May 2016 was the 13th month in a row that broke a record for worldwide average temperature. It's hot out there.

May 2016 was also when it was abundantly clear that the two major-party candidates for president would be Donald Trump (R) and Hillary Clinton (D).

That means, for Americans, we have in one corner Trump, a candidate who believes that "global warming was created by and for the Chinese in order to make U.S. manufacturing non-competitive." And in the other corner we have Clinton, a candidate who says "Climate change is real, and threatens us all. We need to act."

You can name any of a thousand issues that divide Clinton and Trump, but I think if I had to choose one that is most representative of the difference between the two of them, both as people and as candidates, climate change would be the top of the list for several reasons.

First, the issue establishes which candidate accepts reality and which candidate rejects it. Trump's position that our warming climate is "a hoax" and "a money-making industry" is at odds not merely with the scientific consensus, but also indicative of his general sense that everything is a conspiracy.

Remember that Trump first came to prominence as a Republican political entity in 2011 when he financed "research" into the legitimacy of President Barack Obama's American birth certificate. Though Trump couldn't produce any evidence that Obama was not born in Hawaii, even in this year's campaign he has refused to say that Obama was born in the U.S.

Trump has also bitten on any number of other conspiracy theories in his campaign, from insinuating Hillary Clinton was behind the death of her friend Vince Foster to suggesting Ted Cruz's father may have been part of a plot to assassinate John F. Kennedy.

He's repeated apocryphal stories of U.S. soldiers shooting Muslim prisoners with bullets dipped in pig blood and that Muslims across the United States celebrated the destruction of the World Trade Center on Sept. 11. And, getting back to the world of science and climate, Trump recently denied that California was in a drought and said, if elected, he would just "open up the water" and solve the problem.

Given Trump's tendency to buy into them, it shouldn't be a surprise that his supporters, too, believe in conspiracy theories about everything from vaccines causing autism to the Sandy Hook Elementary mass shooting being a hoax.

Clinton, on the other hand, is pretty firmly grounded in reality. The closest she has ever come to embracing a conspiracy theory was her proclamation back in the day that there was a coordinated right-wing media attack on her and her husband, a claim that turned out to be true, according to people in the thick of it like David Brock.

In short, Trump is a wacko; Clinton is not.

A second way this issue serves to distinguish between Clinton and Trump is in how seriously they seem to be taking this campaign and this election.

Trump says he would begin his energy work with undoing much of what Obama has accomplished in his administration, like throwing out the historic Paris climate accord and rolling back Obama's Clean Power Plan, which is currently in legal limbo. He told a conference of oil and gas companies in May, "Obama has done everything he can to get in the way of American energy, for whatever reason. If ‘Crooked' Hillary Clinton is in charge, things will get much worse, believe me."

Trump is a big supporter of hydraulic fracking and wants to expand oil and gas production in the United States. He also contrasts himself with Clinton on the continued use of coal – the leading cause of carbon dioxide, a key greenhouse gas, in the U.S. – and the necessity of expanding clean power sources like solar and wind.

But all of this is just based on things he says or tweets; Trump's website lacks any page devoted to energy policy or the climate. It does, however, feature a section on making Mexico pay for "the wall!"

Clinton, on the other hand, has a detailed section on her website about energy and climate change. This includes everything from making American public infrastructure – schools and hospitals, for example – more energy efficient to environmental remediation in poor and minority neighborhoods where the U.S. has tended to dump its waste and toxins for generations.

Clinton regularly makes the climate and clean energy a part of her stump speech and campaign conversation, which sometimes leads to problems: She recently had to clarify to coal miners that, while she would like to end coal mining and coal production, she believes there are clean-energy jobs that could be had in coal country to make up for those lost coal jobs.

There's no question that both coal mining and coal burning are absolutely terrible for the environment, and I personally cannot fathom why we are still using what is essentially a 19th-century fuel source in the 21st century.

Still, the way Clinton and Trump have approached this issue makes clear that only one candidate, Clinton, has taken the time to consider what it means to actually be president and have to lead on issues like climate change and energy. Trump is just not serious about being president.

Finally, the issue of climate change really clarifies what kind of future the two candidates envision for this country.

Clinton sees a world where the U.S. is a leader in the production of clean energy and in reducing pollution and greenhouse-gas production. She has said that the U.S., China or Germany will be the future leader in clean energy, and there's no reason why it can't be us. For her, the green energy sector is a tremendous growth opportunity, and the country is on the cusp of an energy-efficiency revolution. Dealing with the environment is as much a social justice issue as it is an economic one.

Trump calls all of the above "job-killing." For him, the future of the country is the past: oil, coal and – really! – incandescent light bulbs. While he makes hand-wavy noises about clean air being a good thing, Trump does not actually seem to support any positions that would reduce air pollution. And, above all, he seems to be just fine with a future United States where rising sea levels threaten his own real estate holdings.

So as you all enjoy the summer weather this week, think about the climate and what might happen as a result of this November's election. Think about how one of these two is a grounded, thoughtful, serious candidate with a positive vision for a real American future. And think about how the other is a loon who believes in conspiracy theories, wouldn't take being president seriously and, to the extent he has thought about energy policy, is only wishing to regress to a polluted and unlivable past.

Then vote for the better candidate this fall. I'm sure you know which one she is.