Average annual temperature has increased by about 2°F since 1950, with a leveling off of the rate of warming in the last two decades. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
Annual rainfall has decreased throughout Hawai‘i since 1950. The frequency and magnitude of extreme precipitation events have changed in recent years, and these changes are not uniform across the island chain. Geographically, extreme rainfall events have become less frequent for Kauai and Oahu, but become more frequent for the Island of Hawaii. Future annual precipitation is uncertain.
Sea level rise has been, and will continue to be, a major concern, presenting challenges to the state’s coastline through coastal inundation and erosion.
Hawai‘i is the only U.S. state located in the tropics. Almost half of the state’s land area is within five miles of the ocean, which provides a moderating effect on the state’s climate. August is the warmest month, with an average temperature of about 78°F, while the coldest month, February, averages around 72°F. Major geographic variations in temperature occur due to the state’s high elevations. For example, at elevations of less than 1,000 feet, winter temperatures rarely fall below 50°F, whereas lows can reach less than 20°F at the peaks of Mauna Kea and Mauna Loa. Hawai‘i is, however, the only state to have never recorded sub-zero Fahrenheit temperatures.
Since 1950, temperatures across the Hawaiian Islands have risen by about 2°F, but the warming has leveled off in the most recent two decades (Figure 1). Temperatures in Honolulu have increased by 2.3°F over this period and have consistently been above the 1951–1980 average since 1975 (Figure 2). Both the number of hot days (days with maximum temperature above 90°F) and number of warm nights (days with minimum temperature above 75°F) have been near to above average since 1980 (Figures 3 and 4). The rate of temperature increase is greatest at high elevations, far exceeding the global average rate of change. The annual number of days below freezing is decreasing over time, as is the diurnal temperature range, largely due to nighttime warming. Historically, temperatures in Hawai‘i have been tightly coupled to the decadal variability of the atmospheric circulation and underlying ocean in the Pacific Basin (known as the Pacific Decadal Oscillation), however, since the 1970s, increasing temperatures are more consistent with an increase in the frequency of the trade wind inversion, and a decrease in trade wind frequency.
Precipitation varies greatly by both season and location. Hawai‘i experiences a drier season from May through October, in which warm, steady trade winds cause frequent light-to-moderate showers, and a wet season from November through April, with weaker and less frequent trade winds and a significant amount of rain from storms. The mountainous terrain, persistent trade winds, heating and cooling of the land, and other factors interact to result in dramatic differences in average rainfall over short distances. Annual total rainfall sometimes exceeds 300 inches along the windward slopes of mountains, but less than 20 inches in leeward coastal areas and the highest mountain slopes. Despite great variability in precipitation amounts across the islands over the past century, annual rainfall has decreased throughout the island chain (Figure 5), particularly during recent years. The Island of Hawai‘i has experienced the largest significant long-term declines in annual and dry season rainfall, with annual total precipitation in Hilo decreasing the most among four major airports: a decrease of almost 20 inches since 1950 (Figure 6). An increase in the frequency of the trade wind inversion is also linked to a decrease in precipitation at high elevations. The number of consecutive dry days across the major Hawaiian Islands has become longer since 1950s. An increase in drought conditions has been seen in recent years, particularly at high elevations. In 2010, more than 40 percent of the Hawaiian Islands experienced severe, extreme, or exceptional drought conditions. Such conditions lead to a lack of useable water and increased risk of fire. On the other side of the coin, the number of extreme precipitation events has been below average in recent years (Figure 7), with areas at the highest elevations experiencing the strongest downward trend. Regionally, extreme rainfall events have become less frequent for O‘ahu and Kauai, but more frequent for the Island of Hawai‘i.
The North Pacific High, a semi-permanent area of high pressure, has a strong influence on Hawai‘i’s weather. It is responsible for the trade winds which dominate during the dry season. During the wet Hawaiian winter, however, the North Pacific High is diminished, and the middle latitude jet stream shifts southward, providing an occasional opportunity for cool winter storms known as Kona storms. They usually affect the state for a week or more, and occur on average two to three times per year. Kona storms often result in flash flooding (and associated landslides), a common occurrence due to the state’s steep terrain, and the leading cause of direct weather-related deaths in Hawai‘i, far exceeding the toll due to high wind events and tropical cyclones. Kona storms can also produce additional hazards such as hail, heavy mountain snows, waterspouts, and high surf events—the leading cause of indirect weather-related deaths.
Hawai‘i is also susceptible to tropical storms, most often occurring between June and November. Such storms bring heavy rains, high winds, and high waves to the islands. Hurricanes rarely affect the state, with many dissipating into tropical storms or tropical depressions as they approach the islands. Fewer than 40 hurricanes have affected Hawai‘i since 1949, with only a handful making landfall. The number of tropical cyclones formed in the Central North Pacific has been variable over time, with a greater number of tropical cyclones forming during El Niño years. The year 2015 was the most active hurricane season on record in the Central Pacific, with eight hurricanes and six additional tropical storms reported. It is projected that Hawai‘i will see an increase in the frequency of tropical cyclones, due to storm tracks shifting northwards in the Central North Pacific.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records. Rising temperatures will cause future heat waves to be more intense. This warming, accompanied by reduced rainfall in some areas, will stress native Pacific Island plants and animals, especially in high-elevation ecosystems with increasing exposure to invasive species, increasing the risk of extinctions. Increasing temperatures, combined with a growing human population and expanding invasive grass cover, are likely to continue the increase in wildfire occurrence that has been observed since the beginning of the 20th century.
Precipitation projections for Hawaii are particularly challenging due to the state’s high and steep topography, which leads to pronounced small-scale variations in climate. Projections of average annual precipitation are uncertain, with one likelihood that Hawai‘i straddles the transition between wetter conditions in the tropics and drier conditions in the subtropics (Figure 8). It is likely that the currently wet windward sides of the major islands will see an increase in rainfall, while the currently dry leeward sides will experience a decrease in rainfall. Future projections regarding both the frequency and magnitude of extreme precipitation events are also uncertain, with some climate models indicating increases and some decreases in heavy rainfall events. Even if average precipitation remains the same, higher temperatures will increase the rate of loss of soil moisture during dry periods, leading to increased intensity of naturally-occurring droughts.
Increasing temperatures raise concerns for sea level rise in Hawai‘i. Since 1880, global sea level has risen by about 8 inches. It is projected to rise another 1 to 4 feet by 2100 as a result of both past and future emissions due to human activities (Figure 9). Rates of sea level rise in Hawai‘i vary between the islands, ranging from 0.6 inches per decade for Kaua‘i and O‘ahu to 1.3 inches per decade on the Island of Hawai‘i. Sea level rise across Hawai‘i is projected to rise another 1–3 feet by the end of the 21st century. Sea level rise has caused an increase in tidal floods associated with nuisance-level impacts. Nuisance floods are events in which water levels exceed the local threshold (set by NOAA’s National Weather Service) for minor impacts. These events can damage infrastructure, cause road closures, and overwhelm storm drains. As sea level has risen along the Hawaiian coastline, the number of tidal flood days (all days exceeding the nuisance level threshold) has also increased, with the greatest number occurring in 2002–2003 (Figure 10). Continued sea level rise will present major challenges to Hawai‘i’s coastline, through coastal inundation and erosion. Seventy percent of Hawai‘i’s beaches have already been eroded over the past century, with more than 13 miles of beach completely lost. Sea level rise will also affect Hawai‘i’s coastal water management system and could cause extensive economic damage through ecosystem damage and losses in property, tourism, and agriculture.