Many years, depending partly on the chemical conditions in the heap where they are dumped.
It depends on the environment.

In particularly corrosive environments - say on the salty sea shore in a tropical country, just a few months. But in Antarctica, it will never decompose, it'll stay frozen in the ice forever.

The disadvantages of importing food is

• pesticides and other germs on the food.
• transporting food long distances adds to its carbon footprint. Why buy a can of fish that has been flown half way round the world? What a waste of fuel.

It is somewhat more energy intensive to recover oil from oil sands than it is from conventional oil wells. The "oil" in oil sands is closer to tar and will not flow unless heated or diluted with lighter hydrocarbons. Recovery is usually by either strip mining (where the oil sand is closer to the surface) or by injecting steam, solvents, and/or hot air into the sand (where the sands are deeper). Once the tar/oil is recovered, it must be further pre-processed to turn it into a form that can be handled by conventional oil refineries. This usually requires 3 steps:

1. removal of water, sand, physical waste, and lighter products,
2. catalytic purification to remove sulfur, metals, and nitrogen,
3. hydrogenation - usually through hydro-cracking.

These pre-processing steps take large amounts of energy and water, while emitting more carbon dioxide than conventional oil. Most of the oil sands that are currently used as sources of oil use processes that require quite a bit of water for the recovery process. For this reason 'sine' oil sand deposits are even more difficult to process. For example the large oil sand deposits in eastern Utah, which is quite arid, are not being mined.

There are also political considerations:

President Clinton, by executive order, placed most of the oil sand deposits in eastern Utah off limits for development (some have speculated that it was in retaliation for finishing 3rd in Utah in the Presidential election of 1992 behind Bush and Perot - Bush 43.36, Perot 27.34%, Clinton 24.65%). At the very least, order was driven by eco-politics rather than any issues with technology or economics.

One of the largest oil sand deposits in the world is located in Venezuela which nationalized its oil industry in 1975-1976 placing it under the country's state-run oil and natural gas company but allowed for some private development. In 2001, Venezuelan president Hugo Chávez imposed a new Hydrocarbons Law that raised royalties paid by private companies from 1-17% to 20-30%. In 2007, Chávez announced the nationalization of the oil industry and required that all private companies hand over majority ownership to the government of Venezuela. Any company that refused to sign over the majority ownership simply had all their assets seized and turned over to the state-run oil company. This put a damper on oil sand development since the companies with expertise to develop it were no longer willing to risk losing their investment to another seizure by the Chávez government.

Things with large carbon footprints do damage to the earth by releasing large amounts of carbon. Things with small carbon footprints do little damage.

A carbon footprint is just a number, telling you how much carbon is produced in the making, producing, transporting and using of a certain thing. This thing could be a car, or a bottle of beer, or a tin of vegetables, or your house, or even you.

For example, the carbon footprint of a tin of vegetables that was produced on the other side of the world would consider the collection, transport, cooking and canning of the crop. It would then factor in the cost of transportation half way round the world, by ship, air, truck and perhaps your own vehicle. Compare this to vegetables grown in your own garden, or at least in your own local area.

So an item with a large carbon footprint releases more carbon into the atmosphere, adding to the greenhouse gases already there, and contributing to global warming, much more than an item with a small carbon footprint.

An ecological footprint measures how much of the planet's resources you use, and converts this to the amount of land needed to provide the resources and assimilate your waste. It is measured in global hectares (a hectare is about the size of a soccer field). It includes:

* The amount of building materials you use in your home and workplace * The amount of water you use in your home, workplace and garden

* The fossil fuels (oil, coal, wood and natural gas) needed

* to provide the power to run your home and workplace

* to bring your food from all over the world

* to power your vehicles and transport * to carry away and dispose of your waste.

A recent study (October 2008) ranks the top ten countries with the highest ecological footprint per head as:

# United Arab Emirates # United States # Kuwait # Denmark # Australia # New Zealand # Canada # Norway # Estonia # Ireland.
It is what you leave behind such as a foot print at the park. Only in this place the foot print is the resources you use up during a life time.

Nuclear energy has a significant carbon cost of mining uranium fuel. This carbon cost is usually exported to another nation's mines. The actual carbon cost can be reduced by cutting safety in extremely poor countries, which leads to greater worker mortality but doesn't show up in the carbon budget.

The best nuclear ores are all mined. The true carbon cost of additional nuclear energy must be measured in the carbon cost of mining additional ores, not in the current average carbon cost of mining.

The carbon cost of the threat of terrorism incidents at nuclear plants is hard to calculate. Are we invading other countries because terrorist organizations will strike domestic nuclear power plants first? Also, would one successful terrorist incident instantly double the carbon cost of protecting all nuclear power plants?

We don't know how to measure the carbon cost of protecting people from nuclear waste for thousands of years. For example, the current U.S. policy on nuclear waste at the Hanford Military Reservation, which is in the floodplain of the Columbia River, is to leave the waste in place underground until it leaks into the river and is gone, or until a river flood overruns the area. This method of neglect produces a very small carbon footprint.

The decommissioning of reactors has a high carbon footprint. The carbon cost of decommissioning can be amortized over more years of electricity production by extending a nuclear plant's lifetime. However, old nuclear plants tend to have more radioactive leaks, and may have a slightly higher risk of a disaster.

The Ukraine has an issue where authorities have encased the disastrous Chernobyl reactor in a concrete casing and have abandoned the nearby city and region. Nuclear radiation has helped destroy the casing, and a new concrete tomb is planned. The entire region of Chernobyl is now threatened by fuel buildup on the forest floor. A forest fire would pump huge amounts of radiation into the air, which would cross national boundaries. Again, this storage problem's carbon footprint is low, simply because in case of a fire much of the isotope radiation would blow into some other country.

What we find is that the carbon footprint involved in generating nuclear energy and nuclear safety efforts are inextricably linked. The carbon costs in keeping people extremely safe from radiation would be enormous. Nuclear energy would be a net carbon sinkhole, where it would be more practical just to burn fossil fuels for electricity. If, however, many public health and safety shortcuts are taken by a government, especially by exporting safety problems or ignoring the mining and nuclear waste problems, nuclear energy has a much lower carbon footprint than just burning fossil fuels.

To put this into context, the following are average estimates of total greenhouse gasses by production type with numbers of grams of CO2e/kWh:

1000 - coal

900 - oil

750 - open cycle natural gas

580 - closed cycle natural gas

(closed cycle natural gas combined with co-generation might bring this down to 400 g. CO2e/kWh)

500 coal plant burning 50% coal with 50% miscanthus

110 - old solar photovoltaics

95 - biomass from miscanthus

85 - nuclear

40 - concentrated solar thermal with thermal storage

35 - new solar photovoltaics

25 - biomass from gasification of wood chips (used to fuel conventional natural gas turbines)

21 - wind

15 - hydroelectricity

<10 - geothermal doublet

These numbers come mostly from the Wikipedia article cited below. The figure for nuclear is extracted from the Sovacool article cited by using only studies dated after 2004. The figures for solar come from current solar literature as solar technology has changed a lot in the last ten years. The figures for biomass come from the UK Parliamentary Office of Science and Technology.

This places the carbon footprint of nuclear as 400% to 1600% of wind, hydro, solar, but about 15% of natural gas, and 8.5% of coal. Bear in mind that some estimates for the nuclear are much higher.

Finally, the carbon footprint spent fuel disposal or reprocessing still does not change the BTU ratio significantly.