Imaging: M. Iqbal Shaikh

Here’s a simple solution to global warming: vacuum carbon dioxide out of the air.

Klaus Lackner, a physicist at Columbia University, said placing enough carbon filters around the planet could reel the world’s atmosphere back toward the 18th century, like a climatic time machine.

After a decade of work, his shower-sized prototype whirs away inside a warehouse in Tucson, Arizona, each day capturing about 10 pounds (nearly 4.5kg) of the heat-trapping greenhouse gas as air wafts through it.

Only a few billion tonnes to go.

But Lackner’s machine is a clunky reminder of how distant that dream is. He estimates that sucking up the current stream of emissions would require about 67 million boxcar-sized filters at a cost of trillions of dollars a year.

The orchards of filters would have to be powered by complexes of new nuclear plants, dams, solar farms or other clean-energy sources to avoid adding more pollution to the atmosphere.

So while the science of dialling back the planet’s thermostat is straightforward, the execution is fabulously expensive, complex and grandiose on a scale that boggles the mind.

The 1997 Kyoto accords were supposed to bring the world together to address the problem, but the two biggest polluters, the United States and China, have refused to cap their emissions, and Europe is failing to meet even its modest targets.

Worldwide annual emissions of carbon dioxide — the main culprit in global warming — have climbed 28 per cent over the last decade, according to the US Department of Energy.

The rise has been driven largely by industrialising countries, such as China and India, which claim that they have the right to exploit their coal reserves to catch up with the West.

Among options, carbon filtering is the most direct and best understood. If industrialisation is a process of transferring carbon stored in the earth to the atmosphere, filtering seeks to put it back.

The process for removing atmospheric carbon involves putting one compound, usually a hydroxide, in contact with the air, setting off a reaction that grabs carbon dioxide and incorporates its carbon atoms into a carbonate compound. Then, in a reaction that requires a large input of heat, the carbonate compound is broken apart, reconstituting and trapping the carbon dioxide.

Researchers propose pumping the captured carbon dioxide into the ground. Geologists say there is room in subterranean rock formations to lock it away forever.

This carbon capture scrubs the planet without intruding on it, unlike artificial volcanoes and sun reflectors, which could cause enormous planetary damage in the form of acid rain or giant shadows that stunt crops.

The filters could be placed anywhere in the world, because carbon dioxide disperses throughout the atmosphere.

For all its appeal, the process is hideously inefficient. Carbon dioxide makes up less than 0.04 per cent of the atmosphere, and removing climate-changing quantities of it requires filtering massive amounts of air.

Lackner calculated that sucking up all 28 billion tonnes of carbon dioxide released worldwide each year would require spreading out his machines over a land area the size of Arizona (nearly 300,000 sq km).

That seems reasonable until you consider the expense.

Experts estimate that it would cost as much as $200 a tonne to filter and store carbon dioxide from the air. That means the yearly vacuuming bill could reach $5.6 trillion.

The enormous cost raises the question: Who would pay?

The impasse remains. China claims that the West should foot the bill because it created the problem over the last two centuries. The US says China must accept its share of responsibility as the world’s new top polluter.

The cost will surely fall over time, but without government action that is unlikely to happen soon enough to stave off the worst effects of climate change.

Without at least a 50 per cent cut in emissions by mid-century, the United Nations’ Intergovernmental Panel on Climate Change predicts that the temperature rise will exceed 2 degrees, resulting in worsening drought, a dangerous sea level rise and widespread extinction of species.

Paul Crutzen, a Nobel Prize-winning atmospheric chemist at the Max Planck Institute for Chemistry in Mainz, Germany, said that only out of a “sense of despair” had he come to favour the last-ditch option of spewing more than a million tonnes of sulphur a year into the air.

It’s a dirty proposition that, in some ways, is its own environmental crime.

But it works, as shown by the 1991 Mount Pinatubo eruption in the Philippines, which temporarily cooled the planet by almost 1 degree Fahrenheit (0.56 degree Celsius). “It might be the last escape route from the problem,” he said.

The power to re-engineer the planet raises another question: Who gets to control the thermostat?

With enough carbon filters, a single country or even several rich individuals would have the power to set the world’s temperature.

“No matter how you go about it, there will be a lot of politics,” Lackner said.

For now, his machine, a solitary prototype, continues to hum away in the Tucson warehouse. With no good place to store the carbon dioxide it traps, the gas is simply released back into the air.