"As I mused, the fire burned"

Reflection on life as a person of faith.

Seeking the TRUTHS about climate change

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A very interesting presentation from the UK’s NPL (National Physical Laboratory) on the challenges of measuring climate on a global scale. The first 30 minutes are very worthwhile as the presenter talks about the level of uncertainty in present satellite measuring systems. His conclusion – the level of uncertainty in present satellite measurement is such that we can’t project climate models beyond about 30 years out. His presentation is to support a call for traceable satellite observation systems.

As a former instrumentation engineer, I really appreciated his overview of the measurement challenges. It also highlites the present uncertainty about climate models, something that is rarely included in some of the emotional discussions on climate change. What this means, is that before we spend billions on halting climate change, we need to determine if what we are measuring is actually reflecting reality. As a present parallel, we hear police argue that the long gun registry is a “useful tool” for law enforcement…but how does that “useful tool” measure up cost-wise about other possible tools. Before we spend money, we need to understand if the data set provides us any support for that expenditure.

Most concerning is his description of the present strategy, which is to rely on relative measurements only, versus the desired measurement of an absolute value versus a traceble standard.

The task facing the developers of measuring systems is complex. Here’s an example.

You want to measure the wear on the second step of a stone staircase over the course of several decades, to validate a wear model you have produced. How do you perform this measurement, over 30 years, in a manner which allows you to absolutely measure the change in height of the surface of that step?

It may sound like a trivial problem, but it is far from it. Some thoughts…how do you demonstrate that the ruler you are using to measure the step’s surface is unchanging over those 30 years? Each time you use the ruler, you risk changing it, or causing wear to it, which changes its measuring characteristics. What do you measure the height of the step against? What in the environment is unchanging, or is changing in a way that is known well enough to compensate for that change? Do you use the step below (which is also wearing), a survey reference point nearby, or a global positioning reference? How is each of those changing with time, and how do you track those changes? What is the uncertainty in each reference (for example, differential GPS may be able to resolve in the millimeter range, but you are seeking to measure in the tenths of a millimetre) 30 years is a long time, what if your measuring technology changes over that period, how do you ensure absolute continuity over the overlapping periods of measurement?

A general rule of measurement requires a truth source at least one order of magnitude more precise than the standard you are measuring to. If you are trying to measure to a tenth of a millimetre, your truth source needs to be good to the hundredth of a millimetre.

It just leaves me wondering about the factual basis for much of the climate modeling which is projecting change out to 2100…

An absolute measurement is one that represents reality as close as is possible within the limits of the measurement system. A relative measurement represents only the difference between what is being measured and a reference source. An absolute measurement is portable – if it is traceable to an SI standard, you can apply that measurement in comparison with any other SI traceable measurement with confidence. With relative measurements, it is very difficult to make any comparisons as the measurement is not based on an objective, external, standard.


Written by sameo416

October 26, 2011 at 8:00 pm

Posted in Uncategorized

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