Proxy climatic and environmental changes of the past 1000 years
“Proxy climatic and environmental changes of the past 1000 years” by Willie Soon and Sallie Baliunas is one of the most notable climate change denial articles ever. The paper was published in 2003 in Climate Research, a journal known for publishing respectable articles on its titular subject. However, Soon and Baliunas, 2003 was not one such respectable paper.
The article intended to “compare the 20th century objectively with more extended past changes” and climate anomalies using a variety of proxies (90). The authors define anomalies as a “period of more than 50 yr of sustained warmth [or cold], wetness or dryness” (90). Specifically, the authors analyze two notable periods of climatic change, the Medieval Warm Period, which occurred between about 900-1300 BCE, and the Little Ice Age, which occurred between 1550-1700 BCE, and compare changes in those periods to 20th century changes. Ultimately, they ask whether climate anomalies occurred during the LIA and MWP, and whether those changes were more extreme than climate anomalies experienced during the 20th century.
Notably, this article is a literature review, not a study, so no new data is being collected. The article also relies on local trends across several regions (Western Europe, North Atlantic and other oceans, Asia and Eastern Europe, North America, New Zealand, South Africa, South America, and Antarctica), rather than regional ones. To perform their analysis, the authors synthesize a large number of proxies, including borehole, cultural, documentary, glacier, geomorphology, instrumental, core, plant cellulose, coral, fossil, ice, dust/chemical count, aquatic sediment, melt layer, pollen, isotope, tree ring, and tree stump data. Much of the paper goes into describing specific local changes in these regions based on these proxies. The authors conclude that climatic anomalies did indeed occur locally during the LIA and MWP, and that these anomalies were often more extreme than those observed during the first half of the 20th century. This conclusion informs the paper’s most prominent finding: that “the 20th century does not contain the warmest anomaly of the past millennium…Past researchers implied that unusual 20th century warming means a global human impact. However, the proxies show that the 20th century is not unusually warm or extreme” (104).
Upon its publication, the paper was promptly criticized for three main flaws.
- Conflating disparate regional trends with simultaneous global trends: Soon and Baliunas, 2003 relied predominantly on local or regional observations, allowing them to falsely portray local anomalies as global trends. They also ignored whether different anomalies occurred at the same time when analyzing historical data, making it difficult to prove any trend during the LIA or MWP.
- Using proxies not proven to be equivalent to temperature: Many of the proxies used in the analysis were portrayed as being a direct stand-in for temperature, when in reality, they were simply linked to temperature changes. For instance, while precipitation change is linked to temperature change, it is impacted by many other factors, especially at local scales, which were not accounted for in the analysis before being directly portrayed as a proxy for temperature.
Using proxies which fail to distinguish between decadal changes: Several proxies used in the analysis, such as cultural, documentary, coral, and sediments, do not show fine-scale data, meaning decadal trends would be obscured. Meanwhile, they compared these proxies to average warming across the whole of the 20th century, rather than just the especially anomalous late-20th century. This is relevant because the changes being observed during the late-20th century are on decadal timescales, so when comparing to historical anomalies, a fair comparison would also be on decadal timescales.
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