Daniel Hill - Academia.edu (original) (raw)

Papers by Daniel Hill

2008 IEEE/LEOS International Conference on Optical MEMs and Nanophotonics, 2008

Concern about climate change and greenhouse gas (GHG) emissions has brought about renewed attenti... more Concern about climate change and greenhouse gas (GHG) emissions has brought about renewed attention to energy conservation, with a particular focus on energy efficiency of buildings. Economic literature of the past 30 years has identified both market and non-market barriers concerning energy efficiency, with one in particular affecting the residential sector: the principal-agent (PA) problem. Involving transaction costs, asymmetrical information, and split incentives, PA problems are thought to keep economically sound investments in energy efficiency from being realized. This problem is prevalent within the landlord-tenant relationship in the private rental housing segment. And since it is generally acknowledged that energy use in buildings can be significantly reduced through cost effective investments in efficient technology, it is important to understand the magnitude of PA problems that keep economically sound investments from being realized. The aim of this study, therefore, is...

Global and Planetary Change, 2015

Climate of the Past Discussions, 2014

ABSTRACT Intervals in deep time (i.e. the pre-Quaternary) are being increasingly used as a means ... more ABSTRACT Intervals in deep time (i.e. the pre-Quaternary) are being increasingly used as a means to quantify longer term climate sensitivity (hereafter referred to as Earth System Sensitivity). Earth System Sensitivity (ESS) differs from equilibrium climate sensitivity (CS) by including additional feedbacks to a change in atmospheric carbon dioxide (CO2) concentration from slow responding components of the climate system, such as vegetation and ice-sheets. Warm intervals of the Pliocene epoch (2.7 to 5.2 million years ago), lasting for thousands of years, were likely characterised by CO2 concentrations 80 to 120 ppmv higher than the pre-industrial era, and therefore provide a potential way to calculate ESS and determine how it differs from equilibrium CS. This task is aided by the availability of geological reconstructions of ice sheets and vegetation that can be used as boundary conditions for global climate models. An initial calculation from Lunt et al. (2010) suggests that on the basis of examining the Pliocene, ESS may be 30 to 50% greater than equilibrium CS. However, this study used geological reconstructions of the ice-sheets and vegetation that have now been improved. Furthermore, the Lunt et al. (2010) study only used a single coupled ocean-atmosphere climate model to produce the initial ESS estimate, so the degree of model dependency in the calculated ESS was unknown. Here we revise estimates of ESS based on the Pliocene using a new ensemble of climate simulations produced by the Pliocene Model Intercomparison Project (PlioMIP), which has provided a multi-model ensemble of Pliocene climate utilising the latest geological reconstructions for vegetation and the ice-sheets as boundary conditions. In the PlioMIP ensemble there is a large spread in the ratio ESS/CS from 1.04 (the IPSL model) to 2.0 (the HadCM3). The ratio for the ensemble mean is 1.5. Therefore, the PlioMIP simulations provide a similar result to the Lunt et al. (2010) study, and show that ESS is greater than equilibrium CS. The new estimates indicate that the ESS/CS ratio is between 1.04 and 2, and imply a best estimate of 1.5. Caveats to these new calculations of ESS based on the PlioMIP are numerous and include the degree to which models within the ensemble have reached an equilibrium climate condition with imposed Pliocene boundary conditions, the consistency between each groups Pliocene simulation and the simulation used in published estimates of equilibrium CS, and the degree to which the geological record for Pliocene ice sheets and vegetation reflects a response to higher CO2 concentrations, rather than other factors (i.e. orbital forcing). The latter issue is particularly complex and difficult to assess due to the uncertainties in chronology and correlation inherent within the proxy data that underpin the boundary conditions used by the climate models. We conclude by presenting a new strategy for proxy data collection and modelling designed to overcome some of these stated weaknesses.

ABSTRACT Ice sheet retreat in a warmer world is the largest uncertainty in predictions of future ... more ABSTRACT Ice sheet retreat in a warmer world is the largest uncertainty in predictions of future sea level rise. Warm intervals in Earth history provide an opportunity to study ice sheets under similar conditions. Due to atmospheric CO2 concentrations and temperatures being similar to those predicted for the 21st century the Pliocene epoch is receiving considerable attention. The mid-Pliocene Warm Period (mPWP ca. 3.3 to 3 Ma BP) has been the focus of major data synthesis and palaeoclimate modelling efforts. In terms of global sea level variation, and hence ice volume change, there are numerous sources of geological data which suggest sea level high-stands above present-day of around 25 m. This implies a reduction in the Earth's ice sheets. However, each estimate has inherent uncertainties and provides little indication of how the ice was globally distributed. Therefore, climate and ice sheet models have been used to indicate the likely distribution of mPWP ice sheets. Nevertheless, the model dependency of these results remains untested. Here, we present results from the international Pliocene Ice Sheet Modelling Intercomparison Project (PLISMIP). The aim of this project is to test and compare the performance of existing numerical ice sheet models (ISMs) under prescribed climatic forcing in simulating Greenland and Antarctic Ice sheets of the mPWP. Initially, we compare the simulated modern ice sheets from seven ISMs, forced with (1) climate model output from the Hadley Centre atmosphere-only model (HadAM3) and (2) observational data (NCEP2 reanalysis data). These tests highlight ice sheet model dependent biases when simulating present-day ice sheets on Greenland and Antarctica. We also present results from mid-Pliocene ISM simulations. This addresses the dependency of the simulated ice sheets on the both the ISM used and on the assumptions made within the modelling framework (boundary and initialisation conditions). When compared with observed ice sheet geometry, most of the ISMs simulate acceptable present-day ice sheets, although small differences in the absolute ice volume, ice area and distribution are evident. ISM simulations of the mPWP ice sheets are less consistent in size and volume, especially over the Greenland Ice Sheet. This suggests that the ISMs may have a different sensitivity to climate forcing in a warmer world. We present an inter-model mean representation of the Greenland and Antarctic Ice Sheets, which has the potential to be a more robust overall representation of the ice sheets than can be attained through the use of a single model. This study contributes to our understanding of the uncertainties in existing Pliocene ice sheet reconstructions due to the use of a single ISM. We provide new benchmarks in the simulation of ice sheets in a past warm period, which could directly inform the uncertainty predictions of future ice sheet behaviour and sea level response.

