Research


Response of the global hydrate stability zone volume and hydrate inventory to IPCC AR5 RCP future scenarios.

Significant concentrations of methane are stored within marine sediments in the form of hydrate deposits (methane gas trapped within a "cage-like" structure consisting of H-bonded water molecules). The global inventory of methane hydrate is believed to be compareable to estimates of the present-day reserve of potentially-recoverable hydrocarbons. This hydrate forms and exists within a delicate low-temperature, high-pressure system. Therefore changes in bottom water temperature (due to ocean warming and changes in circulation) and hydrostatic pressure (due to sea-level change) can lead to the dissociation of these hydrate deposits, leading to the release of methane into the sedimentary column. If conditions allow this methane to leave the ocean floor, it can lead to changes in ocean chemistry and atmosphere composition. Although relatively short-lived, atmospheric methane is a powerful greenhouse gas. It is therefore important to understand more about this potential source of atmospheric methane.

This multi-climate model study was designed to investigate how the global Hydrate Stability Zone (HSZ) volume and hydrate inventory will respond to the four Representative Concentration Pathways (RCPs) modelled within the Coupled Model Intercomparison Project (CMIP5).

We began by evaluating the performance of 12 climate and earth system models against Bottom Water Temperatures (BWT) of the WOA05 observational database. This allowed each climate model to be allocated a relative performance metric. From initial pre-industrial conditions (~1860) we modelled the sediment-column propagation of bottom water temperatures through the historical (1860-2005) and RCP scenarios (2005 - 2100; ECP to 2300). Incorporating simple models of global sea-level change we then modelled the temporal evolution of the extent of the HSZ on a global scale. To derive the initial hydrate inventory and its response we used 1) a simple model of hydrate saturation and 2) a 1-D time-dependent hydrate model. This allowed potential upper-limit rates of in-situ hydrate dissociation to be modelled. Knowing the relative performance of each climate model allowed us to generate a multi-model mean prediction of the HSZ and hydrate evolution for each RCP scenario.

Publications
Hunter, S. J., Goldobin, D. S., Haywood, A. M., Ridgwell, A. and Rees, J. G. (2013) Sensitivity of the global submarine hydrate inventory to scenarios of future climate change. Earth and Planetary Science Letters 367, 105-115 link
Hunter, S. J., Goldobin, D. S., Haywood, A. M., Ridgwell, A. and Rees, J. G. Response of the oceanic methane hydrate inventory to future climate change (AR5 RCP 4.5 - 8.5). AGU 2012 Fall Meeting (Presentation OS34A-08)


Exploring changes in the Marine Methane Hydrate Reservoir during the Last Interglacial-Glacial Cycle

Investigating how the volume and stability of the global methane hydrate reservoir has changed since the last interglacial period (~ 120 kyr BP) allows us to assess and put into perspective the potential for the large-scale destabilisation of methane hydrates in the future. To define the bottom-water temperature boundary condition we use a series of atmosphere-ocean general circulation model (HadCM3 and FAMOUS) snap-shot and transient type experiments covering the last 120 ky (HadCM3 experiments introduced within Singarayer and Valdes (2010) ). In combination with global sea-level reconstructions we model how the volume of the global Hydrate Stabillity Zone (HSZ) evolved through the last 120 kyr. We then used these boundary conditions to drive a methane hydrate model, based upon the time-dependent and steady-state models of Davie and Buffett (2001 and 2003) and Archer and Buffett (2005).

