Peering into the Past

After examining the recent changes in Arctic sea ice last week I wanted to consider whether such trends are indeed a mark of human-induced climate change or, as climate sceptics argue, part of a cycle of natural variability.

Understanding past climatic trends is fundamental for putting recent warming into context. The problem is direct instrumental records of temperature are often brief and geographically sparse, particularly in high latitude regions. The few records that do exist suggest the Arctic has warmed by ~0.6°C since the early 20^th century, peaking in 1945 (Comiso and Hall, 2014). However, this is not enough to derive the natural cyclic trends and forcing mechanisms characterising Arctic climate. Proxies in palaeo records, including tree rings, ice cores and lake sediments, are valuable and widely applied tools for reconstructing past environmental changes. To account for limitations in individual records, multi-proxy analysis is increasingly being incorporated into palaeo studies to expand spatial and temporal coverage and improve the reliability of reconstructions.

One such study was conducted by McKay and Kaufman (2014) (revised version of original study by Kaufman et al., 2009) who reconstructed Arctic summer temperatures over the last 2000 years at a decadal resolution. The study incorporated proxies from glacier ice, lake sediment and tree rings, extending the climatic record beyond the previous 400-year-long record (Overpeck et al., 1997). The results indicated recent warming that is anomalous when compared to past variability. The beginning of the 2000-year record shows decreasing Arctic summer temperatures at a rate of ~0.2°C per 1000 years until levelling out during the Little Ice Age (16^th to mid-19^th century). This cooling trend corresponds with the shift in the timing of the perihelion (when the Earth is closest to the Sun) from September to January. Despite the continued low summer Arctic insolation induced by precessional forcing, summer temperatures rose to ~1.4°C higher by 2000 than would otherwise be expected.

Comparison between temperature reconstruction of Kaufman et al. (2009) and McKay and Kaufman (2014)

In response to critics, this revision by Mckay and Kaufman is far more spatially extensive than the previous study, although bias remains toward Greenland. Moreover, in validating the temperature reconstruction, uncertainty exists (particularly in the mid-20^th century) between the calculated proxy values and measured instrumental data because of differences in their locations.

Location and archive of proxy temperature records (McKay and Kaufman, 2014)

A more recent study conducted by Florian et al. (2015) reconstructed climate based on algal pigments, stable isotopes (δ¹⁵N and δ¹³C) and diatoms. The use of sedimentary pigments meant that taxonomic groups morphologically absent from sediment cores could be reconstructed and differentiated between. Climatic change can be inferred from these biological proxies as alterations in the extent and persistence of ice cover impact nitrogen cycling and thus ecology. Reconstruction of the proxies revealed that modern conditions are comparable to those during the Holocene Thermal Maximum (HTM). However, it was also shown that confounding factors, such as atmospheric nitrogen deposition, have further altered lake ecology beyond what was recorded during the HTM.

It is evident that arctic environments have experienced significant natural variations in climate throughout the Holocene in response to changes in orbital parameters and climate feedback mechanisms. Although the recent warming trend may initially appear to be part of this natural cyclicity, analysis of past trends from multiple sources suggest that the present-day Arctic climate is responding in a fundamentally different way, largely in response to human-induced climate change.