DEVELOPING A LISTENING POST IN THE TROPICAL PACIFIC: SENSITIVITY OF HAWAIIAN HIGH-ELEVATION AND AQUATIC ECOSYSTEMS TO GLOBAL CHANGE

Principal Investigators: Lloyd L. Loope (BRD-PIERC, Haleakala, Maui); David Foote, Dennis A. LaPointe, James D. Jacobi (BRD-PIERC, Volcano, Hawaii); Thomas W. Giambelluca (Geography Department, University of Hawaii); Sara C. Hotchkiss (Geology and Geophysics Department at University of Wisconsin); and Dan A. Polhemus (Smithsonian Institution).

BRD PRINCIPAL CONTACT: Lloyd L. Loope, USGS-BRD, Pacific Island Ecosystems Research Center, Haleakala Field Station, P.O. Box 369, Makawao, Maui, HI 96768 Phone:808-572-4470 Fax: 572-1304 Email: Lloyd_Loope@usgs.gov

Project Duration: 1999-2004.

HaleNet Climate Data

Background

As the source area for atmospheric and oceanic energy, the global tropics are of fundamental importance within the earth's coupled climate system. Knowledge of tropical climate variability and the sensitivity of tropical climate to global climatic change is a crucial element of current efforts to understand past climates, predict future climates, and anticipate ecological responses to global warming. While recent advances in understanding past glacial cycles (Webb et al. 1997) have led to an increased awareness of the importance of the tropics, climatic conditions in the tropics during and since the last glacial maximum (LGM) remain poorly understood, with huge contradictions in the literature of the past two decades (numerous references cited in Loope 1995 and Hotchkiss 1998). For example, whereas evidence is accumulating for a large depression of snowlines and glaciers, with at least 4-6 C of cooling in the tropics (including Mauna Kea, Hawaii) at LGM, paleoclimatic reconstruction and model prediction of tropical sea surface temperature generally indicate cooling of 0-2 C. Increasing evidence suggests that in fact the tropics were much cooler at LGM than previously thought, even at low elevations.

The Pacific Ocean, stretching 20,000 kilometers from Panama to Singapore, encompasses almost half of the earth's tropical latitudes. The Pacific is also the primary domain of the El Niño/Southern Oscillation phenomenon, which underlies the dominant global climate variability signal at interannual to decadal time scales. Terrestrial ecosystems of tropical Pacific islands are extremely diverse (cf. Mueller-Dombois and Fosberg 1998), highly vulnerable to relatively small shifts in global weather patterns (Loope and Giambelluca 1998), and provide potential "listening posts" to assess climatic stability from the present into the near future. Polynesia is the largest of Pacific geographic areas; its land area within the tropics (i.e., excluding New Zealand) is slightly more than 30,000 km2. Located at the northern apex of the Polynesian triangle, the Hawaiian Islands have by far the highest mountains in the central Pacific and comprise over half the land area of tropical Polynesia. The Hawaiian islands of Maui and Hawaii are the largest islands (1864 km2 and 10,240 km2 in area, respectively) in tropical Polynesia and have relatively undisturbed montane ecosystems. We have argued elsewhere (Loope and Giambelluca 1998) that tropical cloud forests in general and Hawaiian cloud forests in particular are among the most sensitive of the earth's ecosystems to global climate change. The position of the North Pacific subtropical anticyclone and the altitude of the trade wind inversion (TWI) are fundamental drivers of local climate in Hawaii (Giambelluca and Nullet, 1991). The TWI is influenced by the strength of the Hadley cell circulation; stronger circulation should tend to locally depress the inversion. Relatively small shifts in patterns of atmospheric circulation are likely to trigger major local changes in rainfall, cloud cover and humidity, which in turn would provide additional stresses on island biota which are already vulnerable to disturbance-related invasion of non-native species.

