Weather intensities are driven by natural variations and overall climate forcing. Forcing agents vary such as surface reflectivity, greenhouse gases and natural variation in atmospheric and ocean cycles as well as longer term forcing agents such as the Milankovitch cycles that drive long term climate change. Even longer term forcing agents have to do with tectonic shifts over millions of years.
Increased Radiative Forcing
First, the total radiative forcing has been increased by increases in industrial emissions to the atmosphere of Earth.
Source: Hansen et al 2008 PDF
Positive and Negative Forcings
Other changes have a cooling effect but the net change is positive:
Temperature Rises Due to Positive Forcing
This net positive forcing forces the temperature higher:
Source: IPCC AR4 WG1 Ch6 Fig-6-10
Increased Weather and Climate Event Intensity Increases
The higher temperatures interact with natural variation cycles which cause anomalous weather and climate behavior:
Source: IPCC AR4 WG1 Ch3 Fig-3-39