Decade-to-Century-Scale Climate Variability and Change A Science Strategy (1998) / Chapter Skim
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5 Climate-System Components
5 Climate-System Components
Pages 48-110
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From page 48... ...
The first two, which are closely related, involve two aspects of the atmosphere: atmospheric circulation and the hydrologic cycle. (Of course, the latter section's scope involves more than just the atmosphere, since it discusses the storage of water and its movement through the atmosphere and boundaries.)
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From page 49... ...
In the process, it influences all of the climate attributes discussed in Chapter 2. These feedbacks include changing the atmospheric concentration of water vapor, itself the major greenhouse gas; changing cloudiness; changing the surface albedo due to changes in snow, ice, and vegetative cover; changing source and sink rates for carbon dioxide, methane, and nitrous oxide; changing the formation rates for tropospheric ozone and aerosols; and changing the transport and storage of heat in the oceans.
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From page 50... ...
Sea level responds to the heat content of the oceans and the distribution of heat in the oceans, as well as reflecting the proportion of the Earth's total water mass that resides in the oceans. Changes in the radiation budget also affect ocean transport and storage of heat and carbon, further modifying surface temperatures and the hydrologic cycle.
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From page 51... ...
Although the precise nature of the mechanisms that have caused temperature and methane to co-vary in the past are somewhat uncertain, these paleorecords indicate the possibility that temperature and methane may also co-vary in response to future climate changes. Changes in tropospheric ozone, a third greenhouse gas, are not well documented.
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From page 52... ...
, they do constitute a significant anomaly for that region. Evidence of stratospheric ozone depletion over dec-cen time scales is also indicated in other records.
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From page 53... ...
Periods of peak aerosol loading appear to correlate better with additional ozone depletion than with a trend fitted to the dominant driving force in ozone depletion, stratospheric chlorine levels. It is difficult to interpret this trend in ozone over the 15 years of TOMS data without including the concurrent variations in stratospheric aerosols.
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From page 54... ...
. By comparison, a doubling of CO2 in the atmosphere would generate a radiative forcing equivalent to a 1.8 percent increase in solar irradiance.
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From page 55... ...
The relationship between solar wind and solar irradiance has been calibrated for the last two solar cycles; the extrapolation for conditions outside the range of direct observations of total solar irradiance if applicableimplies a dec-cen solar irradiance variation with periods in which irradiance may be lower by as much as 0.25 percent. Mechanisms The sun's radiation, volcanic eruptions, and human emissions of greenhouse gases and aerosols are sources of variability and change that are external to the climate system.
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From page 56... ...
In addition, it has an indirect effect on the stratosphere, because, once oxidized, it is an important source of stratospheric water vapor. The most important sources of atmospheric methane are wetlands, rice agriculture, cattle and sheep, biomass burning, fossil fuels, landfills and waste, and termites (see, e.g., Fung et al., 1991~.
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From page 57... ...
The fast processes associated with this mechanism, such as cloud formation and the related intraand inter-cloud radiative impacts, influence cloud nucleation, longwave radiation, albedo feedbacks, and ultimately the surface energy balance. On dec-cen time scales, the impact of increased water vapor is realized through alterations in large-scale cloud distribution (shown earlier in Figure 212)
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From page 58... ...
One of the most important feedback processes is the interaction between atmospheric water vapor, clouds, and the surface radiation balance. The details of this complex interaction are still poorly understood.
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From page 59... ...
. Dec-cen changes in methane can be expected to alter the vertical distribution of water vapor in the upper troposphere and lower stratosphere, thereby changing radiative forcing.
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From page 60... ...
Our present understanding of how water vapor, clouds, and radiation balance interact is poor; until this fundamental set of feedbacks is better known, one of the most basic questions regarding anthropogenically induced climate change cannot be properly addressed. Stratospheric Ozone · How does the coupling between chemistry, dynamics, and radiation in the lower stratosphere and upper troposphere operate on dec-cen time scales?
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From page 61... ...
A better understanding of the sun's influence on dec-cen climate variability requires improving reconstructed solar irradiance. · What feedbacks govern climate and ecosystem response to changes in solar UV on dec-cen time scales?
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From page 62... ...
How do aerosols contribute to cloud formation, precipitation, and radiative interaction? Cloud processes in general and their relationship to atmospheric water vapor and the radiation balance, although they occur on time scales far shorter than decadal, remain a major uncertainty in the prediction of future radiation balances; parameterizations need to be improved for cloud formation and distribution as a function of water-vapor distribution, surface boundary conditions, and rate of the hydrologic cycle.
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From page 63... ...
Atmospheric circulation strongly influences the other three climate attributes (solar irradiance, sea level, and ecosystems) as well.
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From page 64... ...
