Sea level Rise in Auckland?

You may have seen something of the reported findings of the Climate Change part of CSIRO in Australia. (The intro to the report includes: The Australian Government’s Bureau of Meteorology has been observing, reporting and researching Australia’s weather since 1908. CSIRO has been undertaking atmospheric and marine research for more than 60 years. Together our scientists continue to build the body of knowledge that allows people to understand the changes in our climate that we are observing and prepare for any future changes.) The findings were circulated earlier this year. 

Made me wonder what was happening – or being recorded in New Zealand.

For example, the summary says this:

State of the Climate 2012 provides an updated summary of long-term climate trends. It notes that the long-term warming trend has not changed, with each decade having been warmer than the previous decade since the 1950s…. 2011 was the world’s 11th warmest year and the warmest year on record during a La Niña event. The world’s 13 warmest years on record have all occurred in the past 15 years.

There has been a general trend towards increased spring and summer monsoonal rainfall across Australia’s north during recent decades, and decreased late autumn and winter rainfall across southern Australia. The summary shows that the very strong La Niña event in 2010 followed by another in 2011 brought the highest two-year Australian-average rainfall total on record.

State of the Climate 2012 also highlights the increase in global sea level and notes sea-level rise around Australia since 1993 is greater than, or equal to, the global average. Our observations show that sea-surface temperatures around Australia have increased faster than the global average. The concentrations of long-lived greenhouse gases in the atmosphere reached a new high in 2011. Annual growth in global fossil-fuel CO₂ emissions between 2009 and 2010 was 5.9 per cent, reversing a small decline of 1.2 per cent recorded between 2008 and 2009 during the global financial crisis.

It was the sea level rise information that intrigued me. You can see for yourself here. Further into the summary we read the following Key points:

• Global average mean sea level for 2011 was 210 mm above the level in 1880.
• Global average mean sea level rose faster between 1993 and 2011 than during the 20th century as a whole.

And then this paragraph, and associated map (below) about rising sea level around Australia

  

Rates of sea-level rise are not uniform around the globe and vary from year to year. Since 1993, the rates of sea-level rise to the north and northwest of Australia have been 7 to 11 mm per year, two to three times the global average, and rates of sea-level rise on the central east and southern coasts of the continent are mostly similar to the global average. These variations are at least in part a result of natural variability of the climate system.

This was the first time I knew that sea level rises vary across the planet. I had assumed sea level rise was uniform. But not so. Here’s how sea level changes vary across the Earth:

 

You can check for yourself with this useful tool (remember to expand out so you can see the whole world, or zoom in on your country of interest, click ‘global’):

You can also see data for 4 measuring stations in New Zealand. 

These give the sea level change in mm/year, with a variability:

Auckland            1.29mm.yr    +/- 0.2mm

Wellington           2.45mm/yr   +/- 0.29mm

Lyttleton              2.36mm/yr    +/- 0.29mm

Bluff                    1.57mm/yr     +/- 0.24mm

What is also interesting is that some areas of sea are dropping in level, while others are rising. On average the sea level is rising mainly because of warming. (For at least the last 100 years, sea level has been rising at an average rate of about 1.8 mm per year. Most of this rise can be attributed to the increase in temperature of the sea and the resulting slight thermal expansion of the upper 500m of sea water. Additional contributions, around 25% of the total expansion, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.)

So why does it vary? Why some increases and some decreases? I have summarised my research here:

To understand sea-level change at a particular coast, we must know the sum of global, regional and local trends related to changing ocean and land levels.

Local sea level change can deviate from the global mean sea level change for a number of reasons:

- Ocean water moves around driven by winds and other factors, so that even if global water volume does not change (constant global-average sea level) there will be regional sea level changes. This can happen due to both natural oscillations in the climate system (such as El Niño / Southern Oscillation) and forced anthropogenic changes.
- The gravitational pull of land ice is reduced as the ice melts, which has a surprisingly large effect on the sea surface. For example, as ice on Greenland melts this will cause a global sea level rise, but a regional drop in sea level in a circular area around Greenland (you can see that in the map above).
- There are vertical land movements that make the sea level change locally relative to the land - which is relevant for the impacts of sea level rise. This can have both natural reasons (i.e. plate tectonic or earth’s crust processes as well as the still ongoing response to the massive ice loss at the end of the last ice age, called glacial isostatic adjustment), and anthropogenic reasons (like groundwater or oil extraction causing the coast to subside).
Changes in ocean circulation are difficult to predict. Modelling ice melt is a challenge in the context of global sea level. To compute the gravity effects, one also needs to know where the ice melts, not just the overall total. And while long-term local land motions can be assumed to occur at a constant rate and thus extrapolated from past measurements, such measurements are not available everywhere, and local land movement can change due to recent effects (like local melting of glaciers, or changes in groundwater pumping, or sudden earthquakes).

Overall, sea level changes can locally differ by some tens of centimeters (or even more in some special cases) from the global mean sea level change. This makes some locations, like low-lying delta cities on subsiding ground, particularly vulnerable.

So there you have it. Some facts and food for thought.

And it looks like we need to pay attention to what's happening in Auckland, because it might not behave in a nice straight line. Unexpected or unpredictable events elsewhere can significantly affect our local situation.