This is the second article on the importance of ice in tracking the effects of climate change.Â If you haven't already, please read the first article here, which introduces the different issues and terminology related to the study of ice.Â As noted, there are many ways to examine ice, including sea vs land ice, and physical measurements such as the extent, area, mass, volume, and age of ice.Â In this article we will focus on one area - the Arctic - and one measurement - sea ice extent.Â In general, the trends seen here are also seen with the other measurements.
Sea ice extent is a rather unusual measurement in that it is defined as area of ocean covered by at least 15% ice.Â So it could be 100% covered with ice or only 15% covered with ice, and obviously the actual ice cover and its characteristics is important for a total understanding of the situation. Climate scientists, of course, look at sea ice extent in conjunction with all of those other factors.
The bottom line on sea ice extent in the Arctic is that it is decreasing.Â But before we get into that let's first understand how sea ice expands and retreats over the year.Â As you might expect sea ice tends to expand during the winter months and melt back during the summer months.Â This is the normal process and it happens every year.Â Some years the winter growth is larger and some years the summer melt is larger, but the summer melt has historically never been complete (there is always ice still there, just not as much).Â So scientists look at the year to year averages.Â And the result has been striking.
The following graph comes from the National Snow and Ice Data Center (NSIDC).Â The NSIDC collects data and does research all over the world on, you guessed it, snow and ice, and makes those data available via their web site.
As you can see, 2011 (blue line) shows ice extent levels that are close to, and even at times below, the record low year of 2007.Â The blue 2011 line is currently slightly above 2007 as variable weather conditions allowed the slowing of the ice loss in recent weeks.Â However, this doesn't mean things are getting better. On the contrary, these weather changes likely just spread the ice pieces further apart, which means thinner coverage even as the extent increased (remember extent is defined as anything more than 15% area but doesn't consider thickness of the ice).Â Even more troubling is that the changes in weather already observed suggest that the latter half of August will result in even faster ice loss (warmer temperatures and thinner ice equals faster melting).Â So the NSIDC graph next month is likely to show the line for 2011 has dropped even below the record low of 2007.
There is one more very important piece of information in the graph above.Â The grey line and the light grey area signify the average from 1979-2000, that is, where the ice used to be.Â All of the yearly ice extent lines since 2001 (including the ones for the years not shown) are below the average.Â That means that we've been in a long period of decreased sea ice extent.Â And that is not good.
I'll illustrate this with one more graph below, also from the NSIDC.
The graph shows the extent of sea ice in the Arctic in the month of July from 1979 to 2011.Â For obvious reasons you can't compare sea ice extent from July to, let's say, December.Â So scientists look at what the ice does in the same month every year.Â As anyone can clearly see, the sea ice extent has been decreasing substantially (check out that blue line). Some years the ice melts a bit less than others, and some years it melts a bit more (jagged black line).Â But the data clearly show that the Arctic sea ice extent is decreasing dramatically.Â And not just for July.Â If you show the graph for each month, comparing it with the same month for previous years and decades, you get exactly the same trend.Â Even the winter months show that the growth in winter is less than it was previously. Which, of course, means less ice to melt in the summer (and less ice extent).
And remember that this is only sea ice extent.Â Ice volume, mass, and age are also decreasing (more on those in future articles).
So what does this all mean?Â Well, the more the sea ice melts the more open water is exposed, and open water decreases albedo, which speeds up warming and that speeds up melting (check out previous articles in the The Truth About Global Warming series for more information).Â Decreasing sea ice could also have significant impacts on polar bears because they rely on the availability of sea ice for hunting during the summer.Â Decreasing sea ice extent also means that parts of the Arctic will be open to commercial and military shipping, which could have serious impacts on our national security and economic viability. There are even more ramifications of this decreasing sea ice extent, but we'll leave those for future posts.
Â© David K, August 2011
I also have a separate group called "Exposing Climate Denialism - A Guide to Tactics and Tall Tales," located at climatelies.gather.com for those who want to read about some of the intentional disinformation used by climate denialists to confuse the public about the state of climate science.