Measuring and accounting for sea-level rise
John Church explains how sea-level is measured. One method uses tide gauges. The other employs a satellite 1,300 km above the earth. Sea-level is currently rising at the average rate of 3.1 mm/year. This compares with the average rise for the 20th century of 1.7 mm/year. The higher sea-level is partly explained by thermal expansion. Hotter sea water takes up more volume than colder water. As the Earth is warming, so too is the ocean and so this contributes to sea-level rise. The other factor is more water in the ocean as the result of melting ice from sheets at the poles and from glaciers. An additional impact is the extraction of ground water for irrigation. John Church says the increase in sea-level over the 20th century cannot be fully explained by natural phenomena. The concern is when thresholds are passed, such as the one at 3 degrees of global warming, which will guarantee the melting of the Greenland icesheet, pushing sea-levels up by up to 7 metres. This melting will occur over some centuries, and any chance of stopping it, while with us now will be soon lost.
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This week Dr John Church was one of those given our highest honour in science in Australia, an election to the Academy. Tim Flannery was elected as well. But Dr John Church of CSIRO studies sea-level rise. They use two methods; tide gauges going back to the 19th century, and satellites, in operation since 1993. And the precision of these measurements is quite remarkable.
John Church: We're estimating global average sea-level or the rate of change to a couple of tenths or a few tenths of a millimetre per year in the global average rates of change.
Robyn Williams: I see. So I'm thinking of when the signal comes down and you're looking at waves...because I had heard that they are unbelievably accurate when it comes actually to measuring the surface of something.
John Church: They are, it's an incredible instrument. You are measuring individual point measurements to an accuracy of centimetres. The current accuracy is about three centimetres on a spot measurement. Much of that is large-scale variations associated with the Earth's orbit. As you say, this is just an incredible accuracy from 1,300 kilometres in space.
Robyn Williams: And when you're doing that, do you find that paradoxically instead of imagining that the ocean is more or less at the same level, it does vary across the face of the globe?
John Church: Yes, sea-level varies. The good analogue to think about is the weather map that you see every night after the news. You see highs and lows on that weather map associated with a high pressure systems, low pressure systems in the atmosphere. The ocean has exactly the same things. You have currents on the gradients of those high and low pressure systems. The difference in the ocean is that much of the variability, the highs and lows that you see in the atmospheric pressure chart, are much smaller-scale. But you also have global phenomena associated with the global circulation patterns in the ocean, and these come out very clearly in the altimeter measurements of sea surface height.
Robyn Williams: You wouldn't imagine that the ocean would be lumpy, you'd think it would all flatten off, more or less. Where is it highest and where is it lowest?
John Church: The highest parts are in what are called the subtropical gyres. So these are the wind-driven ocean current systems. So you have in the high latitudes westerly winds blowing the current east, in low latitudes you have the trade system blowing the currents west, and in between that you have a convergence of mass. So the order of 20 degrees north and south is somewhere near the maximum sea-level in the western half of each of the basins.
Robyn Williams: So those gyres, we think of them in terms of plastics floating in the ocean, sometimes the size of France.
John Church: Yes, I guess that's right. The winds cause the surface waters to converge and of course carrying any pollutants with them.
Robyn Williams: What are the recent measurements of the rising of sea-level?
John Church: The current rate of rise as measured by satellite altimeter is about 3.1 mm per year since the start of 1993 up to the present. When we estimate it with tide gauges we get a quite similar rate, just fractionally less than 3 mm per year from the tide gauge measurements. But both of these measurements are significantly larger than what we have for the 20th century alone which is about 1.7 mm per year.
Robyn Williams: So the level is rising faster.
John Church: So it is rising faster since 1993. Whether this is a sustained acceleration or a sustained increase in the rate of rise or just some natural variability is as yet unclear.
Robyn Williams: Has that been found by other people doing similar work elsewhere?
John Church: Yes, but the 20th century rate is agreed pretty well by most people now. Our number of 1.7 mm per year is sort of the accepted standard, and the satellite altimeter rate is obtained by about four or five groups around the world, ours being one of them.
Robyn Williams: To the average bystander, the layperson, that seems to be quite small. Does it seem the same to you?
John Church: Yes, in individual years that's a small amount. But it's the cumulative amount over years, year-on-year rising, and the increase that we've already observed from the 19th to the 20th century and what we expect through the 21st century, an increase in the rate which drives the impacts on coastal societies.
