This is Section 2.1 of the Arctic Climate Impact Assessment Lead Author: Gordon McBean. Contributing Authors: Genrikh Alekseev, Deliang Chen, Eirik Førland, John Fyfe, Pavel Y. Groisman, Roger King, Humfrey Melling, Russell Vose, Paul H.Whitfield
The Arctic is the northern polar component of the global climate system. The global climate system has been thoroughly examined in the recent reports of the Intergovernmental Panel on Climate Change, which include discussion of the impacts of climate change in the Arctic. Arctic climate is characterized by a low amount or absence of sunlight in winter and long periods of daylight during summer. Although these solar inputs are a dominant influence, arctic climate exhibits significant spatial and temporal variability. As a result, the Arctic is a collection of regional climates with different ecological and physical climatic characteristics.
The cryosphere is a prominent feature of the Arctic, present as snow, ice sheets, glaciers, sea ice, and permafrost.The physical properties of snow and ice include high reflectivity, low thermal conductivity, and the high latent heat required to convert ice to liquid water; these contribute significantly to the regional character of arctic climate.
The arctic climate interacts with the climates of more southern latitudes through the atmosphere, oceans, and rivers. Because of these regionally diverse features, an exact geographic definition of the Arctic is not appropriate and this chapter focuses on the northernmost areas (usually north of 60º N), while acknowledging interactions with more southerly areas.
The observational database for the Arctic is quite limited, with few long-term stations and a paucity of observations in general.The combination of a sparse observational data set and high variability makes it difficult to distinguish with confidence between the signals of climate variability and change.
With respect to the polar regions, the Intergovernmental Panel on Climate Change stated:
Changes in climate that have already taken place are manifested in the decrease in extent and thickness of Arctic sea ice, permafrost thawing, coastal erosion, changes in ice sheets and ice shelves, and altered distribution and abundance of species in polar regions (high confidence).
Climate change in polar regions is expected to be among the largest and most rapid of any region on the Earth, and will cause major physical, ecological, sociological, and economic impacts, especially in the Arctic, Antarctic Peninsula, and Southern Ocean (high confidence).
Polar regions contain important drivers of climate change. Once triggered, they may continue for centuries, long after greenhouse gas concentrations are stabilized, and cause irreversible impacts on ice sheets, global ocean circulation, and sea-level rise (medium confidence).
The arctic climate is a complex system and has multiple interactions with the global climate system.The sensitivities of snow and ice regimes to small temperature increases and of cold oceans to small changes in salinity, both of which can lead to subsequent amplification of the signal, are processes that could contribute to unusually large and rapid climate change in the Arctic.The Arctic Oscillation (AO) is an important feature of the arctic atmosphere and its connections with global climate (section 2.2).The phase of the AO was at its most negative in the 1960s, but from about 1970 to the early 1990s there was a general trend toward a more positive phase and it has remained mostly positive since. It is possible that this is the result of increased radiative forcing due to anthropogenic greenhouse gas (GHG) emissions, but it is also possible that it is a result of variations in sea surface temperatures.The Arctic Ocean (section 2.3) forms the core of the Arctic. Sea ice is the defining characteristic of the marine Arctic and is an important means by which the Arctic exerts leverage on global climate.This leverage occurs through mediation of the exchange of radiation, sensible heat, and momentum between the atmosphere and the ocean. Terrestrial hydrology (section 2.4) and arctic climate are intricately linked. In terrestrial areas, temperature increases over the past 80 years have increased the frequency of mild winter days, causing changes in the timing of river-ice breakups; in the frequency and severity of extreme ice jams, floods, and low flows; and in aquatic ecosystems.The increased frequency of mild winter days has also affected transportation and hydroelectric generation.
There are both positive and negative feedback processes in the Arctic, occurring over a range of timescales. Positive feedbacks include snow and ice albedo feedback; reduction in the duration of time that sea ice insulates the atmosphere from the Arctic Ocean; and permafrost– methane hydrate feedbacks. Negative feedbacks can result from increased freshwater input from arctic watersheds, which makes the upper ocean more stably stratified and hence reduces temperature increases near the air–sea interface; reductions in the intensity of the thermohaline circulation that brings heat to the Arctic; and a possible vegetation–carbon dioxide (CO2) feedback that has the potential to promote vegetation growth, resulting in a reduced albedo due to more vegetation covering the tundra. Polar amplification (greater temperature increases in the Arctic compared to the earth as a whole) is a result of the collective effect of these feedbacks and other processes. The Arctic is connected to the global climate, being influenced by it and vice versa (section 2.5).
Based on the analysis of the climate of the 20th century (section 2.6), it is very probable2 that arctic temperatures have increased over the past century, although the increase has not been spatially or temporally uniform. The average surface temperature in the Arctic increased by approximately 0.09 ºC/decade during the past century, which is 50% greater than the 0.06 ºC/decade increase observed over the entire Northern Hemisphere (IPCC, 2001b). Probably as a result of natural variations, the Arctic may have been as warm in the 1930s as in the 1990s, although the spatial pattern of the warming was quite different and may have been primarily an artifact of the station distribution.
Evidence of polar amplification depends on the timescale of examination. Over the past 100 years, it is possible that there has been polar amplification, however, over the past 50 years it is probable that polar amplification has occurred.
It is very probable that atmospheric pressure over the Arctic Basin has been dropping, and it is probable that there has been an increase in total precipitation over the past century at the rate of about 1% per decade. Trends in precipitation are hard to assess because precipitation is difficult to measure with precision in the cold arctic environment. It is very probable that snowcover extent around the periphery of the Arctic has decreased. It is also very probable that there have been decreases in average arctic sea-ice extent over at least the past 40 years and a decrease in multi-year sea-ice extent in the central Arctic.
Reconstruction of arctic climate over thousands to millions of years demonstrates that the arctic climate has varied substantially. There appears to be no natural impediment to anthropogenic climate change being very significant and greater in the Arctic than the change on the global scale. Section 2.7.2 examines the variability of arctic climate during the Quaternary Period (the past 1.6 million years) with a focus on the past 20,000 years. Arctic temperature variability during the Quaternary Period has been greater than the global average. Especially during past cold periods, there have been times when the variability and transitions in temperature have been quite rapid – from a few to several degrees change over a century. There have also been decadal-scale variations due to changes in the thermohaline circulation, with marked regional variations.
Chapter 2: Arctic Climate - Past and Present
2.2 Arctic atmosphere
2.3 Marine Arctic
2.4 Terrestrial Water Balance in the Arctic
2.5 Influence of the Arctic on global climate
2.6 Arctic climate variability in the twentieth century
2.7 Arctic climate variability prior to 100 years BP
2.8 Summary and key findings of ACIA on Arctic Climate - Past and Present
^IPCC, 2001a. Climate Change 2001: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. McCarthy, J.J., O.F. Canziani, N.A. Leary, D.J. Dokken and K.S.White (eds.). Cambridge University Press, 1032 pp.
IPCC, 2001b. Climate Change 2001: Synthesis Report. A Contribution of Working Groups I, II, and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change.Watson, R.T., and the CoreWriting Team (eds.). Cambridge University Press, 398 pp.IPCC, 2001c. Climate Change 2001:The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Houghton, J.T.,Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell and C.A. Johnson (eds.) Cambridge University Press, 881 pp.
- ^IPCC, 2001a. Op. cit.
- ^IPCC, 2001a. Op. cit