Flooding is complicated in that it is impacted by both climatic and non-climatic factors. Flooding is worsened by regional climate trends such as increases in heavy rain and snow (see precipitation section), early snowmelt, and increased seasonal precipitation. Flooding is also affected by non-climatic factors such as land development, deforestation (Paix et al. 2011), levee placement and local topography, making it challenging to determine an overarching climate signal in flood trends.

Heavy precipitation is one of the most direct ways that climate change contributes to increased flooding around the world. Very heavy precipitation has increased over the past 50 years in the U.S (NCA Chpt 2 Fig 2.16, p.50). The extreme precipitation during both the Nashville flood of 2010 and Hurricane Irene provides an example of what we can expect to see more often in a warmer world. Climate change may also have provided the extra boost to precipitation totals that allowed flood levels to exceed the limits of drainage infrastructure and begin causing serious damage ($3 billion and $15.8 billion, respectively).

In contrast to flooding driven by short-term extreme precipitation, flooding in large river basins is caused by seasonal precipitation persisting for weeks or even months. The frequency of great floods (100-year floods in large basins) around the world has increased over the course of the 20th century (Milly 2002). Very heavy, sustained rains drove record-breaking Mississippi River flooding in 2011.

Overall, flooding in the Midwest and Northeast has been increasing (NCA Chpt 2, p.47). In the areas of greater flooding, increases in both total precipitation and extreme precipitation contribute to this troubling trend (NCA Chpt 2, p.55).