9-HI HAWAI I Key Messages Average annual temperature has increased by about F since 95, 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 st century. Annual rainfall has decreased throughout since 95. 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 Kaua i and O ahu, but become more frequent for the Island of. 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. Hawaiʻi 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 7 F. Major geographic variations in temperature occur due to the state s high elevations. For example, at elevations of less than, feet, winter temperatures rarely fall below 5 F, whereas lows can reach less than F at the peaks of Mauna Kea and Mauna Loa. Hawaiʻi is, however, the only state to have never recorded sub-zero Fahrenheit temperatures. Since 95, temperatures across the Hawaiian Islands have risen by about F, but the warming has leveled off in the most recent two decades (Figure ). Temperatures in Honolulu have increased by.3 F over this period and have consistently been above the 95 98 average since 975 (Figure ). Both the number of hot days (days with maximum temperature above 9 F) and number of warm nights (days with minimum temperature above 75 F) have been near to above average since 98 (Figures 3 and ). The rate of temperature increase is greatest at high elevations, far exceeding Observed and Projected Temperature Change Figure : Observed and projected changes (compared to the 95 98 average) in nearsurface air temperature for. Observed data Observations Modeled Historical are for 95. Projected changes for 6 8 Higher Emissions are from global climate models for two possible Lower Emissions futures: one in which greenhouse gas emissions 6 continue to increase (higher emissions) and another in which greenhouse gas emissions increase at a slower rate (lower emissions). Temperatures in (orange line) have risen about F since the beginning of the th century. Shading indicates the range of annual temperatures from the set of models. Observed temperatures are generally within the envelope of model simulations of the historical period (gray shading). Historically unprecedented warming is projected during the st century. Less warming is expected under a lower emissions future 9 95 95 975 5 5 75 (the coldest years being about as warm as the hottest year in the historical record; green shading) and more warming under a higher emissions future (the hottest years being about 8 F warmer than the hottest year in the historical record; red shading). Source: CICS-NC and NOAA NCEI. Temperature Change ( F) Higher Emissions Lower Emissions Technical details on models and projections are provided in an appendix, available online at: https://statesummaries.ncics.org/hi.
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 Hawaiʻi 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 97s, 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. Hawaiʻi 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 3 inches along the windward slopes of mountains, but less than inches in leeward coastal areas and on 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 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 inches since 95 (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 95s. An increase in drought conditions has been seen in recent years, particularly at high elevations. In, more than 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 Oʻahu and Kaua i, but more frequent for the Island of Hawaiʻi. Temperature Change ( F) 3 3 95 Observed Temperature Change 96 97 98 99 Līhu e, Kaua i Honolulu, O ahu Kahului, Maui Hilo, Island of Figure : Observed changes (compared to the 95 98 average) in annual near-surface air temperature for four stations in : Līhu e, Kaua i (red line), Honolulu, O ahu (blue line), Kahului, Maui (yellow line), and Hilo, Island of (green line). Data are for 95 5. Temperatures across the islands have increased since 95, at rates of between. F and. F per decade. Temperatures in Honolulu have increased by.3 F over this period and have consistently been above the 95 98 average since 975. Source: CICS-NC and NOAA NCEI. Number of Days with Maximum Temperature Above 9 F 8 6 95 5 Observed Number of Hot Days 96 6 97 7 The North Pacific High, a semi-permanent area of high pressure, has a strong influence on Hawaiʻi 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 98 8 Figure 3: The observed number of hot days (annual number of days with maximum temperature above 9 F) for 95, averaged over 5-year periods; these values are averages from seven longterm reporting stations. The number of hot days has increased since the 95s, being consistently high throughout the 98s and 99s. The dark horizontal line is the long-term average (95 ) of 6.5 days per year. Source: CICS-NC and NOAA NCEI. 99 9 HAWAI I
