Winter is Coming

Before delving into the major impacts of Arctic climate change I just wanted to share some new research that looks at an indirect effect that's a bit closer to home and a conversation favourite, the British weather. New research conducted by Overland et al. (2016) has linked recent cold winters in the UK and US to Arctic warming amplifying the effects of the jet stream's position. This has added to previous studies linking severe mid-latitude cold snaps to when the jet stream is wavy and is vital to improving our understanding of complex atmospheric interactions for predicting long-term winter weather.

Could we be expecting colder winters amid Arctic warming?

Global Teleconnections

As we have seen, Arctic climate has fluctuated over the last few millennia in response to orbital forcing and climatic feedback with recent warming trends significantly diverging from natural cyclicity. Focusing in on temporal scale, it is also apparent that the past century does not follow a steady, uniform temperature rise but instead variations characterise the increase, reflecting the Atlantic Multidecadal Oscillation (AMO). Past surface air temperatures reveal two distinct warming events in northern latitudes: one during the 1920s-40s (the early 20^th century warming event or ETCW) and the ongoing temperature increase that began in the 1980s. As mentioned earlier in the introductory post, the enhanced rate of warming in the Arctic compared to the global average is a result of the phenomena 'Arctic Amplification'.

Annual mean (Oct-Sept) surface air temperature (SAT) at 60-90°N compared to the global average from 1900-2015 (Overland et al., 2015)

Internal variability

The mechanisms forcing ETCW have been widely debated and the relative role of external forcings (greenhouse gases, volcanic and solar activity), which are often considered the main drivers of climatic fluctuations, is still under debate (Zhang et al., 2013). In a study incorporating observations and global climate model simulations, decreased volcanic activity and enhanced solar radiation during the early 20^th century was suggested to be the main cause of ETCW (Suo et al., 2013). However, considerable uncertainty surrounds solar activity reconstructions and volcanic aerosol estimations, which greatly influences model results.

Beitsch et al. (2014) analysed an unperturbed climate simulation covering the last 3000 years and found that internal variability within the Northern Hemisphere is sufficient to produce ETCW-like events. ETCW may, therefore, have been initiated through warming in the North Atlantic and associated feedback through melting ice, enhancing albedo in the Barents Sea and warming the atmosphere above. The fact that ETCW was concentrated at high latitudes in the Atlantic Arctic, in comparison to the more Arctic-wide warming of recent decades, also favours the dominance of internal variability.

Recent warming

In contrast, despite being so distant in both location and climate, changes in sea surface temperatures in the tropics may explain the recent Arctic warming experienced since the 1980s. This is all down to global teleconnections and interactions between the atmosphere, oceans, and cryosphere.

Ding et al. (2014) investigated warming during the past three decades over northeastern Canada and Greenland. Previously, this had been attributed anthropogenic climate change, however, the spatial inconsistency of warming suggests that natural climate variability may play a larger role. The North Atlantic Oscillation (NAO) is a mode of climate variability in which the pressure difference between the Azores high-pressure system and the Icelandic low-pressure shifts. In the positive phase, the pressure zones are strengthened enhancing the difference between them, whereas during the negative phase both systems are weakened. It is thought that alterations in tropical sea surface temperatures can influence the NAO by altering convection in the low latitude troposphere, stimulating atmospheric Rossby waves.

The authors argue that Arctic surface and tropospheric warming is more likely remotely forced and stimulated in the tropics, rather than initiated locally. Declining sea surface temperatures in the tropical Pacific and associated Rossby-wave activity induced a negative NAO phase and thus warming in Greenland and northeastern Canada. With increases in greenhouse gas emissions in the future, external forcing could become increasingly dominant as a determinant of Arctic climate (Bader, 2014).

Annual mean surface and near-surface temperature per decade from 1979-2012 (Bader, 2014)

The role of pollution

Recently, it has been suggested that Arctic climate is also influenced by global pollution trends. Model simulations conducted by Acosta Navarro et al. (2016) illustrate recent warming connected to declines in European sulfur emissions. Following scientific observations of acid rain and resultant pH declines in freshwater environments that raised public awareness and led to the implementation of several clean air policies in Europe and America, sulphur emissions have declined significantly. In addition to their acidifying effects, sulphur dioxide molecules form small sulphate aerosol particles in the atmosphere that effectively scatter light and cool the planet by reflecting some solar radiation back to space. Although controversial, sulphate aerosol particles may also contribute to cooling through cloud formation. 

As air and ocean currents transported to the southern Arctic latitudes pass America and Europe, the warming of the upper ocean and atmosphere at these lower latitudes has strengthened heat transport to the Arctic. The model shows that it is possible that as much as half of the recent warming trend to be a response to reduced aerosol cooling through sulphur reductions. However, this study was based on a single model, which was rerun nine times to produce a statistically significant signal, so the relative contribution of sulphate aerosol particles may have been a lot more or less than 0.5°C.

Following such illustrations of the importance of pollution in contributing to climatic change, the role of black carbon aerosols is increasingly being investigated. Black carbon aerosols absorb solar radiation and reduce albedo once deposited on snow or ice, contributing to net warming. Therefore, by reducing black carbon emissions, a reduction of 0.2 +- 0.17K could be realised by 2050 (Sand et al., 2016).

