Our changing climate How will rainfall change in Northern Australia over this century? Climate change means that the past climate of Northern Australia is no longer a reliable indicator of future climate conditions.
Our warming world is affecting a range individual wet season (‘sub-seasonal’). also be influenced by mid-latitude of climate processes, including those For example, the total amount of weather systems (e.g. cold fronts) responsible for rainfall in Northern rainfall received each wet season bringing moisture from the mid- Australia. Rainfall change and variability can vary by up to a factor of three. latitudes into the monsoon region. can have huge environmental, social Periods of relatively wet or dry The arrival of the eastward-moving and economic impacts across Northern conditions within a wet season MJO as well as the intrusion of middle Australia, especially when linked are known as monsoon ‘bursts’ latitude systems are identified as the to extreme events. Water, energy, and ‘breaks’, respectively. There primary triggers of the onset of the infrastructure, transport, industry, is large variability in the number of Australian monsoon. The average regional development, agri-business monsoon bursts from year to year monsoon onset date across all years and tourism will all be affected. and in the time interval between in the vicinity of Darwin is around 25 Understanding how these processes consecutive monsoon bursts. December. However, the onset is also are changing, and what this means influenced by the El Niño–Southern Monsoon bursts are more likely to for the timing and amount of rainfall, is Oscillation (ENSO), typically occurring occur when the active phase of the important information for development later during El Niño years and earlier Madden–Julian Oscillation (MJO) and planning decisions. during La Niña. is in the vicinity of Australia. The MJO What influences rainfall is as an eastward moving ‘pulse’ of Several recent studies1 suggest in Northern Australia? cloud and rainfall near the equator that the Indian-Ocean Dipole that typically recurs every 30 to (IOD) – an east-west see-sawing of The vast majority of rainfall over 60 days. It originates mostly in the temperatures in the Indian Ocean – also northern Australia falls in the summer equatorial Indian Ocean and travels affects the monsoon development, ‘wet season’, due to the Australian eastward across tropical Australia especially the onset of the Australian monsoon. The monsoon onset then on into the Pacific Ocean. The monsoon which often happens occurs when the easterly trade winds MJO influences tropical weather on following the peak of the IOD. reverse, becoming moisture-laden a timescale of weeks to months. In Tropical cyclones often originate westerlies. This usually happens in late between these pulses of the MJO, the over the ocean near the ‘monsoon December and the monsoon persists monsoon westerly flow can become trough’ and can then travel towards until April. The winter months (May to quite weak or even reverse and periods northern Australia, bringing extreme November) are climatologically dry. of no or little rain (monsoon breaks) rainfall to surrounding areas. For some occur before the next burst brings However, there is enormous variability regions across tropical Australia, another significant rainfall event. in rainfall both from year to year rainfall from tropical cyclones can (‘interannual’) and within each The changes in atmospheric circulation on average contribute up to 30% at the start of the rainfall burst can to the annual rainfall amount.
Northern Australia’s climate can vary greatly from one year to the next due to a number of factors.
1. See references in Zhang H and Moise AF (2016) The Australian Summer Monsoon in Current and Future Climate. In: The Monsoons and Climate Change – Observations and Modeling. (Eds LMV de Carvalho and C Jones), Springer, pp 67–120 Rainfall in Northern Australia over the 20th century
Northern Australia as a whole experienced an overall slight mm/decade increase in rainfall during the 20th century but changes 50 have not been geographically uniform. There have been 40 marked increases across north-western regions during 30 recent decades, but decreases in the eastern 20 sector over Queensland. The increase in rainfall in 10 the north-west has influenced the temperature 5 trend over this region, reducing the warming 0 trend over parts of northern Australia -5 compared with the surrounding -10 region.2 Superimposed on these -20 longer-term changes is large -30 year-to-year variability, which is -40 strongly influenced by the -50 El Niño–Southern Oscillation.
Trend in rainfall December–February 1970–2016
Since 1970, the north-west of Australia has become wetter while total rainfall in the north-east has decreased.
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Northern Australia’s rainfall varies a great deal from year to year.
