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 20th 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'.
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| 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 20th 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.
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).
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).
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| 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.
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).
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