Uncertainty Ranges, Error Bars, and CIs

Guest Post by Kip Hansen

This post does not attempt to answer questions – instead, it asks them. I hope to draw on the expertise and training of the readers here, many of whom are climate scientists, both professional and amateur, statisticians, researchers in various scientific and medical fields, engineers and many other highly trained and educated professions.

The NY Times, and thousands of other news outlets, covered both the loud proclamations that 2014 was “the warmest year ever” and the denouncements of those proclamations. Some, like the NY Times Opinion blog, Dot Earth, unashamedly covered both.

Dr. David Whitehouse, via The GWPF, counters in his post at WUWT – UK Met Office says 2014 was NOT the hottest year ever due to ‘uncertainty ranges’ of the data — with the information from the UK Met Office:

“The HadCRUT4 dataset (compiled by the Met Office and the University of East Anglia’s Climatic Research Unit) shows last year was 0.56C (±0.1C*) above the long-term (1961-1990) average. Nominally this ranks 2014 as the joint warmest year in the record, tied with 2010, but the uncertainty ranges mean it’s not possible to definitively say which of several recent years was the warmest.” And at the bottom of the page: “*0.1° C is the 95% uncertainty range.”

The David Whitehouse essay included this image – HADCRUT4 Annual Averages with bars representing the +/-0.1°C uncertainty range:

The journal Nature has long had a policy of insisting that papers containing figures with error bars describe what the error bars represent, I thought it would be good in this case to see exactly what the Met Office means by “uncertainty range”.

In its FAQ, the Met Office says:

“It is not possible to calculate the global average temperature anomaly with perfect accuracy because the underlying data contain measurement errors and because the measurements do not cover the whole globe. However, it is possible to quantify the accuracy with which we can measure the global temperature and that forms an important part of the creation of the HadCRUT4 data set. The accuracy with which we can measure the global average temperature of 2010 is around one tenth of a degree Celsius. The difference between the median estimates for 1998 and 2010 is around one hundredth of a degree, which is much less than the accuracy with which either value can be calculated. This means that we can’t know for certain – based on this information alone – which was warmer. However, the difference between 2010 and 1989 is around four tenths of a degree, so we can say with a good deal of confidence that 2010 was warmer than 1989, or indeed any year prior to 1996.” (emphasis mine)

I applaud the Met Office for its openness and frankness in this simple statement.

Now, to the question, which derives from this illustration:

(Right-click on the image and select “View Image” if you need to see more clearly.)

This graph is created from data directly from the UK Met Office, “untouched by human hands” (no numbers were hand-copied, re-typed, rounded-off, krigged, or otherwise modified). I have greyed-out the CRUTEM4 land-only values, leaving them barely visible for reference. Links to the publically available datasets are given on the graph. I have added some text and two graphic elements:

a. In light blue, Uncertain Range bars for the 2014 value, extending back over the whole time period.

b. A ribbon of light peachy yellow, the width of the Uncertainty Range for this metric, overlaid in such a way as to cover the maximum number of values on the graph.

Here is the question:

What does this illustration mean scientifically?

More precisely — If the numbers were in your specialty – engineering, medicine, geology, chemistry, statistics, mathematics, physics – and were results of a series of measurements over time, what would it mean to you that:

a. Eleven of the 18 mean values lie within the Uncertainty Range bars of the most current mean value, 2014?

b. All but three values (1996, 1999, 2000) can be overlaid by a ribbon the width of the Uncertainty Range for the metric being measured?

Let’s have answers and observations from as many different fields of endeavor as possible.

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Authors Comment Policy: I have no vested opinion on this matter – and no particular expertise myself. (Oh, I do have an opinion, but it is not very well informed.) I’d like to hear yours, particularly those with research experience in other fields.

This is not a discussion of “Was 2014 the warmest year?” or any of its derivatives. Simple repetitions of the various Articles of Faith from either of the two opposing Churches of Global Warming (for and against) will not add much to this discussion and are best left for elsewhere.

As Judith Curry would say: This is a technical thread — it is meant to be a discussion about scientific methods of recognizing what uncertainty ranges, error bars, and CIs can and do tell us about the results of research. Please try to restrict your comments to this issue, thank you.

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