Change in Sea Level Rise Estimates

In the SF Chronicle, this information rates a page 14 treatment. Where was the article in your paper?

The 2001 Intergovernmental Panel on Climate Change reports estimated sea level rise of 0.5 m, give or take 0.4 m (up to about a yard), this century. This would primarily result from water expansion in warming oceans, just like the liquid in a thermometer expands when warmed. The addition of Greenland’s water to the oceans, which would raise sea level by 7 m, was expected to take a thousand years, or thousands of years. New results show Greenland melt will be an important portion of sea level rise this year.

A sea level rise of 0.5 m could have enormous consequences. The sea moves in about 50 – 100 m for every m increase in sea level, and ocean surges during high tide in a powerful storm would further devastate coastal communities, push salt water inland, affecting rivers and ground water, and generally make life unpleasant for much of mankind. It is not encouraging to learn that this may be an underestimate.

Melting icebergs don’t raise sea level, just like melting ice in a full glass of water doesn’t cause spillage. However, estimates of sea level rise due to land ice melt have increased in recent years. The latest study shows that it’s increasing much more rapidly than had been thought.

The current rate of ice melt, just from Greenland, is increasing sea level at the rate of 0.5 mm/year, or 5 cm a century. However, this is expected to increase even if atmospheric carbon levels remain at 380 ppm.

Scientists will continue their attempts to understand mechanisms of increased melt, such as increased flow, to better understand what is happening.

And then there’s Antarctica – many tens of meters of sea level increase there.

From James Hansen’s article in the Independent

His complaints about Bush

Yet, a few weeks ago, when I – a Nasa climate scientist – tried to talk to the media about these issues following a lecture I had given calling for prompt reductions in the emission of greenhouse gases, the Nasa public affairs team – staffed by political appointees from the Bush administration – tried to stop me doing so. I was not happy with that, and I ignored the restrictions. The first line of Nasa’s mission is to understand and protect the planet.

About the ice:

Our understanding of what is going on is very new. Today’s forecasts of sea-level rise use climate models of the ice sheets that say they can only disintegrate over a thousand years or more. But we can now see that the models are almost worthless. They treat the ice sheets like a single block of ice that will slowly melt. But what is happening is much more dynamic.

Once the ice starts to melt at the surface, it forms lakes that empty down crevasses to the bottom of the ice. You get rivers of water underneath the ice. And the ice slides towards the ocean.

Our Nasa scientists have measured this in Greenland. And once these ice streams start moving, their influence stretches right to the interior of the ice sheet. Building an ice sheet takes a long time, because it is limited by snowfall. But destroying it can be explosively rapid.

How fast can this go? Right now, I think our best measure is what happened in the past. We know that, for instance, 14,000 years ago sea levels rose by 20m in 400 years – that is five metres in a century. This was towards the end of the last ice age, so there was more ice around. But, on the other hand, temperatures were not warming as fast as today.

How far can it go? The last time the world was three degrees warmer than today – which is what we expect later this century – sea levels were 25m higher. So that is what we can look forward to…

And the important question:

How long have we got? We have to stabilise emissions of carbon dioxide within a decade, or temperatures will warm by more than one degree. That will be warmer than it has been for half a million years, and many things could become unstoppable. If we are to stop that, we cannot wait for new technologies like capturing emissions from burning coal. We have to act with what we have. This decade, that means focusing on energy efficiency and renewable sources of energy that do not burn carbon. We don’t have much time left.


Yesterday I talked to some people who felt that climate change could be stopped or slowed if we go a fairly short period of time without fossil fuels. Put aside for now the idea that people who can’t cut back slightly can go on a crash diet. There are two reasons this won’t work:

• greenhouse gases take a while to leave the atmosphere, primarily by absorption as Ca CO3. It is safe to assume that some of the greenhouse gases we have added since the beginning of the industrial revolution will still be there in 1,000 years. In part, this is because of fast removal of some from the atmosphere, and slow removal of others. In part, this is because positive feedback is causing more greenhouse gases to be released.

• Hysteresis is the lag in a system moving in one direction compared to it moving the other direction. Suppose a stable atmospheric level of greenhouse gases equivalent to 400 ppm cause the climate to tip. Lowering carbon to 399 ppm will not cause it to switch back. Systems with a large hysteresis, such as a changing climate at high carbon levels, will only switch back if carbon levels fall considerably below the level that tipped them in the first place.

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