variability
large
Very
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2. Grose, M.R., Risbey, J.S. and Whetton, P.H. (2017) Tracking regional temperature variations from the early 1990s in the light of variations in regional warming, including ‘warming holes’. Climatic Change, 140: 307-322, doi:10.1007/s10584-016-1840-9. Northern Australian rainfall in the 21st century
Will the wetting trend How will the warming 60 continue? climate affect rainfall variability and extremes? The increasing rainfall observed over 1986–2005 40 of north-western Australia over the 20th We have high confidence that rainfall % century is thought to be mainly due to
variability will increase, and that extreme in natural variability, with some possible rainfall events will become more 20 influence from greenhouse gases intense. Rainfall will be more variable
and aerosols (tiny airborne particles 2080–99 across all time scales – from day to
of soot and dust) from industrial 4 by day, month to month and year to year. 0 pollution across Asia and globally. This is mainly because the warming
atmosphere will hold more moisture, change
Northern Australian rainfall will continue to be strongly influenced by natural so when it rains it will get heavier. -20 year-to-year and decade-to-decade Given the importance of ENSO for variability over the coming century. As Relative rainfall in northern Australia, it is worth global warming due to greenhouse noting that there is some indication that gases increases, beyond around mean extreme El Nino and La Nina events will Maximum rainfall 2030, changes in average rainfall may be more frequent in a warmer climate5 Annual precipitation 1-day occur in response to this warming. and that the rainfall extremes due to 6 However, projections of average rainfall ENSO will become more intense. Mean rainfall change is uncertain for both over northern Australia are inconclusive Tropical cyclones are projected, with low (blue bar) and high (red bar) emissions – some climate models show increases medium confidence, to occur less scenarios. Increases in extremes, such as maximum one-day rainfall, are much more while others show decreases. By 2090, often, but with a greater proportion annual rainfall differences of up to 25% confidently projected for low and high of high intensity storms (bringing emissions cases. The grey bars show the are projected under the high emissions stronger winds and greater rainfall). year-to-year variability. scenario (RCP8.5), but some climate models indicate this will be a decrease while others indicate an increase. We can improve our confidence in climate projections by analysing the model simulations to determine which are most realistic. For example, we have more confidence in models that show a wetter or little changed Australian monsoon because the climate models that show a drier Australian monsoon in the future tend to have sea-surface temperatures that are too cold in the western Pacific in the current climate.3 (Sea- surface temperatures influence rainfall patterns, particularly in the tropics.) Mike Rosel, Bureau of Meteorology
3. Brown J.R., Moise A.F., Colman R. and Zhang H. (2016), Will a warmer world mean a wetter or drier Australian monsoon? Journal of Climate, 29, 4577–4596, doi:10.1175/JCLI-D-15-0695.1. 4. Brown J.R., Moise A.F. and Colman R.A. (2017), Projected increases in daily to decadal variability of Asian-Australian Monsoon Rainfall. Geophysical Research Letters, 44, 5683-5690, doi:10.1002/2017GL073217. 5. Cai, W. and co-authors (2014), Increasing frequency of extreme El Nino events due to greenhouse warming, Nature Climate Change, 4, 111-116, doi:10.1038/nclimate2100; Cai, W. and co-authors (2015), Increased frequency of extreme La Nina events under greenhouse warming, Nature Climate Change, 5, 132-137, doi:10.1038/nclimate2492. 6., Power S., F. Delage, C. Chung, G. Kociuba and K. Keay (2013), Robust twenty-first century projections of El Nino and related precipitation variability, Nature, 502, 541-545, doi:10.1038/nature12580. Why is it difficult to model northern Australian rainfall?
There are several reasons that Modelling the balance between direction, may also change where climate models disagree about different influences on future the bands of heaviest rainfall occur. the direction of future changes in monsoon rainfall is also difficult. Some of these factors will lead northern Australian rainfall (wetter Higher temperatures will lead to to increased Australian summer or drier). Firstly, models are not able more moisture in the atmosphere, monsoon, while others will have the to reproduce all the details of the which supports heavier rainfall on opposite influence – and the relative current climate correctly, including the average. However, the warming importance of each factor may be spatial pattern and timing of rainfall. climate also tends to slow the different in different climate models. For example, models may have circulation of the atmosphere in While climate models do not show overly cold sea surface temperatures the tropics, weakening the upward clear agreement on future changes nearby in the tropical Pacific, which motion that produces rainfall in the in total summer monsoon rainfall, causes rainfall amounts to be too monsoon regions. In addition, future almost all models agree on changes low over northern Australia. The reductions in aerosols (tiny particles in extremes and variability. Scientists details of tropical convection, which produced by industrial pollution) are working to better understand is the main source of monsoon are likely to cause regional shifts in the sources of uncertainty in rainfall, are also not easy to simulate rainfall and temperature that favour rainfall changes, and to narrow in models which have grid spacing a drier Australian monsoon. Other the range of likely future change. of around 100-300 km, larger than shifts in the atmospheric circulation, many tropical storm systems. such as changes in wind strength or
Northern Australia’s warming climate