ABSTRACT Evidence for the state of the East Antarctic Ice Sheet during the Pliocene is sparse, di... more ABSTRACT Evidence for the state of the East Antarctic Ice Sheet during the Pliocene is sparse, difficult to interpret and often contradictory. Recent provenance measurements from ice-rafted debris (IRD) at ODP Site 1165 in Prydz Bay, East Antarctica suggests major iceberg release events from Wilkes Land during the warmer periods of the Pliocene. While this data provides unprecedented geographical information and important constraints on Pliocene ice sheets, the exact nature of the ice sheets they represent and the cause of the measured variability in the source of Prydz Bay icebergs remain unclear. Here we present an integrated Pliocene climate, ice sheet and iceberg modelling approach to simulate the conditions under which these changes in East Antarctic icebergs could have occurred. Scenarios with little variability in the East Antarctic Ice Sheet cannot reproduce the data from the early Pliocene IRD data at Site 1165, although transition to a modern-like state may explain changes in the latter part of the record. Careful data-model comparison shows that much of the variability in the record can be explained by an East Antarctic Ice Sheet that is sensitive to changes in the orbital forcing during the Pliocene. However, even with an ice sheet that varies by several metres of sea level equivalent volume, the extreme lows in Wilkes Land IRD seen in the record are not reproduced. This suggests there may be additional forcing producing large retreats of the ice sheet in the Wilkes Land region. IRD data and iceberg modelling provides us with a unique window into the changes and oscillations in the East Antarctic Ice Sheet that must have occurred during the Pliocene. While this approach does not give a complete picture of ice sheets during the Pliocene it helps to evaluate existing reconstructions and provides important constraints on any future reconstructions.

ABSTRACT During the mid-Pliocene Warm Period (3.264 to 3.025 million years ago), global mean temp... more ABSTRACT During the mid-Pliocene Warm Period (3.264 to 3.025 million years ago), global mean temperature was similar to that predicted for the next century, and atmospheric carbon dioxide concentrations were higher than pre-industrial levels (400 ppmv). Sea level was also higher than today (high-stands of ~+22m), implying a reduction in the extent of the ice sheets. Thus, the mid-Pliocene Warm Period provides a natural laboratory in which to investigate the long-term response of the Earth's ice sheets and sea level in a warmer-than-modern world. A combination of climate and ice sheet models can be used to enhance our understanding of ice sheet stability. At present, our understanding of the Greenland and Antarctic ice sheets during the warmest intervals of the mid-Pliocene is based upon predictions using generally only one climate model and one ice sheet model. Therefore, it is essential that the model dependency of these results is assessed. The Pliocene Model Intercomparison Project (PlioMIP) has brought together 14 international modelling groups to simulate the warm climate of the Pliocene. Here we use the climatological fields derived from the results of both the atmosphere-only climate models (Experiment 1) and the coupled atmosphere-ocean climate models (Experiment 2) to force an offline shallow ice approximation ice sheet model (BASISM). This will test the climate model dependency of ice sheet simulations over Greenland. We show that climate model dependency is high over Greenland, with Pliocene reconstructions ranging from an ice-free state to a near modern Greenland ice sheet. An analysis of the surface albedo differences between the models over Greenland offers some insights in to the reasons for inter-model discrepancies. Although these results are unable to identify the causes of the major differences between modelled reconstructions, they do serve to highlight the differences between model predictions of climate over the ice sheet regions during the mid-Pliocene Warm Period.

Sensors and Actuators B: Chemical, 2014

Nature Climate Change, 2013

Nature Climate Change, 2012

Silicon Photonics and Photonic Integrated Circuits II, 2010

Journal of High Energy Physics, 2012

We present a measurement of two-particle angular correlations in proton- proton collisions at $ \... more We present a measurement of two-particle angular correlations in proton- proton collisions at $ \sqrt {s} = 900 $ GeV and 7 TeV. The collision events were collected during 2009 and 2010 with the ATLAS detector at the Large Hadron Collider using a single-arm minimum bias trigger. Correlations are measured for charged particles produced in the kinematic range of transverse momentum p T > 100 MeV and pseudorapidity |η| < 2.5. A complex structure in pseudorapidity and azimuth is observed at both collision energies. Results are compared to pythia 8 and herwig++ as well as to the AMBT2B, DW and Perugia 2011 tunes of pythia 6. The data are not satisfactorily described by any of these models.

Photonics and Nanostructures - Fundamentals and Applications, 2012

Perceptual and Motor Skills, 1980

For 89 white children 6 to 11 yr. old and of upper middle class, scores on Piagetian tasks of con... more For 89 white children 6 to 11 yr. old and of upper middle class, scores on Piagetian tasks of conservation of length and volume were related to age but field independence was correlated significantly to these scores.

Nature Communications, 2011