Global Environmental Change Research Pathways for the Next Decade (1999) / Chapter Skim
Currently Skimming:
4 Changes in the Climate System on Decade-to-Century Timescales
4 Changes in the Climate System on Decade-to-Century Timescales
Pages 127-190
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 127... ...
Recent advances in understanding climate prediction on timescales of decades to centuries include the following, among others: documentation and recognition of the scope of natural variability; documentation by calibrated satellite observations that clouds have a net global radiative cooling effect on the Earthatmosphere system by about 15 to 20 W/m2; achievements in understanding water vapor behavior and in feedback analysis, proposed and to some degree realized, on theoretical, observational, modeling, and methodological grounds; and understanding the role of volcanic eruptions as a climate-forcing factor, as seen in measurement and assessment of the impacts of recent eruptions. This area of research has underscored the complexities and uncertainties of detecting and projecting climate change.
|
From page 128... ...
, providing a foundation on which future policy decisions and infrastructure management can be rationally based. A number of Research Imperatives must be met to understand climate change on decadal to centennial timescales: • Natural climate patterns.
|
From page 129... ...
Heavy reliance on these methods and assumed forcing curves, without the benefit of real-time observations for constant model validation and improvement, implies a considerable effort for model validation through alternative means, improved understanding of the limits and implications of proxy indicators constituting the paleoclimate records, and detailed monitoring of emissions to help track actual rates. As for future observations, we can only now begin collecting the data to aid future generations of scientists in understanding dec-cen climate variability and change.
|
From page 130... ...
The first case reviews findings from Greenland ice cores about the natural variability of the climate system. The second illuminates human responses to climate variability in Mesopotamia, as deduced from the paleorecord.
|
From page 131... ...
can be rigorously pursued. a GISP2 successfully completed drilling through the base of the Greenland ice sheet and another 1.55m into bedrock in central Greenland on July 1, 1993, recovering the deepest ice core record in the northern hemisphere (3053.44m)
|
From page 132... ...
Since these cores were obtained, additional investigations, involving large numbers of proxy indicators of past climate change, from all of the different climate zones on Earth, have reinforced these initial findings and more clearly driven home the vulnerability of the Earth's climate system to natural variability. Consequently, these findings have changed our way of viewing the climate system and fundamentally undercut the notion that we live in a relatively stable climate system.
|
From page 133... ...
In regions on the ice margins, such events as the disappearance of the Norse colonies in Greenland during the mid- to late fourteenth century appear to be chronologically correlated at some sites with the occurrence of a few extremely cold winters and at others with the general amelioration of climate produced at the onset of the Little Ice Age.1 By utilizing climate-linked paleoclimate records, it was found that periods of decreased atmospheric circulation intensity in the North Atlantic, developed from the GISP2 ice core, could be correlated with discontinuous Dead Sea level records of drying,2 which are a reasonable indicator for west Asian aridity.3 The more detailed record reveals a close correlation between major periods of drying and major social disruptions in west Asian civilization.4 Other research5 has found that the driest period represented by a late Holocene lake sediment record from Mexico correlates closely with the collapse of the classic Mayan civilization around 750 to 900 AD.
|
From page 134... ...
Flood insurance programs rely on a similar analysis. The variability of flood risk at decadal to centennial timescales and its implications for flood control are discussed here in the context of the American River near Sacramento, California.
|
From page 135... ...
Others8 argue that earlier snowmelt in California may be caused by a trend toward warmer winters there and a concurrent long-term fluctuation in winter atmospheric circulation over the North Pacific Ocean and North America. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation in winter circulations shifted to a part of the central
|
From page 136... ...
These shifts in atmospheric circulation are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures and with earlier snowmelt and increased spring moisture fluxes in the American River basin. Gridded (5o *
|
From page 137... ...
, with a strong warming over northern hemisphere land, but some small cooling over the northern hemisphere oceans.13 This is the so-called COWL pattern: cold oceans and warm land pattern that is readily apparent in the global surface temperature data when comparing the past 20 years to the previous 20 years (see Plate 5)
|
From page 138... ...
