After 195 nations agreed to commit nearly all of the world’s countries to cut greenhouse gas emissions, heads of state praised the accord and the people who made it happen.

President Obama said the deal “sends a powerful signal,” deeming it a possible “turning point for the world.” Brazilian President Dilma Rousseff called the final version balanced and long-lasting, and U.K. Prime Minister David Cameron said “the nations of the world have shown what unity, ambition and perseverance can do.”

However, the agreement has at least two conspicuous holes: ships and planes.

“What didn’t go in is regulating as if you left out 164 nations,” James Corbett, a professor of marine science and policy as well as civil and environmental engineering at the University of Delaware, said in an interview.

Taken together, he explained, emissions from the aviation and shipping industries represent about 5 percent of humanity’s total emissions—the approximate equivalent of the collective carbon footprint from the planet’s least-polluting 164 nations, he explained.

The final text from the Paris summit, approved Saturday, following days of syntactical sanding, buffing, tweaking and negotiating, requires no action from the shipping and aviation industries, experts said.

Earlier drafts circulating in the months before and during the Paris conference included language on shipping and aviation.

One rough draft urged the “reduction of greenhouse gas emissions from international aviation and marine bunker fuels,” according to Transport & Environment, a Brussels-headquartered advocacy group. That language was removed.

Carbon pollution from both industries, already a substantial contributor to man-made climate change, is expected to rise sharply as international trade accelerates.

“Global efforts to stay well below 2 degrees [Celsius of warming], and especially 1.5 degrees, will be severely compromised if international aviation and shipping emissions continue to increase,” Mark Lutes, senior global climate policy adviser at the World Wide Fund for Nature’s global climate and energy initiative, said by email.

“These sectors,” he said, “are the fastest-growing source of emissions globally.”

According to the Organisation for Economic Co-operation and Development, shipping emissions could climb between 50 and 250 percent in the next 35 years if unchecked. At 2 percent of emissions worldwide, airplane emissions are rising 2 to 3 percent every year, according to the International Civil Aviation Organization, the U.N. body that oversees plane emissions and implements policies.

Where is home?
Negotiators at the Kyoto climate conference in December 1997 addressed the shipping and airplane footprints and assigned the sICAO and the International Maritime Organization, both U.N. agencies, to regulate the emissions of their respective industries. But the Kyoto Protocol only asked wealthy industrial nations to “pursue limitation or reduction of emissions” of “aviation and marine bunker fuels” that weren’t covered by the Montreal Protocol, a separate international accord signed in 1987 to address ozone depletion.

“They have tried to claim success in addressing emissions, but the reality is that they have made very little progress, and if nothing is done, emissions will continue to increase, putting climate objectives at risk,” Lutes said of the ICAO and the IMO.

Curbing emissions from planes and ships is challenging because it’s difficult to attribute those gases to a particular nation.

Panama, Liberia, Hong Kong and the Marshall Islands are some of the most common nations where ships are registered, or flagged, but ships registered there, as well as their crews and owners, often call at ports in different countries home and spend the bulk of their time sailing elsewhere. Airlines, planes and their crews follow a similar pattern.

In 2012, the Marshall Islands was the third-largest ship registry in the world, covering 2,700 vessels, but today is responsible for a sliver of the world’s greenhouse gases.

The issue for both modes of transportation is that regulating international trade and business transactions is complex: Plane and ship emissions, essentially fungible and without a clear nation of origin, occur worldwide.

“It’s basically layers and layers,” Corbett said of the challenge of attributing emissions to a specific country.

study published in November, which the European Parliament commissioned, said a market-based solution, like a carbon tax, is necessary to “satisfactorily reduce” greenhouse gases from international shipping.

“If, as in the past, the ambition of these sectors continues to fall behind efforts in other sectors and if action to combat climate change is further postponed, their emission shares in global CO2 emissions may rise substantially to 22 percent for international aviation and 17 percent for maritime transport by 2050,” the report said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500