Category: Climate Change

Storms are becoming more intense as the climate changes. This is just one reason why the signing of the Paris Agreement by 170 nations in New York last week is great news. Finally, there is a sense of momentum; a shared purpose worldwide that the rate of climate change urgently needs to be slowed down. The next—and most important—step is for countries to ratify the treaty and enshrine the agreement in national legislation. Let’s all hope this progresses smoothly and quickly.

 

This collective enthusiasm to reduce humanity’s greenhouse gas emissions cannot come fast enough. The first three months of this year have been record-breakers in terms of global temperature, causing  heatwaves, extensive coral bleaching, and continued declines in the Arctic sea ice.

Implications of climate change on rainfall intensity

Potentially equally important to these direct effects of increasing temperature are the indirect effects of greenhouse gas emissions on the world’s weather patterns. And of these, changes to extreme storms are particularly important. Alarmingly, the evidence is suggesting that storms have started becoming much more intense.

The basic logic is that a warmer atmosphere can hold more water. If this seems unexpected, then consider why we use warm air for hand-driers, or why on warm humid days you can get water droplets forming on the outside of cold water bottles. Or why storms in the tropics are often much more intense than those in the higher latitudes.

Because of this, we have good reason to expect that storms should become more intense as the climate warms. In fact, scientists who have looked into this expect the rainfall intensity from storms to increase by between 7% and 14% for every degree of global temperature increase.

This is not just some theoretical concept about what scientists expect in the future; it’s something we’ve already started to observe.

The changing size of storms

But is a change in the intensity of storms the only thing we can expect? This was the question we asked when I began collaborating with PhD student Conrad Wasko and his supervisor Professor Ashish Sharma at the University of New South Wales. Here, we did a comprehensive analysis of data from 1300 rain gauges and 1700 temperature stations across Australia to see how air temperature affects the spatial organisation of storms.

What we found was surprising: not only do storms intensify with temperature, but they also become more concentrated over a smaller area. This is because as the storm cells intensify, they also become more effective at drawing in moisture toward the storm centre.

This is illustrated in the image below, where we looked at the 1000 most intense storms that occurred when the atmosphere was relatively cool (about 18 degrees Celsius or below, shown as the blue curves), and compared them to 1000 most intense storms when the atmosphere was relatively warm (above 25 degrees Celsius).

But what was really surprising was how uniform the changes were around Australia. In fact, regardless of whether we were looking at storms in temperate Tasmania or the tropical north of Australia, the humid east coast or the arid interior, the results were pretty much the same. A warmer atmosphere is associated with more intense rainfall, occurring over a smaller geographic area.

If we combine this with the large number of other studies that have recently been published on changes to extreme rainfall under climate change, it is clear that we are beginning to have a much better understanding of these changes. But there is still much to learn about what climate change will mean for changes in storms, and how this is  related to possible changes in flooding, hail damage and other meteorological hazards.

Nevertheless, the sorts of changes our team and others around the world have been documenting are cause for concern. So let’s hope that Australia starts following the chorus of international support for stronger action of climate change and takes the action needed to ratify the Paris Agreement as soon as possible.

Further reading

There are literally hundreds of scientific papers that have been published describing historical and expected future changes in extreme rainfall. I’ve provided a selection of papers from our own group on the topic below. The first paper provides a review of nearly 250 scientific papers on the topic, and can be used as an entry point to the broader literature.

• A review of the international literature: Westra S, Fowler HJ, Evans JP, Alexander LV, Berg P, Johnson F, Kendon EJ, Lenderink G, Roberts NM, 2014, Future changes to the intensity and frequency of short-duration extreme rainfall, Reviews of Geophysics, 52, 3, 522-555.
• The discovery that extreme rainfall intensity increases with atmospheric temperature: Hardwick-Jones R, Westra S, Sharma A, 2010, Observed relationships between extreme sub-daily precipitation, surface temperature, and relative humidity, Geophysical Research Letters, 37, L22805, 1-5
• Evidence that daily-scale extreme rainfall is increasing around the world: Westra S, Alexander L, Zwiers F, 2013, Global increasing trends in annual maximum daily precipitation, Journal of Climate, 26, 11, 3904-3918
• Evidence that shorter-duration (e.g. hourly) rainfall is increasing more quickly than longer duration rainfall: Westra S, Sisson S, 2011, Detection of non-stationarity in precipitation extremes using a max-stable process model, Journal of Hydrology, 406, 1-2, 119-128
• The surprising finding that it is possible to have opposing trends in summer and winter rainfall: Zheng F, Westra S, Leonard M, 2015, Opposing local precipitation extremes, Nature Climate Change, 5, 5, 389-390
• An evaluation of whether regional climate models are capable to reproduce the physical processes leading to extreme rainfall: Cortés-Hernández VE, Zheng F, Evans J, Lambert M, Sharma A, Westra S, 2015, Evaluating regional climate models for simulating sub-daily rainfall extremes, Climate Dynamics, 1-16
• The finding that the spatial extent of extreme rainfall decreases at higher temperatures: Wasko C, Sharma A, Westra S, 2016, Reduced spatial extent of extreme storms at higher temperatures, Geophysical Research Letters, doi: 10.1002/2016GL068509

 

In this article we look at the most important questions to ask about climate change, so we can make better decisions. Those questions are: Under what circumstances will a system fail? And can we extend a system’s breaking point?

