Wind speeds and wave heights over the world's oceans have been rising for the past quarter-century.
It's unclear if this is a short-term trend, or a symptom of longer-term climatic change.
Either way, more frequent hurricanes and cyclones could be on the horizon.
Ian Young at the Australian National University in Canberra and colleagues analysed satellite data from 1985 to 2008 to calculate wave heights and wind speeds over the world's oceans.
They found that winds had strengthened – speeding up over most of the world's oceans by 0.25 to 0.5 per cent, on average, each year.
Overall, wind speeds were 5 to 10 per cent faster than they had been 20 years earlier.
The trend was most pronounced for the strongest winds.
For instance, the very fastest 1 per cent of winds were getting stronger by 0.75 per cent per year, says Young.
Average wave height was also on the rise, but less so; and the highest waves showed the strongest trend.
The results were compared against conventional measurements taken from deep-water buoys and numerical modelling.
"There is variability, but the same general features are observed," Young says.
From space to sea
Previous attempts to investigate these phenomena used observations from ships and buoys, but these could generally provide only a regional picture.
Using altimeter data from satellites allowed the team to detect decadal trends on a global scale for the first time.
Satellite altimeters use radar to measure the height of points on the Earth's surface, and can measure wave height very precisely.
Measuring the amount of backscattering from the radar signals, meanwhile, can help calculate wind speed.
The global view afforded by the satellites reveals stronger trends in some areas than in others.
For example, both wave height and wind speed have been increasing more rapidly in the oceans of the southern hemisphere than in the north.
Young can only speculate on what is causing the increases.
"If we have oceans that are warming, that energy could feed storms, which increase wind speeds and wave heights," he says.
But with a data series that covers just two decades, it's too early to tell whether there's a long-term trend at work. "We don't know the driving force."
Considering there are so many regional forces influencing waves and wind, "it's surprising that there is such a uniform trend", says Mark Hemer, a wave researcher at the Centre for Australian Weather and Climate Research in Hobart, Tasmania.
Variability in winds and waves associated with weather systems such as El Niño and La Niña, the North Atlantic Oscillation and the Southern Annular Mode could all help to explain it, he says.
In either case, if winds continue to strengthen and waves to rise – even if only for a few years – it suggests more intense storms, hurricanes and cyclones are on the horizon, says Young.
However, Tom Baldock, a coastal engineer at the University of Queensland in St Lucia, Australia, says that although there is no reason to doubt the analysis, it doesn't mean more coastal natural disasters will necessarily ensue.
"Tornados, hurricanes and cyclones occur through complicated regional weather conditions, and are not just related to wind speed and wave height," he says.
For example, there are higher wind speeds at high latitudes, but most cyclones hit around the equator.
The new study may be more relevant to the burgeoning offshore gas, wind, wave and tidal power industries, Baldock thinks.
"Larger waves are a hazard for any offshore construction."