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Tag Archives: sea-level rise

Is your ‘Vision’ the same as your climate science colleagues?

by Liz Kalaugher

In climate science, projections tend to be long-term and patience is required to discover their accuracy. But by entering the Vision Prize polls you can find out in just a few months how well you predicted your colleagues’ views, as well as how much you agree with majority thinking.

vision-prize-160x60In this quarter’s polls you can comment on when sea-level will rise 1 metre above 2000 levels, which regions will see most weather disasters in the 2030s, the likelihood of geoengineering deployment in the form of solar radiation management, which technologies could most slow climate change this century, and whether burning all currrent fossil fuel reserves would bring more than 3 degrees of warming.

It’s free to sign up, participants are vetted to ensure they have relevant expertise, the poll takes around 5-10 minutes to complete, and the best predictors win prizes for the charity of their choice.

  • environmentalresearchweb has set up a collaboration with VisionPrize.

 

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GRACE and the dance of the kilograms

By Graham Cogley

The GRACE satellites have transformed our understanding of how kilograms dance around on and beneath the Earth’s solid surface, but nobody would claim that analyzing what they are telling us is a simple job. A recent analysis by Riccardo Riva and co-authors exemplifies this point.

The problems start with a list of technical details to do with processing of the raw observables. “Observables” is jargon, short for “observable quantities”, but it is a valuable clue to how to think about the “inferables” that we are concerned about.

The point is that an “inferable”, such as relative sea-level change, may be quite some distance down the chain of reasoning from the observable, which in this case is the rate at which the two satellites are accelerating away from or towards each other. This rate depends directly on all the gravitational attractions they feel at the time of each measurement. We want to remove the technical noise so that we can infer the signal of the fluctuating gravity field experienced by the satellites, and so infer the transfers of mass that explain the gravitational fluctuations.

One of the technical details, for example, has to do with spatial resolution, which for GRACE is about 300 km. But the regions between which mass is being transferred generally have quite sharp boundaries, for example the coastline. The jargon for this part of the problem, “leakage”, is quite expressive. It hints that part of the signal we want has strayed out of our study region and into neighbouring regions.

Riva and co-authors have two study regions, the land and the ocean. Signal could be leaking either way across the coastline, but they argue that the oceanic signal of mass gain, expressed as relative sea-level change, is probably much smoother than the terrestrial signal of mass loss. So they simply define a 250-km wide buffer in the offshore waters and “unleak” all of its supposed signal back onto the landmasses.

There then follow a number of other corrections, including a correction for movements of mass within the solid Earth and a trial-and-error phase that seeks to undo the addition of some oceanic signal to the land signal during the unleaking phase.

Now the geophysical part of the problem can be addressed. Riva and co-authors reckon that +1.0 mm/yr of equivalent sea-level rise moved from the continental surfaces to the oceans between 2003 and 2009, give or take 0.4 mm/yr. This surprises me.

My estimate for the transfer from small glaciers (those other than the ice sheets) is about +1.2 mm/yr for the same period. Several recent estimates for the transfer from the Greenland Ice Sheet lie between about +0.5 and +0.7 mm/yr, and for the Antarctic Ice Sheet at about +0.5 mm/yr. (All of these abouts are partly because of the uncertainty of the measurements, or rather of the inferables, but also because of the difficulty of matching the different time spans of the analyses.) The glaciers, then, seem to be adding more than twice the mass to the ocean that is estimated by the Riva analysis.

It gets worse. Yoshihide Wada and co-authors, in a paper to appear shortly, argue that the mining of groundwater is running at present at a rate equivalent to +0.8 mm/yr. This addition is partly offset by the filling of reservoirs, estimated at —0.5 mm/yr over the past 50-60 years. The rate during the past decade is probably lower, because the frenzy of dam-building has abated somewhat recently. But it is not possible to get all of the continental surface contributions to add up to less than, say, +2.6 to +2.8 mm/yr, give or take perhaps 0.4 mm/yr.

What we have here is stark discord, well outside the error bars, between several “inferables”, and we haven’t even got to the sea-level rise due to thermal expansion and the estimated sea-level rise itself. This is a classic example of unsettled science in a context of settled science. We can draw a diagram to depict the water balance of the ocean, or write down a little equation. A balance is, after all, simple arithmetic. The boundary between the settled and unsettled parts of the problem lies somewhere beyond the diagram, and indeed beyond the signs, + or —, attached to the various terms in the equation. But at the moment it is definitely before we get to the first decimal digits of the numbers, at least one of which must be wrong.

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Reservoirs and the dance of the kilograms

By Graham Cogley

Suppose you have a kilogram of something, and you know where it is, somewhere near the surface of the Earth. And suppose it has been there for quite a long time.

It will have been obeying Newton’s laws of gravitation, like all the other six trillion trillion kilograms. They will all have got used to each other, and will be relatively at rest, because all of the gravitational accelerations will have decreased to zero (pretend).

Now suppose you take your kilogram and put it somewhere else. It will attract all the other kilograms towards its new location, more strongly the nearer they are. Remember, Newton says that the acceleration drawing any two bodies together is inversely proportional to the square of the distance between them.

As kilograms move around, they induce other kilograms to move around as well. Recently Julia Fiedler and Clinton Conrad identified the steps in one part of this dance of the kilograms: the removal of about 10,000 trillion kilograms from the ocean into reservoirs since 1950. These kilograms represent a hypothetically uniform lowering of sea level by 28 mm, and a nearly equivalent displacement of fresh air by reservoir water. (Only nearly, because almost a quarter of the sea water has seeped into the aquifers beneath the new reservoirs.)