Publications
Hunter, S. J. et al., (in prep) A model of the global methane hydrate stability zone through the last glacial cycle (120 kyr BP to pre-industrial). Hunter, S. J., Goldobin, D. S., Haywood, A. M., Rees, J. G., Ridgwell, A. J. Brilliantov, N., Jackson, P. D., Levesley, J., Rochelle, C., and Lovell. M. Modelling Changes In the Global Methane Hydrate Inventory. AGU 2010 Fall Meeting.
Hunter, S. J., Haywood, A. M., Goldobin, D. S., Brilliantov, N., Levesley, J., Rees, J. G., Rochelle, C., Lovell. M. and Ridgwell, A. J. Modelling Changes In the Global Methane Hydrate Inventory. Proc. of the 7th Intl. Conf. on Gas Hydrates (ICGH7), Edinburgh, July 17-21, 2011. (Poster P7.008)
Goldobin, D. S., Hunter, S. J., Haywood, A. M., Brilliantov, N. V., Levesley, J., Lovell, M. A., Ridgwell, A. J. Rochelle, C. A., Jackson, P. D. and Rees. J. G. Forecasting diffusive formation of free-gas methane layers in sea sediments. Proc. of the 7th Intl. Conf. on Gas Hydrates (ICGH7), Edinburgh, July 17-21, 2011
Goldobin, D. S., Brilliantov, N. V., Levesley, J., Lovell, M. A., Rochelle, C. A., Jackson, P. D., Haywood, A. M., Hunter, S. J. and Rees, J. G. Non-Fickian Diffusion and the Accumulation of Methane Bubbles in Deep-Water Sediments (2011); arXiv:1011.6345.

Davies, D. J., Thatcher, K. E., Armstrong, H., Yang, J., Hunter, S. (2012) Tracking the relict bases of marine methane hydrates using their intersections with stratigraphic reflections, Geology, 40, pp.1011-1014. link
Lunt, D., Ridgwell, A., Sluijs, A., Zachos, J., Hunter, S. and Haywood, A. A model for orbital pacing of methane hydrate destabilization during the Palaeogene (2011); Nature Geoscience, vol 4., pp. 775 - 778


Modelling Antarctic Ice-sheets under Greenhouse Earth Conditions (PhD research).

The greenhouse world of the Cretaceous has traditionally been considered warm and ice-free. Geological evidence from the Maastrichtian stage of the Late Cretaceous suggest that large and geologically rapid, eustatic sea-level changes occurred. The formation and decay of continental-scale ice sheets has been suggested as a likely mechanism. The aim of my PhD thesis was to explore this 'Cretaceous paradox' by simulating the Maastrichtian Earth system using a combination of climate and ice sheet models.

A database of climatologically-sensitive geological proxies was created to provide boundary conditions and evaluation data for a series of HadCM3L (AO-GCM) climate model experiments. These experiments were designed to investigate uncertainties and changing boundary conditions associated with the Maastrichtian. Once evaluated, the predicted climates were then used to drive an ice-sheet model to determine an envelope of possible Antarctic and Arctic ice sheet confgurations.

Comparison of the GCM predicted climatological envelope against geological proxies was encouraging. However, a number of regional data-model inconsistencies were apparent. Within the Asian continental interior a range of proxies suggested warmer, more equable climate than GCM predictions. In the high latitudes, proxy-model discrepancies were evident but it was recognised that this could equally be result of proxy interpretation.

Ice-sheet modelling results suggest that during intervals of reduced CO2 and favourable orbits, ice sheets could form and persist on the highlands of the Bering Strait region and East Antarctica. With shallow seaways providing connectivity with the Arctic basin, global climatic conditions were more favourable for ice sheet growth. The uncertainty in Arctic and Antarctic topography was explored with a a series of sensitivity experiments. Retrodicted sea-level envelopes derived from CO2 reconstructions and the open-Arctic ice sheet envelope do not accommodate a Campanian-Maastrichtian sea-level fall but tentatively support a possible mid Maastrichtian sea-level excursion of >15 m. Uncertainty in these results are driven primarily by uncertainties in the high latitude orography, concentrations of greenhouse gasses, the palaeogeography (e.g. continental configuration and bathymetry) and the land surface scheme.

Further reading
Hunter, S. J. (2010). "Modelling Antarctic Ice Sheets under Greenhouse Earth Conditions." PhD thesis, 323p
Hunter, S. J., Haywood, A. M., Valdes, P. J., Francis, J.E., Pound, M. J. (2013) "Modelling equable climates of the Late Cretaceous: Can new boundary conditions resolve data–model discrepancies?", Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 392, Pages 41-51, link
Hunter, S. J., et al (in prep) "Modelling Late Cretaceous Ice-Sheets".