Despite the push for clean energy to wean us from petroleum, the nation’s carbon addiction is unlikely to end by the middle of this century.

Natural gas is poised to become the standard bearer for fossil fuels in the near future. Natural gas is the cleanest-burning fossil fuel. It produces only half as much carbon dioxide as the energy equivalent of coal and contains almost none of the heavy metals that coexist with coal.

The rock that traps the natural gas is not so clean.

Hydraulic fracturing, or "fracking," began in the 1940s to release natural gas from sandstone thousands of feet below the surface. Forcing 1 to 10 million gallons of water under high pressure mixed with fine sand and chemical additives shatters the rock and forces the oil and gas out.

Fracking proved successful in sandstone, which fractures easily, but was much less efficient in shale, another kind of sedimentary rock.

Shale forms in friable layers. It is "garbage can" rock containing notable amounts of toxic metals like uranium, barium, strontium, chromium, zinc and arsenic.

Because shale has a lateral structure, vertical drilling reaches only a small fraction of the trapped gas.

In the 1990s new technology allowed the drill bit to turn 90 degrees to bore parallel to the shale layers and expose more of the gas deposits. It also allows drilling multiple wells in any direction from a single drilling pad, creating a honeycomb that can extract gas from hundreds of acres.

Fracking has drawn criticism from environmental groups, homeowners and teams of lawyers who blame it for polluting rivers, turning farmland into industrial zones and leaking enough methane to make tap water as flammable as lighter fluid.

Recent research shows that the explosive force of high-pressure fluids can contaminate ground water by dislodging radioactive uranium and strontium from the shale.

The reported that sewage treatment plants that treat fracking waste water discharge radioactive and other toxic fluids into public waterways, meaning that current methods for cleaning fracking waste water are inadequate.

Fracking booms have invaded shale beds across the country.

Large-scale fracking began in the Barnett Shale, which underlies 5,000 square miles around Fort Worth, Texas, in 2002.

Residents have complained of contaminated water, poor air quality and health problems such as headaches, dizziness, nosebleeds, nausea and blackouts.

Beginning in 2008, fracking in the Marcellus Shale, which underlies 95,000 square miles in the Northeast, also began to affect residents in Pennsylvania.

The EPA declared in 2004 that fracking posed little or no threat to underground sources of drinking water, but is now reconsidering that conclusion.

Solutions are under way to make fracking more efficient and less toxic, but fracking is not going away. The economic and environmental components of natural gas policies are too powerful.

These two lessons that we must learn keep coming back to haunt us: Nonrenewable resources will become more and more scarce and more and more expensive, and no source of energy is perfect.

Richard Brill is a professor of science at Honolulu Community College. Email questions and comments to rickb@hcc.hawaii.edu.