[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Under a Green Sky



" The Greenland Icecap is not going to melt in a few hundred years. It
 takes 
much longer. At the end of the last glaciation 11600 years ago
 climate in 
northern Europa became as warm or warmer than today and much drier..."

I am not sure how this played out in reality, but it is theoretically, if 
counter-intuitively, possible for wetter, cooler summers to melt ice much 
faster than warmer dry ones. This is because evaporative cooling is increased 
in a condition of low relative humidity, and snow/ice has high albedo. In other 
words, the thin layer of ice melted by sunshine evaporates rapidly, cooling the 
ice below; whereas rain transmits heat to the icepack without significant 
evaporation taking place. This could be exacerbated where summer air is dry, 
and most precipitation occurs in winter (slow melting), or where summer air is 
wet and most precip occurs in summer (fast melting). As a meteorological 
phenomenon, familiar to those who shovel snow.

Don

----- Original Message ----
From: David Marjanovic <david.marjanovic@gmx.at>
To: DML <dinosaur@usc.edu>
Sent: Monday, November 19, 2007 8:35:54 AM
Subject: Re: Under a Green Sky


> The Greenland Icecap is not going to melt in a few hundred years. It
 takes 
> much longer. At the end of the last glaciation 11600 years ago
 climate in 
> northern Europa became as warm or warmer than today and much drier,
 but 
> even so it took 2000 years for the residual Scandinavian icecap to
 melt, 
> even though it could calve into the Bothnian Gulf for much of the
 time. 
> The only way an icecap can melt fast is if it can calve out into the
 sea. 
> The Greenland icecap can't since its bed is above sealevel. Note that
 not 
> even the southern dome of the Greenland icecap melted during the
 previous 
> (Eemian) interglacial when arctic temperatures were about 6 degrees 
> centigrade warmer than now.

Are you sure they were that much warmer? The following "brief
 communication" 
says that when the annual average temperature in Greenland increases by
 more 
than 2.7 °C (which is going to happen according to most of the
 scenarios of 
the IPCC report of 2001 -- the latest one is said to have generally
 warmer 
scenarios), melting exceeds snowfall, so that "the ice-sheet must
 contract, 
even if iceberg production is reduced to zero as it retreats from the 
coast", which is estimated to take at least about 1,000 years. Don't
 the 
huge amounts of meltwater produced every summer flow into the sea? It
 does 
in West Antarctica: 
http://www.sciencemag.org/cgi/content/abstract/sci;315/5818/1544.

Jonathan M. Gregory, Philippe Huybrechts & Sarah C. B. Raper:
 Threatened 
loss of the Greenland ice sheet, Nature 428, 616 (8 April 2004)

Unfortunately, the citation for the 2.7 °C is a paper from 1991...

More recent papers -- which I have been too stupid to save, and here I
 don't 
have full-text Science access (in the lab I have, but the public
 transport 
is on strike) -- find that the ice sheet is melting faster than
 expected 
under any of the scenarios considered above, and emphasize that, in 
interglacials, it depends on itself to survive by creating its own cold
 
weather around itself: if it is removed from a climate model even under
 
preindustrial CO2 levels, it doesn't come back. At least one of these
 papers 
says the ice sheet will likely be gone in a few hundred years.

However, searching for papers with "Greenland" in the title on the
 Science 
website brings up almost only papers on the melting of the ice-sheet.
 Here's 
one http://www.sciencemag.org/cgi/content/abstract/311/5768/1751 that 
simulates the conditions of the last interglacial and correctly
 concludes 
that the ice-sheet did not melt completely, even though the sea level
 did 
rise several meters above today's. Here's another 
http://www.sciencemag.org/cgi/content/abstract/sci;311/5768/1747 which
 looks 
at the last interglacial and whose abstract ends in: "The record of
 past 
ice-sheet melting indicates that the rate of future melting and related
 
sea-level rise could be faster than widely thought." Some 
http://
lso
 worry 
that "Satellite data show that ice sheets can change much faster than 
commonly appreciated, with potentially worrying implications for their 
stability"...

