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.

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

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

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As global food demand rises, climate change is hitting our staple crops

Farmers face falling crop yields and growing food demand.
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Andrew Borrell, The University of Queensland

Climate change and extreme weather events are already impacting our food, from meat and vegetables, right through to wine. In our series on the Climate and Food, we’re looking at what this means for the food chain.

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While increases in population and wealth will lift global demand for food by up to 70% by 2050, agriculture is already feeling the effects of climate change. This is expected to continue in coming decades.

Scientists and farmers will need to act on multiple fronts to counter falling crop yields and feed more people. As with previous agricultural revolutions, we need a new set of plant characteristics to meet the challenge.

When it comes to the staple crops – wheat, rice, maize, soybean, barley and sorghum – research has found changes in rainfall and temperature explain about 30% of the yearly variation in agricultural yields. All six crops responded negatively to increasing temperatures – most likely associated with increases in crop development rates and water stress. In particular, wheat, maize and barley show a negative response to increased temperatures. But, overall, rainfall trends had only minor effects on crop yields in these studies.

Since 1950, average global temperatures have risen by roughly 0.13°C per decade. An even faster rate of roughly 0.2°C of warming per decade is expected over the next few decades.

As temperatures rise, rainfall patterns change. Increased heat also leads to greater evaporation and surface drying, which further intensifies and prolongs droughts.

A warmer atmosphere can also hold more water – about 7% more water vapour for every 1°C increase in temperature. This ultimately results in storms with more intense rainfall. A review of rainfall patterns shows changes in the amount of rainfall everywhere.

Maize yields are predicted to decline with climate change.
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Falling yields

Crop yields around Australia have been hard hit by recent weather. Last year, for instance, the outlook for mungbeans was excellent. But the hot, dry weather has hurt growers. The extreme conditions have reduced average yields from an expected 1-1.5 tonnes per hectare to just 0.1-0.5 tonnes per hectare.

Sorghum and cotton crops fared little better, due to depleted soil water, lack of in-crop rainfall, and extreme heat. Fruit and vegetables, from strawberries to lettuce, were also hit hard.

But the story is larger than this. Globally, production of maize and wheat between 1980 and 2008 was 3.8% and 5.5% below what we would have expected without temperature increases. One model, which combines historical crop production and weather data, projects significant reductions in production of several key African crops. For maize, the predicted decline is as much as 22% by 2050.

Feeding more people in these changing conditions is the challenge before us. It will require crops that are highly adapted to dry and hot environments. The so-called “Green Revolution” of the 1960s and 1970s created plants with short stature and enhanced responsiveness to nitrogen fertilizer.

Now, a new set of plant characteristics is needed to further increase crop yield, by making plants resilient to the challenges of a water-scarce planet.

Developing resilient crops for a highly variable climate

Resilient crops will require significant research and action on multiple fronts – to create adaptation to drought and waterlogging, and tolerance to cold, heat and salinity. Whatever we do, we also need to factor in that agriculture contributes significantly to greenhouse gas emissions (GHGs).

Scientists are meeting this challenge by creating a framework for adapting to climate change. We are identifying favourable combinations of crop varieties (genotypes) and management practices (agronomy) to work together in a complex system.

We can mitigate the effects of some climate variations with good management practices. For example, to tackle drought, we can alter planting dates, fertilizer, irrigation, row spacing, population and cropping systems.

Genotypic solutions can bolster this approach. The challenge is to identify favourable combinations of genotypes (G) and management (M) practices in a variable environment (E). Understanding the interaction between genotypes, management and the environment (GxMxE) is critical to improving grain yield under hot and dry conditions.

Genetic and management solutions can be used to develop climate-resilient crops for highly variable environments in Australia and globally. Sorghum is a great example. It is the dietary staple for over 500 million people in more than 30 countries, making it the world’s fifth-most-important crop for human consumption after rice, wheat, maize and potatoes.

‘Stay-green’ in sorghum is an example of a genetic solution to drought that has been deployed in Australia, India and sub-Saharan Africa. Crops with stay-green maintain greener stems and leaves during drought, resulting in increased stem strength, grain size and yield. This genetic solution can be combined with a management solution (e.g. reduced plant population) to optimise production and food security in highly variable and water-limited environments.

Other projects in India have found that alternate wetting and drying (AWD) irrigation in rice, compared with normal flooded production, can reduce water use by about 32%. And, by maintaining an aerobic environment in the soil, it reduces methane emissions five-fold.

Climate change, water, agriculture and food security form a critical nexus for the 21st century. We need to create and implement practices that will increase yields, while overcoming changing conditions and limiting the emissions from the agricultural sector. There is no room for complacency here.

Andrew Borrell, Associate Professor, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland; Centre Leader, Hermitage Research Facility; College of Experts, Global Change Institute, The University of Queensland

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

Climate Change and Sea Level Rises

The link below is to an article that looks at the impact of global warming on island nations around the world.

For more visit:
http://www.businessinsider.com/islands-threatened-by-climate-change-2012-10

Article: Australia – Old Bar Under Threat

The link below is to an article reporting on coastal erosion at Old Bar, New South Wales, Australia. This town is just up the coast from where I live. It is a similar situation to Winda Woppa, which is only a suburb away from me. During intense storms the ocean erodes the sandy coastline rapidly and homes are increasingly at threat from storm surges.

The article below suggests that the situation at Old Bar is being caused by sea level rises as a consequence of climate change. This is the sort of reporting that is bringing a lot of discredit to climate change advocates, as it is not an honest report on the actual situation being reported on. I would not dispute that climate change is bringing us more severe weather events and this is certainly increasing pressure on coastal areas like Old Bar and Winda Woppa – but it is not sea level rises that is the problem. Factual and honest reporting is what is needed.

To view the article visit:
http://www.mmail.com.my/story/sea-rise-threatens-paradise-down-under-23507

Article: Sea Level Rises Can’t Be Stopped

The link below is to a disturbing article concerning sea level rises into the future. In short, they can’t be stopped.

For more visit:
http://www.reuters.com/article/2012/07/01/climate-sealevel-idUSL6E8HSIDA20120701