Forest

The direct effects of climate change on forests will be variable and strongly dependent on interactions with other disturbances, especially novel fire regimes that are expanding into new areas because of invasion by fire-prone exotic grass and shrub species, such as fountain grass and common gorse in Hawaii and guinea grass across the region. Combined with warmer and drier conditions, these invasions have the potential to alter or even eliminate native forests through conversion of forested systems to open, exotic-dominated grass and shrub lands.

Existing climate zones on high islands are generally projected to shift upslope in response to climate change. The ability of native species to adapt to shifting habitats will be affected by ecosystem discontinuity and fragmentation, as well as the survival or extinction of pollinators and seed dispersers. Some (perhaps many) invasive plant species will have a competitive edge over native species, as they disproportionately benefit from increased carbon dioxide, disturbances from extreme weather and climate events, and an ability to invade higher elevation habitats as climates warm.

Sea level in Hawai'i is projected to rise an additional 1-4 feet by 2100, with increases accelerating strongly after mid-century. In the most extreme scenarios, it is possible that sea level may rise by as much as 8 feet by 2100. Sea-level rise and related increases in storm surges pulsing farther inland will result in land loss in low-lying coastal areas and may result in excessive saltwater inundation of coastal forests.

The El Niño–Southern Oscillation (ENSO) causes year-to-year changes in climate, with impacts to sea level, precipitation, and storm frequency. Climate models suggest a possible doubling of these extreme events in the 21st century, with El Niño-like conditions projected to occur more frequently. After an El Niño, Hawai'i generally experiences drier conditions from January-March. A La Niña often brings wetter conditions from December-February. An El Niño phase is characterized by decreased trade wind activity, which allows warm waters to flow west to east.

By 2100, under a high emissions scenario, rainfall in wet areas is expected to increase (from small increases to more than 30% increases), while rainfall in dry areas is expected to decrease (from small decreases to decreases of up to 60%). Future precipitation patterns are difficult to project for Hawai'i, due to its steep topography and small-scale variations in climate. Overall precipitation has been trending downward in recent decades, with the sharpest downward trends occurring on western Hawai'i island.

Temperatures in Hawai'i have warmed by 0.76 degrees F over the past 100 years. Periods of warming and cooling have occurred over the past century, with a statistically significant trend toward warmer temperatures. Warming is occurring at both high and low elevations, although higher elevation areas have been warming faster than lower elevations over the past 30 years. Much of the warming can be attributed to higher minimum temperatures.

Changing temperature and precipitation regimes may shift suitable habitat for many tree species and ecosytems, and the effects of wildfires, pests, and diseases will be more harmful to some species than to others. Also, fundamental changes may make some areas less suitable for forests. For example, permafrost degradation is widespread in central Alaska, shifting ecosystems from birch forests to wetland types such as bogs and fens. If broad-scale water balances become increasingly negative, peatlands may begin to support upland forest species.

Alaska cedar is declining and experiencing elevated mortality in large areas. Alaska cedar decline has been attributed to earlier snowmelt, which exposes roots to damage from lower temperatures. Projected future warming is expected to cause additional mortality from freezing-induced root damage, because more snow will be delivered as rain and winter snowpacks will be thinner and more variable. Additionally, warmer winters may allow pests and diseases to expand into previously unavailable territory in Alaska.