Liam Reinhardt

Landscape reorganization under changing climatic forcing: Results from an experimental landscape

Understanding how landscapes respond to climate dynamics in terms of macroscale (average topographic features) and microscale (landform reorganization) is of interest both for deciphering past climates from todays landscapes and for predicting future landscapes in view of recent climatic trends. Although several studies have addressed macro-scale response, only a few have focused on quantifying smaller-scale basin reorganization. To that goal, a series of controlled laboratory experiments were conducted where a self-organized complete drainage network emerged under constant precipitation and uplift dynamics. Once steady state was achieved, the landscape was subjected to a fivefold increase in precipitation (transient state). Throughout the evolution, high-resolution spatiotemporal topographic data in the form of digital elevation models were collected. The steady state landscape was shown to possess three distinct geomorphic regimes (unchannelized hillslopes, debris-dominated channels, and fluvially dominated channels). During transient state, landscape reorganization was observed to be driven by hillslopes via accelerated erosion, ridge lowering, channel widening, and reduction of basin relief as opposed to channel base-level reduction. Quantitative metrics on which these conclusions were based included slope-area curve, correlation analysis of spatial and temporal elevation increments, and wavelet spectral analysis of the evolving landscapes. Our results highlight that landscape reorganization in response to increased precipitation seems to follow “an arrow of scale”: major elevation change initiates at the hillslope scale driving erosional regime change at intermediate scales and further cascading to geomorphic changes at the channel scale as time evolves.

Landslides and synoptic weather trends in the European Alps

Landslides present a substantial geomorphological hazard in Alpine regions and there are expectations that climate change will alter their frequency and magnitude in the future. Understanding the spatial distribution and timing of landslides in the context of past change is therefore necessary if we are to assess their future behaviour. Using a regional landslide inventory for the European Alps we analyse the influence of weather types, specifically the COST733 database, on landslides. Monte Carlo permutation tests are used to assess which weather types are most likely associated with landslides. Weather types with high precipitation are consistent with more landslides, although there are also seasonal differences. Over the duration of the COST733 catalogue there has been a significant decrease in the number of days with weather types associated with low frequencies of landslides. During the spring and autumn months, the trend in observed landslide frequency and weather types are well matched. However while there is potential for weather typing to be used as a proxy for future landslide frequency, other external factors must be carefully considered.

The emergence of topographic steady state in a perpetually dynamic self-organized critical landscape

We conducted a series of four physical modeling experiments of mountain growth at differing rates of uplift and three distinct climates ranging from relatively wet to relatively dry. The spatial and temporal pattern of landscape behavior is characterized by ∼f−1 scaling in sediment discharge and power law scaling in the magnitude and frequency of ridge movement in all four experiments. We find that internally generated self-organized critical (SOC) processes generate dynamically stable catchment geometries after ∼1 relief depths of erosion: these regularly spaced catchments have an average outlet-spacing ratio of 2.16, well within the range of values reported in field studies. Once formed, large catchment bounding ridges oscillate about a critically balanced mean location, with occasional large-scale changes in catchment size. Ridge movement appears to be driven by the competition for discharge as landslides push ridges back and forth. These dynamics lead to the emergence of a complex twofold scaling in catchment dynamics that is fully established by 1.8 relief depths of erosion; at this stage, a clear threshold has emerged separating two distinct scaling regimes, where large ridge mobility is insensitive to relief and small ridge mobility is relief dependent. Overall, we demonstrate that the development of dynamically stable large-scale landforms is related to the emergence of a complex-system hierarchy in topographic dynamics. Once formed, these landscapes do not evolve; statistical properties such as average topography and discharge become stationary while topography remains highly dynamic at smaller length scales.

