Foreign policy, national security, and taxes typically emerge as the biggest issues in presidential races, pushing seemingly "smaller" issues to the side. Climate change is no small issue, though. In the State of the Union address, President Obama said, "No challenge — no challenge — poses a greater threat to future generations than climate change." It's no longer a mystery what's causing the extreme warming — 97 percent of climate scientists agree that climate change is caused by human activity. So what are the current 2016 presidential candidates planning to do about climate change?

The presidential candidates from both parties cover a wide range of opinions on the issue, and who thinks it's a real threat and who believes it's a hoax doesn't necessarily split down party lines. Despite the overwhelming amount of scientific data and explanation, some Republican candidates still deny the existence of global warming altogether, while others just disagree with scientists' projections for the future. The candidates that not only believe in climate change, but also want the country to take action to slow it down, are in much fewer numbers.

Here's a definitive ranking of how green each candidate is on a scale of zero to 10 (zero being the least and 10 being the most environmentally friendly).

Bernie Sanders: 10

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Bernie Sanders has already been called "the best candidate on climate change," based on his ranking as the top climate leader in the Senate by the Climate Hawks Vote super PAC. Sanders has written, sponsored, or introduced six bills aimed toward reducing America's harmful impact on the environment since 2007.

Martin O'Malley: 9

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Martin O'Malley was very influential in making Maryland more green while he was governor, signing a law to reduce the state's greenhouse gas emissions by 25 percent from 2006 levels by 2020, cleaning up the Chesapeake Bay, and expanding public transportation. He just launched his presidential campaign a few days ago, so it's too soon to tell how much he'll push climate change issues in his run for president.

Hillary Clinton: 7

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Hillary Clinton believes wholeheartedly that climate change is real, saying in 2014, "The science of climate change is unforgiving, no matter what the deniers may say, sea levels are rising, ice caps are melting, storms, droughts, and wildfires are wreaking havoc." She co-wrote a bill with Sanders in 2007 that allocated funds for jobs in clean energy. However, she promoted fracking abroad while she was Secretary of State, and the Clinton Foundation receives large donations from big oil companies like ExxonMobil and ConocoPhillips.

George Pataki: 7

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George Pataki is also a global warming believer and even co-chaired the Council on Foreign Relations' task force on climate change in 2007, but he thinks emissions should be reduced by private and market-based incentives, so he's not likely to push any government regulations.

Lindsey Graham: 6

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Lindsey Graham rides the middle ground when it comes to climate change theory and policy. His voting record illustrates that he doesn't support government regulation, but rather tax incentives for companies in the clean energy sector. In 2010, he opposed the Environmental Protection Agency's emission regulations, saying, "Carbon is bad. Let’s do something about it in a common sense way."

Carly Fiorina: 4

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Carly Fiorina thinks climate change exists, but doesn't think government regulations or international deals will solve the problem. In April, she said, "The only answer to this is innovation," explaining that scientists should focus on technological advances that would reduce emissions. This approach means doing nothing while waiting for a technological fix.

Ted Cruz: 3

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Ted Cruz doesn't necessarily believe climate change is a hoax, but he doesn't trust scientists' projections of what will happen in the future because of global warming and thinks the warming might be caused by a natural process no one has discovered yet.

Marco Rubio: 3

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Marco Rubio has successfully remained non-committal to any stance on climate change. On Face the Nation in April, he conceded that humans have played some role in global warming, but not as much as "some of these people out there are trying to make us believe." He also doesn't think anything the U.S. does will have a positive impact on the problem.

Mike Huckabee: 2

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While Mike Huckabee hasn't taken a real stance on climate change, he doesn't think scientists' predictions are accurate. At the Iowa Freedom Summit in January, he mocked President Obama's plans to fight global warming, saying, "Not to diminish anything about the climate at all, but Mr. President, I believe that most of us would think that a beheading is a far greater threat to an American than a sunburn." He's really out of touch if he thinks the worst effects of climate change are sunburns.

Ben Carson: 2

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Ben Carson gives pretty vague answers when asked about climate change, but he clearly doesn't believe it's a threat. In a 2014 interview, he said, "We may be warming. We may be cooling."

Rand Paul: 1

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Although Rand Paul voted for an amendment this year that said climate change is real and was caused by human activity, he's said in the past that he's "not sure anybody exactly knows why" the globe is warming. Was he aware of what he was voting on? He also lashed out against the Environmental Protection Agency's attempts to reduce greenhouse gas emissions, calling them "illegal" and promising to repeal them. Actively fighting environmentally friendly efforts is worse than just not believing in it.

Rick Santorum: 0

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Rick Santorum has said that the idea that human activity caused global warming is "patently absurd" and that it's due to natural fluctuations in temperature. "It's just an excuse for more government control of your life," he said on The Rush Limbaugh Show in 2011. "And I've never been for any scheme or even accepted the junk science behind the whole narrative."