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Canadian Prime Minister Stephen Harper, left, jokes around as he talks with United States President Barack Obama during dinner at the G7 meeting at Schloss Elmau near Garmisch, Germany on Sunday, June 7, 2015. THE CANADIAN PRESS/Adrian WyldAdrian Wyld/The Canadian Press

It's quite the thought: a "decarbonized" world by the end of century. No more fossil fuels being burned 85 years hence? No more reliance on internal combustion to move goods and people across oceans? Hard to imagine.

Yet that's the commitment the G7 announced Tuesday. "Urgent and concrete action is needed to address climate change," the leaders, Prime Minister Harper included, said in their closing declaration. "Deep cuts in global greenhouse-gas emissions are required with a decarbonization of the global economy over the course of this century."

The declaration includes a commitment to reduce global greenhouse-gas emissions by 40 to 70 per cent by 2050, compared with 2010 levels. But it's the "decarbonization" that is eye-catching. It is supremely ambitious, but also somewhat obvious. If global warming is real (it is), then over time the economy must dramatically cut emissions, and maybe even move to zero emissions, to limit the damage of climate change.

But there is more required than a simple declaration. As Mr. Harper said, "Nobody's going to start to shut down their industries or turn off the lights. We've simply got to find a way to create lower-carbon-emitting sources of energy – and that work is ongoing."

Ongoing, but burdened by the reality of the moment. With new oil reserves being discovered and exploited, and the price falling, there is little incentive for companies and consumers to develop and use new, low-emissions technologies, or to change behaviour in ways that reduce carbon output. The incentive will have to come from governments willing to impose carbon taxes on companies and consumers, thereby raising the price of carbon. Those carbon-tax revenues can then be used to lower other taxes, rather than to simply line the coffers of government. Stephen Harper is right to worry about the second possibility, but wrong to reject the first.

British Columbia's carbon tax is the best Canadian example of how to do it right, with higher taxes on pollution paying for lower taxes on everything else. It appears, so far, to have led to lower carbon emissions without harming economic growth. Mr. Harper appears to loathe carbon-pricing schemes, but a world safe from climate change's worst outcomes won't happen without government action. The best way to do that is with an idea that is as conservative as can be: Put a price on carbon, and let the market figure out the rest.