15 years from now, our impact on regional sea level will be clear

Sea-level rise is also not uniform across the world’s oceans. It can therefore be difficult to separate natural variability from the signal of climate change at a regional scale. But it is this combination of the long-term rise and the natural variability that impacts coastal regions.

A simple home experiment to demonstrate the difference between two signals is to gently slosh the water in a bath tub backwards and forwards with your hand (equivalent to the natural variability in sea level) and at the same time keep the tap running (equivalent to the climate change signal). At any instant in time, the change in height of water will mostly depend on the sloshing, but over time the additional water from the tap will cause the bath tub to overflow.

Emerging evidence

Our Nature Climate Change paper published Sunday provides clear evidence that, at a regional scale, sea-level rise due to anthropogenic climate change will likely exceed natural variability within the next two decades for many areas of the globe.

To estimate this time of emergence, we examined regional sea level in seventeen state-of-the-art climate models and recently published regional sea-level projections, and compared them to the average sea level between 1986 and 2005.

We considered two future climate scenarios — one in which greenhouse gas emissions continue to increase at a rapid rate (business as usual, called RCP8.5) and the other in which greenhouse gas emissions are stabilized by 2100 (a moderate mitigation scenario, called RCP4.5).

We focused on annual mean sea level. We didn’t consider tides and extreme sea level events over short periods (such as storm surges).

From the models we calculated the probability that climate-driven sea level will emerge from natural variability by a certain time. We found that the sea-level change signal is likely to emerge over 80 percent of the ocean before 2030 for the business-as-usual scenario.

The date is pushed back by less than a decade to before 2040 for the moderate mitigation scenario.

The date varies between Australia’s east and west coasts. Under the business as usual scenario, sea-level rise is likely to emerge on the east coast before 2030, and the west coast before 2040. The later time of emergence on the north and west coast is due to larger natural variability, associated with El Niño and La Niña and Pacific Decadal Oscillation.

In fact, since 1993 when we have had satellite observations of sea level over the global ocean, sea-level rise on the north and west coasts of Australia has been significantly larger than the global average rise, mainly because of this natural variability.

Sea-level rise clear before warming

With the same methodology and models, we also calculated the emergence for surface air temperature, and found that the surface warming signal is likely to emerge over 80 percent of the Earth’s total area by 2070. Thus, sea-level rise will be clear generally before surface air warming.

Similar studies have been done on temperature by others, including research from the University of Hawaii, though their estimates are slightly different due to different calculation and reporting methods, and different climate models.

It’s not far away — the time to prepare is now

The projected sea-level changes discussed here add to those already observed during the twentieth century.

In the business as usual scenario, the sea-level rise signal on the order of 14 (with a possible range of 9 to 18) centimeters is likely to emerge for the east coast of Australia by 2030, while it’s about 18 (possible range of 12 to 26) centimeters for the west coast of Australia by 2040.

The results imply the importance of local risk assessment and adaptation planning for sea-level change. This should be undertaken in anticipation of a sea level that within the next two or three decades is likely to be significantly different to the past two or three decades.

Coastal communities and industries require information on regional sea-level change to develop strategies for reducing the risk to population, infrastructure and the environment.

This requires modelling projections of sea-level rise, estimating the costs and benefits of adaptation options, and understanding the impacts on coastal ecosystems.

Inundation maps that can be used to identify areas that are most vulnerable to rising sea levels are particularly valuable.

Adaptation measures may include land-use planning such as preventing building in low lying areas, increasing or maintaining a vegetated coastal margin that serves as a buffer zone against extreme sea levels, or using protective sea walls in the long run if certain sea level rise thresholds are exceeded.

— Read more in Kewei Lyu et al., “Time of emergence for regional sea-level change,” Nature Climate Change (12 October 2014) (doi:10.1038/nclimate2397)

John Church is CSIRO Fellow at CSIRO; Xuebin Zhang is Senior Research scientist, sea level rise at CSIRO. This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).