Sunday, July 08, 2007

The meaning of the Greenland DNA discovery

I've read some immensely confused responses to the discovery of ancient ice layers beneath the more modern Greenland ice. The usual whackos seem to think this somehow changes global warming models, but the real climate discovery is not that Greenland was once warm and largely ice free -- we knew that already. The real climate discovery is that even during periods of moderate to strong global warming the ice doesn't necessarily melt. That finding decreases the probability of large sea level rise over the next 50 years, and it reinforces the projections for a moderate rise that came from the recent UN climate consensus statement.

In other words, it strengthens the consensus climate models.

The non-climate discovery is of very old DNA, which will be very interesting indeed. The Boston Globe coverage is better than most, but it's better as edited below (honest!). Emphases mine:
Greenland ice yields hope on climate - The Boston Globe
The Boston Globe
DNA hints warm era didn't melt entire cap
By Colin Nickerson, Globe Staff | July 6, 2007

An international team of scientists, drilling deep into the ice layers of Greenland, has found DNA from ancient spiders and trees, evidence that suggests the frozen shield covering the immense island survived the earth's last period of global warming.

The findings, published today in the journal Science, indicate Greenland's ice may be less susceptible to the massive meltdown predicted by computer models of climate change, the article's main author said in an interview...

....the discovery of organic matter in ice dating from half-a-million years ago offers evidence that the Greenland ice shield remained frozen even during the earth's last "interglacial period" -- some 120,000 years ago -- when average temperatures were 9 degrees Fahrenheit warmer than they are now. That's slightly higher than the average temperatures foreseen by most scientists for the end of this century, although some environmentalists warn it might get even hotter.

... "We should remain very worried about rising sea levels," he said. "We know that during the last interglacial, sea levels rose by 5 meters or more. But this must have come from sources additional to Greenland, such as Antarctic ice. It does not appear the whole [Greenland] sheet will melt."

.... Some scientists not involved in the study drew a conclusion very different from that of the Danish-led team .

"The raw results of this study are very impressive -- southern Greenland was unglaciated sometime during the last million years or so," said Gavin Schmidt, a climatologist with National Aeronautics and Space Administration 's Goddard Institute for Space Studies. "I would argue that this implies a more unstable ice sheet, not the opposite."

Schmidt said that during the more recent warming period, most but not all of the ice might have disappeared from southern Greenland, leaving a thin layer that would have been sufficient to preserve the DNA.

The organic matter found in ice cores taken from a southern Greenland drilling site known as Dye 3 represents the oldest-ever authenticated DNA found by scientists to date, with fragments of beetles, tree bark, prehistoric spiders, and other life verified independently in laboratories in Denmark, Canada, and Germany.

... Scientists were able to isolate only about 1.6 ounces of organic matter from the ice at the bottom of the core sample, according to Enrico Cappellini, a researcher specializing in ancient proteins at Britain's University of York.

....Analysis of the insect mitochondria, cellular components that contain genomes that can be used to date DNA, as well as amino acids, indicate d that the creatures were at least 450,000 years old. Uncertainties with dating, however, leave the possibility that the DNA dated only as far back as the last interglacial period.

The identification of such relatively well-preserved genetic material beneath ice sheets was exciting news for biologists. Ten percent of the earth's surface has been covered by deep ice for tens of thousands of years. "We could be opening up a frozen world of new discoveries," said Cappellini, the University of York researcher.

....The Nordic settlements -- the first European presence in the New World -- survived for nearly 500 years before mysteriously disappearing. Historians speculate that either the inhabitants starved as Greenland grew colder or they were killed by Inuit, who appeared on the scene around 1200 AD.
I kept the last fragment because I didn't know that the Inuit arrived on Greenland relatively late. In summary:
  1. The study suggests the Greenland ice layers may be more stable than thought, which supports the consensus sea level rise model. If the DNA is found, however, to only be 500,000 years old, this will actually argue for a large sea level rise.
  2. It's possible that lower layers of the ice are resistant to melting even if the top layers melt, in which case we're back to thinking Greenland will raise sea levels in 50 to 100 years.
  3. Since sea levels rose lots in the last interglacial, and since the water had to come from either Greenland or Antarctica, if Greenland didn't melt that means Antarctica did. We don't know why however, current models predict Antarctica won't melt.

Thinking it over I'm leaning a bit towards #2. I sure hope Antarctica won't be melting, that would push us more firmly into a large sea level rise ...

Update 7/9/07: See the interesting comment by "JC". This makes me worry a bit more about #3 now ...

Update 7/11/07: RealClimate has a very good review. It sounds like the researchers themselves were responsible for the confused interpretations. I'm used to medical researchers garbling their own findings (often by overstating them, usually wrongly), unsurprisingly the failing is not limited to the medical domain. The scientists were evidently soundly flogged, for they've issued a clarification with is gratifyingly close to what I ended up with after a savvy commeter corrected my understanding of Antarctic contribution to sea level rise in the last interglacial:

The scientists do not want to put into question the rise in sea level during a global warming. During the last interglacial period 125.000 years ago, temperatures in Greenland were 5 degrees higher and global sea level was 4-5 meters higher than it is today. However, since the new scientific results show that the ice sheet also covered southern Greenland, the melting of the Greenlandic ice cap can only have caused a sea level rise of about 2 meters. Therefore some of the ice contributing to the sea level rise must have come from other sources, for instance the Antarctic. Furthermore, thermal warming of the oceans will cause expansion of the sea water and result in a sea level rise of half a meter, and the melting of small glaciers around the globe will result in an additional half meter rise.

1 comment:

JC said...

Just a clarification re Antartica not melting - the studies show *East* Antartica won't melt anytime soon but West Antartica will and is. It's a weird regional phenomena where the ozone hole over the pole causes circular winds that warms the West, cools the East. In fact, West Antartica is one of the fastest warmest regions of the planet and overall, Antartica is losing ice.