Chalk it up to natural fluctuations in weather patterns, said Trenberth. In this case, the cause is the periodic warming and cooling cycles in the Pacific Ocean known as El Niño and La Niña.
In the winter of 2004-2005, warmer water temperatures brought on by a weak El Niño meant lighter winds and sunny skies in the tropical Atlantic, where hurricanes form.
"The sun was beating down.There was less heat energy being sucked out by trade winds," said Trenberth. "The water warmed up and that set the stage for the 2005 season."
But this season the opposite occurred. A La Niña cooling of the Pacific Ocean led to stronger trade winds that pulled more heat energy out of the ocean while also helping prevent hurricane-strength winds from forming. Cloudier skies meant the sun didn't heat the oceans as much.
"There was very little heat in the ocean" compared with 2005, Trenberth said.
Despite the natural fluctuations, a growing body of scientific research argues that human-induced global warming is having an increasing effect on the intensity of hurricanes.
Scientists said it is difficult to predict just how future hurricane seasons will stack up, but according to a 2005 study by scientists at Georgia Tech University and the National Center for Atmospheric Research, the number of Category 4 and 5 hurricanes -- packing winds up to 155 miles an hour -- has doubled in the past 35 years.
Up in the Arctic, the amount of summer sea ice that melted in the summer of 2006 may not have broken the record set in 2005, but that doesn't necessarily mean good news, Serreze said.
Throughout the early summer, for example, the amount of sea ice was well below the 2005 record. An unusually cooler, cloudy August that reflected the sun's energy away from the ice helped reverse the steady downward trend.
The big picture, researchers said, shows that Arctic sea ice is "sharply declining" and has melted back at almost 10 percent per decade over the last 30 years. If that trend continues, some scientists believe the Arctic could be completely ice free by 2060.
"In terms of the long-term trends in the Arctic sea ice cover, I'm certainly pessimistic," Serreze said.
We'll still need weather forecasters in coming decades, said scientists, to tell us how tomorrow will be different from today. Even as Earth's overall temperature -- its climate -- gets warmer and warmer.