This paper http://ppg.sagepub.com/cgi/reprint/30/6/785, which is freely
 
accessible, mentions the recent increase in Greenland melting:

"Alternative observational data provided by´radar interferometry
 suggests 
that, while the interior of the Greenland ice sheet is in approximate
 mass 
balance, there is rapid thinning around the periphery close to certain 
outlet glaciers (Rignot and Kanagaratnam, 2006). Many of these glaciers
 have 
accelerated, increasing Greenland's estimated contributionto sea-level
 rise 
from 0.23 +- 0.08 mm/yr in 1996 to 0.57 +- 0.1 mm/yr in 2005. This 
acceleration is also recorded by increased glacial seismicity (Ekström
 et 
al., 2006). Interestingly, these data are strongly seasonal, with
 seismicity 
increasing nearly fivefold during the summer months. This may indicate 
accelerations are linked to bed lubrication by increased meltwater 
penetration, suggesting that even modest changes in temperature (~1°C)
 can 
produce large changes in discharge (Zwally et al., 2002; Joughin,
 2006)."

"This variability poses new challenges to existing conceptual and 
mathematical models of how sea-level/cryosphere/climate linkages
 operate 
during warm intervals, both at the suborbital (millennial) and
 subdecadal 
timescales. For example, the recent changes in Greenland revealed by
 seismic 
and satellite data cannot be explained by melting mechanisms alone
 (Rignot 
and Kanagaratnam, 2006). The physical processes associated with dynamic
 
glacier change, perhaps linked to ocean warming and the retreat of
 tidewater 
glaciers (Joughin et al., 2004; Alley et al., 2005; Payne et al., 2004;
 
Bindschadler, 2006), are not included in the current models used to
 predict 
future sea-level contributions (Marshall, 2005). Consequently, these
 models 
do not display the sensitivity to change indicated by recent remote
 sensing 
data and may underestimate the magnitude of future sea-level rise 
(Dowdeswell, 2006; Rignot and Kanagaratnam, 2006; Velicogna and Wahr,
 2006). 
This is particularly interesting in light of a recent modelling study
 that 
proposes existing contributions from mountain glaciers and ice caps may
 have 
been overestimated (Raper and Braithwaite, 2006)."

"An alternative is to examine records from previous interglacials,
 which 
provide a longer-term context against which more recent changes can be 
assessed. The last interglacial (LIG) has attracted particular
 attention as 
Arctic temperatures were warmer, sea levels higher and the Greenland
 Ice 
sheet smaller than present. For example, Overpeck et al. (2006) use a 
coupled ocean-atmosphere climate model to simulate LIG climate and
 compare 
this with simulations of the next 140 years to examine potential
 ice-sheet 
contributions to sea-level rise. Their results suggest that by 2100 the
 
Greenland region will be at least as warm as it was during the LIG and,
 by 
implication, warm enough to melt large portions of the Greenland Ice
 Sheet."

Overpeck et al. (2006) is the first paper to which I provide a link
 above.

Lastly, there's this paper 
http://www.sciencemag.org/cgi/content/abstract/313/5795/1958 which
 reports a 
high probability for "accelerated melting since the summer of 2004".

I give up here, this off-topic post is long enough...!

> As for MIS 11 (Holstein) when the Greenland ice and at least part of
> the West Antarctic ice melted, the worrying thing about is that it
> was apparently NOT an extremely warm interglacial.

Does this hold for the whole interglacial, or only for the earlier
 part? 
IIRC the sea-level highstand came late in the interglacial.

> Concerning Bangladesh, that country will be gone in a few centuries,
> climate warming or not. The Ganges and Brahmaputra are dammed now,
> little sediment therefore comes into the delta and the sea is going
> to erode it away pretty quickly. The same thing is happening to the
> Nile Delta since the Ass

------------------------

> > > Right now, the current state of the climate follows suit as it
> > > warms from its recent glacial state.
> >
> > Ouch!!! This is nonsense. The _end of_ the last ice age is over,
 and
> > has been over for 11,000 years.
>
> We still have glaciers. We still have ice caps. I didn't say Ice Age.
 I
> said glacial state. Ouch!!!

I've never seen "glacial state" used like this, and because there's no
 real 
chance that East Antarctica will become ice-free, I didn't get the idea
 that 
you may have meant the article was talking about an exit from the
 current 
"icehouse" state and the return of the middle Eocene and earlier 
"greenhouse" state. So, while wrong, my unfortunate knee-jerk reaction
 was 
inevitable, and further increased by the fact that I've several times
 come 
across the supposed argument (elsewhere on the Internet) that the
 current 
warming is still part of the end of the last ice-age.