Understanding Climate Risk

Science, policy and decision-making

2012 Marine Report Card

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Warmer oceans, tropical species being found further south, decline in temperate species, the first signs of CO2 effects on shell production in Australian waters …

These are a few of the headlines from the Marine Climate Change in Australia, Impacts and Adaptation Responses 2012 Report Card (download pdf). Put together by the Marine Biodiversity and Resources Adaptation Network (NCCARF), Fisheries Research and Development Corporation, and CSIRO’s Climate Adaptation Flagship. The reporting comprehensive,  covering the report card itself and six chapters on marine climate and thirteen on marine biodiversity. Alistair Hobday, summarising the report card on The Conversation.

Here’s a summary with some of my own conclusions about observed and projected changes. The latter you can take or leave as they’re based on my personal views about how climate changes. For recent and near future changes, I place a greater emphasis on how climate is likely to change rather than by how much. This places the emphasis more on the diagnosis and understanding of change rather than prediction. There’s a fair bit in doing this, so amongst other things I’m into today (like gardening, cooking and cleaning), I’ll update these sections as I go (Sunday 11 am, SST; Wednesday, SLR).

Summarised below, the main headings are:

Sea Surface Temperature (SST): high confidence in current observations and future projections (the latter subject to emissions).

Ocean temperatures around Australia have warmed by 0.68°C since 1910–1929, with south-west and south-eastern waters warming fastest. The rate of temperature rise in Australian waters has accelerated since the mid-20th century; from 0.08°C/decade in 1910–2011 to 0.11°C/decade from 1950–2011.

New results based on a relatively high scenario for greenhouse gas emissions (RCP8.5 ) indicate greatest warming in south-east (>3°C) and north-west waters (~2.5°C) by the end of this century.

The latter can be considered as a business as usual growth scenario with limited GHG mitigation.

An accompanying paper by Sen Gupta, Lough and Hobday gives a really good summary of observations and changes. Annual and seasonal observations of spatial trends since 1950 are below.

Linear SST trend (°C/decade) for a) annual, 1950–2010, b) summer, c) autumn, d) winter, and e) spring, 1950-2011. Data source: Australian Bureau of Meteorology (Sen Gupta et al., 2012).

But the analysis is of linear trend. How has SST actually changed? Lets have a look for Australian average SST (same data set). Analysis using two tests, the bivariate test and Rodionov’s STARS suggests significant shifts in mean in 1933, 1969 and 1997; and 1936, 1969 and 1995, respectively. This is shown below.

Observed Australian SST 1900–2011 with step-change analysis. Data source: Australian Bureau of Meteorology

What’s more, these can be compared to the global zonal average air temperature from GISS, also shown. Here the shifts are 1936/7, 1972 and 1997 using both tests.

Observed zonal average air temperature 24–44°S 1880–2011 with step-change analysis. Data source: Goddard Institute of Space Studies

These graphs clearly show the non-linear evolution of SST is related to larger spatial changes (air temperature across the largely oceanic zone of 24–44°S will be closely related to SST across the same zone). Decadal climate variability and anthropogenic climate change are interacting with each other in the way that warming is distributed over space and time. Impacts in related chemical and  biological systems, in addition to containing their own non-linear response, will clearly follow.

So, what of the future? The following chart shows changes from the models, showing the largest warming generally follows the same pattern in the observations. Hot-spots are off the north-west and south-east in the Tasman Sea. The latter is essentially the southward extension of the East Australian Current. Both these changes will affect Australian climate on land in the future. My take is that tropical Australia will become wetter from the north-west following changes over the past four decades, and that occasional breakouts will affect the south-east and that much warmer waters in the south-east will make coastal lows along the NSW/Victorian coast that much stronger. Oh, and non-linear warming consisting of step changes and trends will continue.

Projected change in SST for end of 21st century based on RCP8.5 scenario. Areas where at least 10/11 models agree that warming is greater or less than the region average are mottled (Sen Gupta et al., 2012).

Sea Level Rise: high confidence in current observations and future projections.

Sea levels are rising around Australia, with fastest rates currently in northern Australia. New analyses of sedimentary records from the east coast of Tasmania confirm slow sea-level change over 1000s of years until the early 20th century, when there was a significant acceleration in the rate of sea-level rise. High sea-level events on annual to decadal timescales have increased by a factor of three during the 20th century.

Sea level will continue to rise during the 21st century and beyond, and result in inundation of low-lying coastal regions and coastal recession.

Confidence that sea level will rise in high, but at the high end of risk, the chance of rapid rise due to ice-shelf instability, confidence in the timing and consequent rate of rise is low. High rates of continuing emissions means that these futures may be committed to before they are properly understood. This is one area where scientific confidence cannot be well translated into risk. Very high confidence can be attached to the statement above (it’s virtually certain) but lower confidence – high risk outcomes are also relevant.

The paper accompanying the report, by John Church, Neil White, John Hunter and Kathy McInnes is excellent, updating recent observations. They have this to say about the high risk end of sea level rise in 2100:

Note that some of the projections are approaching the value of 2 m and that Pfeffer et al. (2008) argued that rises above this value were physically untenable. In summary, although semi-empirical models give a warning that larger sea-level rises than suggested by current process-based models may be possible, they should be used with caution until the concerns about their value are adequately evaluated and addressed.

Global mean sea level continues to rise with the recent acceleration shown by the satellite data.

Global mean sea level from 1880 to 2011 with one standard deviation error estimates (blue), and the satellite altimeter records from 1993 to 2011(red) (Church et al., 2012).

In Australia, sea level has also risen over the continent, but recent rises have been much greater in the north. In their paper, Church et al. (2012) show that tide gauge rises have been faster than offshore rises indicating relative shore-level decline, due either to sediment compaction or tectonics, but the regional rise remains faster than in the south.

Sea-level rise during 1993-2011 in the Australian region.

They also show potential change in return periods of high sea levels events, where with mean rises of around 0.5 m, extremes can change by as much as a thousand-fold. That is assuming that the pattern of extremes around the mean remains the same.

Left: estimated multiplying factor for the increase in the frequency of occurrence of high sea-level events caused by a sea-level rise of 0.5 m. Right: sea-level rise allowance (m) for 1990 to 2100 based on preserving the frequency of flooding events as sea level rises, based on global-average projections from the IPCC AR4 and the A1FI emission scenario.

Which may not be the case. Taking three SEAFRAME (high quality) records from southern Australia, there is some divergence between the mean and extremes, though the relatively short  record means this phenomenon may be transitory. The Burnie peak monthly tide gauge measurement decreases relative to the rising mean (though is constant relative to the land) while the Lorne record shows larger increase in peak monthly measurements.

Mean and peak monthly tide gauge measurements from three sites in southern Australia 1993-2011

The final point is, and I know I keep banging on about it, is that regional mean sea level rise is non-linear, occurring of a series of step changes and trends due to non-linear changes in regional circulation and ocean temperature. This will have important implications for both coastal risks and for marine ecosystems.

Fremantle annual average tide gauge data analysed for step and trend changes from 1920, also shown with a linear line of best fit.

 

Fort Denison tide gauge records with step and trend analysis, combined with simple trend and 11-year running mean.

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