Courtesy of the National Climatic Data Center. When the monthly averaged northern hemisphere surface temperature time series for this century is adjusted to eliminate the influence of the COWL pattern, two things become apparent:16 a large fraction of the month-to-month variability, particularly apparent in the cool-season months, is no longer seen, and a significant fraction of the accelerated hemispheric warming observed since the mid-1970s, again concentrated in the cooling-season months, is also removed, making the summer and winter trends comparable (see Figure 4.5)
|
From page 139... ...
Is this contribution of the NAO and PNA to the COWL warming a result of natural variability? That is, simply by chance will there likely be extended periods of time in which they display similar and relatively persistent polarity, or is this the manifestation of anthropogenic warming influencing the polarity of the natural climate modes?
|
From page 140... ...
(3) What are the relationships among the COWL pattern, greenhouse fingerprint, and natural climate patterns?
|
From page 141... ...
further reveals the degree to which the pattern is shared by the North Atlantic, the northern part of Europe, and northern Asia.19 Typically, when the index is high, the Icelandic low is strong, which results in the increased influence of cold Arctic air masses on the northeastern seaboard of North America and enhanced westerlies introducing warmer, moister air masses to western Europe in winter.20 Thus, NAO anomalies are related to wintertime temperature and precipitation downstream over Europe and across Russia and Siberia.21 They have also been linked22 to changes in the thermohaline circulation in the North Atlantic,23 the cod stock in the northwest Atlantic,24 and the mass balance of European glaciers.25 The Pacific-North American Teleconnection The PNA is a large-scale teleconnection between the North Pacific Ocean and North America that appears as four distinct cells in the 500 mb geopotential height field. An index of this teleconnection pattern was created26 through a
|
From page 142... ...
(b) Difference in sea level pressure between high and low NAO index years, showing region of NAO influence.
|
From page 143... ...
. However, the PNA also appears in SLP27 as well and can be depicted by the North Pacific Index.28 The North Pacific Index is expressed as the areally averaged SLP over a large area of the North Pacific Ocean near the center of the Aleutian low.
|
From page 144... ...
(b) Time series (solid line)
|
From page 145... ...
Tropical Atlantic SST The tropical Atlantic Ocean shows a coherent structure in SST. There, the dominant empirical orthogonal function pattern of SST often shows a warm pool in the tropical North Atlantic and a complementary cool pool in the tropical South Atlantic, or vice versa.
|
From page 146... ...
(b) Time series of the pattern displayed in (a)
|
From page 147... ...
For example, a number of regional atmospheric patterns have been analyzed, such as the North Pacific Oscillation,39 West Pacific Oscillation,40 West Atlantic Pattern,41 and Pacific Decadal Oscillation.42 A completely different kind of pattern, involving sea ice, has been found in the Southern Ocean. The Antarctic Circumpolar Wave is characterized by deviations in the Antarctic sea ice extent from monthly climatological averages, although it is also apparent in surface wind, SST, and SLP anomalies along the Antarctic polar front, near the winter marginal ice zone.43 It is also highly coherent with temporal variations in ENSO44 and Indian Ocean monsoons.45 Other atmospheric patterns have been identified in the Southern Hemisphere,46 though the data are frequently too few to allow detailed analyses.
|
From page 148... ...
For example, the decadal variability of ENSO and its relationship to the PNA and other North Pacific climate patterns has been noted, and ENSO's wintertime 500 mb height anomaly pattern of decadal variability is very similar to its interannual variability.52 In the other fields (SST, SLP, and wind stress) the leading empirical orthogonal function pattern of decadal and longer timescale variability looks very similar in shape to ENSO (with a slight extension into the extratropical North Pacific)
|
From page 149... ...
Improve knowledge of decadal- to century scale natural climate patterns, their distributions in time and space, their optimal characterizations, mechanistic controls, feedbacks, and sensitivi ties, including their interactions with, and responses to, anthropogenic climate change. Meeting this goal -- and the related objectives discussed in the sec tion on climate components below -- requires long-term calibrated obser vations.
|
From page 150... ...
