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Hawaii’s climate is changing in ways that are consistent with the influence of global warming.

In Hawaii:

• 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

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 groundwater 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?

To elaborate on just a few of the issues:

Rainfall

Perhaps nothing is as critical to life in the islands as rain, and in Hawaii there are two principal sources: trade winds and Kona storms. Cloud formation by trade winds 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, 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, although to some extent they indicate that we should anticipate decreased rainfall. Studies of rainfall records corroborate this. Rainfall in Hawaii has steadily declined about 15 percent over the past twenty years.

Rain Intensity

Between 1958 and 2007, the amount of rain falling in the very heaviest downpours (defined as the heaviest 1 percent of all events) has increased approximately 12 percent in Hawaii. Heavy rainfall means more than simply getting wet; it is a major challenge for civil defense agencies and emergency responders. Intense rains trigger a domino effect of other impacts, including flash flooding, mudslides and debris flows, road and business closures, infrastructure damage, and loss of public services to isolated communities.

It was heavy rain that caused over $80 million dollars of damage in Manoa Valley, Oahu; isolated Hana, Maui for weeks; flooded homes in Laie, Oahu; and swept houses off their foundations in Hilo, Hawaii. When intense rains struck on New Year’s Eve 1987, forty thousand people in Hawaii Kai, Waimanalo, Aina Haina, and other east Honolulu communities went without power, emergency aid, communication, and road access for up to twenty-four hours. While these events cannot be directly tied to global warming, they are consistent with expectations in a warmer world, and they illustrate the severe impacts associated with intense rains.

Ocean Acidification

As rising carbon dioxide in the atmosphere mixes with seawater, the ocean acidifies. Measurements at Station ALOHA over two decades document that the surface ocean around Hawaii has grown more acidic at exactly the rate expected from chemical equilibration with the atmosphere. Continued acidification may have a host of negative impacts on marine life, and has the potential to alter the rates of ocean biogeochemical processes.

When carbon dioxide reacts with seawater it reduces the availability of dissolved carbonate. Carbonate (CO3) is vital to shell and skeleton formation in corals, marine plankton, some algae, and shellfish. In coming decades, ocean acidification could have profound impacts on some of the most fundamental biological and geochemical processes of the sea. Plankton is a critical food source that supports the entire marine food chain.

Declining coral reefs will impact coastal communities, tourism, fisheries, and overall marine biodiversity. Abundance of commercially important shellfish species may decline, and negative impacts on finfish may occur. This rapidly emerging scientific issue, and its potential ecological impacts, have raised concerns across the scientific and fisheries communities.

Climate Risk Management

Hawaii’s risk of damage from climate change is increasing because humankind refuses to take meaningful steps to counteract global warming. However, local vulnerability is increasing because Hawaii leaders have not recognized the problem as worth planning for. If we are vulnerable to climate impacts, it is our fault.

The danger is evident in all sectors. Water is key to agriculture, and efforts to advance local agriculture will be set back by declining rainfall, base flow, and stream discharge. Sea-level rise threatens much more than beaches. Intense rainfall events will have nowhere to drain when high ocean waters have flooded the storm drain system, and damaging marine events such as storm surge, tsunami, and high waves will penetrate further landward with each year. Acidification and sea surface temperature increases threaten our marine economy and tourism.

Like other science-based planning challenges, managing climate risk involves complex assessment based on the best available information originating, in part, with scientific research. Effective progress on the problem is facilitated by a strong partnership between scientists and planners; but how to proceed? The roadmap must come from an informed and involved community working to leave a resource-rich Hawaii to future generations.

Conclusions

The effects of global warming are evident in Hawaii: air temperature is rising, rainfall and stream flow have decreased, rain intensity has increased, sea level and sea surface temperatures have increased, and the ocean is acidifying. Because these trends are likely to continue, scientists anticipate growing impacts to Hawai‘i’s water resources and forests, coastal communities and environments, and marine ecology.

Now is the time to increase climate monitoring and assessment activities, and to produce skillful models of future climate changes and impacts. Climate change education and public awareness of the problem should expand in order to open a community discussion and implement place-based strategic planning.