With all of the burning and clearing happening in the Amazon rainforest, it was only going to be a short matter of time before a tipping point was reached and now a tipping point appears on the horizon. It would seem only a matter of 1 or 2 years before the Amazon is unable to sustain itself through rainfall. The link below is to an article reporting on the threat posed to the Amazon.
Over the past decade we have seen a substantial increase in our scientific understanding of how climate change affects extreme rainfall events. Not only do our climate models suggest that heavy rainfall events will intensify as the atmosphere warms, but we have also seen these projections start to become reality, with observed increases in rainfall intensity in two-thirds of the places covered by our global database.
Given this, we might expect that the risk of floods should be increasing globally as well. When it comes to global flood damage, the economic losses increased from roughly US$7 billion per year in the 1980s to US$24 billion per year in 2001-11 (adjusted for inflation).
It would be natural to conclude that at least some of this should be attributable to climate change. However, we know that our global population is increasing rapidly and that more people now live in flood-prone areas, particularly in developing countries. Our assets are also becoming more valuable – one only needs to look at rising Australian house prices to see that the values of homes at risk of flooding would be much greater now than they used to be in decades past.
So how much of this change in flood risk is really attributable to the observed changes in extreme rainfall? This is where the story gets much more complicated, with our new research showing that this question is still a long way from being answered.
Are floods on the rise?
To understand whether flood risk is changing – even after accounting for changes in population or asset value – we looked at measurements of the highest water flows at a given location for each year of record.
This sort of data is easy to collect, and as such we have reasonably reliable records to study. There are more than 9,000 streamflow gauges around the world, some of which have been collecting data for more than a century. We can thus determine when and how often each location has experienced particularly high volumes of water flow (called “large streamflow events”), and work out whether its flood frequency has changed.
We found that many more locations have experienced a decrease in large streamflow events than have experienced an increase. These decreases are particularly evident in tropical, arid, and humid snowy climate regions, whereas locations with increasing trends were more prevalent in temperate regions.
To understand our findings, we must first look closely at the factors that could alter the frequency and magnitude of these large streamflow events. These factors are many and varied, and not all of them are related directly to climate. For example, land-use changes, regulated water releases (through dam operations), and the construction of channels or flood levées could all influence streamflow measurements.
We looked into this further by focusing on water catchments that do not have large upstream dams, and have not experienced large changes in forest cover that would alter water runoff patterns. Interestingly, this barely changed our results – we still found more locations with decreasing trends than increasing trends.
The Australian Bureau of Meteorology and similar agencies worldwide have also gone to great lengths to assemble “reference hydrological stations”, in catchments that have experienced relatively limited human change. Studies that used these sorts of stations in Australia, North America and Europe are all still consistent with our findings – namely that most stations show either limited changes or decreases in large streamflow events, depending on their location.
What can we say about future flood risk?
So what about the apparent contradiction between the observed increases in extreme rainfall and the observed decreases in large streamflow events? As noted above, our results don’t seem to be heavily influenced by changes in land use, so this is unlikely to be the primary explanation.
An alternative explanation is that, perhaps counterintuitively, extreme rainfall is not the only cause of floods. If one considers the 2010-11 floods in Queensland, these happened because of heavy rainfall in December and January, but an important part of the picture is that the catchments were already “primed” for flooding by a very wet spring.
Perhaps the way in which catchments are primed for floods is changing. This would make sense, because climate change also can cause higher potential moisture loss from soils and plants, and reductions in average annual rainfall in many parts of the world, such as has been projected for large parts of Australia.
This could mean that catchments in many parts of the world are getting drier on average, which might mean that extreme rainfall events, when they do arrive, are less likely to trigger floods. But testing this hypothesis is difficult, so the jury is still out on whether this can explain our findings.
Despite these uncertainties, we can be confident that the impacts of climate change on flooding will be much more nuanced than is commonly appreciated, with decreases in some places and increases in others.
Your own flood risk will probably be determined by your local geography. If you live in a low-lying catchment close to the ocean (and therefore affected by sea level rise), you’re probably at increased risk. If you’re in a small urban catchment that is sensitive to short sharp storms, there is emerging evidence that you may be at increased risk too. But for larger rural catchments, or places where floods are generally caused by snow melt, the outcome is far harder to predict and certain locations may see a decrease in flooding.
