Globally, floods seem to be decreasing even as extreme rainfall rises. Why?


Seth Westra, University of Adelaide and Hong Xuan Do, University of Adelaide

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.

A streamflow gauging station in Scotland.
Jim Barton/Wikimedia Commons, CC BY-SA

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.

The ConversationOnly 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

This article was originally published on The Conversation. Read the original article.

Was Tasmania’s summer of fires and floods a glimpse of its climate future?


Alistair Hobday, CSIRO; Eric Oliver, University of Tasmania; Jan McDonald, University of Tasmania, and Michael Grose, CSIRO

Drought, fires, floods, marine heatwaves – Tasmania has had a tough time this summer. These events damaged its natural environment, including world heritage forests and alpine areas, and affected homes, businesses and energy security.

In past decades, climate-related warming of Tasmania’s land and ocean environments has seen dozens of marine species moving south, contributed to dieback in several tree species, and encouraged businesses and people from mainland Australia to relocate. These slow changes don’t generate a lot of attention, but this summer’s events have made people sit up and take notice.

If climate change will produce conditions that we have never seen before, did Tasmania just get a glimpse of this future?

Hot summer

After the coldest winter in half a century, Tasmania experienced a warm and very dry spring in 2015, including a record dry October. During this time there was a strong El Niño event in the Pacific Ocean and a positive Indian Ocean Dipole event, both of which influence Tasmania’s climate.

The dry spring was followed by Tasmania’s warmest summer since records began in 1910, with temperatures 1.78℃ above the long-term average. Many regions, especially the west coast, stayed dry during the summer – a pattern consistent with climate projections. The dry spring and summer led to a reduction in available water, including a reduction of inflows into reservoirs.

Left: September-November 2015 rainfall, relative to the long-term average. Right: December 2015-February 2016 temperatures, relative to the long-term average.
Bureau of Meteorology, Author provided

Is warmer better? Not with fires and floods

Tourists and locals alike enjoyed the clear, warm days – but these conditions came at a cost, priming Tasmania for damaging bushfires. Three big lightning storms struck, including one on January 13 that delivered almost 2,000 lightning strikes and sparked many fires, particularly in the state’s northwest.

By the end of February, more than 300 fires had burned more than 120,000 hectares, including more than 1% of Tasmania’s World Heritage Area – alpine areas that had not burnt since the end of the last ice age some 8,000 years ago. Their fire-sensitive cushion plants and endemic pine forests are unlikely to recover, due to the loss of peat and soils.

Meanwhile, the state’s emergency resources were further stretched by heavy rain at the end of January. This caused flash flooding in several east coast towns, some of which received their highest rainfall ever. Launceston experienced its second-wettest day on record, while Gray recorded 221 mm in one day, and 489 mm over four days.

Flooding and road closures isolated parts of the state for several days, and many businesses (particularly tourism) suffered weeks of disruption. The extreme rainfall was caused by an intense low-pressure system – the Climate Futures for Tasmania project has predicted that this kind of event will become more frequent in the state’s northeast under a warming climate.

Warm seas

This summer, an extended marine heatwave also developed off eastern Tasmania. Temperatures were 4.4℃ above average, partly due to the warm East Australian Current extending southwards. The heatwave began on December 3, 2015, and was ongoing as of April 17 – the longest such event recorded in Tasmania since satellite records began in 1982. It began just days after the end of the second-longest marine heatwave on record, from August 31 to November 28, 2015, although that event was less intense.

Anatomy of a marine heatwave. Top left: summer sea surface temperatures relative to seasonal average. Top right: ocean temperature over time; red shaded region shows the ongoing heatwave. Bottom panels: duration (left) and intensity (right) of all recorded heatwaves; the ongoing event is shown in red.
Eric Oliver

As well as months of near-constant heat stress, oyster farms along the east coast were devastated by a new disease, Pacific Oyster Mortality Syndrome, which killed 100% of juvenile oysters at some farms. The disease, which has previously affected New South Wales oyster farms, is thought to be linked to unusually warm water temperatures, although this is not yet proven.

