In short, people keep betting him that the world will cool or warm slightly, rather than continue on its accelerating warming trend. Annan hasn't lost.
Most recently, Annan won $10,000 from two solar physicists at the Institute of Solar-Terrestrial Physics in Russia — Galina Mashnich and Vladimir Bashkirtsev — on a wager agreed upon 10 years ago.
But now that the results have come in (showing that 2012-2017 was warmer than 1998-2003), his fellow gamblers won't pay up.
"I was pretty confident in winning," Annan said in an interview. "Now, they're refusing to reply — I'm a little disappointed."
"They had 10 years to save up," he added.
There are no climate scientists, Annan included, arguing that such wagers will help solve the considerable political hurdles needed to dramatically lower modern civilization's greenhouse gas emissions.
Yet, it's a continuance of a rich history of researchers making similar scientific antes, and underscores that the climate isn't just warming at an accelerating pace, but that humans — not the whims of the sun or other natural processes — are now to blame.
For some scientists, it's an easy bet.
"It’s like taking candy from a baby," Bill Patzert, a former NASA climatologist who spent decades researching the rising trends of both sea levels and global temperatures, said in an interview.
“I’m not adverse to taking candy away from [climate] skeptics," said Patzert. “Often in my public lectures, I have offered to take all comers on sea level rise and temperature. At this point, I'll definitely take all bets."
In the past, prominent scientists have both won and lost wagers on a wide variety of scientific topics.
"There is a long history of people using bets to encapsulate their beliefs," said Annan.
Famed theoretical physicist Stephen Hawking once conceded a light-hearted defeat to physicist Kip Thorne on a matter of black holes. In 1990, biologist Paul Ehrlich lost a wagerto economist Julian Simon after betting that certain valuable metals would grow more scarce, and expensive.
Today, however, the extreme minority of scientists who remain skeptical about the overwhelming consensus that the Earth will continue warming at an accelerating pace are quite careful about how they might place a bet, if they choose to bet at all.
Over a decade ago, Annan attempted to bet MIT atmospheric physicist Richard Lindzen — who, broadly speaking, doubts climate change forecasts are scientifically plausible — that temperatures would rise, not cool.
But Lindzen asked for 50 to 1 odds in his favor — meaning that if Lindzen won, Annan would have to pay 50 times more than Linzen. Annan, not pleased to be put at such a profound disadvantage, declined.
Still today, Lindzen would take such odds — which puts him at little risk.
"At 50-1, I would certainly bet that 2018-2023 will be cooler than 1998-2003," Lindzen said over email.
Lindzen, who has debated Bill Nye about climate change, maintains the world's future warming will be only slight, and that global temperature will always fluctuate in small ways. For this reason, he's adverse to taking lower odds.
"Betting on small changes is pure gambling," said Lindzen.
A wager on how much temperatures might rise, however, seems more amenable to Roy Spencer, a meteorologist at the University of Alabama in Huntsville who continually asserts the current warming trend is insignificant and will continue to be insignificant.
"The question is, how much warming?" Spencer said over email. "That would have to be part of any bet I'd participate in. I believe future warming will be weak and possibly even beneficial."
There's no question Lindzen and Spencer are in an extreme climate science minority, and they're well aware of it. Their analysis or research efforts, for instance, aren't seriously considered by either the U.S. Climate Assessment or the United Nations' Intergovernmental Panel on Climate Change (IPCC) — the global agency tasked with providing objective analyses of the societal impacts of climate change.
But even mainstream scientists, as recently as 2008, have argued in peer-reviewed scientific literature that the world would likely experience a temporary cooling trend. They were quickly challenged.
In 2008, a group of six climate scientists including Penn State's Michael Mann challenged the researchers to a bet.
In the study published in the journal Nature, the cooling forecasters concluded that both intervals of 2000 to 2010 and 2005 to 2015 would be slightly cooler than conditions between 1994 to 2004.
But would the world actually cool during these periods, even just as a temporary cooling blip?
"We think not — and we are prepared to bet serious money on this," Mann and company wrote.
But this 2008 bet, underscored Mann, was far different than the bet Annan recently won against the Russian solar scientists, who argued the sun's activity — not human activity — is currently responsible for the globe's warming.
The scientists who published the study challenged by Mann and others "weren’t denying human-caused climate change — they are mainstream researchers and they were doing honest science," Mann said over email. "However, they were making an extraordinary claim."
"And as Carl Sagan famously said, extraordinary claims require extraordinary evidence," noted Mann. "They were unable to provide that evidence and we didn’t think their prediction was good science.
Mann and company were never taken up on the scientific wager. But Mann would have won.
"Subsequent time and data proved them wrong and us correct," said Mann.
As climate scientists are always quick to point out, short-term changes in climate can be telling, but longer-term trends are the gold scientific standard.
“Continuing warming, when averaged over sufficient time span variability, like of 10-20 years, seems like a great bet to me,” David Archer, a geophysicist at the University of Chicago, said over email. Archer was part of Mann's group who previously challenged the Natureresearchers in 2008.
There's also another, somewhat obvious trend: Annan keeps winning.
He's won a variety of smaller wagers, he said. But this doesn't exactly make up for his not getting paid $10,000 for a clear win.
"Those guys should pay up — that's not right," said Patzert.
Also tellingly, the few climate skeptics out there might just not take bets at all.
In May 2005, Nature reported that British environmental writer George Monbiot challenged climate skeptic Myron Ebell — who led President Trump's Environmental Protection Agency (EPA) transition team — to a $9,000 bet.
Ebell, who has confidently stated that climate change "is nothing to worry about" — would not take the bet.
But Annan, like Patzert, is still very much keen on taking wagers that the planet will continue its warming trend.
"If anyone wants to argue otherwise, I would be happy to take their money," said Annan.