All of this means that a one-size-fits-all approach is unlikely to be suitable if we are to allocate our resources wisely in adjusting to future flood risk. We must also think about the effects of climate change in a broader context that includes changes to land-use planning, investment in flood protection infrastructure, flood insurance, early warning systems, and so on.
Only by taking a holistic view, informed by the best available science, can we truly minimise risk and maximise our resilience to future floods.
Seth Westra, Associate Professor, School of Civil, Environmental and Mining Engineering, University of Adelaide and Hong Xuan Do, PhD candidate in Civil and Environmental Engineering, University of Adelaide
The Millennium Drought ended more than five years ago, but several years of below-average rainfall and El Niño have brought drought back to many parts of Australia. Our latest report on water in Australia shows rainfall is continuing to decline in eastern Australia and increase in the north.
However in urban areas, where water use has not changed significantly since the Millennium Drought, more water is available for use thanks to technologies such as desalination and recycling.
In a recent article on The Conversation, the Bureau of Meteorology put the case that Australia can now better manage water resources using new water information capability.
Last week the Bureau released a new assessment report on our national water availability and use. Water in Australia 2013–14 examines climatic conditions and the physical hydrology to create the most recent national assessment of Australia’s water resources.
The main findings are outlined below.
Since 1950, rainfall has increased in Australia’s north and northwest, but declined along the west coast and most of eastern Australia. This pattern was reflected in 2013–14.
Rainfall affects streamflow and groundwater replenishment, which in turn affects our available water resources.
In southern Queensland and northern New South Wales, a severe drought, which started in 2012, continued in 2013–14. As expected, streamflow in these areas was very much below average in drought-impacted areas.
The volume of groundwater stored in aquifers is very large compared with surface water and responds more slowly to external influences. As a result, most bores across the country had average groundwater levels in 2013–14.
In South Australia and Queensland, more than one-third of all bores had an above-average level. Below-average groundwater levels were present in 5–20% of bores in each State and Territory.
Australia has extensive water supplies and their use is managed by various institutional arrangements. Water availability is being increased by using recycled and desalinated water. At the same time, greater protection is being afforded to the environment through the purchase of entitlements from water users and investments in water-saving infrastructure.
Since the Millennium Drought, Australia’s water market has thrived, facilitating the buying and selling of water rights to allow water to be moved and put to more effective use. Entitlement trade increased in 2013–14 to about 2400 gigalitres (GL), and can partly be attributed to entitlements being transferred to the Commonwealth for the environment and partly to declining water storage levels that prompt buyers into the market to secure more water. Allocation trade in 2013–14 was around 5500 GL.
As well as ensuring sustainable water supply for human needs, water resources are managed to ensure that environmental needs are met. Environmental water holders in the Murray–Darling Basin held 3192 GL of surface water entitlements at the end of 2013–14 (increasing from 3160 GL at the end of 2012–13). Of the total allocated environmental water available in 2013–14, 68% was delivered for environmental purposes and 27% was carried over to 2014–15.
Total water use across Australia was estimated at 23 500 GL in 2013–14. Irrigation (57% of total use) and urban consumption (17% of total use) were the top two water uses.
The main irrigation use in 2013–14, at just over 9500 GL, was in the Murray–Darling Basin. The estimated total surface water use for irrigation in the Murray–Darling Basin decreased from 11 000 GL in 2012–13 to about 8400 GL in 2013–14—a drop of 24%.
Groundwater use for irrigation increased by 18%, to 1100 GL, because of drier conditions and limited surface water allocation announcements — particularly in the northern Basin.
Outside the Basin, 3900 GL were used for irrigation, mainly in the Queensland and Victorian coastal regions, the coastal regions surrounding Perth and Adelaide, northeastern Tasmania, and in the Ord irrigation scheme in northern Australia.
Urban residential use in 2013–14 was 185 kilolitre per property, up 3% from 2012–13. However, use per property has not increased significantly from the levels at the end of the Millennium Drought. Total water use in 2013–14 in the major cities showed no significant changes from the recent past.
Though water availability exceeded use in 2013–14, shortages were experienced across large parts of the continent. With Australia’s highly variable and changing climate there will be ongoing challenges to meet the water needs of our nation.
The links below are to articles concerning changing rainfall patterns due to changes in ocean salinity – very interesting reading.