The surface of the sea is an equipotential, a surface on which the gravitational potential is a constant. The value of the constant is of no interest, except that it is just right for accommodating all the sea water there is. Take some water out of the sea and the new sea surface is still an equipotential, but a different, lower one (the new constant is smaller).

Sea level, though, has been rising steadily. As the ocean warms, it expands — each of its kilograms takes up more space. And as the glaciers melt — which is where I come in — they add kilograms to the ocean. Since the early 1990s we have been able to track this rise with satellite altimeters, but for times before then we have to rely on tide gauges. A tide gauge measures RSL or relative sea-level, the distance between the sea-surface equipotential and the part of the solid Earth to which it is attached.

In the present context the solid Earth is more like toothpaste than rock. It moves because the reservoirs squeeze the toothpaste, which flows away towards where the kilograms came from. The solid surface falls beneath the reservoirs, so relative sea level rises there. There is a compensating relative fall, spread over the oceanic source of the kilograms.

But here comes a new arabesque of the dance. The dammed kilograms are busily attracting all the others — including the ones still in the ocean — towards the reservoirs. They have changed the shape of the sea-surface equipotential, which is higher (further from the Earth’s centre of mass) near the reservoirs than it used to be, and lower over the oceanic source. For practical reasons we can only install tide gauges on coastlines, so they give us a biased view: no sampling at all of the open ocean, and an index of coastal RSL that deviates from the global average, gauge by gauge, depending on the number of kilograms we have moved into reservoirs nearby.

Fiedler and Conrad estimate that some gauges, in southern locations far from reservoirs, have been recording less than the global-average change of RSL that is due to the reservoirs. Others have been recording more, and at some the sea level has actually gone up simply because they are close to big reservoirs. Gauges in Ghana, not far from the 148 trillion kilograms that we moved into Lake Volta beginning in 1965, are good examples. But, based on a sample of 200 gauges, Fiedler and Conrad reckon that the tide gauges have been seeing only about —0.3 mm/yr instead of the true average reservoir signal, —28 mm over 58 years or about —0.5 mm/yr.

So the tide-gauge estimates of global-average sea-level rise are too high by +0.2 mm/yr. There are reasons for thinking that the necessary correction might be smaller, but the total rate (over the past few years) is in the neighbourhood of +2.5 to +3.0 mm/yr. Looking on the bright side, we have reached the stage of worrying about tenths of a millimetre. All the same, people like me, who try to estimate contributions to the water balance of the ocean, now have to learn new dance steps because the band is playing a subtly different tune.

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Climate Change Congress: from art to gloom

By Liz Kalaugher

The Danish passion for design came to the fore at the
Climate Change Congress opening session this morning. Not only was there an
unusually artistic backdrop at the front of the hall – a massive cut-out
version of the conference iceberg logo – but around 2500 delegates, including
Danish royalty, were also entertained with some virtuoso recorder playing.

 Once the conference kicked off for real, however, the
outlook was more bleak. A wide range of climate and other scientists have come
together to discuss their discoveries since the IPCC report of 2007. Because of
the way that report was produced, that means any results from the last 4-5
years. In a nutshell, the news is not good.

 Carbon emissions are now at the upper bound of those
projected by the IPCC, sea level rise could well top one metre by the end of
the century, and it appears that tropical forest carbon sinks are likely to
decline as the planet warms, to name just a few.

“The good news is in the social sciences and the
human sciences,” said Katherine Richardson of the University of Copenhagen
and chair of the conference scientific steering committee. “In those
fields you will find we have a lot of tools in our toolbox, things we can do
already.”

 For once, the credit crunch is arguably good news as it’s
likely to see a slowdown in world carbon emissions. Although, according to
Terry Barker of the University of Cambridge, it could also lead to a collapse
of the European emissions trading scheme as declining demand for electricity
leads to a plummeting price for emissions credits.

 “Politicians have refocused on jobs because of the
economic crisis,” said John Ashton of the UK Foreign and Commonwealth
Office. “If we want a successful response to climate change we have to
reframe it in terms of jobs. We need to build the prospect of a low carbon
recovery.”

 The plan is for the output from the conference to feed
into the climate negotiations for the follow-on treaty to the Kyoto Protocol to
be held in the same venue in December. “We are looking for things to
happen from this conference, not just more talk,” said Ian Chubb of
Australian National University

 With that in mind, organizers will produce a 30 page long
synthesis report by June 1st while next year will see the release of a book.
What’s more, at the conference closing ceremony on Thursday, Danish prime
minister Anders Fogh Rasmussen will receive a summary of a handful of key
results presented at the event. He’ll then discuss these with a panel of
leading researchers, including Dan Kammen of the University of California,
Berkeley, and Nicholas Stern of the London School of Economics and Political
Science. Watch this space for more.

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Sea level rise flies high

By Liz Kalaugher

Last year Stefan Rahmstorf of the University of Potsdam, Germany, wrote a paper over the weekend. Later the simple relationship he developed between sea-level rise and temperature change appeared in Science. This year he has surpassed himself by doing the calculations for his talk at the AAAS Meeting on the plane on the way over. While they still need checking, the sums appear to indicate that sea level rise will be substantially higher than predicted in the IPCC fourth assessment report.

(more…)

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