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鈥淥il鈥� is a deceptively simple word. For most, those three letters conjure up images of a viscous, black liquid gushing from Texan oil wells. But in today鈥檚 evolving energy landscape, oil has come to encompass a vast range of crudes that vary dramatically in color, density, and use. And as the portfolio of oils expands, that range of oils' environmental impact expands, too.

Growing thirst for fossil fuels around the world has driven oil producers to more extreme measures for finding new crude 鈥� from mining Canada鈥檚 oil sands, to drilling unconventional wells to access deposits trapped in North Dakota's shale rock, or deep beneath the ocean floor.

Researchers are beginning to quantify the varying environmental impacts of these different oils, particularly as they relate to climate change. The results could help companies and policymakers better decide when and where to drill in a world increasingly concerned with the cost of carbon 鈥� both financial and otherwise.

鈥淭here鈥檚 room for wringing inefficiency out of the petroleum sector, even as we continue to rely on petroleum,鈥� says David Livingston, an associate in the energy and climate program at the Carnegie Endowment for International Peace, a Washington-based think tank. 鈥淲hat this project reveals is the scope there is to achieve climate mitigation in the existing petroleum sector.鈥� 听

That project is Carnegie鈥檚 produced in collaboration with Stanford University and the University of Calgary. The study finds that the spread between the carbon emissions from the worst and the best kinds of crude is large: A sample of just 30 oils revealed an 80 percent difference in total greenhouse gas emissions per barrel between the lowest- and highest-emitting oils. And the researchers say the spread will likely be larger when more types of crude oil are added to the index, since the 30 oils tested represent only about 5 percent of global production.

The transportation sector 鈥撎齧ore than 90 percent of which is fueled with petroleum 鈥撎齧akes up more than a quarter of US greenhouse gas emissions, according to the US Environmental Protection Agency. That makes cars, trucks, and airplanes second only to power plants as the country鈥檚 biggest emitter of gases that warm the planet and alter the climate.

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Carnegie鈥檚 study aims to document emissions throughout the various oils鈥� entire life cycles 鈥� from extraction, transport, refining, marketing, combustion, to end use. That could come in handy as investors, policymakers, companies, and the public look to slash their emissions.

鈥淭he better and smarter you can design policies to manage these oils, the better you can get us along the path to more efficient use,鈥� Deborah Gordon, director of Carnegie鈥檚 energy and climate program, said in a briefing ahead of the index鈥檚 Wednesday release.

At December climate talks in Paris, negotiators from around the world hope to strike an international climate deal to reduce emissions and avert the worst impacts of climate change. To show the US is making progress ahead of those talks, Obama rolled out his Clean Power Plan in 2013, aiming to slash US power plant emissions 30 percent below 2005 levels by 2030. The hope is that other nations will join the US in making the steep cuts necessary to scale back global emissions.

In the last 20 years, nearly 75 percent of human-caused carbon emissions came from burning fossil fuels like oil, coal, and natural gas, to the Department of Energy. And to avert the worst effects of climate change, scientists say countries need to scale back their emissions.

It鈥檚 estimated that 鈥済lobally, a third of oil reserves, half of gas reserves and over 80 percent of current coal reserves should remain unused from 2010 to 2050鈥� to keep the plant from heating up more than 2 degrees C, according to a January published in the journal Nature. International climate talks in the past have called for emissions reductions sharp enough to limit warming to 2 degrees.

鈥淚t鈥檚 companies and countries that hold large coal reserves that are going to suffer under a 2 degree scenario, and also the Arctic oil,鈥� Christophe McGlade, author of the Nature study and a professor at University College London, Mashable.

But if governments can take into account the varying emissions of different types of oil, they can make better decisions about what oils are worth drilling. That would help countries factor the cost of carbon pollution into the price of different oils, and reduce their climate impacts.

鈥淭here's more oil than ever before, and it's not going away,鈥� says Ms. Gordon. 鈥淭here are different types, and we haven鈥檛 been accounting for all the carbon involved.鈥