Key Scientific Questions To address the Research Imperatives above, some difficult Scientific Questions must be answered. Despite the uncertain roles of patterns and coupled modes in global warming and climate change more generally (including those changes important for climate prediction)
|
From page 151... ...
For example, the Bermuda-Azorean High remains relatively stable in its spatial orientation, but the Icelandic low often migrates southward to Newfoundland. Thus, the North Atlantic SST pattern tends to show a rotation around the basin55 that a simple dipole index between two fixed points, such as the NAO, cannot capture.
|
From page 152... ...
What are the rela tionships between the COWL pattern, the greenhouse fingerprint, and natural climate patterns? In other words, how do the natural modes of the climate system respond to different changes in forcing, natural or anthro pogenic?
|
From page 153... ...
In doing so, atmospheric circulation also directly controls the distribution in space and time of temperature and fresh water and thus the nature and distribution of ecosystems, surface radiation (via distribution of aerosols, which influence cloud distribution and formation physics) , and sea level change (via distribution of heat and moisture and ice melt and decay)
|
From page 154... ...
For example, are dec-cen variations of the PNA, NAO, and other climate patterns driven by inherent natural climate system variations, reflecting nonlinear internal interactions, or coupled interactions (in all cases, interactions that would effectively extend the intrinsic atmospheric timescales)
|
From page 155... ...
Moreover, links to tropical sea surface temperature and the decadal variability of ENSO were also drawn, as well as links to variations in the annual cycle of the southern hemisphere. The origin and maintenance of these phenomena and their associations must be the subject of considerable future investigations.
|
From page 156... ...
At seasonal to interannual timescales, the ocean influences climate primarily through its large heat capacity in the relatively thin surface layer, whereas on longer timescales the heat transport over basin and global scales predominates. Issues in Ocean Circulation A key issue is the oceans' role in the longevity and long-term variability of climate patterns.
|
From page 157... ...
seem rather similar, but the participation of the oceans beneath their action centers may differ because of the presence of deep overturning in the North Atlantic. • What are the processes of formation and sequestering of water masses and of their subsequent modification and eventual return to the surface?
|
From page 158... ...
What is the role of salinity advec tion feedback to surface freshwater anomalies when heat transport and SST anomalies exist? • What feedback and coupling mechanisms maintain SST, heat, fresh wa ter, sea ice, and chemical anomalies on dec-cen timescales?
|
From page 159... ...
of the polar regions through vast areas of highly reflective ice and snow fields. The cryosphere also plays a predominant role in sea level; the most vulnerable ice sheet susceptible to potentially rapid destruction is the West Antarctic ice sheet, which contains enough water to raise sea level by 18 m should it melt.
|
From page 160... ...
It is also essential to identify the sensitivity and dependencies of the patterns on crysophere characteristics, particularly the fairly mutable and mobile sea ice cover and snow fields. Finally, because the Antarctic Circumpolar Wave and its covariation with ENSO seem to suggest a hemispherewide teleconnection between the tropics and extratropics in the poorly examined southern hemisphere, we need to refine our understanding of the extent, nature, linkages, and controls of the Antarctic Circumpolar Wave with regard to extrapolar climate patterns (and climate in general)
|
From page 161... ...
Do changes in the patterns of large-scale planetary waves or ocean circulation alter polar conditions so much that they in turn drive additional changes in other parts of the climate system? For example, could variations in NAO influence the volume of fresh water exported from the Arctic in the form of sea ice to influence thermohaline circulation significantly?
|
From page 162... ...
Given the direct impact of this budget on sea level, we must better quan tify the mass balance of continental ice sheets, alpine glaciers, and per manent snow fields. In particular, the balance at the base of floating ice shelves is in considerable question, and whether the Greenland and Ant arctic ice sheets are gaining or losing mass is still uncertain.
|
From page 163... ...
