Thanks to its controversial nature, most people have some knowledge of shale gas and fracking, the method used to extract it. As fracking has boomed in recent years, energy experts have woken up to another unconventional source of natural gas. It is methane hydrate, and its potential reserves are huge. Methane hydrate is formed naturally when methane gas, produced by microbial action or the effects of earth's heat and pressure on buried organic matter, combines with water at moderate pressures and temperatures ranging from below zero to nearly 25 degrees Celsius. After it is formed, it becomes fairly stable and looks like ice. About 99 per cent of methane hydrate deposits are found in deep-water environments and 1 per cent occurs on land. It may be found in nodules and mounds on the seafloor and in spaces such as pores, cracks, and seams within sediment on the seabed that is hundreds of metres thick, as deep as 2,000 metres down. It occurs on land mainly in sediment under thick permafrost in Arctic latitudes in Alaska, Canada, and Russia. In the US, in the deep sediment of the northern Gulf of Mexico alone, potential methane hydrate resources are estimated to be as large as 605 trillion cubic metres (tcm). A more modest 2.4 tcm is estimated to be present on the Alaskan North Slope in sandy sediment beneath thick permafrost. These two deposits represent a tenfold increase over the US' future natural gas supply as estimated in 2008. Natural gas production in the US in 2008 was about 0.5 tcm, and the extra methane hydrate could potentially extend that supply for another 1,000 years. New optimism in the US over indigenous natural gas resources thus comes independent of expected advances in fracking that will allow significantly more shale gas to be extracted than is currently possible. Between 2006 and 2008, the US experienced an unprecedented boom in shale gas supply, which leapt 35 per cent from 43.4 tcm to 58.7 tcm. New fracking methods alone can reasonably be expected to continuously and significantly increase the world's natural gas supply over time. And the high hopes for methane hydrate as a vast energy resource have been borne out by more than a decade of successful field work, conducted initially in Canada's Northwestern Territories, then on Alaska's North Slope. More recently, work has been done in the Nankai Trough, off the southeast coast of Japan's main island, Honshu. Other countries currently conducting field studies include China, South Korea, and Russia. When natural gas is used for electricity generation, it puts out about half the carbon dioxide emissions of coal per unit of energy produced. So if a fossil fuel must be used for power generation, natural gas is the least bad choice in terms of climate change. In the few years up to 2012, as US electricity generation gradually shifted from coal to natural gas amid the abundant and inexpensive supply made possible by fracking, the country's carbon dioxide emissions fell to about 5.3 billion tonnes - the lowest level since 1994. That's why natural gas is often touted in the US as a "blue bridge to a greener world". But even as the use of gas becomes increasingly widespread, it is still a fossil fuel that emits carbon dioxide when burned, and its existence in vast quantities around the world may prove to be a mixed blessing. Methane leaks into the atmosphere from large-scale drilling and pumping operations, and from fracking, and these leaks can do considerable harm. That's because methane is about 20 times more potent than carbon dioxide as a greenhouse gas. It also stays in the atmosphere for about 10 years before it is oxidised and becomes carbon dioxide, which itself remains in the atmosphere as a greenhouse gas. With technological advances, potentially huge methane hydrate resources around the world will become economical to exploit, resulting in increased use of the gas as a fuel. If the costs of using gas remain low relative to the costs of using renewable energy sources, and if carbon taxes are not levied on the use of fossil fuels as a primary energy source in order to level the playing field for renewables, then investment in renewables and infrastructure such as smart, distributed electrical grids will be held back. And that's not likely to do much in the way of building a bridge to a greener world. Patrick Lui, a US trained scientist, is a co-founder of Savantas Policy Institute in Hong Kong and was a long-time staff member at Stanford University.