Sunday :: Feb 4, 2007

What the IPCC Fourth Assessment Report Missed

by Mary

Dr. Christina Hulbe gave us a preliminary review on the The Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report when it was published last Friday. As she said, this report is an amazing body of work by a great many scientists from all over the world who working together have contributed to our current understanding of global warming and what lies ahead. The consensus that the scientific community has reached is remarkable and the conclusion is unequivocal that humans are the cause of what we are seeing and that humans must urgently address greenhouse gas emissions or the earth we face will be almost unlivable.


Greenland Glacier
Courtesy of the NOAA Photo Library

However, there was one aspect of global warming that was understated in the report and Dr. James Hansen, director of NASA Goddard Institute for Space Studies and notable climate scientist, thinks that because it was left out, the critical urgency of addressing the problem is not clear enough. Dr. Hansen believes that the problem of rising sea levels was under reported because frankly, we don't understand the science of what will happen in the event of a catastrophic failure of the ice sheets. Here's Hansen's discussion about the report on this week's Living On Earth.

CURWOOD: So with that in mind how should we view predictions from this latest IPCC assessment? What did the report leave out or underplay from your view?

HANSEN: Well I think that there is a natural reticence which may derive from the scientific method which says you should be skeptical. You should check all sides of a story and before you make a very strong conclusion. The difficulty here is that I think we have very limited time to get on a different path with our energy use and greenhouse gas emissions or we're going to end up with unstoppable problems in the future. And I would have preferred an even clearer statement about the dangers of future sea level rise if the ice sheets begin to disintegrate. And I think that a business as usual scenario will guarantee future disintegration of West Antarctica and parts of Greenland.

CURWOOD: So your concern about the chance of loss of large sheets of ice and the considerable impact that that could have on sea level rise is echoed by, um, Ohio State University scientist Lonnie Thompson and he says that those issues are gorillas in the room that the IPCC isn't paying as much attention to as he thinks it ought to.

HANSEN: Well that's a very good point because IPCC addresses a lot of things but there are a small number which deserve very great attention. And this, I think, is the number one item just because of the inertia of the system. If we get it started it will become very difficult to stop it, perhaps impossible to stop it.

Hansen goes on to describe what makes something like rising sea levels so hard to predict.

The last time a large ice sheet melted sea level went up at a rate of five meters per century. That's one meter every 20 years. And that is a kind of sea level rise, a rate which the simple ice sheet models available now just cannot produce because they don't have the physics in them to give you the rapid collapse that happens in a very nonlinear system.

CURWOOD: Could you explain what you mean by a nonlinear event?

HANSEN: Well, it's one in which the response begins slowly but you reach a certain point and suddenly things collapse. So you get a very rapid change. And you can imagine that in the case of large ice sheets. There are several processes that contribute to more rapid loss of ice. As you have melt on the surface of the ice sheet it descends through holes and crevasses to the surface and lubricates the base so that the ice sheet begins to move faster. There's ice at the edge of the continent, both Greenland and especially West Antarctica, which holds back the ice streams but as the ocean warms and those ice shelves melt that's like taking the cork out of a bottle and stuff can come out much more rapidly. And even as the ice sheet begins to decrease in size the surface lowers to a lower altitude where it's warmer and so it melts faster. So you get these multiple positive feedbacks and then you get a sudden collapse and sea level goes up very rapidly. We're going to guarantee that that happens eventually. If we get warming I would argue of more than 1 degree Celsius additional above that of year 2000.

Think about that. The report says that in the very best case scientists now believe that by 2100 the average yearly temperature will be 3 degrees Celsius warmer than the pre-industrial earth and if we don't address the problem with the urgency it needs, 3 degrees could be wildly optimistic. And for those who are not used to working in the metric system, the level of sea rise if it follows the model of the previous melting of the major ice sheets could be 1 meter per 20 years, which is a bit more than 3 feet per 20 years and by 100 years, the rise in sea levels would be more than 16 feet. As Dr. Hulbe noted in her article on the rising sea, during the 20th century, the observed sea rise was approximately 0.15 to 0.2 meters. And she noted that many places in the world, including large areas of many coastal cities will be underwater. What happened when Katrina hit New Orleans is just an example of what is coming to the entire earth.

Dr. Hansen believes that the melting ice sheets should have been given a much greater emphasis because it is the most critical consequence of not getting our greenhouse gases under control. Our job as citizens of the earth is to make sure our representatives address this problem with the urgency that it demands.

Mary :: 6:06 PM :: Comments (33) :: Digg It!