GRL - Shifting Climate Zones For Australia's Tropical Marine Ecosystems

[hatrack]

J. M. Lough

Australian Institute of Marine Science,
Townsville, Queensland, Australia

Abstract

<1> Sea surface temperatures (SSTs) are significantly warming along the northwest (NW) and northeast (NE) coasts of Australia - regions containing well-protected and internationally significant tropical marine ecosystems. The magnitude and spatial distribution of observed warming of annual, maximum and minimum SSTs is examined, 1950–2007. Observed warming is comparable along the NE and NW coasts although greater along the NE coast south ~15°S, greater at higher than lower latitudes, and greater for annual minimum than annual maximum SSTs. Average climate zones have also shifted >200 km south along the NE coast and about half that distance along the NW coast. If current trends continue, annual average SSTs in northern parts could be ~0.5°C warmer and those of more southern parts ~2.0°C warmer within the next 100 years. These rapid changes in oceanic climate are already causing responses in Australia's tropical marine ecosystems and these responses, if present rates of warming continue, can only intensify.

Received 8 May 2008; revised 16 June 2008; accepted 26 June 2008; published 29 July 2008.

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<15> Extrapolation of current warming trends suggest northern SSTs could be ~0.5°C warmer and southern SSTs ~1.5–2.0°C warmer within the next 100 years. The lower rate of warming at lower latitudes may also be related to suggested lower warming rates and less coral bleaching in the Western Pacific Warm Pool to the NE of Australia . Southward shifts in marine climate regimes (defined by annual average SST) are evident along both coasts, but the magnitude of the shift is about twice as great (~>200 km) on the NE than on the NW coast (~100 km). These differential shifts have also resulted in losses and gains in size of climatic zones which are most marked on the NE coast with a maximum loss of ~9% in size of the 23–24°C SST band.

<16> This paper has identified current rates of SST warming along Australia's east and west coastlines. It is beyond the scope of this study to identify proximate causes but these are likely to include both modified air-sea heat fluxes and changes to local and large-scale ocean dynamics. Studies in the Indian Ocean, for example, note that widespread surface warming is associated with subsurface temperature changes related to a modelled 0.5° southward shift of the Indian Ocean subtropical gyre . Indian Ocean water temperatures and ocean dynamics are also modulated, via the Indonesian Through Flow, by ocean-atmosphere conditions in the Pacific Ocean and, possibly, larger-scale impacts on SH ocean circulation of increased NH aerosols and changes in the Southern Annular Mode induced by Antarctic ozone depletion . On Australia's Pacific coast, a long-term (1944–2003) observational record off eastern Tasmania (~43°S) shows a warming (~0.23°C/decade) consistent with a southward advance of the East Australian Current ~350 km and increased southward penetration of lower-latitude marine organisms .

<17> These recent observed significant changes in Australia's tropical coastal climates have a number of implications, particularly if current rates and patterns of warming continue. Evidence is mounting for significant widening of the global tropical belt and, at least in the terrestrial sphere, some new climate zones may appear and others disappear . Greater rates of warming and greater shifts in climate zones in the more southerly parts of each coast suggests that these will show earlier evidence of climate change impacts than more northerly regions. Southern regions may, therefore, be the best areas to identify the nature of climate impacts on Australia's tropical marine ecosystems and responses of marine organisms sensitive to climate change . The corollary of this is that lower rates of change in northern areas suggests that these maybe the best focus for improved protection from local stresses that may increase marine ecosystems' resilience to climate change. Tropical coral reefs are, however, very sensitive to relatively small temperature increases and recent terrestrial evidence suggests that tropical species may indeed be at greater risk of the relatively smaller temperature rises than higher latitude species . Greater warming rates in more southerly regions suggests that their ecological processes may already be compromised by the significant climate changes already observed and studies of such processes may not be characteristic of processes in a more stable climate regime. A possible indicator of greater thermal stress in the southern GBR is the evidence from three recent large-scale coral bleaching events: those of 1998 and 2002 affected large parts of the reef , whereas the most recent 2006 event was largely confined to the southern GBR (Great Barrier Reef Marine Park Authority, http://www.gbrmpa.gov.au/corp_site/key_issues/climate_change/management_responses/current_condition_reports/conditions_report.html).

<18> The rapidity and magnitude of warming along Australia's tropical coastal regions is of great concern for maintenance of the integrity of their diverse tropical marine ecosystems, especially, coral reefs. Monitoring of regional climate trends and verification against improved regionally-downscaled climate scenarios will increase confidence in model output but, as demonstrated here, useful insights into rapidly changing marine climates can be obtained from instrumental climate records.

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