Evaluating the Impacts of Global Warming on Geomorphological Systems

Abstract

Ongoing climate change (global warming) is a major forcing factor on Earth’s ecological services including agricultural production, biodiversity, and carbon cycle. Climatic regime and climate change is also a major driver of the dynamics of Earth’s geomorphological systems, including its glaciers, rivers, mountains and coasts, especially over longer (103–105 year) time scales that correspond to climate forcing by orbital cycles. Many studies have considered how geomorphological systems have responded to climate forcing over long time scales, where system responses are approximately in phase with forcing (Lal 2004; Lowe et al. 2008). Over shorter time scales, however, geomorphological systems do not respond in phase with climate forcing, are affected by human (anthropogenic) activity, and yield nonlinear responses that cannot be fully predicted based on their previous behaviour (Perry 2002; Murray et al. 2009). The response of geomorphological systems to climate forcing can be examined by monitoring changes to their morphology and geomorphological processes during the recent past (last <150 years) for which instrumental climate data are available. This comparison allows for a more complete understanding of the relationship between climate forcing and geomorphological response. Global warming has given a new impetus to studies of climate forcing of geomorphological systems. This is because, despite the ability of global climate models (GCMs) to predict future temperature, precipitation and sea level, they do not consider likely responses of geomorphological (land surface) systems despite there being important feedbacks between surface processes and climate. These feedbacks include variations in snow/ice cover that result in changes to albedo and energy balance; changes in terrestrial ecosystems and land surface erosion that result in changes in carbon storage; and variations in continental weathering that lead to variations in cation flux to the ocean that influences its capacity for carbon dioxide (CO2) downdraw. Despite feedbacks being a significant source of uncertainty in GCMs (Boer and Yu 2003), the land surface conditions and geomorphological processes that give rise to these feedbacks are generally poorly known. GCMs do not consider the nature of the land surface, except at very broad scales, and cannot incorporate these feedbacks. Studies of geomorphological processes are usually based on small geographic areas, over short time scales, and are not closely linked to inputs or outputs from GCMs. Identifying the relationships between climate forcing and geomorphological response can yield a better understanding of the sensitivity of geomorphological systems to such forcing. Ongoing global warming in combination with landuse change, urbanisation and geoengineering, is now making geomorphological systems work at rates and within structural limitations unprecedented throughout human history. It is therefore critical to know how, where and at what rates these geomorphological systems will respond to climate and anthropogenic forcing. In this article, we describe how and why ongoing global warming is causing changes to the workings of geomorphological systems. We argue that the concept of geomorphological sensitivity is a useful means by which to understand why these changes occur, and provide a context for monitoring and modelling these changes into the future. This article first explains climate sensitivity from which we derive the concept of geomorphological sensitivity. We then apply this concept to consider how sediment yield changes as a result of climate forcing. A useful analogue for geomorphological system behaviour under present global warming is that of paraglacial processes under conditions of ice retreat. We argue that understanding the response of geomorphological systems to global warming has major implications for climate policy and adaption strategies.