Wednesday, 19 August 2009

Sea Change (P4P*)

The kind of plant we might display in the botanic gardens later this century? (It's a picture I've used before but too good to resist for this story.)

The land seaward of the Gardens Restaurant in the Royal Botanic Gardens was reclaimed from the sea over a period of 30 years in the mid nineteenth century. In the next 30 years some of it could well be claimed back.

Modelling by the Department of Environment and Climate Change, our parent Department, predicts a 40 cm rise (above 1990 levels) by 2050, and a further 50 cm by 2100. So we’d expect the harbour to be nearly a foot higher in 30 years time.

Our Seawall should cope with this kind of sea level rise, although king tides are likely to reach further into the Royal Botanic Gardens. Already they dump seaweeds on the paths, and occasionally fish and other marine life, but these all arrive through the deliberately cut slots in the wall.

Even centimetre rises will increase the likelihood of flooding, and of the soil becoming even more saline. The bottom lake in the Royal Botanic Gardens has always been brackish and water flows back and forth from Farm Cove, depending on the sea level.

Mullet and long-finned eels also travel through this channel between Farm Cove and the pond, although the eels can do it overland on a rainy night (in fact the mature eels have to leave the pond to spawn, migrating all the way to New Caledonia).

Accelerated climate change in the Sydney region is also likely to increase minimum and maximum temperatures by up to a couple of degrees. Summer rainfall on the coast is likely to increase (but not necessarily in the catchments), but winter rainfall is more likely to decrease.

These changes to the climate will eventually affect what we can grow in the Royal Botanic Gardens. Some plants can tolerate wide variations in temperature and water availability, while those grown at the extreme of their range may succumb. Because we display many different plants, from all around the world, some will much harder to keep alive.

If we have more, or more severe, storms and winds, that will obviously affect how we care for existing trees and the kind of trees we plant over coming years.

So what can we do? Already, when we replaced the stone wall around our two islands in the Main Pond recently, we took the opportunity to make then 50 cm higher. In our reviewing our ‘thematic plans’ (our guides to what grows where and why), we’ll take into account likely changes in salinity along Farm Cove and perhaps require that all new trees planted in the lower garden are in raised beds or mounds. In any repairs and maintenance to the sea wall we’ll take into account likely sea level increases and the increased threat of storm and tidal damage.

In Sydney generally, our gardens will change. You should look at the effects of the 40+ degree day in coastal Sydney on New Year’s Day in 2006. Tree ferns growing out in the open may be a thing of the past, as might be box hedges in planter boxes. Consider bromeliads, succulents or local native plants, but also water-loving gardens in seepage areas and near your rainwater tank.

On the positive side, our gardens have already adapted a little due to the new watering regime (recently relaxed so we can water every day, early in the morning and later in the afternoon). Although most gardeners would prefer some more flexibility in the system, most of us probably over watered our gardens in the past. We’ve all lost a few plants but we are planting and preparing more wisely now. Even though we may get more summer rain, other seasons could be quite dry and the higher temperatures will lead to more evaporation.

Meanwhile, do all you can to reduce your carbon footprint and you may save, among other things, the Royal Botanic Gardens.

*This Passion for Plants posting will also appear on the ABC Sydney website (possibly under 'gardening'), and is the gist of my radio interview with Simon Marnie on Saturday Morning sometime between 9-10 am on 702AM.


Anonymous said...

so if the seal level rise prediction is correct - i.e. a rise of 400mm by 2050 above 1990 levels - it should be clear that the harbour level has already risen by 28.5mm since 1990.

Can you show the evidence for this?

Tim Entwisle said...

It's probably not going to be a linear increase, but I based my comments on modelling done by the NSW Department of Environment, Climate Change and Water. They note that the average sea-level rise (including land movement) around Australia has been about 1.2mm/year between 1920 and 2000. The trend is upwards but difficult to track over a small number of years due to annual variations. They note that in Sydney, the frequency of extreme sea-level events of up to 2 m has at least doubled since 1950.

Have a look at for further information

Anonymous said...

Sure - but we are one third of the way towards 2050 from 1990, surely there be some level change showing by now.

The report you posted said: "The sea level has risen 1.8 mm per year since
1950 and that rate is accelerating."

That is just not true as it's NOT accelerating. Sea levels have been rising at around 1.2mm/annum since the last ice age yet recent reports show that the rate of sea level rise is in fact falling. sea level

Tim Entwisle said...

Thanks for the link. It will interesting to watch sea level changes over the next few decades to see how they track. I'm aware that effective sea level is slightly lower around Australia than other places but I must emphasise I'm not a climate scientist, just in this case a botanic garden manager preparing for possible future changes to my patch.

Anonymous said...

I understand - it's a wonderful patch you have to manage. :)