James Hansen - Without Action Now, Sea Level Rises Massive, Likely Non-Linear - New Scientist

[hatrack]

EDIT

I believe there is pressure on scientists to be conservative. Caveats are essential to science. They are born in scepticism, and scepticism is at the heart of the scientific method and discovery. However, in a case such as ice sheet instability and sea level rise, excessive caution also holds dangers. "Scientific reticence" can hinder communication with the public about the dangers of global warming. We may rue reticence if it means no action is taken until it is too late to prevent future disasters.

So why do I think a sea level rise of metres would be a near certainty if greenhouse gas emissions keep increasing? Because while the growth of great ice sheets takes millennia, the disintegration of ice sheets is a wet process that can proceed rapidly. Sea level is already rising at a moderate rate. In the past decade, it increased by 3 centimetres, about double the average rate during the preceding century. The rate of sea level rise over the 20th century was itself probably greater than the rate in the prior millennium, and this is due at least in part to human activity. About half of the increase is accounted for by thermal expansion of ocean water as a result of global warming. Melting mountain glaciers worldwide are responsible for several centimetres of the increase.

Greenland and Antarctica are also contributing to the rise in recent years. Gravity measurements by the GRACE satellites have recently shown that the ice sheets of Greenland and West Antarctica are each losing about 150 cubic kilometres of ice per year. Spread over the oceans, this is close to 1 millimetre a year, or 10 centimetres per century. The current rate of sea level change is not without consequences. However, the primary issue is whether global warming will reach a level such that ice sheets begin to disintegrate in a rapid, non-linear fashion on West Antarctica, Greenland or both. Once well under way, such a collapse might be impossible to stop, because there are multiple positive feedbacks. In that event, a sea level rise of several metres at least would be expected.

As an example, let us say that ice sheet melting adds 1 centimetre to sea level for the decade 2005 to 2015, and that this doubles each decade until the West Antarctic ice sheet is largely depleted. This would yield a rise in sea level of more than 5 metres by 2095. Of course, I cannot prove that my choice of a 10-year doubling time is accurate but I'd bet $1000 to a doughnut that it provides a far better estimate of the ice sheet's contribution to sea level rise than a linear response. In my opinion, if the world warms by 2 °C to 3 °C, such massive sea level rise is inevitable, and a substantial fraction of the rise would occur within a century. Business-as-usual global warming would almost surely send the planet beyond a tipping point, guaranteeing a disastrous degree of sea level rise.

EDIT

http://environment.newscientist.com/article.ns?id=mg19526141.600&feedId=climate-change_rss20

On edit: I thought this was also an interesting graphic.

[phantom power]

1. Thank you, Mr. Hansen.

:applause:

[philb]

4. sea level rise 3.1 cm/decade since 1993; with increasing rate over time

Tidal gauges in the Wellington region showed a sea level rise of 1mm-2mm a year for the past 150 years, apparently speeding up to 1.78mm a year over the past 50 years, and 3.1mm a year since 1993.
http://www.stuff.co.nz/stuff/4124438a7693.html

consistent with exponential growth rate

anyone seen an estimate for the last 5 years?

[Dead_Parrot]

5. I think it's too early to say

The change gets buried in multiple periodic changes, so you can't see it until some years afterwards: there's a blurb at http://www.niwascience.co.nz/pubs/wa/09-4/move you mind find interesting.

[ayeshahaqqiqa]

2. Let's also take into account

the hurricanes. They can play havoc on beaches, destroy soil preserving trees, etc. It would seem to me that a couple of Cat 5 hurricanes hitting Florida would only help with destroying the coastine--after all, the US Government isn't going to be helping much with recovery.

[Systematic Chaos]

3. I think his estimates are too conservative....

It is also important to note that I am a walking tub of optimism. :(