Story by Lin Sutherland 

reproduced with permission from Explore, the Australian Museum magazine, vol 30 (2) pp 24-27 © Australian Museum 2008.

CLIMATE CHANGE
A GEOLOGIST’S VIEW

Climate change is not new to Australia. Our ‘rocky’ records hold many clues to past climates, which is why debates about climate change need geology on the agenda, according to Museum geologist Lin Sutherland.

Many converging and conflicting factors control Earth’s climates to create variable cooling or warming effects over the global surface. These forces are monumental indeed. Consider for example Earth’s axial tilt in its elliptical orbit around the Sun, which leads to summer and winter weather changes. Imposed on this basic cycle is an ever-shifting interplay between atmosphere, land masses, ice caps, seas and oceans and biological inhabitants, including people, all operating at different time scales.

The biosphere is sensitive to these and other powerful events from Earth’s planetary setting. Depending on who you listen to, either we have neglected these forces or have underestimated how much impact human activity has had on climate change. Join me now for a bird’s-eye view of the major geodynamic forces over the past 65 million years and how they have shaped, and continue to shape, Earth’s climates.

The solar system is a constant climate changer for Earth, sometimes by sudden and severe means or through more regular eccentricities. For example, collisions with asteroids and comets have delivered instant global molten showers, firestorms, acid rains and prolonged winters followed by greenhouse blooming. Such a climatic trauma 65 million years ago – a meteorite strike – wiped out over 65 per cent of the planet’s living species. At other times, when land and water masses reach critical distributions, variations in Earth’s polar wobble, axial tilt and orbit around the sun can usher in ice age cycles. Sea levels fall as water is taken into the expanding ice caps. Glacial climates set in for up to 100,000 years, interspersed with warmer inter-glacial periods (as at present) with higher sea levels lasting between 10,000 and 50,000 years.

Some scientists think that increased solar or cosmic flares cause episodes of climate change. Certainly these form significant inputs of energy into Earth’s climatic system and deserve to be considered, but their role in climate change relative to other factors is disputed.

Earth tectonics is a slow but sure driver of long-term climate changes. Earth’s surface plates move forward by centimetres a year (at about the same rate as your fingernails grow, as US astronomer Jay Pasachoff puts it), forming new ocean floors and uplifting mountain ranges in collision zones, with inevitable consequences for climate. As Antarctica moved to the South Pole, and the Alps and Himalayas rose in the mid- and high northern latitudes 50 million years ago, it led to global cooling and finally glaciations. Meanwhile, Australia, moving north as the Southern Ocean opened, left its southern roots and travelled towards more tropical zones, becoming warmer against the global cooling trend.

Volcanism in stupendous eruptions regularly delivers ash and gas-charged clouds into the stratosphere, masking the Sun’s radiation and inducing global cooling. Spectacular documented eruptions, such as Krakatoa in 1883 and Tambora in 1815 in the Indonesian islands and Laki in Iceland in 1783, produced ozone holes and years ‘without summer’. Larger past eruptions would have had much greater disturbances: Huaynaputina, Peru in 1660 AD; Taupo, New Zealand in 230 AD; Thera, Greece about 1620 BC – all caused longer periods of cooling. A super-volcano at Lake Toba in Indonesia nearly 800,000 years ago ejected 1000 cubic kilometres of material into the global atmosphere. There were climatic consequences for the expanding human race, but no prolonged deterioration. More immense still, the Deccan lava floods in India at the Cretaceous–Cenozoic boundary injected billions of tonnes of sulphur dioxide into the atmosphere in decade-long eruptions for over a million years. Such inputs placed severe climatic stress on the biosphere and no doubt assisted mass extinction when a large meteorite struck Earth 65 million years ago.

New waterways can change local and even global climates. Oceans breaching into depressed valley floors provide new circulations, such as the drowning of the Mediterranean by the Atlantic Ocean, or the formation of the Southern Polar Current as Antarctica and Australia separated. Breakouts from huge, coldwater lakes into adjacent oceans, such as from the Great Lakes of Canada into the Atlantic around 16,000 years ago, interfered with circulating currents and affected the climate. Such an event created an icy spell in Europe while the climate generally was warming.