Are montane cloud forests of Hawaii indeed among the earth's most sensitive ecosystems to global change or will the zones simply shift upward on the high mountains? Although no true consensus yet exists on expectations of the extent or direction of shifts in local climatic variables in Hawaii in response to global warming, plausible scenarios are emerging. The "Workshop on the Consequences of Climate Variability and Change: Hawaii-Pacific Region" (Water Resources Working Group) adopted a frequent/prolonged-El Niño regime as the best scenario for climate change impacts assessment in Hawaii and the Pacific (workshop organized under the auspices of the White House Office of Science and Technology Policy and the U.S. Global Change Research Program, March 1998). Climate models and recent trends support this view (Sun and Trenberth 1998; Meehl 1996). In Hawaii, a trend toward more persistent El Niño-like conditions would have many negative consequences. Winter drought in Hawaii is highly associated with El Niño (Chu 1989); the ten driest years in the past century in Hawaii have all coincided with El Niño events (Schroeder 1993). Temporal rainfall variability in Hawaii is strongly correlated with fluctuations in the height and strength of the TWI (Tran 1995), and El Niño is associated with a lower level of the TWI (Tran, pers. comm., 1998). Lowering the TWI would reduce rainfall in Hawaii in general, but especially at high elevations, and almost certainly depress the forest-grassland ecotone and heavily impact endangered native forest birds. A scenario in which El Niño serves as an analog of Hawaii's climate under global warming portends serious hydrological, ecological, and human impacts. For the Pacific in general, such a scenario leads to collapse of sustainability for human populations of atolls, causing migration and increased urbanization, with all the attendant problems, on larger high islands (such as the Hawaiian Islands) with more stable water supplies (Meehl 1996).

On the other hand, some striking palynological evidence suggests the opposite of the above -- that warmer climate is associated with a higher TWI and upper forest limit (Selling 1948; Burney et al. 1995; Hotchkiss 1998). During the Holocene climatic optimum, the postglacial time when radiative forcing was maximum, the treeline increased in elevation in the area of Flattop Bog on East Maui, suggesting that warming results in a rise in the TWI with upward shifting of vegetation zones (Burney et al. 1995).

For the purposes of this study, we are assuming that, of the two likely scenarios, the scenario involving increased frequency and intensity of El Niño (combined with lowering of the TWI) with global warming is the more likely one to happen for Hawaii. We will begin to quantify the sensitivity of Hawaiian montane rain forest and aquatic ecosystems to this regime of increasing drought. However, we think it wise to simultaneously pursue the alternative hypothesis of a rising TWI and rising treeline.

We are focusing the major biological components of this project on 1) developing and implementing a methodology to detect incipient change at treeline and in the montane rain forest, and 2) testing the use of a diverse Hawaiian complex of endemic and alien aquatic and semi-aquatic species as sensitive and reliable indicators of global change.

Literature Cited:

Burney, D.A., R.V. DeCandido, L.P. Burney, F.N. Kostel-Hughes, T.W. Stafford, and H.F. James. 1995. A Holocene record of climate change, fire ecology and human activity from montane Flat Top Bog, Maui. Jour. Paleolimnology 13:209-217.

Chu, P.-S. 1989. Hawaiian drought and the Southern Oscillation. Intl. Jour. Climatology 9:619-631.

Giambelluca, T.W., and D. Nullet. 1991. Influence of the trade-wind inversion on the climate of a leeward mountain slope in Hawaii. Climate Research 1:207-216.

Hotchkiss, S.C. 1998. Late-Quaternary climate history from Hawaiian pollen records. Chapter 4, p. 146-188 in S.C. Hotchkiss, Quaternary Vegetation and Climate of Hawai'i. Ph.D. thesis, University of Minnesota.

Loope, L.L. 1995. Climate change and island biological diversity. Pages 123-134 in P. Vitousek, L. Loope, and H. Adsersen (eds.), Biological Diversity and Ecosystem Function on Islands. Springer-Verlag. New York.

Loope, L.L., and T.W. Giambelluca. 1998. Vulnerability of island tropical montane cloud forests to climate change, with special reference to East Maui, Hawaii. Climatic Change 39:503-517.

Meehl, G. A. 1996. Vulnerability of fresh water resources to climate change in the tropical Pacific region. J. Water, Air and Soil Pollution 92:203--213.

Mueller-Dombois, D., and F.R. Fosberg. 1998. Vegetation of the Tropical Pacific Islands. Springer-Verlag, New York. 733 p.

Schroeder, T. 1993. Climate controls (Chapter 3), p. 12-36. In M. Sanderson (ed.), Prevailing Tradewind Climate and Weather in Hawaii. University of Hawaii Press. Honolulu.

Selling, O.H. 1948. Studies in Hawaiian Pollen Statistics. Part III. On the Late Quaternary History of the Hawaiian Vegetation. Bernice P. Bishop Museum Special Publ. 39.

Sun, D.-Z., and K. E. Trenberth, 1998: Coordinated heat removal from the tropical Pacific during the 1986-87 El Nino. Geophysical Research Letters 25:2659-2662.

Tran, L.T. 1995. Relationship between the Inversion and Rainfall on the Island of Maui. M.S. thesis, Department of Geography, University of Hawaii at Manoa, Honolulu, 115 p.

Webb, R.S., D.H. Rind, S.J. Lehman, R.J. Healy, and D. Sigman. 1997. Influence of ocean heat transport on the climate of the Last Glacial Maximum. Nature 385:695-699.