Archive for the ‘projecting’ Category

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Arctic Sea Ice Gone by Summer 2012?

March 21, 2008

You have heard and read it all over the media in the last several months. An Associated Press article picked up by newspapers and the web (See National Geographic version here) reported that one scientist “speculated that summer sea ice could be gone in five years.”  The article warns…

“”The Arctic is screaming,” said Mark Serreze, senior scientist at the government’s snow and ice data center in Boulder, Colorado. “

and

“The Arctic is often cited as the canary in the coal mine for climate warming,” said Zwally, who as a teenager hauled coal. “Now as a sign of climate warming, the canary has died. It is time to start getting out of the coal mines.”

and

This week, after reviewing his own new data, NASA climate scientist Jay Zwally said: “At this rate, the Arctic Ocean could be nearly ice-free at the end of summer by 2012, much faster than previous predictions.”

Wow!  Where did this ominously close date of 2012 come from?  The sea ice extent in the Arctic (and Antarctic) has been monitored by satellite for almost 30 years, since 1979.  The extent of the ice rises and falls, as you would expect, as the year cycles through its seasons.  It reaches its yearly minimum by late September or early October.  On the average, this minimum has been declining for the last 30 years.  After October the northern sea ice extent increases until it reaches a maximum in late March or early April  each year. 

The yearly cycle is huge.  Typically, about 60% of the total sea ice extent melts away as is goes from yearly maximum to the yearly minimum.  Figure 1, below, shows the northern sea ice extent from 1979 to November 2007.  Note the large drop in the yearly minimum sea ice extent by October 2007.  This is the canary that Zwally was talking about.  The prediction of an ice free arctic summer by 2012 comes by simply extrapolating the change in the minimum ice extent between 2006 and 2007 into the future, as shown.

nothern-sea-ice-to-nov-07.gif

 

Figure 1.  Northern Hemisphere Sea Ice Extent from 1979 to November 2007.  Rapid drop in minimum sea ice extent from 2006 to 2007 is extrapolated to show total loss of ice by 2012. Click on image to enlarge.

Nature, ever the jokester, has had her fun with the climate alarmists since last November.  The Arctic has had more than its usual run up of ice this winter.  Today’s (3/20/08) version of the Northern Hemisphere Sea Ice Extent shows an unprecedented (Ever heard that word before?) run up of ice in the Arctic in the last five months 

nothern-sea-ice-to-mar-08.gif

 

Figure 2.  Same as figure 1, but updated to March 20th, 2008.  The last five months have shown a rapid ice increase in the Northern Hemisphere. Click on image to enlarge.

The 11 million square kilometer gain in sea ice extent is the greatest seasonal ice gain in history (where history, according to alarmist rhetoric, began in 1979 when satellites started tracking the ice extent).  You can read about a little bit of “pre-history” here.  Take a look at this long list of peer reviewed journal articles showing that the “pre-history” or the Arctic was warmer than the present.

 While we are at it, lets not forget about the Southern Hemisphere.  After all, they didn’t put the word “global” in “global warming” for nothing.  Figure 3, below shows the sea ice extent for the Southern Hemisphere going back to 1979 from the same satellites tracking the northern ice.  The peak at the upper right corner of the graph shows that at about the same time that the Arctic ice was at its lowest “historic” extent, the Antarctic ice was at its highest “unprecedented” extent.

southern-sea-ice-to-mar-08.gif

 

Figure 3.  Sea ice extent in the Southern Hemisphere.  Click on image to enlarge.

I would not  predict that the Southern Hemisphere sea ice extent will reach 17 million square kilometers in 2012 by simply extrapolating the increases of the last four years out another four years.  The changes are to complex.  But that never seems to stop the alarmists.

All the graphs in this post are from the University of Illinois Polar Research Group.  They got the data from the National Center for Environmental Prediction/NOAA

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Critique of "A Semi-Empirical Approach to Projecting Future Sea-Level Rise" by Rahmstorf

August 12, 2007

A recent article in Science by Stefan Rahmstorf (2007) predicted extreme sea level rise during the 21st century. Rahmstorf’s predictions went as high as 140 cm (55 inches), far beyond even the high edge of the uncertainty of the IPCC’s unlikely A1Fl scenario (see here, page 820). This high estimate by the IPCC was 59cm (23 inches), with other other scenarios yielding considerably lower estimates. Following is a critique of Rahmstorf’s method and conclusions.

This post has a quick summary of Rahmstorf’s approach to to projecting sea-level rise for this century. Following that summary is a quick list of problems that I have identified in his paper, each with a link to subsequent posts with more detailed information.

Rahmstorf’s Simple Model

Rahmstorf’s simple model of sea level rise consists of a system in equilibrium, where the sea level and the temperature start out as constants. Then an instantaneous step occurs in the temperature, causing the sea level to rise. Eventually the sea level will rise to a new equilibrium, as shown below.

It is very important to note that the time required to arrive at the new equilibrium is, according to Rahmstorf, “to be on the order of millennia.” This long time scale provides the other important point of this simple model. That is, over a short enough time scale the rate of sea level rise can be considered a constant (as illustrated in the above graph during the time where dH/dT is proportional to delta T). Rahmstorf posits that “this linear approximation may be valid for a few centuries.”

Therefore, in this model, a temperature jump in the 1920s, for example, would result in a sea level rising at a constant rate for several hundred years, even without any subsequent temperature increases. Of course, subsequent temperature rises would each result in a greater sea level rise rate, but there would never be any drop in the rise rate for several hundred years, assuming no significant drops in the temperature. The following section puts this model on a mathematical footing.

Rahmstorf’s Mathematical Strategy

1) Assume that the rate of sea level rise rate at any given time is proportional to the deviation form some global equilibrium temperature at that time. He expresses this in the following formula…

where H is the sea level, dH/dt is the sea level rise rate, T is the temperature, To is the the equilibrium temperature, and a is the constant of proportionality.

2) To and a can be derived by simply plotting dH/dt vs T and fitting to a line.

3) Once To and a have been determined, then the sea level for any given time, H(t), can be calculated by integrating equation (I), above, with respect to time…

4) By applying various temperature rise scenarios for the 21st century to equation (II), Rahmstorf predicts the sea level for the hear 2100 (H(2100)).


Problems with this model

1) Sea level rise rate vs. temperature is displayed in a way that erroneously implies that it is well fit to a line, as expressed in equation I, above. More…

2) The assumption that the time required to arrive at the new equilibrium is “on the order or millennia” is not borne out by the data. More…

3)Rahmstorf extrapolates out more than five times the measured temperature domain. More…

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