Robyn Williams: And I suppose the causes...well, the obvious one is the melting of the ice caps and the ice sheets, but also expansion. Which is the main force?
John Church: The two largest contributions are expansion of the oceans; as the oceans warm, the water expands, sea-level rises, exactly the same phenomena as in a thermometer. The second, about equal actually, both oceans and melting of glaciers contribute about 40%. So these are glaciers in places like Alaska, South America, so they are about 40% since 1970. And we can do the budget pretty well from 1970 up until the present. Ice sheets are a smaller component, but they are a component that has accelerated over recent years. And there's a small amount in terrestrial storage, so this is the storage of water in dams, and also the depletion of groundwater, extracting groundwater, using it for irrigation and it ultimately making its way into the ocean.
Robyn Williams: Has it been a steady rise, or is it interrupted up and down or whatever?
John Church: There are many perturbations in the rate of rise. One of the interesting ones that we stumbled on was the impact of volcanoes on sea-level. Volcanoes inject aerosols into the stratosphere, stratospheric aerosols reflect the sunlight, cooling the Earth, and we see that in the cooler surface temperatures and decrease in the heat content in the ocean, and sea-level falls.
Interestingly, just over the last two years Australia has experienced major floods. Several other areas around the world have also experienced major floods. As a result of this, sea-level actually fell, it fell by about 5 mm, and you can measure this both from the satellites and also from the gravity measurements which determine the mass stored in the ocean and the mass stored on land. And these agree that the prime reason for the sea-level fall is transferring of mass from the oceans to the land. We are actually seeing sea-level coming back to the trendline as the water runs off from the land back into the ocean.
Robyn Williams: Is it possible to see an unmistakable human influence here?
John Church: You cannot explain the 20th century rate of rise from natural phenomena alone. You do require a human contribution from greenhouse gases and warming of the earth to explain the observed sea-level rise.
Robyn Williams: As you look at your models and the increasing sea-level rise going year after year, how do you see the century then ending up? Is it going to be really a problem, as you said, with the coastline and the rise locally?
John Church: I think particularly if we are at the upper end of the projections it will be a problem, it will be increasing during the period. The impacts will be felt most acutely through extreme events. Pick a particular threshold, we will be crossing that threshold more frequently. We can ameliorate a lot of these impacts if we plan appropriately. One of the long-term issues which I think is of major concern is we could this century cross thresholds which will lead to a long-term sea-level rise of metres. Thresholds such as the decay of the Greenland ice sheet, for about a three degrees warming, then increased melting of the Greenland ice sheet precedes precipitation, and that means that the ice sheet will decay over a long time frame, but once we get to that point and even before we get to that point, once we are committed to that level, there's probably not a lot we can do about it.
Robyn Williams: A rise of metres, you said?
John Church: That will be a rise of metres.
Robyn Williams: How many metres, do you know?
John Church: The Greenland ice sheet contains about seven metres of sea-level equivalent, whether you get all of that out, but you'd probably get several metres, plus contributions from thermal expansion and a small contribution from glaciers in the longer term.
Robyn Williams: That's within the century?
John Church: No, that is not within the century. We could commit to that happening within the century, but it won't actually occur for many centuries.
Robyn Williams: But it's something that you can actually plan for?
John Church: It's something we can plan for, and if we want to actually avoid those large rises, it's something that we can work towards now, but it means now. We can't postpone this decision until the end of the century. Once we get to 3°C it will be too late.
Robyn Williams: How do you feel (and I can't resist asking this) about the number of people who criticise this sort of work and say it's exaggerated, there is essentially no problem?
John Church: What can you say? Clearly sea-levels are rising, all the evidence points to a significant increase in the rate of rise from the 19th to the 20th century. You can see the impacts of that rise occurring. We now have a quantitative explanation for the amount of sea-level rise, at least since 1970, and we are working on that for the whole of the 20th century. And it's inevitable that there will be continued rise. There is still significant uncertainty about how much rise there will be, whether we're at the top of the projections or at the bottom, particularly for local adaptation because we will have to adapt. We cannot mitigate all sea-level rise, we can't stop it. So we will have to adapt, and knowing where we are in that spectrum will be important, and understanding ice sheets in particular and the ocean ice interaction which leads to the ice sheet response will be challenging and perhaps one of the most important issues there is to address.
Robyn Williams: Dr John Church at the CSIRO in Hobart. He was elected a fellow at the Australian Academy of Science this week.
- John Church
- CSIRO Fellow
Marine and Atmospheric Research
- Robyn Williams
- David Fisher