Figure a b DECADE-TO-CENTURY-SCALE CLIMATE VARIABILITY AND CHANGE: A SCIENCE STRATEGY 5-12 shows the change in global-mean land and sea surface temperatures recorded by instruments since the 1850s, and their hem~sphenc distribution. The overall increase in temperature over time is unequally distributed between land and ocean (IPCC, 1996a)
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From page 65... ...
In the mid-latitudes, the low-frequency variability of atmospheric circulation-as reflected, for example, by anomalous distribution of sea level pressure and geopotential height-displays the distinct spatial patterns described in Chapter 3. Most clearly discernible during winter, the pressure fluctuations associated with such teleconnection patterns as the PNA and the NAG regulate the meridional distribution of temperature, and moisture advection and convergence, in their region of influence (Trenberth and Hurrell, 1994~.
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From page 66... ...
1967- 1976 INSUFFIC~NTDATA ~ ~ ~_ _ ~ ~ INS~ENT DATA ~ ~-~ (c) 1980- 1989 INSUFFICIENT DATA - ,, _ _ ~ _= INSUFFICIENT DATA - ~: A: -l Em, 0 1L~ INSUFFICIENT DATA INSUFFICIENT DATA ~ ~ INSUFFICIENT DATA =e'~ ~ W ~.oo__ ~ 1 ~ £~ DECADE-TO-CENTURY-SCALE CLIMATE VARIABILITY AND CHANGE: A SCIENCE STRATEGY the "Atmospheric Composition and Radiative Forcing" section of this chapter)
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From page 67... ...
Because the vertical and latitudinal distributions of these constituents influence the radiative forcing of the planet, changes in atmospheric circulation directly affect the global mean temperature and its horizontal and vertical distribution. Changes in temperature in turn affect the distribution of winds and their convergence patterns, leading to changes in evaporation, in the atmospheric moisture content and its distribution, and ultimately to changes in radiative , .,,,,,, .
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From page 68... ...
While this assertion may be disputed (James and James, 1989; Lorenz, 1990) , fluctuations in the atmospheric circulation on the deccen time scale that do not arise from changes in external forcing are most likely influenced by atmospheric interactions with the more slowly varying parts of the climate system.
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From page 69... ...
Remaining Issues and Questions Many of the remaining issues and questions about the role of atmospheric circulation in dec-cen variability are related to climate patterns, interaction with other climate-system components, and the role of changing boundary conditions in triggering and maintaining long-term circulation anomalies (see Chapter 3 and other sections of Chapter 5~. Here we highlight the issues especially important for atmospheric circulation.
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From page 70... ...
The processes controlling the evolution of all these important components must be improved in atmospheric-circulation models so that the dominant interactions driving long-term climate change in the atmosphere can be properly evaluated. In particular, the processes controlling the feedbacks and interactions between water vapor, cloudformation processes, and atmospheric circulation must be better understood and parameterized.
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From page 71... ...
In fact, aside from the local sea-level changes associated with the coastland response to the weight of the waxing and waning continental ice sheets, the largest changes in sea level arise from the addition or subtraction of freshwater as those ice sheets change in size. During the last ice age, enough freshwater was removed from the oceans and stored in continental ice to lower the sea level by more than 120 m.
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From page 72... ...
Climate variability at decadal to centennial time scales translates into significant impacts on surface and subsurface hydrology and, thus, on water-resource management. Regional flood potential, water-quality trends, hydropower and recreation potential, and irrigation and municipal-water demands and supply are modulated at these time scales as a function of climatic variability.
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From page 73... ...
The hydrologic cycle also influences ecosystems, because the supply of freshwater is central to terrestrial life. Because much of the evaporation from the land's surface is in fact water transpired from the soil to plants through their root systems and then evaporated from the stomata in their leaves, the total vapor leaving the land surface is referred to as evapotranspiration.
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From page 74... ...
He argues that, in the tropics at least, upper-level water vapor will decrease, not increase, with the addition of greenhouse gases. Thus, in the tropics, the net water-vapor feedback from the entire atmospheric column could be weak or negative, rather than strongly positive (which would be the case if upper-level water-vapor concentrations were to increase)
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From page 75... ...
It is currently believed that discharges from the melting ice pack that covered much of the mid- to high latitudes of North America during the last glacial maximum significantly influenced thermohaline circulation, and thus the temperature, of the North Atlantic. Modulations by iceberg discharges (Heinrich events)
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From page 76... ...
Understanding the decadal variability of SST in the Pacific offers the best hope for skillful predictions of U.S. precipitation over medium and long time scales.
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From page 77... ...
