Publications

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Journal Article
Wagner, TJW, Dell R, Eisenman I.  In Press.  An Analytical Model Of Iceberg Drift. Journal of Physical Oceanography.
Wagner, TJW, Eisenman I.  2015.  False alarms: How early warning signals falsely predict abrupt sea ice loss. Geophysical Research Letters. 42   10.1002/2015gl066297   AbstractWebsite

Uncovering universal early warning signals for critical transitions has become a coveted goal in diverse scientific disciplines, ranging from climate science to financial mathematics. There has been a flurry of recent research proposing such signals, with increasing autocorrelation and increasing variance being among the most widely discussed candidates. A number of studies have suggested that increasing autocorrelation alone may suffice to signal an impending transition, although some others have questioned this. Here we consider variance and autocorrelation in the context of sea ice loss in an idealized model of the global climate system. The model features no bifurcation, nor increased rate of retreat, as the ice disappears. Nonetheless, the autocorrelation of summer sea ice area is found to increase in a global warming scenario. The variance, by contrast, decreases. A simple physical mechanism is proposed to explain the occurrence of increasing autocorrelation but not variance when there is no approaching bifurcation. Additionally, a similar mechanism is shown to allow an increase in both indicators with no physically attainable bifurcation. This implies that relying on autocorrelation and variance as early warning signals can raise false alarms in the climate system, warning of "tipping points" that are not actually there.

Wagner, TJW, Vella D.  2011.  Floating Carpets and the Delamination of Elastic Sheets. Physical Review Letters. 107:44301.: APS Abstract
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Wagner, TJW, Eisenman I.  2017.  How climate model biases skew the distribution of iceberg meltwater. Geophysical Research Letters. 44:GL071645.   10.1002/2016GL071645   AbstractWebsite

The discharge of icebergs into the polar oceans is expected to increase over the coming
century, which raises the importance of accurate representations of icebergs in global climate models
(GCMs) used for future projections. Here we analyze the prospects for interactive icebergs in GCMs
by forcing an iceberg drift and decay model with circulation and temperature fields from (i) state-of-the-art
GCM output and (ii) an observational state estimate. The spread of meltwater is found to be smaller for the
GCM than for the observational state estimate, despite a substantial high wind bias in the GCM—a bias
that is similar to most current GCMs. We argue that this large-scale reduction in the spread of meltwater
occurs primarily due to localized differences in ocean currents, which may be related to the coarseness of
the horizontal resolution in the GCM. The high wind bias in the GCM is shown to have relatively little impact
on the meltwater distribution, despite Arctic iceberg drift typically being dominated by the wind forcing.
We find that this is due to compensating effects between faster drift under stronger winds and larger
wind-driven wave erosion. These results may have implications for future changes in the Atlantic meridional
overturning circulation simulated with iceberg-enabled GCMs.

Wagner, TJW, Eisenman I.  2015.  How Climate Model Complexity Influences Sea Ice Stability. Journal of Climate. 28(10):3998–4014.   10.1175/JCLI-D-14-00654.1   Abstract

Record lows in Arctic sea ice extent have been making frequent headlines in recent years. The change in
albedo when sea ice is replaced by open water introduces a nonlinearity that has sparked an ongoing debate
about the stability of the Arctic sea ice cover and the possibility of Arctic ‘‘tipping points.’’ Previous studies
identified instabilities for a shrinking ice cover in two types of idealized climate models: (i) annual-mean
latitudinally varying diffusive energy balance models (EBMs) and (ii) seasonally varying single-column
models (SCMs). The instabilities in these low-order models stand in contrast with results from comprehensive
global climate models (GCMs), which typically do not simulate any such instability. To help bridge
the gap between low-order models and GCMs, an idealized model is developed that includes both latitudinal
and seasonal variations. The model reduces to a standard EBM or SCM as limiting cases in the parameter
space, thus reconciling the two previous lines of research. It is found that the stability of the ice cover vastly
increases with the inclusion of spatial communication via meridional heat transport or a seasonal cycle in
solar forcing, being most stable when both are included. If the associated parameters are set to values that
correspond to the current climate, the ice retreat is reversible and there is no instability when the climate is
warmed. The two parameters have to be reduced by at least a factor of 3 for instability to occur. This implies
that the sea ice cover may be substantially more stable than has been suggested in previous idealized
modeling studies.

