THOUGHT LEADERSHIP

Changing the climate future

The future isn’t what it used to be. The future we now expect is one of even more intense rainfall. What can we do about it?

In Australia, there is now expected to be a 41–88 % increase in intense rainfall assuming a fossil-fuel development emission scenario by 2090, working from a 1961–90 climate base. In Tasmania, our previous vision of 2090 was an expected intense rainfall increase of only 16.3 %. So the future is looking different, with more intense rainfall. New projections are making the present and near future look different too. We now understand that there will be a 16 % increase to the current climate (2021–40) for 3-hour-duration rainstorms (since the 1961–90 period). In other words, the ‘old future’ is now and the ‘new future’ is different from what we thought.

In December 2023, draft changes to the Australian Rainfall and Runoff (ARR) climate change advice were released, changing many of our projections. Between the 1961–90 rainfall data used to calculate the intensity-frequency-duration of most rainstorms and the ‘current’ climate (2021–40), there is expected to be a 1.3 °C rise in global temperature (noting that this comes on top of the already 0.3 °C increase in global temperature from the 1850–1900 pre-industrial period to 1961–90). So for a fossil-fuel development emissions scenario (SSP5–8.5, Meinshausen et al 2020), what we previously projected for intense rainfall by 2090 is now our projection for some storms in the ‘current’ climate (2021–40).

If the ‘old future’ is our new reality, what could the actual future be?

As of March 2024, the future is projected to be hotter than previously expected, and intense rainfall is expected to increase proportionally more for every degree of temperature rise. There could be a small increase in catchment losses, but these are expected to be overwhelmed by the increases in intense rainfall. There is also a better understanding of the uncertainty in the modelled projections.

An example in Tasmania

In Tasmania, water is fundamental for the environment and community, and the importance of our understanding of water is heightened by our reliance on hydropower for the bulk of our electricity. However, the climate changes discussed here are less about longer term water and energy yields than about the intense rainfall associated with flooding.

For Tasmania:

Prior to the draft December 2023 ARR advice on climate change (Engineers Australia, 2023), with the SSP5 emission scenario with 8.5 W/m² radiative forcing there was projected to be a 16.3 % increase for all rainfall durations by 2090. The December 2023 draft advice for this scenario is that by 2090 the increase in intense rainfall is expected to be 41–88 % over the 1961–90 climate base (that is, the data you can get from the Bureau of Meteorology as the 2016 intensity-frequency-duration rainfall data). This means 41 % for 24-hour and longer duration rain storms, and up to 88 % for durations of 1 hour and shorter. These apply across Australia for rarities from an exceedance per year to the probable maximum precipitation event. There are several papers on the subject, for example Visser et al (2022) and Wasko et al (2024).
For 1 hour and shorter duration storms, which are important for drainage from building roofs and for most town local stormwater systems, the current period (2021–40) has a 20 % increase in intense rainfall over the climate base (1961–90). This means that all designs made over the last few years using a 20 % increase in rainfall to allow for a future climate will still work as expected for the time being. But after about 2040, these designs are unlikely to perform as expected.
For 3-hour-duration storms there is expected to be a 16 % increase over the climate base (1961–90) for the 1.3 °C rise in temperature to the ‘current’ period (2021–40). This means that what we thought would only happen in the more distant future is expected to be occurring now. The reasons we say this is ‘expected’ is that we won’t know for sure until we look back on this period with hindsight.
For the 24 hour and longer durations, the current period (2021–40) has an 11 % increase in intense rainfall over the 1961–1990 climate base. With the non-linear relationship between rainfall and runoff, the increase in peak stream flood flow is expected to be higher than 11 % for most larger rivers, such as those that flow to our dams.

Impacts on decision-making and design

Following the Sixth Assessment Report in 2023 (AR6) by the United Nation’s Intergovernmental Panel on Climate Change (IPCC) (https://www.ipcc.ch/assessment-report/ar6/), and anticipating the next one due around 2029 – and with science and engineering understanding increasing all the time – it’s likely that our projections of the future scenarios and understanding of the past will continue to evolve. Obviously, infrastructure that’s built stays built, but can be augmented. Standards and methods can’t change every year for practical reasons, but those of us impacted by climate and water are wise to remain up to date and always use the most contemporary knowledge. Asset owners, regulators, consultants and the community need to pay attention as the climate changes.

When the goal posts shift, we need to take stock of previous advice provided with old rainfall data, and consider how to include current rainfall data in new advice. As we go forward, we also need to be more careful in our language about how we reference the past, current and future.

