Software Nerd

Saturday, February 10, 2007

Global Warming: Why do Scientists disagree?

Most global warming presentations have at least one graph that shows temperature rising recently. "Global mean temperature" is a fuzzy and its significant is unclear. Nevertheless, putting that aside for the moment, this type of global mean appears to have risen ever so slightly over the last 100 years. Since the rise is so tiny (less than a degree) the global warming theorists have to show more, they have to show that these 100 years are anomalous, when one looks back at the history of the earth. So, to support their argument, they need a graph that looks something like this:

If you aren't familiar with the way this argument is made, this short snippet featuring Al Gore is worth looking at. (Don't worry, it's really short.)

The problem that confronts a layman is that some scientists show us graphs like this from their studies while others show us relatively flat graphs.

As a result, some laymen shrug off the anomalous-looking graph as being the product of enviro-scientists with an agenda, while other laymen shrug off the flat-line graphs as being the product of scientists sponsored by the oil-industry. Nothing in either graph tells us why we should believe one over the other. So, most laymen use their personal political evaluations as a way to make judgement (and this is not totally wrong when it comes to a layman), while other laymen simply look for which graphs have the larger number of scientists rooting for it or which one has the agreement of the United Nations. (Aside: To some, the UN approval is additional proof of truth, to others it is additional proof of falsehood; so, we're really back to politics.)

While a layman may not be able to critique the various studies, anyone with a basic education can understand the issues involved. One can understand why the studies would show such varied results. Here is a brief explanation:

Since there is no way to measure temps from years ago, scientists measure things that they think would have been (at least partly) caused by those temperature changes (so-called "proxies"). This is a perfectly valid approach. However, the approach does raise some issues:

  • what exactly is the relationship of the proxy (e.g. distance between two rings) to thing being measured (e.g. temperature). With many proxies, there is scientific debate about the method of going from the proxy meausrement to the inferred measurement. Different scientists want to use different equations
  • how to handle different results from a particular site (e.g. 4 trees where one shows a history different from the others). Taking a mean of all is often not the right approach, because when confronted with the data it may strike you that that tree is different in some other way that was not accounted for before (e.g. it's position and orientation on the site). There are scientific debates on how best to account for these variations and extract the "signal" from this.
  • when trying to combine data across sites, how does one combine it? For instance if there are three ice-core samples in one area and 5 from another about a mile away, should one take a mean of all 8 or should one take the mean for each site and then compute a mean across the two sites? (For some proxies, using different methods has been shown to yield quite different results)
  • the issue above is compounded when one is speaking of data from sites all over the world (as in tree-ring cases). How does one combine the data to really arrive at a global picture, particularly when the data from different hemispheres and from far-away sites in the same hemisphere show vastly different patterns. Is a mean meaningful? Should the statistical technique of "principal component analysis" be used, and if so what should be the different weights assigned to each site?
  • the issues are further compounded when one tries to combine different proxies. For instance the tree-ring proxies tell a different story from the ice-core proxies. So, how to combine them?
The question, then is: what is the nature of the "deniers" objections? It seems clear to me that the popular ones, many of whom are scientists themselves, are not calling for scepticism (as in "we can never know the truth"); rather they are raising questions about methodology.

Those who want to understand the positions of the Global Warming folks, should look at If anyone here wants to understand the nature of the objections, a good site is one called ClimateAudit.Org, that has a mission of auditing scientific studies that claim global-warming is taking place. Nobody who is not a scientist will be able to make a judgement about which formula to use or what weighting will best portray the causal connections. However, it is worthwhile to get a flavor of the types of objections.

There is a single "anti-GW" paper that acts as a good introduction to the objections. It is titled "What is the Hockey Stick debate about?" by Ross McKitrick. I suggest that anyone who is sceptical about Global Warming ought to at least view the brief Al Gore video that was linked to above, and better still view "An Inconvenient Truth". On the other hand, anyone who thinks the "sceptics" are fools, should read McKitrick's essay and also check out the site.


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