Fact: According to the National Hurricane Center, storms are no more intense or frequent than they have been since 1850.
Short-term natural cycles and variations dominate the historical record of extreme storm events. There has been no long-term trend in strong storms such as hurricanes or tornadoes.
The perception that hurricanes and tornadoes are getting worse is driven by a combination of a greater (and wealthier) population living in harm's way, better technologies directed towards detecting and tracking these storms, and nearly ubiquitous media coverage of extreme weather and its impacts. So when storms do strike, they cause seemingly more damage and suffering that is on display for the world to see.
But, when you take an unemotional looks at storm numbers themselves, nothing unusual stands out. There have been no long-term changes that could be associated with climate change from CO2 emissions.
The figure below shows the historical occurrence of strong-to-violent tornadoes across the U.S. since 1950 during the height of the annual tornado season (March-August). Note the large degree of year-to-year and decade-to-decade variability. The 1960s and 1970s were a very active period of strong tornadoes, the most recent decade has been relatively inactive.
Hurricanes show large annual to decadal fluctuations in their activity level. The figure below shows the level of hurricane activity summed across the global and the Northern Hemisphere for the past 50 years. Hurricane activity levels fluctuate widely, but there is little evidence for long-term behavior changes.
Conclusion: CO2 omissions are not leading to detectable changes in the behavior or characteristics of extreme storm systems like hurricanes and tornadoes.
Maue, R., 2009 Northern Hemisphere Tropical Cyclone Activity. Geophysical Research Letters, 36, L05805, doi: 10.1029/2008GL035946. Updates available at http://www.coaps.fsu.edu/~maue/tropical
National Climate Data Center, 2008. U.S. Tornado Climatology. Updates available at http://www.ncdc.noaa.gov/oa/climates/severeweather/tornadoes.html