Rainfall changes are not occurring rate of warming will depend on above 35°C is projected to increase in isolation. They are happening the concentration of greenhouse from 11 in the current climate to 43 against a backdrop of broader gases in the atmosphere. In the by 2030, and 111 (RCP4.5) or 265 climate change that they influence near term (that is, to 2030), the (RCP8.5) by 2090 (depending on and are influenced by. warming will be 0.5°C to 1.3°C with greenhouse gas concentrations). only small differences for different For example, increases in mean emissions scenarios. In contrast to temperature allow the atmosphere Northern Australia’s future climate the case for rainfall, the warming to hold more moisture, leading to is large compared with year-to- • Average temperatures heavier rainfall events. At the same will continue to increase year variability in temperature. time, this increase in temperature in all seasons modifies the larger circulation The effect of greenhouse gas • More hot days and patterns in the atmosphere, which concentration is much more warm spells could lead to a change in strength pronounced by the end of the • Changes in average and location of wind regimes. All this century. Under a high emissions rainfall are possible, but will influence how water is distributed scenario (RCP8.5) projected warming uncertain; large decreases across tropical Australia with follow- for 2090 is 2.8°C to 5.1°C (compared are less likely than on effects on humidity, evaporation, to the period 1986–2005). Under a increases or little change storms, flooding and bushfires. lower emissions scenario (RCP4.5), • Increased intensity of extreme daily rainfall events expected warming is 1.3°C to 2.7°C. Mean temperatures in northern Australia have increased by around Northern Australia can also expect • Mean sea level will continue to rise. Height of extreme 0.9°C between 1910 and 2013, changes to temperature extremes sea level will also increase although areas of north-west with substantial increases in the Australia have seen a decrease in number of warm spell days and in • Fewer but more intense mean temperature since 1960, due the hottest days of the year. There tropical cyclones in part to increases in cloudiness. will also be an increase in the number of days with maximum temperatures More detailed climate projections for Temperatures in Northern Australia above a threshold such as 35°C and Northern Australia are available at are projected to continue to rise 40°C. For example, for Darwin, days www.climatechangeinaustralia.gov.au. throughout the 21st century. The Implications for policy and decision making
Impacts of climate change Planning for a detailed studies, such as impacts in Northern Australia changing climate assessments, output from global or high resolution models may be required. Many economic, social and Because of the wide-ranging impacts, Many different climate models are environmental aspects of Northern climate change needs to be considered used to develop projections for the Australia are likely to be sensitive to as an integral part of a risk analysis future. More than 40 climate models climate change. Impacts may include: in policies and decisions about are available, each with slightly different Northern Australia. In considering • increased human health risks due components and resulting differences climate risk, it is vital to bear in mind to the increase in the number of hot in the simulated future climate. For that projected changes of different days and heatwaves. Under a high variables or regions where all or most aspects of the climate have different emissions scenario, Darwin and models agree on future changes, the levels of scientific certainty. Broome may experience temperatures results have a higher level of confidence. over 35°C for two-thirds of the year The scientific community has extremely However, models showing possible by the end of the century. This will high confidence in further warming but less likely future states may also be increase heat stress, particularly and sea level rises over Northern useful in a risk management context. for the outdoor workforce. Australia in all seasons: these are The Climate Futures Tool on the virtually certain. The size and rapidity Climate Change in Australia website • damage to cultural sites and loss of of the changes, however, will depend (www.climatechangeinaustralia. natural resources that are important on future greenhouse gas emissions. gov.au) can be used to help select to Indigenous communities. Due internally consistent climate projections to poorer health and socioeconomic Projected rainfall changes are less datasets for impact assessments disadvantage Indigenous Australians certain. There is very high confidence and adaptation planning, based on are likely to be disproportionately that rainfall variability (from daily to combinations of key variables. These vulnerable to climate change. seasonal timescales) will increase, and datasets can be applied to impact that heavier rainfall extremes will occur. • grazing, cropping and plans for models, for example in simulating the However, changes in average rainfall agricultural expansion may be effect of climate change on different (i.e. seasonal totals) are less clear. affected by changes in summer rainfall crops, or on future bushfire risk. Therefore, rainfall-sensitive projects and the incidence of droughts. and industries in Northern Australia Climate change science • damage to settlements and will need to include the possibility to support policy and infrastructure due to increased of both increases and decreases in frequency and intensity of floods future average rainfall amounts. decision making • damage to the composition and Different types of climate change Research undertaken through the structure of the Great Barrier Reef projections information may be Earth Systems and Climate Change and significant loss of biodiversity due needed for different purposes. In many Hub is improving confidence in climate to ocean warming and acidification. instances, decision makers may want to change projections as a result of This will have flow on impacts know the likely direction and magnitude improved understanding of climate for many tourism operators. of change in a few key climate variables, feedbacks and key climate processes. such as mean temperature, humidity, • changes in ecosystems, such as the The Hub is also working with a sea level or extremes such as heat expansion of monsoon rainforest at range of stakeholders to improve waves. Ranges and probabilities may the expense of eucalypt savannah the usability and uptake of also be needed, and these can be and grassland, with associated climate change information for obtained by considering the full set of biodiversity and economic impacts. adaptation and related activities. climate model projections. For more With greater access to and confidence in climate change projections, policy For more information, and management decisions can please contact: be effectively informed by climate Dr Jo Brown change science, ensuring the best Project 2.6 possible response to Australia’s [email protected] variable and changing climate. www.nespclimate.com.au
The Earth Systems and Climate Change Hub is funded by the Australian Government’s National Environmental Science Program.