Compounding the damage

Tasmania is often seen as having a mild climate that is less vulnerable to damage from climate change. It has even been portrayed as a “climate refuge”. But if this summer was a taste of things to come, Tasmania may be less resilient than many have believed.

The spring and summer weather also hit Tasmania’s hydroelectric dams, which were already run down during the short-lived carbon price as Tasmania sold clean renewable power to the mainland. Dam levels are at an all-time low and continue to fall.

The situation has escalated into a looming energy crisis, because the state’s connection to the national electricity grid – the Basslink cable – has not been operational since late December. The state faces the prospect of meeting winter energy demand by running 200 leased diesel generators, at a cost of A$43 million and making major carbon emissions that can only exacerbate the climate-related problems that are already stretching the state’s emergency response capability.

Is this summer’s experience a window on the future? Further study into the causes of climate events, known as “detection and attribution”, can help us untangle the human influence from natural factors.

If we do see the fingerprint of human influence on this summer, Tasmania and every other state and territory should take in the view and plan accordingly. The likely concurrence of multiple events in the future – such as Tasmania’s simultaneous fires and floods at either end of the island and a heatwave offshore – demands that governments and communities devise new strategies and mobilise extra resources.

This will require unprecedented coordination and cooperation between governments at all levels, and between governments, citizens, and community and business groups. Done well, the island state could show other parts of Australia how to prepare for a future with no precedent.

The Conversation

Alistair Hobday, Senior Principal Research Scientist – Oceans and Atmosphere, CSIRO; Eric Oliver, Postdoctoral Fellow (Physical Oceanography and Climate), University of Tasmania; Jan McDonald, Professor of Environmental Law, University of Tasmania, and Michael Grose, Climate Projections Scientist, CSIRO

This article was originally published on The Conversation. Read the original article.

500 years of drought and flood: trees and corals reveal Australia’s climate history


Patrick Baker, University of Melbourne; Chris Turney, UNSW Australia, and Jonathan Palmer, UNSW Australia

Australia is the land of drought and flooding rains, and in a recent paper we’ve shown that’s been the case for more than 500 years. As part of our Australia and New Zealand Drought Atlas we’ve published the most detailed record of drought and wet periods (or “pluvials”) since 1500.

The data reveal that despite the severity of the Millennium Drought, the five worst single years of drought happened before 1900. But 2011 was the wettest year in our 513-year record.

The dominant theme of Australia’s drought history is variability. We may get one year of extremely wet conditions (for example in 2011) or we might get six years of extremely dry conditions (such as 2003-2009).

North Queensland may be flooded out while Victoria suffers with drought. Or in extreme circumstances, the entire eastern half of Australia might be bone dry.

Even as people change the climate by adding greenhouse gases to the atmosphere, variability will continue to play a large role in Australia’s climate. This year one of the strongest El Niños on record is kicking into high gear in the tropical Pacific, driving global temperatures higher still.

To tease out these complex patterns we need to look deep into the past.

It’s in the trees (and coral)

The existing drought records are relatively short and geographically patchy. Measurements from weather stations rarely extend beyond the early 1900s and informal historical records from diaries and ships logs — some of which go back to the first days of European settlement in Australia — are relatively uncommon and limited to a few sites. This has limited our understanding of drought variability to what has been directly observed over the past 120 years.

To extend the drought record beyond 1900, we used 177 tree ring and coral records from Australia, New Zealand, and Indonesia to reconstruct summer (spanning December to February) drought conditions in New Zealand and most of Australia.

Trees and corals are sensitive to their environments. For example, trees grow less in dry years and more in wet years. We carefully examined, dated, and measured each growth ring in thousands of trees and then compared the patterns of growth to an index of drought variability, the Palmer Drought Severity Index.

