A high-resolution, 35,000-year record of the Australian monsoon and ENSO-variability as reconstructed from Papua New Guinea speleothems

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Subsistence farmers in the remote highlands of Papua New Guinea need no convincing that climate change is occurring. Their survival depends on what they can produce from their gardens and recently they have been able to grow crops at higher and higher elevations. The Papua New Guinea highlands are one of the planetary canaries warning us of future climate change. This sensitivity to climate change is why Papua New Guinea is a key site for understanding our future greenhouse world. It is also why I explored caves in Papua New Guinea last year to look for speleothems (cave deposits such as stalagmites) that would capture a record of how climate in this critical region varied in the past. Papua New Guinea sits at the heart of the Western Pacific Warm Pool. This region has the warmest ocean surface waters and the most intense atmospheric convection on Earth. Often referred to as the Earth's heat engine, the Western Pacific Warm Pool is the site of massive energy exchange that drives global atmospheric circulation. A change in the warm pool, for example its temperature or position, can have profound effects on climate around the globe. We see this influence every 2 to 7 years in El Niño events, when the Western Pacific Warm Pool shifts eastward, taking the centre of atmospheric convection with it. The 1997-98 El Niño event was the most extreme in recent history. It caused catastrophic flooding in Peru and Ecuador, and drought in Bolivia, Brazil, Indonesia, New Guinea and Australia. In Papua New Guinea over 40% of the predominantly rural population suffered severe food shortage because of drought, frost and forest fires. At reefs across the tropics, warmer than normal sea surface temperatures caused mass bleaching, killing 16% of the world's coral. One of the key unknowns in predicting future climate variability is how the El Niño Southern Oscillation (ENSO) phenomenon and the background state of the Western Pacific Warm Pool will respond. ENSO also interacts with the Asian-Australian monsoon circulation, but the relationship is both inconsistent and poorly understood. With over 800 million people from across India, South-east Asia to Australia directly dependent on the stability and onset of these monsoonal rains, predicting how it will behave in a warmer world is also critical. As the monsoon system swings seasonally between the Northern and Southern Hemispheres, the trade winds run into a barrier - the 1600 km long mountain chain of peaks up to 5000m high that forms the central spine of New Guinea. This creates strong seasonality as the trade winds swing from NW to SE - the slopes facing the trade winds experience a wet season, while the opposite slopes are in a rain shadow. I have collected speleothem samples from a coastal mountain range (the Sarawaget Range; 6 deg.S, 147 deg.E), which receives rain during the NW trade wind season when the monsoon system is in the Southern Hemisphere. The samples were collected in caves at altitudes ranging from 850m to 3650m (possibly the highest elevation speleothems ever collected). Speleothems form by slow calcite precipitation from drip-water, recording changes in rainfall and the environment above the cave. It is this signal of rainfall variations and the vegetation response that I want to investigate with geochemical tools and tree-ring research techniques. For my PhD I used coral from the Great Barrier Reef to show how the Australian monsoon strength changed over the last 370 years, and its shifting relationship with ENSO. With a NERC fellowship I will be able to document changes in ENSO and the Australian monsoon over the last 35,000 years, which will improve our understanding of how they will impact our future climate.