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

A king tide in New Zealand, part of a project documenting what future sea level rise might look like. Witness King Tides/Flickr, CC BY-NC-SA

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

Human activity is driving sea levels higher. Australia’s seas are likely to rise by around 70 centimetres by 2100 if nothing is done to combat climate change. But 2100 can seem a long way off.

At the moment, regional sea-level rise driven by warming oceans and melting ice is hidden by natural variability such as the El Niño, which causes year-to-year changes in sea level of several centimetres.

So at any particular place, the sea level might go up in one year, and down in the next. On Australia’s northwest coast, for example, the sea level was three centimetres below normal during 1998, but four centimetres above normal the following year.

At the same time, human-caused climate change is driving sea level relentlessly upwards in most regions, eventually pushing it far outside the bounds of historical variation. But when will the difference become clear?

Our new analysis of sea-level projections published in Nature Climate Change today indicates that regional sea-level rise will be generally noticeable before 2030. By then the average sea-level rise globally will be about 13 centimetres higher than the average sea level calculated between 1986 and 2005.

The blue curve shows projected sea level, the red curve shows the same projections once year-to-year variations have been removed. The grey and black lines show the range of natural variability. The asterisk denotes the time of emergence when sea level moves beyond the realm of natural variability.

Sea-level rise: depends on your perspective

First, it’s important to note that global sea-level rise is already attributed to anthropogenic climate change.

Like temperature changes, the sea-level changes are not uniform across the world. One region may experience a very different sea-level change from other regions.

When averaged around the globe, sea level has been rising at a rate of about 1.7 millimetres per year between 1901 and 2010, and about 3.2 millimetres per year between 1993 and 2014.

This is a clear signal of climate change, driven by expansion of ocean waters as they warm and from the increase in the mass of the ocean as water is added from glaciers and ice sheets. Over recent decades, these changes are largely a result of increasing concentrations of greenhouse gases.

As well as this gradual and relatively steady rise in global sea levels, in any part of the ocean there are also natural variations in sea level. This is associated with climate phenomena such as El Niño and La Niña, storm surges and tides that can dominate any sea level variations over short periods.

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 today 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 17 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 stabilised 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% of the ocean before 2030 for the business-as-usual scenario.

The likely Time of Emergence (year) for regional sea-level change for a business-as-usual scenario. The warm (cold) colours represent rising (falling) sea level. John Church & Xuebin Zhang, Author provided

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% 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 20th 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) centimetres is likely to emerge for the east coast of Australia by 2030, while it’s about 18 (possible range of 12 to 26) centimetres 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.