Past Climate Variability and Change in the Arctic and at High Latitudes

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36 The U.S. Climate Science Program Chapter 3 Changes in the seasonal and areal distribution of snow and ice will exert strong influences on the planetary energy balance. 3.2 FEEDBACKS INFLUENCING ARCTIC TEMPERATURE AND PRECIPITATION The most commonly used measure of the climate is the mean surface air temperature (Figure 3.3), which is influenced by climate forcings and climate feedbacks. As discussed with references in Chapter 2, Paleoclimate Concepts, section 2.2, important forcings during the past several millennia have been changes in the distribution of solar radiation that resulted from features of Earth’s orbit, volcanism, changes in atmospheric greenhouse-gas concentrations and, to a lesser extent, small variations in solar irradiance. On longer time scales (tens of millions of years), the long-term increase in the solar constant (a 30% increase in the past 4,600 m.y.) was important, and the redistribution of continental landmasses caused by plate motions also affected the planetary energy balance. How much the temperature changes in response to a forcing of a given magnitude (or in response to the net magnitude of a set of forcings in combination) depends on the sum of all of the feedbacks. Feedbacks can act in days or less or endure for millions of years. The focus here is Mean Surface Air Temperature AIRS data, January 2003 DEGREES KELVIN 263 273 283 293 303 on faster feedbacks. For example, a warming may have many causes (such as brighter Sun, higher concentration of greenhouse gases in the atmosphere, less blocking of the Sun by volcanic emissions). Whatever the cause, warmer air moving over the ocean tends to entrain more water vapor, which itself is a greenhouse gas, so more water vapor in the atmosphere leads to a further rise in global mean surface temperature (Pierrehumbert et al., 2007). The discussion below focuses on those feedbacks especially linked to the Arctic. Several processes linked to iceage cycling are included here, because of the dominant role of northern land in supporting ice-sheet growth, although ice-age processes (like some of the other processes discussed below) clearly extend well beyond the Arctic. 3.2.1 Ice-Albedo Feedback Ice and snow present highly reflective surfaces. The albedo of a surface is defined as the reflectivity of that surface to the wavelengths of solar radiation. Fresh ice and snow have the highest albedo of any widespread surfaces on the planet (Figure 3.4), so it is apparent that changes in the seasonal and areal distribution of snow and ice will exert strong influences on the planetary Figure 3.3. Global mean observed near-surface air temperatures for the month of January 2003, derived from the Atmospheric Infrared Sounder (AIRS) data. Contrast between equatorial and Arctic temperatures is greatest during the Northern Hemisphere winter. The transfer of heat from the tropics to the polar regions is a primary feature of Earth’s climate system. (Color scale is in Kelvin degrees such that 0°C=273.15 Kelvin.) [Source: http://daac.gsfc.nasa.gov/data/datapool/AIRS/index.html.]
–112.50 –90 –67.50 SURFACE BROADBAND ALBEDO, JUNE. –135 -45 –180 180 –157.50 157.50 energy balance (Peixoto and Oort, 1992). Open ocean, on the other hand, has a low albedo; it absorbs almost all solar energy when the Sun angle is high. Changes in albedo are most important in the Arctic summer, when solar radiation is at a maximum, whereas changes in the winter albedo have little influence on the energy balance because little solar radiation reaches the surface then. In general, warming reduces ice and snow whereas cooling allows them to extend, so the changes in ice and snow act as positive feedbacks to amplify climate changes (e.g., Lemke et al., 2007). 3.2.2 Ice-Insulation Feedback In addition to its effects on albedo, sea ice also causes a positive insulation feedback, primarily in the wintertime. Ice effectively blocks heat transfer between relatively warm ocean (at or above the freezing point of seawater) and cold atmosphere (which, in the Arctic winter, averages –40°C (Chapman and Walsh, 2007). If sea ice is thinned by warming, then the ocean heats the overlying atmosphere in winter months, amplifying that warming. 90 85 80 75 70 –22.50 22.50 65 60 Past Climate Variability and Change in the Arctic and at High Latitudes 135 45 112.50 67.50 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 ALBEDO I. Open ocean α=0.06 II. Bare ice α=0.5 III. Ice with snow Figure 3.4. Albedo values in the Arctic. A) Advanced Very High Resolution Radiometer (AVHRR)derived Arctic albedo values in June, 1982–2004 multi-year average, showing the strong contrast between snow- and ice-covered areas (green through red) and open water or land (blue). [Image courtesy of X. Wang, University of Wisconsin-Madison, CIMSS/NOAA.] B) Albedo feedbacks. Albedo is the fraction of incident sunlight that is reflected. Snow, ice, and glaciers have high albedo. Dark objects such as the open ocean, which absorbs some 93% of the Sun’s energy, have low albedo (about 0.06). Bare ice has an albedo of 0.5; however, sea ice covered with snow has an albedo of nearly 90% [Source: http://nsidc.org/ seaice/processes/albedo.html.] 90 α=0.9 Feedbacks involving snow insulation of the ground are also important, through their effects on vegetation and on permafrost temperature and its influence on storage or release of greenhouse gases, as described in the next subsections (e.g., Ling and Zhang, 2007). 3.2.3 Vegetation Feedback 0.94 A related terrestrial feedback involves changing vegetation. A warming climate can cause tundra to give way to shrub vegetation. However, the shrub vegetation has a lower albedo than tundra, and the shrubs thus cause further warming (Figure 3.5) (Chapin et al., 2005; Goetz et al., 2007). Interactions involving the boreal forest and deciduous forest can also be important. When, as a result of warming, deciduous forest replaces evergreen boreal forest, then winter surface albedo increases—an example of a negative feedback to the warming climate (Bonan et al., 1992; Rivers and Lynch, 2004). Alternatively, if warming allows evergreen boreal forest to advance northward replacing tundra or shrub vegetation, then the lower 0.5 0.1 If sea ice is thinned by warming, then the ocean heats the overlying atmosphere in winter months, amplifying that warming. 37
Page 1 and 2: Past Climate Variability and Change
Page 3 and 4: Past Climate Variability and Change
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CHAPTER 6 ABSTRACT Past Climate Var
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second, and the Transpolar Drift, t
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EXTENT, IN MILLION SQUARE KILOMETER
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Until recently, and for logistical
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1990; Fyles et al., 1994), the nort
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FREQUENCY 50 45 40 35 30 25 20 15 1
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Seas around icelAnd provide a rare
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at least 14 Ma (Darby, 2008). Sever
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T. quinqueloba, IN SPECIMENS PER GR
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7 6 5 4 3 2 1 Past Climate Variabil
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SEA-ICE COVERAGE, IN NUMBER OF MONT
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QUARTZ WEIGHT, IN PERCENT 6 5 4 3 2
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6.5 SUMMARY Geological data indicat
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GLOSSARY GLOSSARY 8 ka cold event A
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CAPE Project Circum-Arctic PaleoEnv
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Eccentricity Out of roundness (elli
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Innuitian sector The ice sheet that
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Moraine Landforms (typically ridges
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Positive feedback In climate studie
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Trough-mouth fans Undersea deposits
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The U.S. Climate Change Science Pro
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