Moth Eyes

Navigating a demon-haunted world

Just like my dinner parties!

XCKD - Recipes

Click to embiggen and read the title text. From xkcd.

March 29, 2010 at 8:30 pm Comment (1)

Photo #7 – Rainbow over the Bay of Islands

Rainbow over the Bay of Islands

Photographed in the Bay of Islands, off New Zealand’s North Island.

March 18, 2010 at 10:42 pm Comments (0)

Ministry of Stochastic Walks

Every time someone writes a post about random walks, I feel somewhat motivated to write a little program to generate a few (with adjustable parameters, of course). Anyway, Tamino’s recent discussion motivated me enough to write one.

You can see, for example, the difference between a proper random walk:
Random walk
And a walk that’s been bounded on one side:
Bounded stochastic walk

The code is written in .NET (C#) and licensed under the GPL. The binaries are built for the .NET Framework 2.0, so you’ll need that.

Download

And, of course, because it’s the only reason why anyone clicked on this link:

March 14, 2010 at 7:29 pm Comments (0)

Photo #6 – Whangarei Falls

Whangarei Falls
Whangarei Falls, on New Zealand’s North Island.

March 13, 2010 at 4:50 pm Comments (0)

“Mike’s Nature trick”

Cross-posted on Young Australian Skeptics

One of the most hyped emails from the Climategate hack was this one, sent by Phil Jones:

I’ve just completed Mike’s Nature trick of adding in the real temps to each series for the last 20 years (ie from 1981 onwards) amd from 1961 for Keith’s to hide the decline.

“Proxy records” for temperatures provide a method measuring temperature without having to have a convenient thermometer around. Many natural phenomena occur at different rates depending on climate conditions, and these differences can be observed in, for example, rocks, ice cores, corals and trees. Obviously, Earth has had a climate for a few billion years, rather than the few hundred years for which we’ve had thermometers. Accordingly, if you want to understand long-term climate you’ll need proxies. Tree rings are one common proxy record for spring-summer temperature, and, in general, closely match other proxy records and (when available) the instrumental (thermometer) record.

Until the late 20th century. The tree-ring divergence problem The “decline” refers to the “divergence problem”, documented in Briffa et. al. (1998). Basically, during recent years, the tree ring proxy record has been diverging away from the instrumental record, as shown in the figure on right.

There are several variables which can influence tree growth. At extremely high or low latitudes, temperatures are typically a major factor. If each of these other variables is held constant, changes in temperature will be echoed in the tree rings. However, if another of those variables starts changing, the tree ring trends will no longer reflect the temperature trends.

It seems that, late in the previous century, another of these variables started changing. Several hypotheses as to which variable(s) are changing have been made, and are discussed, for example, in D’Arrigo et. al. (2008).

So what was “Mike’s Nature trick”? The “trick” was used in Mann, Bradley & Hughes (1998). Basically, it involves a diagram of the Northern Hemisphere temperature record from 1610-1995, the “NH” portion of figure 7.
Mike's Nature "trick"
From 1610-1980, they use the tree ring record. However, from 1981, the proxy record diverges away from the instrumental data, and so they use the instrumental data for that period. If you look closely (click to embiggen), you’ll notice that the last part of the diagram is drawn differently – the proxy record (1610-1980) is a dashed line, the instrumental record (1981-1995) is a dotted line. This is explained clearly in the diagram’s caption:

‘NH’, reconstructed NH temperature series from 1610–1980, updated with instrumental data from 1981–95.

And that’s it.

So “Mike’s Nature trick” consisted of a legitimate way of displaying the most accurate available data, clearly documented in Mike’s Nature paper.

Other posts

Here are a few other relevant posts discussing this (feel free to let me know about any other good posts):

References

Briffa, Keith; et. al. (1998) Trees tell of past climates: but are they speaking less clearly today? Phil. Trans. R. Soc. Lond. B January 29, 1998 353:65-73; doi:10.1098/rstb.1998.0191 [fulltext]

D’Arrigo, Rosanne; Wilson, Rob; Liepert, Beate; Cherubini, Paolo (2008). On the ‘Divergence Problem’ in Northern Forests: A review of the tree-ring evidence and possible causes. Global and Planetary Change (Elsevier) 60: 289-305. doi:10.1016/j.gloplacha.2007.03.004 [fulltext]

Mann, Michael; Bradley, Raymond; Hughes, Malcolm (1998). Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392, 779–787. doi:10.1038/nature02478 [fulltext]


March 10, 2010 at 1:23 am Comments (3)