PALEOCLIMATE AND GEOGRAPHY

Current concerns over the potential for global climate change focus largely on the role of human impact. Earth, however, has undergone a long history of climate fluctuations due to natural causes. As the Hawaiian Islands adjusted to global change over the past several million years, local climate variations markedly altered many geographic features . During and between the great glacial ages, sea level changes accompanied the rise and fall of global temperatures as water shifted between continental ice sheets and the oceans. On aver­age, during the past 2 million years global tempera­tures and sea levels were lower than at present, as the record in the graph indicates.

Perhaps the most intriguing geog raphic effect of past lower sea level was the reemergence of Maui Nui -the landmass composed of Maui, Moloka'i, Lana'i, and Kaho'olawe-as the ocean withdrew from the shal-

low basin between the islands. (This area is well known today as a breeding area for humpback whales .) The aerial perspective shows Maui Nui as it would have appeared 21,000 years ago. While Maui Nui existed, the exposed land bridges connecting the islands allowed easier dispersal and migration of plants and animals- a past that left today's remnants ofMaui Nui with a lower density of endemic species (those unique to a given location) than is found on O 'ahu or Kaua'i, which remained isolated. A dramatic increase in land area also occurred during glacial epochs on the atolls of the Northwestern Hawaiian Islands, where now-sub­merged reefs may have once stood as much as 300 feet (100 m) above sea level.

Past sea levels have also occasionally stood higher than at present, as evidenced by ancient reefs underly­ing coastal plains such as 'Ewa and Kane'ohe peninsula

Global Sea Level Change - 900,000 Years Ago to the Present

--------- current sea level ----~--___:_-___.;:_::_~-:---_::;iI~-------_:__

900 800 700 600 500 300 200 present Thousands of years before the present

Late Quaternary Sea Level History

5f 5e 5d 5c 5b 5a 4 3 2 Ox ygen Isotope Stages

o

100

140 120 100 80 60 40 20 present Thousands of years

64

Paleoclimate and Geography 65

Reemergence of Maui Nui, 21,000 Years Ago

V-shaped terminal glacial moraine, Pohakuloa Gulch, Mauna Kea, H awai'i. [J. Juvik}

on O'ahu, and terraces and notches cut by wave action inland from today's shorelines. For example, 126,000 years ago, O 'ahu 's shorelines stood approximately 33 feet (10 m) higher than at present. This shoreline ele­vation probably resulted from the combined effects of absolute sea level rise and island uplift in response to the flexing of Earth's crust under the weight of geo­logically younger Hawai 'i Island to the southeast.

Cooler temperatures in recent geologic time caused another phenomenon unexpected on tropical islands: glaciers. Scattered around the summit of Mauna Kea on Hawai'i Island are glacial features­including moraines, rock striations, and erratics (iso­lated boulders)-that testify to its ice-capped past.

4500m

History of Mauna Kea Glaciation

Pohakuloa Glaciation

Waihii Glaciation

Makanaka Glaciation Present

4000m ---+----O"'----,4-- ---'..,..-- .,<--- --":---,L------'..,..--

3500m ---;1------"-=-:::..:.:....::c=---+_-<..::..:."'-"--:=.c~=~'="~==1------+

level of lowest 3000m _____ +-____ -F=_==---+-------=mc:..:.o::.:.r::.:Bin::..:e:.:=-s------l

2500m

These relict features, as well as buried fossil ice, are preserved in the Mauna Kea Ice Age Natural Area Reserve. The reserve also includes one of the highest lakes in the world, Lake Wai'au, which, at an elevation of 13,020 feet (3,968 m), occupies a shallow depression once filled with ice. Major glacial episodes on Mauna Kea over the past 200,000 years flowed and ebbed in synchrony with the advances and retreats of continen­tal glaciers.

Other clues pointing to local climate variations during geologic time come from a range of sources. For example, the alignment of ancient sand dunes and the asymmetrical shape of cinder cones, both of which depend on prevailing wind direction, indicate that the northeast trade winds apparently persisted throughout the cooler glacial periods . Yet the larger size of ancient dust particles from Asia-carried to Hawai'i by upper­level westerly winds-implies a higher average wind velocity than that of today.

Evidence from pollen analysis, soils, and erosional features, such as the deeply incised valleys of some lee­ward slopes, suggests that past rainfall patterns were altered with the changing climate . During cooler epochs in the past, wetter conditions than at present

Dice cap

66 Paleoclimate and Geography

3000m

2000m

1000m

Sea level

Mountain Ecozone Response to Climatic Change 20,000 years ago (last glacial era)

Composite Hawaiian Mountain

the present

arid alpine desert

dryland scrub Styphelia sp. (piikiawe) Vaccinium sp. ('ahe/o)

upper montane dry forest Sophora chrysophyffa (mamane)

montane forest Acacia koa (koa)

(j) u c ro

"0 c

20%

Acacia koa Pol len Grain. [0. Sel li ngsJ

..... I----- 100 l1m ~

Acacia koa Pollen Abundance Ka'au Crater, O'ahu rain forest

Metrosideros sp. fohi"a)

Cibotiurn sp. (hapu 'u) il 10%

450 m elevation ro c

..Cl! "0 "-Ka 'au Crater (450m)

~A B lowland forest

Pandanus odoratissimus (ha/a) Diospyros sandwicensis (lama)

Stone stripes created by freeze-thaw cycles on Mauna Kea summit, Hawai 'i. [J. Juvik}

apparently prevailed at lower elevations, helping carve leeward O'ahu's Makaha Valley, for example. Higher mountain slopes, however, between 3,000 feet and 6,000 feet (1,000 m and 2,000 m), seem to have expe­rienced drier conditions. The drier climate was proba­bly caused by a lower average altitude of the trade wind inversion, which would limit the penetration of clouds into this elevation range. During warmer periods the reverse situation appears to have predominated, with drier conditions in coastal areas and wetter conditions at high elevation locations. Partial evidence for these conclusions comes from sequences of plant pollen in mountain bogs on Maui, Moloka'i, and Kaua'i. The pollen record from bogs at 4,000-6,000 feet (1,200-1,800 m) documents the dominance of xerophytes (dry-tolerant plants) and mesophytes (plants tolerant

0%

20,000 10,000 present Years before present

Lake Waiau, Mauna Kea, Hawai'i, elevation 13,020 ft. (3,968 m). (J. J uvik}

of average moisture and dryness) at the end of the last glacial period, 10,000 years ago. In contrast, during the postglacial temperature maximum 4,000-6,000 years ago, hydrophytes (plants requiring very wet conditions) abounded at middle elevations . The pollen record also indicates that vegetation zones occupy different eleva­tions in response to temperature and rainfall changes . Based on the current distribution of plants and the pol­len record, during past cooler periods vegetation zones probably occupied a narrower range of elevation lower on the mountain slope.

DENNIS NULLET,

CHARLES H. FLETCHER III, SARA HOTCHKISS, AND

JAMES O. J UVIK