Today, large-scale movements of warm and cold oceanic waters in the Southern Pacific Oscillation give warmer ‘El Niño’ periods characterised by powerful storms, rising sea levels, coastal flooding and inland droughts. Or they can lead to cooler, more equitable ‘La Niña’ conditions.

The loss of waterways also affects climates. When volcanoes joined up North and South America 13 million years ago, the Pacific and Atlantic circulation changed irrevocably and led to cooler climates.

Giant gas bursts from methyl hydrates hidden in seafloor sediments can overwhelm the atmosphere and cause ‘greenhouse’ temperature rises of over 5°C – and may have caused an extinction event 54 million years ago. We blame greenhouse gas emissions from human activities for the current period of global warming, and heated arguments on this topic have engaged many scientists, economists and politicians, leading to calls for action to reduce the rate of change.

Australia’s early climates and biosphere were reset by the meteorite strike 65 million years ago. Held in polar embrace with Antarctica, through umbilical Tasmania, Australia’s Paleocene landmass shook off ‘impact winter’ to develop moist, lush conditions. Antarctica lacked ice then, so no cold wind systems swept north as they do today. A widening rift in the Eocene Epoch 34 million years ago started deep ocean circulation between Australia and Antarctica, and the cold circumpolar current was born. Australia moving ever north entered warmer, wetter climatic belts and developed deeply leached and hard crusted soils. Global warming within the Oligocene and Miocene epochs, between 28 and 15 million years ago, produced warm rising seas that flooded into the Bass, Murray and Nullarbor basins, but this was not ‘greenhouse’ warming, as the concentration of atmospheric carbon dioxide was less than half present levels. Nonetheless, what are now arid inland areas experienced moderate mean annual temperatures of between 14°C and 20°C and mean annual rainfall of up to 600 mm.

Australia’s later climates began cooling when Antarctica became ‘ice cap’ land. East Antarctic ice expanded greatly 15 million years ago and West Antarctic ice joined in 6 million years ago. A collapse of ice sheets by the Pliocene Epoch, 5 million years ago, saw climates warm and seas rise again, forming a shallow Murray Sea. The icing of Antarctica, 3 million years ago, and then of the northern hemisphere, brought in the Pleistocene ice ages and their climate swings, a mere tremor in the overall geological picture but extremely significant for life on Earth.

Australia’s occupation climates extend back 50,000 years, based on dating studies of Indigenous Australian sites. Indigenous Australians survived the last glacial period, when ice caps grew on Tasmanian and Kosciusko highlands. Many places in Australia record ‘recent’ swings in climate, sea level and vegetation and provide data for understanding climate scenarios. Coastal occupation sites, Willandra Lakes and fault-bounded Lake George in New South Wales; volcanic craters like Lynchs Crater and reef systems like the Great Barrier Reef in Queensland; marine embayments like the Gulf of Carpentaria in northern Australia; cave deposits in the Naracoorte and Nullarbor cave systems in southern Australia; marine deposits under the Tasman Sea – these show that wetter climates fluctuated between 55,000 and 40,000 years ago, followed by drier, dustier spells until 30,000 years ago, with a return to wet spells again around 20,000 years ago. The post-glacial sea rose to present levels 10,000 years ago, isolating Tasmania and its inhabitants.

Present global warming trends, whether caused by people or nature or the artefacts of analysis and modelling, must consider the 300 gigatonnes of carbon that humans have added to the air over the last 250 years. Ground temperatures have risen in SW Australia by around 0.5°C in the last century and large growing cities like Sydney form new microclimates around them. Computer modelling suggests several variations in the climates predicted for Australia, with increasing annual temperatures, more severe weather events and a hotter, drier inland being the main scenarios.

From the geological record at least, alternative hypotheses for climate change have not always been considered. For example, glaciers have certainly retreated worldwide since the 1870s, but then they have waxed and waned several times in the last 12,000 years. The fact remains, however, that Earth is warming, glaciers are receding again and the consequences for river systems and the people who depend on them will be severe indeed. Reducing greenhouse gas emissions from human activity and planning for the consequences of change is prudent as world populations increasingly interact with climatic events.

Even so, we should remember that geological forces are always on the agenda, wielding enormous forces that ultimately we are powerless to control.

Dr F Lin Sutherland is Senior Fellow in Geoscience at the Australian Museum.