When making decisions for infrastructure that will last at least 100 years and take 5 to 20 years to plan, design and build, such as sizing dam spillways, a range of risk mitigation strategies are required for managing an uncertain future (for example https://entura.com.au/designing-dams-for-an-uncertain-climate-future/). When reviewing the performance of existing systems, defining the ‘current climate’ is important. Climate change isn’t something just for future scenarios – we’re living it now.

Strategies to support better climate-related decision-making include:

using the current best knowledge
understanding data and model uncertainty
understanding natural climate variability on seasonal to decadal time scales
understanding that future climate scenarios are all possible
applying sensitivity analysis
using multi-criteria assessment
using staging strategies
providing options in design for changing levels of service.

If, for example, we were designing a new building to be built soon near a watercourse, all the following approaches could be considered:

Design the level of the earthworks and finished hardstand levels to meet the level of service in the ‘current’ climate (e.g. 2021–40), considering a freeboard over the raw modelled river levels to account for uncertainty in any modelled results.
Make allowances in the construction to address future climate scenarios (e.g. SSP5 2090) and build now only what’s prudent.
- Allow space for a future flood wall and its footings, potentially building the footings now if integrated into the current site works.
- Consider space for future flood gates on site entry, and consider their storage and other requirements that are best allowed for in the current construction (such as communication and power conduits under hardstand areas, and space in control rooms).
- If a flood wall is not desirable or if construction access for building a flood wall is not going to be practical in the future once the site is developed, it may be better to lift the site levels or build the flood wall as part of the current works.

Uncertainty has always been part of engineering design, as has making decisions with imperfect knowledge. Climate systems in particular are subject to a wide range of natural variability over a wide range of times scales. What’s different now is that the future is more obviously uncertain and changing more rapidly. For example, where once we could use rainfall tables in textbooks for decades, now it seems that every few years there are new projections from the IPCC about large or small changes in our understanding. It’s a dynamic time for making decisions. But this isn’t all bad news.

Looking forward

If you’re planning an improvement project and the future is expected to bring larger floods, your return on investment may be quicker than expected. With the expected increases to rare rainfall intensities and the increasing uncertainty, there should be more confidence in investing in solutions to improve the performance of surface water infrastructure. In the same way, you’ll get a faster return on your investment in improving your engineering skills related to climate, hydrology and hydraulics of surface water systems and associated infrastructure design.

While considering the worst, we hope for the best. The fossil-fuel development emission scenario (SSP5) is based on us continuing the polluting hydrocarbon-based developments of the past. Entura is actively supporting our clients to pursue low emissions developments and more renewable energy for a better future. A best-case scenario is shown in the diagram below as SSP1 (called the sustainability scenario). In this scenario, with 2.6 W/m² radiative forcing, the projection for 2090 would be an increase of 1.7 °C in global temperature over the 1961–90 climate, and only a 14–27 % increase in intense rainfall (for 24 hour and longer to 1 hour and shorter durations respectively).

To prevent the extreme global temperatures projected to arise from polluting the atmosphere, it’s up to all of us to keep changing for a better future.

Figure of projected temperature increases associated with AR6 shared socioeconomic pathways relative to 1961–90 (shaded in grey) and their associated uncertainty (Engineers Australia, 2023)

References

Engineers Australia (2023) Update to the Climate Change Considerations chapter in Australia Rainfall and Runoff, Department of Climate Change, Energy, the Environment and Water https://storage.googleapis.com/files-au-climate/climate-au/p/prj2aec7b7ec59ab390bffc6/public_assets/Draft%20update%20to%20the%20Climate%20Change%20Considerations%20chapter.pdf.

Meinshausen et al. (2020). The shared socio-economic pathway (SSP) greenhouse gas concentrations and their extensions to 2500. Geoscientific Model Development, 13(8), 3571–3605. https://doi.org/10.5194/gmd-13-3571-2020.

Visser, Kim, Wasko, Nathan and Sharma (2022), The Impact of Climate Change on Operational Probable Maximum Precipitation Estimates, Water Resources Research, https://doi.org/10.1029/2022WR032247.

Wasko, Westra, Nathan, Pepler, Raupach, Dowdy, Johnson, Ho, McInnes, Jakob, Evans, Villarini and Fowler (2024), A systematic review of climate change science relevant to Australian design flood estimation, Hydrology and Earth System Sciences, https://doi.org/10.5194/hess-28-1251-2024.

If you’d like to talk with Entura about your water project, contact Phillip Ellerton.

About the author

Dr Colin Terry is a civil engineer at Entura with three decades of experience in Australia and New Zealand. His experience includes modelling, planning, design and construction support. He has worked on multidisciplinary projects across various parts of the water cycle including catchment management, water supply, hydropower, land development, transport, and water quality in natural systems – with a focus on surface and piped water.