We've passed the midpoint of the Atlantic hurricane season, and there's been not a storm to see. What does a hurricane drought tell us about how climate change will impact tropical cyclones?

NASA / Getty Images How will climate change affect storms like Sandy, seen in a satellite image?

It’s been quiet this hurricane season — too quiet. Back in May, the National Oceanic and Atmospheric Administration predicted a “very active” tropical-storm season, one with between seven and 11 hurricanes. The agency lowered that estimate in August to between six and nine hurricanes, but still predicted that three to five of them would become major storms, with winds above 111 m.p.h.

We’re now a day away from the exact midpoint of the Atlantic hurricane season, the period when storm activity is historically at its strongest — Sept. 10 — and so far we’ve experienced … nothing. While there have been a handful of tropical storms, we have yet to see a single hurricane — a tropical cyclone with sustained winds of at least 74 m.p.h. This is extremely unusual, and if the calm continues, 2013 could rival 2002, which didn’t experience its first hurricane until Sept. 11. What gives?

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It’s important to remember that while some of the biggest hurricanes have struck in August or very early September — Andrew in 1994 1992, Dolly in 1996, Katrina in 2005 — the storm season officially lasts until the end of November. Last year Sandy struck at the very end of October, and statistically, the back half of the hurricane season tends to be much stronger than the front half. An October surprise of a storm could still be out there, waiting to strike.

Still, the lack of activity in the first half of the storm season demands explanation. The abundance of warmer, drier air across the Atlantic this summer has made the atmosphere more stable, discouraging the development of strong storms. There’s also a lot of wind shear, when wind at different altitudes occur in different speeds and directions, which tends to snuff out new tropical storms. It’s also possible that dust from North Africa, which can reduce the temperature of the sea surface, may be stalling storms. (Hurricanes are fed by warm ocean waters, which is why they form in the tropics.)

The truth is that scientists aren’t really sure why there hasn’t been a hurricane yet this season, nor do they know why an intense hurricane — Category 3, 4, 5 — hasn’t made landfall in the U.S. since Wilma all the way back in 2005. (Sandy, for all the damage it did, was barely a Category 1 storm by the time it made landfall along the East Coast.) And as Andrew Revkin reported in the New York Times, leaked drafts of the forthcoming Intergovernmental Panel on Climate Change (IPCC) seem to reflect a reduced scientific certainty that global warming will make storms stronger and more frequent. In the 2007 report, the IPCC said that it was more likely than not — a greater than 50% certainty in the panel’s terminology — that human activity was contributing to an observed intensification of hurricane activity in some parts of the world. Now the IPCC — or at least the draft — says it has “low confidence” of that relationship, which means it believes that there is only a 2-out-of-10 chance of being correct. The estimated probability that the 21st century will see more intense hurricane activity has fallen as well.

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What happened? Chris Mooney at Mother Jones notes that some recent studies cast doubt on the connection between warming and stronger storms:

There’s an intense scientific debate going on here, and new research conducted since 2007 has given indications that the hurricane picture under climate change may be more complicated than previously supposed. That’s because even as warmer oceans provide jet fuel for hurricanes, changes in atmospheric wind patterns can still interfere with their formation by preventing storms from forming or, literally, tearing them apart. In particular, the IPCC in its latest round likely took note of a major 2010 paper in the journal Nature Geoscience, authored by no less than 10 hurricane experts, finding that “it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes.” It’s also important to keep in mind how the IPCC works: There’s a cut-off date for assessing the existing scientific literature to be included in each report, and work published after the deadline just doesn’t get taken into account.

Indeed, some more recent studies offer evidence that climate change will indeed soup up storms, including one I reported on in July:

A new study in the Proceedings of the National Academy of Sciences, however, suggest that we may not be so lucky. Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology (MIT) and one of the foremost experts on hurricanes and climate change, argues that tropical cyclones are likely to become both stronger and more frequent as the climate continues to warm — especially in the western North Pacific, home to some of the most heavily populated cities on the planet. But the North Atlantic — meaning the U.S. East Coast and Gulf Coast — won’t be spared either. Bigger bullets, faster gun.

Then there’s another new study that predicts that changing atmospheric conditions created by climate change could actually make storms like Sandy — which struck the East Coast of the U.S., with disastrous consequences — less likely in the decades to come.

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The back-and-forth over hurricanes underscores how heated the scientific debate remains over the exact effects of future warming. If it’s difficult for scientists to understand why there’s been a drought of hurricanes so far this year, you can imagine how challenging it remains to project storm activity decades into the future. But we do know that warming will cause the seas to rise, which in turn will amplify the coastal flooding that tends to cause the most damage when a storm strikes. A study published last week found that the sea-level rise we’ve already experienced has already doubled the chance of a Sandy-like flood in New York City. Depending on how much sea level continues to rise, Sandy-like flooding could occur in New York once every few decades by the end of the century. Add in the fact that the value of property and the number of people living in coastal areas continues to rise, and you can understand why we’ll need to remain on guard for tropical storms — no matter what global warming does.

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