Crawford, AJ, Wadhams P, Wagner TJW, Stern A, Abrahamsen EP, Church I, Bates R, Nicholls KW.  2016.  Journey of an Arctic ice island. Oceanography. 29:254-263.   10.5670/oceanog.2016.30   AbstractWebsite

In August 2010, a 253 km(2) ice island calved from the floating glacial tongue of Petermann Glacier in Northwest Greenland. Petermann Ice Island (PII)-B, a large fragment of this original ice island, is the most intensively observed ice island in recent decades. We chronicle PII-B's deterioration over four years while it drifted more than 2,400 km south along Canada's eastern Arctic coast, investigate the ice island's interactions with surrounding ocean waters, and report on its substantial seafloor scour. Three-dimensional sidewall scans of PII-B taken while it was grounded 130 km southeast of Clyde River, Nunavut, show that prolonged wave erosion at the waterline during sea ice-free conditions created a large underwater protrusion. The resulting buoyancy forces caused a 100 m x 1 km calving event, which was recorded by two GPS units. A field team observed surface waters to be warmer and fresher on the side of PII-B where the calving occurred, which perhaps led to the accelerated growth of the protrusion. PII-B produced up to 3.8 gigatonnes (3.8 x 1012 kg) of ice fragments, known hazards to the shipping and resource extraction industries, monitored over 22 months. Ice island seafloor scour, such as a 850 m long, 3 m deep trench at PII-B's grounding location, also puts subseafloor installations (e.g., pipelines) at risk. This long-term and interdisciplinary assessment of PII-B is the first such study in the eastern Canadian Arctic and captures the multiple implications and risks that ice islands impose on the natural environment and offshore industries.

Wagner, TJW, Stern AA, Dell RW, Eisenman I.  Submitted.  On the representation of capsizing in iceberg models.
Wagner, TJW, James TD, Murray T, Vella D.  2016.  On the role of buoyant flexure in glacier calving. Geophysical Research Letters. 43:232-240.   10.1002/2015gl067247   AbstractWebsite

Interactions between glaciers and the ocean are key for understanding the dynamics of the cryosphere in the climate system. Here we investigate the role of hydrostatic forces in glacier calving. We develop a mathematical model to account for the elastic deformation of glaciers in response to three effects: (i) marine and lake-terminating glaciers tend to enter water with a nonzero slope, resulting in upward flexure around the grounding line; (ii) horizontal pressure imbalances at the terminus are known to cause hydrostatic in-plane stresses and downward acting torque; (iii) submerged ice protrusions at the glacier front may induce additional buoyancy forces that can cause calving. Our model provides theoretical estimates of the importance of each effect and suggests geometric and material conditions under which a given glacier will calve from hydrostatic flexure. We find good agreement with observations. This work sheds light on the intricate processes involved in glacier calving and can be hoped to improve our ability to model and predict future changes in the ice-climate system.

Wagner, TJW, Vella D.  2012.  The sensitivity of graphene “snap-through” to substrate geometry. Applied Physics Letters. 100:3111. Abstract
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Wagner, TJW, Vella D.  2013.  Switch on, switch off: stiction in nanoelectromechanical switches. Nanotechnology. 24:275501.: IOP Publishing Abstract

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Wagner, TJW, Eisenman I, Dell RW, Keeling RF, Severinghaus JP.  Submitted.  Wave inhibition by sea ice enables trans-Atlantic ice rafting of debris during Heinrich Events.
Stern, AA, Johnson E, Holland DM, Wagner TJW, Wadhams P, Bates R, et al.  2015.  Wind-driven upwelling around grounded tabular icebergs. Journal of Geophysical Research - Oceans.   10.1002/2015JC010805  
Wagner, TJW, Vella D.  2013.  The ‘Sticky Elastica’: delamination blisters beyond small deformations. Soft Matter. 9:1025–1030.: Royal Society of Chemistry Abstract
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Wagner, TJW, Wadhams P, Bates R, Elosegui P, Stern A, Vella D, Abrahamsen PE, Crawford A, Nicholls KW.  2014.  The “footloose” mechanism: Iceberg decay from hydrostatic stresses. Geophysical Research Letters. 41:5522–5529. Abstract
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Thesis
Wagner, TJW.  2013.  Elastocapillarity: adhesion and large deformations of thin sheets. : University of Cambridge Abstract
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