Climate Variability and Trends in Australia David Walland Bureau of Meteorology Carbon-Drought Workshop 5-8 June Canberra

Outline An introduction to Australian climate (rainfall) variability The emergence of climate change amongst the variability Projections

A Long History of Rainfall Variability Australia is unique amongst developed countries. Natural and human systems must be adapted to a boom bust climate (100 years of data for Australia and generally also for the other countries)

Australian Rainfall - Droughts and Deluge

Impact on Biosphere Droughts clearly impact on water limited vegetation e.g. tree mortality (Breshears et al 2005), crop yield etc. Bushfires have a major impact on all types of vegetation –Drought associated with the lead up to the 2002/03 bushfires –Extreme event to trigger fires e.g. Wilsons Prom bushfires (April 2005 was nationally hottest April on record (largest temp anomaly on record) & fires occured during a record breaking heatwave)

Significant drivers of Rainfall Variability Natural variability in Australian rainfall is due to many factors, some chaotic. El Nino is one of the more dominant predictable influences and were La Nina and El Nino years respectively. The Walker Circulation - “normal” and El Nino

Emerging Climate Trends High-quality rainfall dataset: Annual trends

High-Quality Annual Temperature trends: Max T: +0.06°C/decade Min T: +0.12°C/decade

Impact of Extremes systems have adaptive capacity so they may be stressed but can often cope with slow changes in the mean extremes events can exceed their adaptive capacity and tip the system over the edge e.g. –hot days …. Bushfires –Tropical cyclones ….wind damage, saltwater intrusion there is much on-going research into frequency of extreme events

High-quality Daily Temperature: Daily Extremes Hot Days: Max T ≥ 35 o C Cold Days: Max T ≤ 15 o C Hot Nights: Min T ≥ 20 o C Cold Nights: Min T ≤ 5 o C Shifts consistent with changes in mean Averaged over “valid” regions only Updated from Collins et al. (2000)

Sea Surface Temperature Trends Smith and Reynolds (2004) Free from urban influence, show rapid warming about SE Australia => interesting insight into the synoptic changes

El-Nino Walker Circulation has been observed to be weaker over the past century which implied a more El Nino like state in the Pacific. –Consistent with observed rainfall trends No consensus about the future evolution and whether this scenario is likely to continue.

Reductions in Southern Australian rainfall are likely due to changes in the dominant weather systems themselves. A southward shift in the frontal/low pressure system genesis region would have a huge impact on Southern Australia Such a change has been mechanically linked to increases in greenhouse gases and reductions in polar ozone. Changing weather systems & rainfall

Impacts on water resources A slow forcing give rise to a rapidly changing resource outcome.

The SE Australian 10 Year Drought An unprecedented and protracted hydrological drought affecting much of southeast Australia Also, warmest period on record.

Temperature and Rainfall

Vegetation health (condition) due to water availability and temperatures, based on satellite data. Drought regions with vegetation under stress are shown in red. (Kogan, 1997) Obtained from NOAA, USA from The drought of 2002 was one of Australia’s most severe due to record high temperatures, not record low rainfall. Increased temperatures have increased moisture stress

Global mean warming C by 2100 Mean sea-level rise 9- 88cm by 2100 The latest IPCC assessment of all scientific literature on Climate Change is due for release next year It concludes that recent warming is highly likely (>90% chance) due to human influences on climate. Modelling centres around the world are running forward projections of climate that is forced with expected increases to greenhouse gases. Future projections of climate change

A synthesis of climate models produces a range of likely rainfall changes in Australia under global warming scenarios Almost all the models agree on reduced rainfall for Southern Australia

Model predicted temperature changes in Australia under global warming scenarios Continental wide warming will result from greenhouse gas increases.

Conclusions Australia is a relatively dry country with a high natural variability in its rainfall regime Natural and human systems have high capacity to deal with such variability but are undoubtedly impacted by droughts, bushfires, storms etc Climate in Australia is changing – temp, rainfall, mean, extremes Extreme events; droughts, record high temps, bushfires impact more obviously on biosphere than changes in mean

Conclusions cont… Changes in rainfall regimes already noted can be interpreted in terms of changing dynamical patterns The combined impact of temperature and rainfall changes can be severe for the biosphere Projections indicate a drier southern Australia with hints of a drier eastern Australia and a wetter north-west Australia but higher temperatures are clear everywhere

Extreme and Mean Rainfall Variations Over Australia Rainfall deficiencies consistent with variations in mean. Some years “drought free” others “drought dominates Australia”

Summary of Current Climate Change/Long-Term Variability Knowledge VariableHQ setObserved ChangesAttribution TemperatureYes... butWarming everywhere Yes RainfallYes... butIncreasingly strong trends Generally no (except SWA) EvaporationNearlyLittle change DroughtNoUncertain... but Extremes (TCs, storms, fire weather, high inflows) For some variables Uncertain/Mixed

Seasonal Signal AUT SPRSUM WIN