The average rainfall of Hawai'i’s island ranges from 10 inches (250 mm) to 445 inches (11,300 mm) [4].  Local climates vary considerably on each island, generally divisible into windward and leeward areas based upon location relative to the higher mountains.  Windward sides face the Northeast Trades and receive much more rainfall; leeward sides are drier and sunnier, with less rain and less cloud cover [8].  Variations in rainfall on the Hawaiian islands are dramatic.  At one extreme (for example, the west side of Hawai'i Island), the annual rainfall averages 20 inches. At the other extreme, the annual average exceeds 300 inches (7,620mm) along the lower windward slopes of the high mountains in Haleakala (Maui), and at the summit of the lower mountains of Kauai, O'ahu, and western Maui [2]. Mount Wai'ale'ale on Kauai, is notable for rainfall, as it has the second highest average annual rainfall on Earth, about 460 inches (38 ft. 4 in., or 11.7 m) [8].

Diamond Head, Oahu, Hawaii. Source: Cheryl Anderson.

Waipio Valley, Hawaii. The Waipio Valley is often referred to as the "Valley of the Kings" because it was once the home to many of the rulers of Hawaii. The valley has both historical and cultural importance to the Hawaiian people.

Hilo side of Hawaii. Source: Steve skipper, NRCS.

Hawai'i has experienced rapid warming, especially since the mid-1970s. Its air temperature trend has diverged from the Pacific Decadal Oscillation (PDO) and local sea surface temperature trends, perhaps signaling increasing influence of global warming [1].  Drought, although it rarely affects an entire island at one time, may occur when there are either no winter storms or no trade winds.  If there are no winter storms, the normally dry leeward areas are most impacted.  A dry winter, followed by a normally dry summer and another dry winter, can have serious effects. The absence of trade winds affects mostly the windward and upland regions, which receive a smaller proportion of their rain from winter storms [4].  Hawai'i’s climate is likely to continue to become drier as warming continues.  Reduced precipitation in combination with a possible increase in potential evapotranspiration due to increased temperature would result in significant reductions in ground-water recharge and stream discharge, and would severely impact vulnerable high-elevation ecosystems [1].

Coastal erosion and beach loss are chronic and widespread problems in the Hawaiian Islands. Shorelines are already affected as a result of island subsidence processes.  Loading of the Pacific tectonic plate by the growth of Hawai'i’s volcanoes, lithostatic flexure (down-bowing) of the plate, as well as compaction of the volcanic products, cause the islands to sink at a measurable rate. Typical erosion rates in Hawai'i are in the range of 15-30 cm/yr or 0.5-1 ft/yr, with some areas reaching annual average erosion rates of up to 5-6 ft /yr (Hwang, 1981; Sea Engineering, Inc., 1988; Makai Engineering, Inc. and Sea Engineering, Inc., 1991) [7].  A sustained sea level rise associated with global climate change will add more threat to the coastal built environment and significantly increase loss of beaches and coastal ecosystems [7].  This is a severe threat to tourism, the largest industry in Hawai'i, contributing 21.5% of the Gross State Product (GSP) in 2001 [3].

Although hurricanes and tsunami are a rare occurrence in Hawai'i, all main islands have been affected.  Tsunamis have accounted for more lost lives than the total of other local disasters [6].  High winds and associated marine flooding from storm events such as Kona Storms and hurricanes, sea level rise, seasonal high surf, stream flooding on coastal plains, all increase the risk exposure along developed coastal lands of Hawai'i [7].