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 Hawaiʻi, 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. Hawaiʻi 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 hurricanes have affected Hawaiʻi since 99, 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 5 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 Hawaiʻi 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 st century (Figure ). 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 st 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, Number of Days with Minimum Temperature Above 75 F.5..5..5..5 35 3 5 5 5 Observed Number of Very Warm Nights 95 5 9 Time Series of HRI Anomolies 9 96 6 93 97 7 Figure : The observed number of very warm nights (annual number of days with minimum temperature above 75 F) for 95, averaged over 5-year periods; these values are averages from seven long-term reporting stations. The number of very warm nights has increased since the 95s, remaining above the long-term average since the 98s. The dark horizontal line is the long-term average (95 ) of 3.9 days per year. Source: CICS-NC and NOAA NCEI. 9 95 96 97 98 99 s Figure 5: Time series of Rainfall Index (HRI) for 95 as derived from 7 long-term gauges from Kaua i, O ahu, and. This index represents rainfall variations over different climate regions of the Hawaiian Islands. A normalization technique is applied to each individual station, and a regional index is then computed as the arithmetic average of all station indices. The vertical axis is the regional standardized anomalies. Source: Chu and Chen 5. are likely to continue the increase in wildfire occurrence that has been observed since the beginning of the th century. Precipitation projections for 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 98 8 99 9 HAWAI I 3
precipitation are uncertain, with one likelihood that Hawaiʻi 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. Precipitation Change (inches) 8 6 6 Observed Precipitation Change Līhu e, Kaua i Honolulu, O ahu Kahului, Maui Hilo, Island of Increasing temperatures raise concerns for sea level rise in Hawaiʻi. Since 88, global sea level has risen by about 8 inches. It is projected to rise another to feet by as a result of both past and future emissions due to human activities (Figure 9). Rates of sea level rise in Hawaiʻi vary between the islands, ranging from.6 inches per decade for Kauaʻi and Oʻahu to.3 inches per decade on the Island of Hawaiʻi. Sea level rise across Hawaiʻi is projected to rise another 3 feet by the end of the st 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. 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 3 (Figure ). Continued sea level rise will present major challenges to Hawaiʻi s coastline, through coastal inundation and erosion. Seventy percent of Hawaiʻi s beaches have already been eroded over the past century, with more than 3 miles of beach completely lost. Sea level rise will also affect s coastal water management system and could cause extensive economic damage through ecosystem damage and losses in property, tourism, and agriculture. 8 95 96 97 98 99 Figure 6: Observed changes (compared to the 95 98 average) in annual precipitation for four stations in : Līhu e, Kaua i (red line), Honolulu, O ahu (blue line), Kahului, Maui (yellow line), and Hilo, Island of (green line). Data are for 95 5. Annual precipitation varies greatly from year to year, however, overall amounts have decreased since 95 at all four stations. The greatest decrease has been seen in Hilo, where annual precipitation has decreased by almost inches across the period of record. Source: CICS-NC and NOAA NCEI. Number of Events with Precipitation Greater Than 3 Inches 3 95 5 Observed Number of Extreme Precipitation Events 96 6 97 7 98 8 Figure 7: The observed number of days with extreme precipitation events (annual number of days with precipitation greater than 3 inches) for 95, averaged over 5-year periods; these values are averages from 33 long-term reporting stations. The number of days with extreme precipitation has been variable over time, with the two most recent decades experiencing a below-average number of events. The dark horizontal line is the long-term average (95 ) of.6 days per year. Source: CICS-NC and NOAA NCEI. 99 9 HAWAI I
Projected Change in Annual Precipitation Observed and Projected Annual Number of Tidal Floods for Honolulu, HI Tidal Floods (Days/) 35 Higher Emissions 3 Lower Emissions Observed 5 5 5 9 9 96 98 6 8 Change in Annual Precipitation (%) < 5 5 5 >5 Figure 8: Projected changes in annual precipitation (%) for the middle of the st century compared to the late th century under a higher emissions pathway. Average annual precipitation is projected to increase slightly across southern parts of the state, with the northernmost islands seeing a decrease in precipitation. These changes are small, however, relative to natural variability in. Source: CICS-NC, NOAA NCEI, and NEMAC. Past and Projected Changes in Global Sea Level 75 5 5 9 9 96 98 Figure : Number of tidal flood days per year for the observed record (orange bars) and projections for two possible futures: lower emissions (light blue) and higher emissions (dark blue) per calendar year for Honolulu, HI. 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, such as road closures and overwhelmed storm drains. The greatest number of tidal flood days (all days exceeding the nuisance level threshold) occurred in and 3 at Honolulu. Projected increases are large even under a lower emissions pathway. Near the end of the century, under a higher emissions pathway, some models (not shown here) project tidal flooding nearly every day of the year. To see these and other projections under additional emissions pathways, please see the supplemental material on the State Summaries website (https://statesummaries.ncics.org/hi). Source: NOAA NOS. Figure 9: Estimated, observed, and possible future amounts of global sea level rise from 8 to, relative to the year. The orange line at right shows the most likely range of to feet by based on an assessment of scientific studies, which falls within a larger possible range of.66 feet to 6.6 feet. Source: Melillo et al. and Parris et al.. WWW.NCEI.NOAA.GOV HTTPS://STATESUMMARIES.NCICS.ORG/HI LEAD AUTHORS: LAURA E. STEVENS, REBEKAH FRANKSON, KENNETH E. KUNKEL, AND PAO-SHIN CHU CONTRIBUTORS: WILLIAM SWEET HAWAI I 5