Past changes in the North Atlantic storm track driven by insolation and sea-ice forcing

Changes in the location of Northern Hemisphere storm tracks may cause significant societal and economic impacts under future climate change, but projections of future changes are highly uncertain and drivers of long-term changes are poorly understood. Here we develop a late Holocene storminess reconstruction from northwest Spain and combine this with an equivalent record from the Outer Hebrides, Scotland, to measure changes in the dominant latitudinal position of the storm track. The north-south index shows that storm tracks moved from a southern position to higher latitudes over the past 4000 yr, likely driven by a change from meridional to zonal atmospheric circulation, associated with a negative to positive North Atlantic Oscillation shift. We suggest that gradual polar cooling (caused by decreasing solar insolation in summer and amplified by sea-ice feedbacks) and mid-latitude warming (caused by increasing winter insolation) drove a steepening of the winter latitudinal temperature gradient through the late Holocene, resulting in the observed change to a more northern winter storm track. Our findings provide paleoclimate support for observational and modeling studies that link changes in the latitudinal temperature gradient and sea-ice extent to the strength and shape of the circumpolar vortex. Together this evidence now suggests that North Atlantic winter storm tracks may shift southward under future warming as sea-ice extent decreases and the mid- to high-latitude temperature gradient decreases, with storms increasingly affecting southern Europe.

Detecting climatic signals in an anthropogenically disturbed catchment: The late-Holocene record from the Petit Lac d'Annecy, French Alps.

Historical and documentary records from the Petit Lac d’Annecy, indicate that human activities have been the dominant ‘geomorphic process’ shaping the catchment during the late Holocene, with deforestation, agriculture and artificial drainage profoundly affecting both the pace and spatial distribution of soil erosion. The impact of past climatic change on the evolution of the catchment is less certain because of the lack of long-term climate records for the site. Previous attempts to use the sediment record from the lake to investigate the role past climate change may have played were hampered by the difficulty in isolating and disentangling the climatic signal preserved within the archive, because of overprinting of human activity. This is a common problem in regions with a long history of human activity in the landscape. In this study we use a range of novel statistical techniques (including cross-correlation and cross spectral analysis) to assess the relative importance of climate in driving landscape dynamics. The statistical analysis is carried out on an updated high-resolution palaeo-environmental data set from the Petit Lac d’Annecy. The results of the statistical analysis indicate that regional climate phenomena such as the Atlantic multidecadal oscillation are partly responsible for landscape dynamics at Petit Lac d’Annecy throughout the late Holocene. We find that the Petit Lac d’Annecy catchment typically requires decades, or longer, to respond to changes in precipitation, reflecting the stochastic nature of river sediment storage and transport. The use of a 4 yr integrated lake core record effectively attenuates the ‘signal shredding’ effect of shorter-term internally generated sediment transport processes. Nonetheless, the lake record of climatically induced geormorphic process–responses is weak compared with the pervasive impact of human activities.

Investigating the maximum resolution of µXRF core scanners: A 1800 year storminess reconstruction from the Outer Hebrides, Scotland, UK

Micro x-ray fluorescence (µXRF) core scanning is capable of measuring the elemental composition of lake sediment at sub-millimetre resolution, but bioturbation and physical mixing may degrade environmental signals at such fine scales. The aim of this research is to determine the maximum possible resolution at which meaningful environmental signals may be reconstructed from lake sediments using this method. Sediment from a coastal lake in the Outer Hebrides, Scotland, has been analysed using calibrated element measurements to reconstruct storminess since AD 200. We find that a Ca/K ratio in lake-core sediments reflects the presence of fine calcium carbonate shell fragments, a constituent of sand in the catchment that is washed and blown into the lake. Variations in this ratio are significantly correlated with instrumental records of precipitation and low pressures, suggesting it is a proxy for storminess. Furthermore, identification of a c. 60-year cycle supports a climatic influence on Ca/K, as this cycle is frequently identified in reconstructions of the North Atlantic Oscillation and North Atlantic sea-surface temperature. Comparison with weather records at different resolutions and spectral analysis indicate that µXRF data from Loch Hosta can be interpreted at sub-decadal resolutions (equivalent to core depth intervals of 3–5 mm in this location). Therefore, we suggest that sub-centimetre sampling using µXRF core scanning could be beneficial in producing environmental reconstructions in many lake settings where sediments are not varved.