The resultant impacts on atmospheric water vapor, cloudiness, and regional precipitation are largely unknown. ·~-% ~ ~ A: 77 Predictability Prediction of the global hydrologic cycle is critical to our ability to predict the magnitude and distribution of anthropogenic greenhouse warming, because the hydrologic cycle influences greenhouse warming through a number of feedbacks, including those involving the distribution and amount of atmospheric water vapor.
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From page 78... ...
At present no research programs are investigating these issues at dec-cen time scales. · How do the distribution of water vapor, precipitation, and clouds respond to and interact with surface boundary conditions and changes in forcings on dec-cen time scales?
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From page 79... ...
Clearly, accurate treatment of upper-level water vapor is essential to realistic modeling of the climate system's radiative response to anthropogenic increases in greenhouse gases (and other external forcing factors) , and to reliably estimating the greenhouse-warming response.
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From page 80... ...
, . ~ without this ocean, the climate would be different in many essential ways: Without the evaporation of water from the sea surface, the hydrologic cycle would be different; without the ocean's heat transport and uptake, the temperature distribution of the globe and thus the atmospheric circulation would be different; and without the biota in the ocean, the total amount of carbon in the atmosphere would be many times its current value.
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From page 81... ...
Since our interest in this report is how the ocean affects climate variability, we concentrate mainly on those processes and those regions of the ocean that can affect the atmosphere on dec-cen time scales. Influence on Attributes The ocean circulation participates in the climate system through three primary agents: · exchange of heat, water vapor, and carbon dioxide with the atmosphere and cryosphere; · sequestering of heat, freshwater, and carbon dioxide at depth in the ocean and sediments for long times before possible return to the atmosphere; · redistribution of heat, freshwater, and carbon dioxide through the action of large-scale ocean currents (surface and deep)
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From page 82... ...
Storms are a critical coastal issue. The position and intensity of atmospheric storms over the oceans depend on atmospheric circulation and ocean temperature distribution (among other things)
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From page 83... ...
involve systematic interannual-to-decadal propagation of winter SST anomalies along the primary circulation pathways in the North Atlantic, which suggests an interplay between ocean dynamics and sequestering of temperature information (making it unavailable to the atmosphere) in the upper ocean.
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From page 84... ...
Therrnohaline Circulation Warm, salty water proceeds northward in the Gulf Stream, cooling as it crosses the Atlantic in a northeastward direction as the North Atlantic Current. Upon reaching the subpolar regions, it enters the GIN Seas and then spreads into the Arctic, where it mixes with resident water masses and further increases in density due to additional cooling and salinization associated with sea-ice formation.
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From page 85... ...
The study of abrupt climate change began relatively recently, and it is not yet fully understood, but the observational evidence is tantalizing in that it suggests the possibility that major climate shifts may occur over relatively short time scales. This possibility underscores the need to improve our understanding of the sensitivities of climate to changes in the thermohaline circulation, and the dependence of the thermohaline circulation on changes in its source areas.
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From page 86... ...
suggest that water subducts from the high-latitude cold pool into the subtropical gyre of the North Pacific on time scales of a decade or more. Analysis of tracerderived ages shows that water subducted into the eastern North Pacific subtropical gyre can be transported on decadal time scales into the equatorial Pacific (Fine et al., 1987)
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From page 87... ...
The Indonesian Seas are the only warm-water pathway between ocean basins. By transporting anomalies between basins, the Indonesian throughflow plays an important role in climate variability.
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From page 88... ...
Changes in temperature and salinity of similar magnitude to those reported in the deep North Atlantic Ocean (Bryden et al., 1996) are also seen in Subantarctic Mode Water and Antarctic Intermediate Water in the Tasman Sea (Bindoff and Church, 1992)
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From page 89... ...
Thus, the net southward flow of 0.3 Sv at 24°N is the difference between a large southward flux in the North Atlantic and a smaller northward flux in the North Pacific (Schmitt and Wijffels, 1992~. This net southward flow compensates for the net atmospheric transport of water vapor from the Atlantic to the Pacific (Broecker, 19911.
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From page 90... ...
Keeling and Peng (1995) estimate that the North Atlantic Ocean takes up and transports about 0.4 Gt of carbon per year across the equator into the Southern Hemisphere.
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From page 91... ...
The ocean may also have active modes of internal variability that force an atmospheric response, or participate actively in coupled ocean-atmosphere modes of variability on the dec-cen time scale. The ocean's active role in dec-cen climate variability involves several local mechanisms.
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From page 92... ...
. A number of climate variability mechanisms involve coupling of the ocean and atmosphere in such a way that the ocean plays a dominant role.
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From page 93... ...
Differences between basins need to be explored; for example, the participation of the oceans beneath the NAO and the PNA action centers may differ because of the presence of deep overturning in the North Atlantic, and its absence in the North Pacific. Determining the predictability of coherent dec-cen variations and the ocean's role in this predictability depends on untangling their exact mechanisms, including the ocean's role in their maintenance and evolution.
