By Dr. Robert Thorson

The fear and dread caused by Hurricane Katrina has put the entire United States on alert for new storms. The most anxious of us despair that we’ve crossed a climate threshold into a scary new world of permanently stronger storms. The most optimistic of us hope we’re simply in a stormy phase that will eventually pass.

I’m stuck in the middle on this one, leaning slightly toward hope rather than despair.

Let’s get the bad news over with first. As the whole Earth warms from greenhouse emissions, so does the subtropical Atlantic, the birthplace of strong storms. If everything were to remain the same, a warmer hurricane nursery should — at least in theory — yield stronger and more frequent storms.

But everything isn’t staying the same. There’s another important control on sea-surface temperature besides greenhouse warming. Something called the Atlantic Multidecade Oscillation toggles the U.S. climate back and forth between a few decades of warmer-than- usual conditions (with greater hurricane frequency) to a few decades of colder-than-usual conditions (with weaker storms). The perfectly natural AMO is so sluggish that it takes about one human lifetime (60 to 80 years) to complete a full cycle.

Now the good news. There’s a good chance that the enhanced greenhouse warming will bring the present hurricane-rich warm phase of the AMO to an earlier end.

Temperature records show that average North Atlantic temperatures were warmer than usual from about 1870 (when good temperature data became available) to about 1900. The region then experienced cooler- than-normal conditions until the late 1920s. Hotter conditions returned around 1930 and the mid-1960s, a time of frequent and strong hurricanes in the eastern United States. By the late 1960s, it was cooler again, a condition that persisted until about 1995, when the climatic tide turned for the last time and the present warm phase commenced. The modern interval of frequent hurricanes in the east, parching drought in the American heartland and the killing heat waves in Europe are taking place within the present warm phase of the AMO, perhaps being amplified by human-caused warming.

Although these observed historic fluctuations suggested a 60- to 80-year temperature cycle, the record was not long enough, nor was our understanding of the ocean’s behavior complete enough, to be certain — until this year, that is. Now, independent results from two of Britain’s sophisticated climate research centers strongly suggest two things: that the AMO has been an important factor in Atlantic circulation for at least 1,400 years; and that it toggles the North Atlantic climate between warm and cool phases.

Here’s how the oscillation works. Under background conditions, warm tropical water moves northward via the Gulf Stream until it sinks near Greenland and Iceland. There it descends to create a south-flowing, deepwater current. During the warm phase of the AMO, an enormous mass of fresher-than-normal water spreads northward, inhibiting the conveyor-belt circulation. This more sluggish oceanic conveyor pulls less warm water northward, cooling the hurricane nursery and, at least theoretically, reducing storm frequency and strength.

Accelerated global warming caused by carbon emissions is still very real, and its health and economic consequences will likely be negative in many places. But on the plus side, greenhouse warming may force an earlier flip from this more volatile phase of the AMO to its cool phase. At that point, and unless greenhouse warming overwhelms the AMO signal, the United States can look forward to at least a decade of less frequent, less powerful storms.

In the meantime, batten down the hatches. We’re not out of this year’s hurricane season yet. Nor are we out of the present warm phase of the AMO. The best we can hope for is that the coming cool phase will bring us some relief.