycles, climate, and physical and chemical erosion (e.g., Riebe et al., 2017). These are just a few examples of the many promising frontiers created by recent advances in our ability to measure and model topographic change on many time scales.
At the same time, the need to understand how topography and topographic change impact human society through geologic hazards and the creation or destruction of natural resources and habitats is more urgent than ever (Davis et al., 2016; NASEM, 2018). For instance, there is a critical need to quantify how topography will influence ecosystem and hydrologic change in a changing climate. In turn, we need to understand how land use, ecosystem, and water cycle changes alter topography. Topography and topographic change also affect the risk to lives and property posed by earthquakes, landslides, floods, mudflows, eruptions, and tsunamis. Observations and process-based understanding of topographic change have great potential to provide essential insight into the processes that underlie these hazards.
Technological advances set the stage for breakthroughs in understanding the causes and consequences of topographic change within the linked system encompassing the deep Earth, surface processes, climate, and the biosphere. For example, the rapid increase in lidar, photogrammetry, InSAR, and drone-based datasets has revolutionized our ability to quantify changes in modern topography (e.g., James and Robson, 2012; Roering et al., 2013; Deng et al., 2019; Fig. 1). In the next decade, a community goal is to reach submeter resolution in modern topography for much of the globe (Davis et al., 2016). Repeated measurements could capture responses to earthquakes, weather events, volcanic unrest, and human activity. Visualizations created from 34 years of Landsat imagery 3 are already transforming how we see the Earth, especially surface processes. River meandering, glacier dynamics, coastline changes, landslide events, and other large-scale surface processes can be observed in ways never before possible, providing new understanding and raising new questions (Schwenk et al., 2017; Dirscherl et al., 2020; Nienhuis et al., 2020).
Fig. 2-10 Topographic change associated with earthquakes in Ridgecrest, California, in July 2019 showed that unexpected slip occurred on many faults around the main rupture. The colors show the amount of ground displacement—land shifting vertically, horizontally, or both—in meters. Blue areas moved roughly northwest (horizontally) and up (vertically), while red and orange areas moved southeast and down. The map shows processed satellite-based SAR (synthetic aperture radar) data and a digital elevation model to show the contours of the land surface. SOURCE: NASA.
Our ability to measure topographic change on geologic time scales has also seen recent advances and is poised to improve dramatically in the near future, creating unprecedented opportunity for progress. New thermochronology approaches (see Fig. 2) provide opportunities to reconstruct the exhumation of rocks from the deep crust to the surface (Huntington and Klepeis, 2018), enabling estimates of the timing and rates of erosion and relief formation across the globe over thousand- to million-year time scales (e.g., Champagnac et al., 2014; Harrison et al., 2015). Refined paleoaltimetry methods, including use of hydrogen isotope archives in volcanic ash and carbonate clumped isotope thermometry, now provide data on topographic change at the scale of watersheds and mountain ranges over geologic time (e.g., Garzione et al., 2017), and are being integrated with climate models and geologic observations to provide 0.5-km-scale paleoelevation estimates (Cassel et al., 2018). Such approaches enable us to explore the significance of topographic change in deep time, for instance connections between plateau uplift, silicate and carbonate weathering, seawater chemistry, and atmospheric circulation and composition (e.g., Farnsworth et al., 2019), and potential links between the evolution of mountainous topography and species richness (Antonelli et al., 2018).
Fig.2-11 A broad range of geochronometers, thermochronometers, and other temperature-time-sensitive tools, highlighting advances in the last 15 years. Thermochronometer temperature ranges from Hodges (2014). Clumped isotope temperature ranges following Passey and Henkes (2012). SOURCE: Figure courtesy of Kip Hodges and Katharine Huntington.
Several emerging challenges must be addressed to realize the opportunities that arise from recent conceptual and technological advances. Improved estimates of the timing and rates of surface uplift, subsidence, and erosion/deposition are needed to directly link near-surface deformation and resulting topographic change with the rheology and dynamics of Earth’s interior. Illuminating rheology and dynamics through integration of seismological observations and dynamical models of mantle flow and lithospheric deformation will be critical for understanding the strength of such links—and the role of Earth’s interior dynamics in present-day sea level and future sea-level predictions. Observations and theory are also needed to quantitatively define the role of chemical and mechanical properties of rocks, and the role of geologically short-term actors such as storms, earthquakes, and rapid glacial retreat in surface processes and landscape evolution. Coupling detailed landscape models to large-scale mantle models (e.g., Braun et al., 2013) remains a challenge owing to their differing spatial and temporal scales and uncertainties in Earth-material properties (e.g., rheology). The role of topography and topographic change in land-atmosphere feedbacks (see Fig. 3), land-ice interactions, coastal and dryland processes, habitat creation, and ecosystem structure in a changing climate is just beginning to be explored, with important implications for Earth’s habitability over geologic time and in the next century.
Fig. 2-13 Photograph showing landscape change due to landslides triggered by earthquakes and storms in Taiwan. Bridge and road infrastructure is also visible, highlighting societal relevance of the landslides. SOURCE: Image courtesy of Kristen L. Cook.
Focused attention on these challenges over the next decade promises new insights into the interactions of Earth’s surface and deep interior, and into the co-evolution of Earth’s solid, fluid, and living components. Progress will require high-resolution repeat measurements of modern surface topography and vegetation cover and cyberinfrastructure to support open access to, and rapid processing and analysis of, imagery and point cloud data; long-term observations of modern weather, hydrology, and geochemistry of surface waters, soils, and sediments; and new records of past climate, elevation, relief, deformation, weathering, erosion, deposition, and ecosystem change through geologic time. Partnerships across GEO and with NASA, and new and refined geochronologic and stable isotopic approaches, will be central to developing these datasets and records. Geophysical methods (e.g., Aster et al., 2015) present exciting opportunities to quantify Earth’s structure and rock mechanical properties (e.g., erodibility, rheology) at depths ranging from meters to thousands of kilometers. Integration of such diverse datasets with high-resolution computer models of landscape evolution, mantle dynamics, and climate is key to enabling process-based understanding of the causes and consequences of topographic change.
National Academies of Sciences, Engineering, and Medicine 2020. A Vision for NSF Earth Sciences 2020-2030: Earth in Time. Washington, DC: The National Academies Press. https://doi.org/10.17226/25761.