Transportation planners and engineers typically extrapolate from historical weather and climate patterns in planning and designing infrastructure. The past will not be a good predictor of future conditions, however, as climate changes bring new weather patterns and climate extremes that exceed current experience. Projections of future climate are often depicted as gradual changes, such as the rise in global temperatures or in sea levels projected over this century. However, climate changes are unlikely to be experienced in such a smooth manner because human-induced changes will be amplified in some years by naturally fluctuating conditions, reflected in potentially sudden and dramatic changes at the regional or local level, where transportation infrastructure is located. Warming temperatures may trigger weather extremes and surprises, such as more rapid melting of the Arctic sea ice or more rapid rise in sea levels than is projected by current climate models.

On the basis of current knowledge, climate scientists have identified five climate changes of particular importance to U.S. transportation and estimated the probability of their occurrence during the 21st century:

• Increases in very hot days and heat waves (very likely),¹

• Increases in Arctic temperatures (virtually certain),

• Rising sea levels (virtually certain),

• Increases in intense precipitation events (very likely), and

• Increases in hurricane intensity (likely).