Wednesday, February 15, 2006

Where to go for real information on climate change.

Here for example, is a good post that shows that consensus temperature measurements do indeed show an increase.

This is what Osborn and Briffa have done in their article "The Spatial Extent of 20th Century Warmth in the Context of the Past 1200 Years", which appears in the Feb 10 issue of the journal Science. The article uses a rigorous statistical methodology to re-examine the question of whether late 20th century warmth is anomalous in the context of the past 1200 years. This is done in a manner that does not require the explicit calibration of the proxy records. In essence, the authors have revisited a question posed earlier in a paper by Willie Soon and Sallie Baliunas (2003: see our previous discussion here), investigating whether or not evidence from past proxy records of temperature support the existence of past intervals of warmth with the widespread global scale of 20th century warming. The Soon and Baliunas (2003) paper was heavily criticized in the scientific literature (e.g. Mann et al, 2003) for failing to distinguish between proxy evidence of temperature and drought or precipitation, and for not accounting for whether temperature anomalies in different regions were contemporaneous or not.

Osborn and Briffa, by contrast, have carefully taken these issues into account. They make use only of those proxy records which demonstrate a statistically significant relationship with modern instrumental temperature records, and which were dated accurately enough that records from different locations could be compared against each other in a chronologically consistent manner. They then standardize the records and look for evidence of simultaneous relative departures that point in the same direction (i.e. "warm" or "cold") using appropriate pre-set thresholds for defining a significant event (they try both one and two standard deviations). There is an important distinction between this careful statistical approach, and the selective cherry picking that is often used by contrarian commentators to misrepresent the available evidence. For example, it is possible to find evidence of significant warmth or significant coldness over literally any century-long interval in at least one of the 14 records used by Osborn and Briffa (see Figure 1 in the article). However, this alone tells us very little. What is of interest, instead, is whether centuries-long intervals can be found over which warm events or cold events tend to cluster.

Osborn and Briffa use Monte Carlo simulations to test the null hypothesis that a given number of simultaneous "warm" or "cold" events should simultaneously emerge among 14 such independent records from chance alone. Where they are able to reject this null hypothesis, they conclude that there is evidence of large-scale warmth or coldness, and they quantify its spatial extent. They also establish that their results are robust with respect to the elimination of any single record, and that the main conclusions are independent of the particular base period (e.g. whether they use the full interval AD 800-1995, or only the modern interval of 1856-1995 which overlaps with the instrumental record). While some of the caveats of some past studies are applicable in this study too (for example, the authors only use 14 proxy sites), the authors do attempt to address them. For example, they show that the modern instrumental record averaged only over their 14 sites captures the full Northern Hemisphere mean temperature variations remarkably well over the available (approximately 150 years) interval. The authors also examine the difference between the number of significant warm and cold events over time, and this tells a similar story.

Osborn and Briffa '06 Figure 2
Figure 2 (from Osborn and Briffa, '06). Fraction of the records available in each year that have normalized values > 0 (red line), > 1 (light red shading), > 2 (dark red shading), <> [source: AAAS] (click to enlarge)

In each case, the authors find that the most widespread warmth by far is evident during the mid and late 20th century. The conclusion is not especially surprising, as nearly all previous peer-reviewed studies over the past decade find that late 20th century warmth is anomalous in a millennial or longer context. Indeed, the curve they produce (Figure 2)--with a modest negative trend over most of the past millennium ending in a dramatic positive 20th century spike--might be likened in shape to a certain implement used in a popular North American winter sport. But we digress...

It is not so much the conclusion, but the approach that the authors use to reach their conclusion, that is most important about this latest study. The authors take advantage of a very straightforward analysis of climate proxy data, avoiding the highly technical and arcane issues of statistical calibration and methodology that are so frequently seized upon by those who dispute that recent large-scale warmth is anomalous in a long-term context. This paper adds to the mounting weight of evidence that such warmth is indeed anomalous in at least a millennial context. We doubt that this, or for that matter, any study will silence the increasingly small but persistently vocal minority of contrarians who continue to challenge this conclusion. But to them, we offer the reminder that paleoclimate evidence comprises only one of many independent lines of evidence indicating a primary role of human activity in modern climate change. If the only line of evidence that remains in dispute pertains to estimated millennial temperature histories, then the case for denialism appears extremely weak indeed.


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