 

Thinking forward prompts the question of when things will change, and by how much. Answering these questions often involves the use of climate projections. These often disagree and don’t provide much certainty on a timeline. This can be an issue when making decisions for systems that are vulnerable to climate change – like open water storages.

Climate change question 1: Under what circumstances will a system fail?

Climate change is gradually impacting our water resource systems. It is changing both demand and supply. Our systems are robust enough to resist some changes, including those seen in the last few decades. While the effects of climate change will continue to be gradual, uninterrupted change will cause our systems to fail, like the proverbial boiled frog.

This is why we need ‘tipping points’.

A ‘tipping point’ is the last point at which you would still consider performance satisfactory. This point has a corresponding climate described by measurements of variables like temperature, precipitation and evaporation. And, importantly, you can identify this climate without the use of climate change projections.

The previous blog post by Danlu Guo is a great example of how to do just that. Taking historical climate conditions – under which a system is performing well – and changing them can allow you to see the point in which the system breaks. Changes might include higher temperatures or different rainfall combinations.

To demonstrate this idea, we considered a simplified model of Lake Como, Italy. The lake regulator has to protect the city of Como from flooding, and ensure there is enough water for irrigating one of the largest agricultural regions in Europe. This creates challenging decisions: The two are competing interests.

After defining some ‘tipping points’ (for both the allowable flooded area and irrigation deficit) we varied annual temperatures and precipitation to see what would cause the system to fail.

The figure shows that when the amount of rain increases the system floods, and when temperature increases, the system doesn’t have enough water for irrigation. But we also found some robustness when temperature and precipitation increase together. The system remains in a healthy balance as more rain occurs with more evaporation. Importantly, we can quantify the changes in climate that result in failure.

The next question to consider is: How far can this breaking point extend?

Climate change question 2: Can we extend the breaking point of the system?

The answer to this question depends on what a decision maker controls. Each of the climate scenarios in the above figure are fixed. The impact is a result of the physical system and its management.

For example, raising the reservoir walls would decrease the danger of flooding, and decrease the amount of failure scenarios in Figure 1. These large infrastructure actions are quite costly, and are not easily reversible. This could lead to regret if the climate were to change in a way that instead threatened irrigation.

Most water storage systems require a daily release decision. Changes to this operation are low cost, fast to implement and are quite flexible. The flexibility allows decision makers to tailor a response to a particular climate. This can be beneficial as the climate changes in unpredictable ways.

To illustrate, we took the same climate scenarios for Lake Como as above, and instead of using the current operation model, designed a new operation as a response to each individual climate. Figure 2 shows this. The green scenarios represent how far the failure boundary extended.

This ‘adaptive capacity’ approach helped Lake Como perform successfully in three times as many scenarios. Compared to Figure 1, it appears to be easier to adapt to irrigation deficit than to flooding.

When will these tipping points occur?

Climate projection models are still the only way of answering this question.

Below are two projections for the The Lake Como scenario. They are in the same categories of the previous figures. For example, in 2025, 3/22 models predict failure, and 6/22 predict a climate that can be adapted to.

Looking at the projections alone doesn’t give you much information about the system. It is difficult to tell how close some of these projections are to failing. That information is important, given the errors they contain.

Identifying the above tipping points can be an important context for looking at projections.

Figure 4 (below) contains 22 climate model projections of the climate of Lake Como in 2025 and 2050. The figure shows that, while the projections change, how the system performs under specific climates does not.

This is where such an approach is useful. You can be more confident in the performance under some projections than others, based on how close they are to the tipping points.

Adapting to a changing climate

How is the future changing thinking?

• water security (e.g. higher demand and reduced rainfall)

• infrastructure (e.g. elevated sea level and associated increase in flooding)

• safety (e.g. exposure of personnel to extreme heat and fire danger)

• inter-dependencies (e.g. power or communications failure leading to service disruption).

Survey of water utilities: How are important decisions being made?

How will the survey help utilities adapt?

• Climate data and information that is used to inform decision-making

• How climate data are analysed, and how resulting information this used

• Rules of thumb and decision processes applied in decision-making

• Formal option evaluation methods to adapt solutions

• Climate change exposure to externalities, such as customers, the urban environment, and businesses.

Who is the survey for?

• Strategy and planning

• Asset management

• Operations

• Communications

• Finance.

What’s in the survey?

• What climate data supports the decision?

• What planning horizons are used?

• Are future scenarios or extreme events considered?

• Are inter-dependencies with other utilities considered (e.g. power, transport)?

Preliminary results