However, the contribution of sulphate and black carbon aerosols to Arctic climate will only be temporary as they continue to decline into the future, with their net contribution becoming more negligible and undetectable. Ultimately natural variability, internal and external forcings will drive future climatic change, as they have done in the past (Mauritsen, 2016).

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.

Arctic Algae

In light of the next few posts, I wanted to share some new research investigating the response of the diatom, Discostella stelligera, to climate change. Diatoms are single-celled algae, with silica cell walls, frequently used as proxies of environmental change due to their sensitivity to geochemical variables and high preservation rate. Palaeolimnological research has often attributed the changing abundance of D. stelligera over the last century in Northern Hemisphere lakes to climate change, however the exact mechanisms were poorly understood.

Saros et al. (2016) investigated the abundance of D. stelligera in two lakes in southwest Greenland in the summers of 2013-14. During the second summer, the thermal structure of one of the lakes was mechanically manipulated. It was found that despite a high natural abundance of D. stelligera in both lakes, the population dramatically declined in the lake subject to thermal manipulation. This research has been fundamental in supporting the use of D. stelligera as a proxy for climate-driven changes in lakes.

Response of the Greenland lakes in the summers of 2013 and 2014 (only the Experimental Lake was manipulated in the summer of 2014) (a and b) thickness of epilimnion (c and d) temperature of epilimnion averaged (e and f) mixed layer average light intensity (g and h) D. stelligera cell densities (Saros et al., 2016)

On Thin Ice

Through early explorations of arctic environments, expeditions to reach the North Pole, and stories narrated by novelists and filmmakers, the polar north has become engrained as much in popular culture as scientific study. Over recent years, images of starving polar bears marooned on increasingly fragmented sea ice and cascading glacial collapses along retreating ice sheets have captured the demise of the Arctic as an emblem of human-induced climate change. What's more, satellite images of declining minimum sea ice extent have set in motion geopolitical feuds over the governance of Arctic regions with the aim of exploiting natural resources and developing new shipping routes.

Stranded polar bear

Fragmenting ice sheets

Alongside such images, the monthly updates on Arctic sea ice extent provided by the National Snow and Ice Data Centre are frequently discussed by scientists and the media to put forward both sobering statistics, as well as optimistic hopes drawing from exaggerated predictions. In this blog, I aim to discover what really is happening in the Arctic with regards to climate change, why this is important and what it means for the future of the region and globally.

Why the Arctic?

The Arctic is the northernmost region of the Earth, located north of the Arctic Circle (66° 33’N) and far removed from the polluting industrial centres inhabited by much of the world's population. So why is the climate change recorded in arctic environments so important for the rest of the planet?

The amount of carbon in the atmosphere continues to rise through the burning of fossil fuels, warming our planet and pushing Earth systems beyond critical tipping points. As a regulator of the Earth's climate, the Arctic 'acts as an early warning system for the entire planet' (Dr. Chip Miller, NASA).

'Arctic amplification' means that the regional climate of the Arctic is altering at a dramatic rate with the surface air temperature rising twice as fast as the global average in the last few decades (Najalfi et al., 2015). Previously, rapid growth of sea ice during autumn produced a negative feedback and initiated cooling. However, the loss of sea ice (particularly old ice) has been staggering. A combination of the surface albedo feedback and ice insulation feedback (due to a warmer ocean in areas that were previously under ice cover) has accelerated Arctic warming (IPCC, 2013). In addition, melting permafrost, changes in sea level, alterations in atmospheric and ocean circulation, and modifications to plant ecotonal boundaries are also all positive feedbacks, amplifying warming in the Arctic (Miller et al., 2010).

What's been happening in the Arctic this year?

This year the seasonal minimum sea ice extent was reached on September 2010 (4.14 million km²), tying with 2007 as the second lowest extent in the 37-year satellite record. All in all, this was 2.56 million km² lower than the 1979-2000 average (that’s equivalent to over 10 times the area of the UK).

Daily Arctic sea ice extent for 2012-2016 against the 1981-2010 average (NSIDC, 2016)

Monthly September ice extent from 1979-2016 (NSIDC, 2016)

What’s even more worrying is the significant decline in Arctic sea ice thickness, which relates to the age of the ice (Tschudi et al., 2016). Thicker 4+ year-old ice made up only 3.1% of the total extent this year (compared to 33% in the mid-1980s!). Not only does this reveal how extensive ice melting has become, it also denotes worrying trends in the future. Young, thin ice is far more prone to melting at the end of the summer and a growing proportion of it brings us closer and closer to experiencing a ‘Blue Ocean Event’ where the Arctic is devoid of ice.

It is clear that the Arctic has undergone considerable change in the past few decades. As the blog progresses, I hope to look beyond changes in sea ice extent and understand more about this altering environment by studying the following questions:

What is the evidence for climate change in the Arctic?

In what ways is increased atmospheric CO2 impacting arctic environments?
To what extent do confounding factors play a role?

What does the future look like for the Arctic and the rest of the world?

My next few posts will explore in further detail the evidence for climate change in the Arctic before investigating the impacts at varying spatial and temporal scales. In the meantime, the video below is a nice illustration of changing ice age and cover in the Arctic through a week to week time lapse since 1990: