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Ottmar Edenhofer, Co-Chairman of the IPCC Working Group III, and Rejendra K. Pachauri, Chairman of the IPCC, from left, pose prior to a press conference as part of a meeting of the Intergovernmental Panel on Climate Change (IPCC) in Berlin on April 13.Michael Sohn/The Associated Press

Over the past six months the UN's Intergovernmental Panel on Climate Change (IPCC) has issued two weighty reports, each accompanied by a strongly worded message that rests on the work of many hundreds of individual scientists.

Together those two messages – that climate change is undeniable and that its impacts are already being felt in every part of the globe – underscores a dilemma for a world in which the strongest drivers of fossil fuel emissions are economic growth and population.

Now comes a third message, politically the most difficult to deliver and consequently the most delicately worded: Without significant near-term action to reduce the output of heat-trapping greenhouse gases, the planet is on track to warm by roughly four celsius degrees by the end of this century. Such an outcome would be costly for the developed world and potentially catastrophic for poorer nations that cannot adjust to a changing planet.

To avoid it, global emissions will have to shrink at least 40 per cent and possibly as much as 70 per cent in the next 35 years, according to the latest IPCC report, released Sunday.

For Canada and other oil-rich nations, that report comes with a troubling corollary. Locking in to carbon-intense industries that anticipate many decades of future growth could prove a costly choice in a world that urgently needs to move its energy consumption in a different direction.

"I think the takeaway message is we should not be investing in long-lived expansion of fossil fuel infrastructure," said Danny Harvey, a University of Toronto climate scientist and one of the Canadian lead authors of the report, accepted by the IPCC last week in Berlin.

Together, the three reports form the IPCC's most comprehensive assessment of climate change to date. The document is expected to be a key point of reference for international negotiators as they work toward a meeting in Paris in late 2015, where a binding agreement on climate change mitigation could be on the table.

The latest report, which encompasses the work of 235 authors, also looks at where progress can be made by curbing forest loss and by improvements to transportation systems, industrial processes and buildings.

"You can design buildings that use one half or less of the energy of buildings recently built for no extra cost," said Dr. Harvey, who worked on the chapter of the report that dealt specifically with building practices.

More broadly, the report estimates that the overall cost of limiting the average global temperature to just two degrees over its pre-industrial level – the current international goal – is relatively modest, and would amount to slowing economic growth by 0.06 per cent annually.

"You don't have to spend the world to save the planet," Ottmar Edenhofer, a German economist and co-chair of the working group behind the report, said at a news briefing on Sunday. But lurking behind the report's diplomatic language is the continuing tension between emerging economies that are still growing and using more fossil fuels – including coal – and the developed world, which is scaling back but is responsible for the bulk of the carbon already released into the atmosphere.

The report shows that emissions are currently flattening or in decline in the developed world but experts note that if emissions are tagged to the countries that consume the products made by generating greenhouse gases, the picture is dramatically different.

Seen in that light, "we have not reduced our emissions at all. We've exported dirty industries to the developing world," said Jake Rice, a lead author on the report and chief scientist with Fisheries and Oceans Canada.

One positive note is that while multilateral climate talks have bogged down for the past several years, "there's been an enormous explosion of other sorts of international agreements," said Matthew Paterson, a professor of political science at the University of Ottawa who contributed to a chapter on international co-operation.

Some of those agreements involve networks of cities, institutional investors and carbon markets that are operating in different sectors and regions of the globe.

"I think the question for policy makers is no longer, 'How do we have a single international treaty that will solve everything?' but 'How do we design international treaties that can maximize the impact of all this effort?'" Dr. Paterson said.