One of the researchers (Kathy Allen) retrieving a tree core from a king billy pine in Tasmania.
Patrick Baker, Author provided

This index takes into account air temperature, rainfall, and soil water-holding capacity to give an indication of the water status of the environment. However, the data only extend back to 1900. By using the statistical relationship between drought and our tree rings and coral, we can translate the growth patterns into data going back hundreds of years.

What we found was a remarkably rich and complex history of wet and dry conditions, particularly across eastern Australia.

A slice of coral from the Great Barrier Reef, photographed under UV light. The lines show periods when sediment from flood plumes affect coastal reefs.
Eric Matson, Australian Institute of Marine Science, Author provided

A history of drought

Over the past five centuries we found extreme droughts similar to the recent Millennium Drought, but we also discovered wet periods that lasted decades.

We found short droughts of brutal intensity that blanketed all of eastern Australia, while other droughts of similar intensity were confined to small pockets across the continent.

The atlas also provides new geographical context for early historical droughts. For example, diaries from early settlers near Sydney documented a crippling drought in 1791-92. Our data demonstrate that this was one of the worst drought years in the past 500 years with extraordinarily dry conditions that stretched from Cape York to eastern Tasmania. The early colony was fortunate to survive.

1792 was one of the worst drought years Australia has experienced since 1500.
Patrick Baker, Author provided

An obvious question is how do our modern droughts and floods stack up against earlier events? Of the five most extreme single years of drought in the past 500 years (when averaged across all of eastern Australia), not one occurred after 1900.

In contrast, two of the five wettest years in our data took place after 1950 (2011 was the wettest year in the 513-year record). The 1700s were particularly dry with three of the five worst drought years, but also notably had the most prolonged wet period (1730-60).

In eastern Australia, wet and dry conditions cycle back and forth over several decades, driven by the oceans around us.

When we compared the data to a recently developed index of changing atmospheric pressure called the Interdecadal Pacific Oscillation (IPO), we found remarkable consistency between the two. The IPO tells us when we have unexpectedly warmer or cooler sea surface temps and air pressures. The IPO also interacts with El Nino and La Nina to make them stronger or weaker.

When the IPO was positive, eastern Australia experienced drought conditions for several decades; when it was negative, eastern Australia experienced pervasive wet conditions. From 1999-2012 we were in a negative phase of the IPO; now it appears we have just entered a strongly positive phase.

You may have noticed that the Millennium Drought happened in a negative IPO phase. Our data show that there is a strong relationship between the phases of the IPO and drought – until around 1976. After that the relationship gets weaker. Why is a question for further research, but one possibility is human-caused climate change.

This new data will help us understand what drives these swings between drought and floods, and help us predict what might happen in the future.

The Conversation

Patrick Baker, ARC Future Fellow and Associate Professor of Silviculture and Forest Ecology, University of Melbourne; Chris Turney, ARC Laureate Fellow and Professor of Earth Sciences and Climate Change, UNSW Australia, and Jonathan Palmer, Research Fellow, Climate Change Research Centre, UNSW Australia

This article was originally published on The Conversation. Read the original article.

Kiribati: Climate Change Threat and Floods


The link below is to an article reporting on the threat to Kiribati by climate change and flooding.

For more visit:
http://www.msnbc.msn.com/id/47432131/ns/us_news-environment/#.T7Nqt-hftnQ

Floods and Holidays


The last couple of times I have been on holidays there has been plenty of rain about the state (New South Wales). It all really started with my trip around the state back in January – it started raining prior to my trip and flooding continued through it. It really hasn’t let up since then. It is probably flooding as much now as it has ever since that trip back in January. For almost 12 months New South Wales has been getting a drenching – which is a change from the drought we had for the best part of ten years prior to the rain.

Once again, as I contemplate the possibility of heading off somewhere for a quick holiday, the vast majority of the state is in flood or being threatened by flooding. The promise is of a lot more rain to come in the coming week or so. So what to do?