The Value of Hawaii: Knowing the Past, Shaping the Future

Climate Change

climate change chip fletcher Hawai‘i’s climate is changing in ways that are consistent with the influence of global warming. In Hawai‘i Air temperature has risen Rainfall and stream flow have decreased Rain intensity has increased Sea level and sea surface temperatures have increased The ocean is acidifying If these trends continue, scientists anticipate growing impacts to Hawai‘i’s water resources and forests, coastal communities, and marine ecology. As a result, there is a compelling need for sustained and enhanced climate monitoring and assessment activities, and for focused research to produce models of future climate changes and impacts in Hawai‘i. Should they continue, these trends point in certain directions and raise questions. Water, already a scarce resource in places on the islands, may grow scarcer. Will we continue building subdivisions in recharge areas? When will privately controlled water return to the public? Flash flooding, presently managed with aquatic-ecosystem-killing channelized waterways, may increase in intensity. How will we manage this threat yet continue attempts to restore native watersheds? Sea-level rise is likely to accelerate, threatening beaches and coastal communities. Who will lead the process to develop a shared vision of what is at risk and what qualities to protect? Rising sea surface temperature and ocean acidification threaten coastal and marine ecosystems. Removing water-shed stressors may improve reef sustainability. What management options lessen the impacts of acidification? Declining stream flow and rising sea level threaten coastal plain agriculture. What options do taro farmers have? 172 The Value of Hawai‘i While researchers work to improve understanding of likely future changes to Hawai‘i’s climate, we need also to begin the process of managing climate risk. Do we love future generations as much as we love ourselves? If the answer is “yes,” then the community discussion of how we manage climate risk in Hawai‘i needs to begin soon. surface air temperature Hawai‘i is getting warmer (Figure 1). Data1 show a rapid rise in air temperature in the past thirty years, averaging 0.3°F per decade—a little less than the global average rate of about 0.36°F per decade. There is stronger warming at high elevations (above 2,600 feet; about 0.48o F per decade) than at low elevations (about 0.16o F per decade). Most of the warming is related to a larger increase in minimum temperatures compared to the maximum—a net warming about three times as large—causing a reduction of the daily temperature range. This response to global warming is consistent with similar trends observed in North America.2 Despite recent years where the rate of global warming was low,3 surface temperatures in Hawai‘i have remained high. Rapid warming at high elevations threatens water resources and may have significant ecological impacts, such as the spread of avian disease, decreased rainfall and cloud water , and threats to native forests.4 Air temperature is rising in Hawai’i, and has accelerated since the 1970s. High elevation temperature stations record greater warming than those at low elevations. FiguRe 1. aiR TemPeRaTuRe in hawai‘i. Fletcher, Climate Change 173 rainfall and stream discharge Perhaps nothing is as critical to life in the islands as rain, and in Hawai‘i there are two principal sources: trade winds and Kona storms. Cloud formation by trade winds (Figure 2) is the most reliable and abundant source of water to the aquifers we rely upon. Although atmospheric circulation in the tropical Pacific has decreased, and global warming is identified as the cause,5 it is not yet clear how the trade winds will respond to global warming. It also remains unclear how future rainfall will respond to global warming. The results of modeling studies have been equivocal,6 although to some extent they indicate that we should anticipate decreased rainfall. Studies of rainfall records corroborate this. Rainfall in Hawai‘i has steadily declined about 15 percent over the past twenty years.7 Streams are one beneficiary of rain. Rainfall feeds streams in two ways: storm flow and base flow. Storm flow responds very quickly to rainfall, and causes stream levels to rise during and immediately after rainfall. Base flow, on the other hand, is supplied by groundwater, and maintains stream flow during periods between rainfall events. Base flow also responds to changes in rainfall over time, but much more slowly than storm flow. Beginning in the early 1940s, base...