Congress is about to confront the challenge of coal, and much of what we hope to do to reduce the threat of global warming hinges on these decisions.

Rep. Ed Markey

There’s a useful test to use whenever the challenges of fossil fuel dependence and global warming come up: We must reduce the threat of global warming without worsening our dependence on foreign oil; and we must reduce the threat of oil dependence without worsening global warming.

When it comes to coal, it’s that second part of the equation that brings up some sticky issues.

Coal has been a big part of our energy mix, providing the majority of our electricity since the invention of the electric light. It has been a principal source of energy since the dawn of the Industrial Revolution — a revolution that provided the basis for our economic prosperity, but also produced exponential pollution growth that was the genesis of the global warming issues we face today.

Now is the time for a new Green Revolution. We must combine the economic reforms of a new industrial revolution based on clean energy development with the moral imperative to protect the planet.

But where does that leave coal? Can our reliance on these carbon-packed nuggets of energy survive while we try to ensure the planet survives as well?

There are two main issues at play here: "coal-to-electricity" and "coal-to-liquids." Capturing carbon pollution from coal and sequestering it deep underground would make it possible to continue to use coal as a major electricity source; turning coal into liquid to replace gasoline, diesel, and jet fuel would, according to the EPA, make global warming worse.

The process of turning coal into liquid transportation fuels is not new. It was first adopted on a large industrial scale by the oil-isolated Nazi Germans in World War II, and later by an embargoed apartheid South Africa.

Without carbon capture and storage, liquid coal fuel contributes more than double the total heat-trapping pollution produced by a simply burning conventional petroleum-based fuels. But even when carbon capture systems are added, it is still worse for the environment than regular gasoline. When scientists tell us we need to be making massive, not minuscule, cuts in global warming emissions, it’s clear that liquid coal would carry us in the wrong direction.

Liquid coal is also incredibly expensive and resource-intensive to create, with small returns compared to the amount of energy and the immense number of new industrial plants needed to create it. Even setting aside the environmental impacts of coal mining, the water resources needed for this sort of undertaking would be staggering: 4.6 billion gallons per year of liquid fuels from coal would require between 21 and 60 billion gallons of water per year. To give some perspective, 60 billion gallons could fill 90,850 Olympic sized swimming pools.

This is why China has reportedly backed off of liquid coal fuels. Instead, they are relying on strong fuel economy standards. Certainly our own country would benefit from much stronger fuel economy standards as well, which is why I have proposed bipartisan legislation — the Markey-Platts bill — to mandate a fuel economy increase to 35 mpg by 2018, and 4 percent a year after that. This legislation would reduce America’s oil dependence by 10 percent. And, unlike liquid coal fuels, that’s without having to build a single new industrial plant.

And compared with the potential of cellulosic ethanol, made not just from crops grown by farmers, but also grasses that grow wild on the American plains and garbage that piles high in America’s cities, liquid coal just doesn’t stack up. Cellulosic ethanol’s total heat-trapping emissions can be as low as a quarter of those from conventional gasoline — even further below those from liquid coal.

The other important emerging coal technology is carbon capture and sequestration (CCS), a process that will allow the heat-trapping pollution from burning coal for electricity to be captured and stored underground. We should never build a power plant we don’t need, and both renewable energy and improved efficiency will continue to allow us to avoid many new expensive, polluting power facilities. But we have a huge stake in solving the CCS problem, because without it we are unlikely to convince China and India that they can grow while still controlling global warming pollution.

In the end, we need to enact real policies to create the market for cleaner fuels instead of dirty ones, and for advanced technology like carbon capture and storage. That means a robust market-based system for capping heat-trapping emissions and giving businesses flexibility to meet those targets — often referred to as a "cap-and-trade" system — is needed. But to really push carbon capture and storage, we’ll also need new standards to ensure coal-fired plants are using this new technology.

Coal is one of the toughest aspects of any energy and climate legislative packages. If Congress is going to spend scarce tax dollars to burn coal cleanly, let’s not waste them on "coal-to-liquids" — a global warming loser — when the world is waiting for American ingenuity to demonstrate one of the most critical global warming winners — carbon capture and storage. Because if we do not solve this challenge, our fight to protect the planet from global warming will be lost before it even started.