A new study directly links increases in the frequency and severity of hurricanes to global warming. From BBC:
This new study, published in Philosophical Transactions of the Royal Society in London, looks at the frequency of these storms from 1900 to the present and it says about twice as many form each year now compared to 100 years ago.
The authors say that man-made climate change, which has increased the temperature of the sea surface, is the major factor behind the increase in numbers.
"Over the period we've had natural variability in the frequency of storms, which has contributed less than 50% of the actual increase in our view," said Dr Greg Holland from the United States National Centre for Atmospheric Research in Colorado, who authored the report.
"Approximately 60%, and possibly even 70% of what we are seeing in the last decade can be attributed directly to greenhouse warming," he said.