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From page 94... ...
How are they modified as they circulate through the interior, and how are water masses re-entrained back into the mixed layer? How do freshwater fluxes (which are influenced by evaporation-minus-precipitation, sea ice, and runoff)
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From page 95... ...
For the longer time scales, interannual through dec-cen, there is an increasing need to include oceanic advection in the predictive models. We need to explore computationally efficient methods to extend to decadal time scales the initialized coupled models that are currently being developed for seasonal-tointerannual forecasting, thereby capitalizing on the decadalscale predictability that has been suggested for the North Pacific and North Atlantic.
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From page 96... ...
These improvements will provide the knowledge of advection and anomaly propagation that is critical for understanding dec-cen climate variability. Continued satellite data are needed for global coverage of sea-surface height, SST, winds, and ocean color, but if they are to be useful, corresponding ground truth must be established through ocean observations.
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From page 97... ...
, but also vast areas of frozen ground and permafrost, as well as seasonal snow fields that lie beyond the limits of glaciers. At present, glacial ice covers about 3 percent of the Earth's surface, while containing nearly 75 percent of its non-ocean water; sea ice covers another 7 percent; and perennially frozen ground covers 20-25 percent of the exposed land surface.
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From page 98... ...
Polar Evidence The waxing and waning of major continental ice sheets during glacial and interglacial stages are recorded in ice cores from both polar regions, as well as those from China, Peru, and Bolivia. However, climate variations during the Holocene apparently were not globally synchronous; they may appear prominently in some records but be wholly absent from others.
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From page 99... ...
note striking examples of decadal-scale variations of sea ice in the North Atlantic during the past hundred years, and century-scale sea-ice variations near Iceland. Several studies have found decadal-scale relationships between Arctic sea ice and atmospheric anomalies (e.g., Dickson et al., 1988; Deser and Blackmon, 1993; and Slonosky et al., 1997~.
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From page 100... ...
The approximate 10 percent reduction in Northern Hemisphere snow cover that occurred in the latter part of the 1980s has likely increased the radiative balance and surface temperature (up to 1.5°C) over the northern extratropical land area, particularly in the spring (Groisman et al., 1994a,b)
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From page 101... ...
The resulting change in freshwater exported to the thermohaline source regions of the North Atlantic influences the effectiveness of the thermohaline circulation system, thereby communicat ing the regional changes elsewhere in the globe. (For instance, large changes may be induced in the Antarctic seaice fields)
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From page 102... ...
Clearly the most important mechanisms influencing cryospheric variability are its couplings to the atmosphere, ocean, and land surface. They lead to a set of geographically unique polar feedbacks such as the ice/snow-albedo feedback, ice-cloud feedback, ice-ocean feedback (the effects of which apply to a variety of scales, from those influencing the sea-ice distribution to those influencing the vigor of the global thermohaline circulation)
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From page 103... ...
It has been speculated that the West Antarctic ice sheet has collapsed in the past, possibly because of the aforementioned instabilities; if such a collapse were to happen again, its impact on global sea level over centennial time scales would be tremendous. Yet another set of polar feedbacks is associated with seaice rheology.
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From page 104... ...
· What are the historical and current global budgets of glacial ice and snow, and what are the primary mechanisms controlling those budgets? Because glacial ice and snow budgets directly affect sea level, we need to better quantify the mass balance of the continental ice sheets, alpine glaciers, and permanent snowfields.
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From page 105... ...
The feedbacks among the hydrologic cycle (including river runoff into the Arctic) , the atmospheric circulation, and the thermohaline circulation must be better understood on a variety of scales, because such larger-scale feedbacks may play a fundamental role in polar climate.
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From page 106... ...
They thus influence the timing of evaporation from the land surface. Therefore, plants indirectly influence surface temperature through their effect on soil moisture, which has a large heat capacity, and thus influences latent heating.
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From page 107... ...
. While land-use changes are important, recent climate variability has probably also led to substantial changes in vegetation-related carbon fluxes.
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From page 108... ...
Schindler and Bailey (1993) estimated that the amount of carbon storage stimulated by anthropogenic nitrogen deposition may be between 1.0 and 2.3 Gt of carbon per year.
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From page 109... ...
Accurate modeling of climate and atmospheric chemistry require the accurate specification of land-surface parameters that are intimately tied to the fluxes of heat, water vapor, and trace gases. Although attempts have been made to predict land-use changes (Zuidema et al., 1994)
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From page 110... ...
In addition, as with greenhouse gases, the transient evolution of land cover (including wetlands) under a slowly changing climate and rapidly exploding population must be monitored to provide the boundary conditions needed for model simulations and assessment of plausible future trends.
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