Changes in land surface characteristics can lead to similar effects, realized mostly, again, through albedo. Vegetation and land surfaces also affect climate through the hydrological cycle, as discussed in the section on the hydrological cycle below.
|
From page 164... ...
Hydrological Cycle The hydrological cycle refers to the origin and fate of water through its many phases in the atmosphere, ocean, land, and biosphere. Water evaporates as vapor from the surfaces of both land and ocean, condenses in the atmosphere, precipitates
|
From page 165... ...
and cooling upper layers, thereby greatly reducing the net warming otherwise anticipated from an increase in total atmospheric water vapor. Regarding patterns of climate variability, the hydrological cycle plays an unknown role.
|
From page 166... ...
In fact, aside from sea level changes associated with plate tectonic spreading rates, the largest changes in sea level result from the waxing and waning of continental ice sheets. During the last ice age, enough fresh water was removed from the oceans and stored in continental ice to lower sea level by more than 100 m.
|
From page 167... ...
Where and at what rate water re-enters the ocean and how precipitation and evaporation redistribute fresh water in the ocean's surface layer play essential roles in ocean salinity, which is the driving factor in high-latitude stability, influencing thermohaline circulation and shallow and deep subduction and ventilation. In short, the role of the hydrological cycle in ocean circulation is critical to understand but is currently poorly known.
|
From page 168... ...
There is considerable controversy about this subject, with one theory suggesting that increased moisture in lower levels of the atmosphere will be offset by decreased levels in the upper troposphere, reducing surface warming (and thus its moisture in crease) and cooling upper layers, thereby greatly reducing the net warm ing otherwise anticipated from an increase in total atmospheric water vapor.62 • By what combination of remote and in situ observations can we measure the large-scale distribution of precipitation on dec-cen timescales?
|
From page 169... ...
It is currently believed that discharges from the melting ice pack that covered the United States during the last glacial maximum had signifi cant influence on the thermohaline circulation of the North Atlantic and therefore the temperature of the North Atlantic. Additionally, modula tions by iceberg discharges (Heinrich events)
|
From page 170... ...
For example, as discussed above, an increased direct warming is thought to drive an increase in the rate of the hydrological cycle, an influence that will have an uncertain net effect on the amount and vertical distribution of water vapor in the atmosphere. Since water vapor is the primary greenhouse gas, being able to predict its response to a direct forcing is essential.
|
From page 171... ...
Changes in tropospheric ozone directly influence the Earth's radiation budget and thus directly influence climate change and variability. Of primary importance in this area is improved understanding of tropospheric chemistry models and improved representation of cloud-chemistry interactions.
|
From page 172... ...
Key Scientific Questions About Atmospheric Composition and Radiation Budget Greenhouse Gases • What are the changes in the spatial distribution of carbon storage and flux on dec-cen timescales? Of the carbon emitted by anthropogenic
|
From page 173... ...
• What are the contributions of various sources and sinks to the recent increase in methane? Although observations document that atmospheric CH4, a potent greenhouse gas, has increased steadily since the nineteenth century, the causes of this rise are not sufficiently quantified.
|
From page 174... ...
Thus, dec-cen changes in methane can be expected to alter the vertical distribution of water vapor in the higher troposphere and lower stratosphere, thereby changing radiative forcing. The ice particles that form at these altitudes also provide sites for heterogeneous chemis try.
|
From page 175... ...
Such changes might include continued large increases in CH4 and stratospheric H2O or stratospheric circulation changes associated with global warming and greenhouse gases. The pre dictive models are based in part on first-principle physics and chemistry and should correctly account for these changes, but we must recognize that observations of the recent past are used to test and calibrate models.
|
From page 176... ...
Understanding the sensitivity of Earth's climate to past changes in solar activity will enable better predictions of future changes in the face of decadally varying solar irradiance superimposed on other radiative forcing trends.
|
From page 177... ...
Radiative forcing, cloud interactions, involvement of the deep ocean, and sea ice dynamics are but a few of the processes that will be major influences on the long-term evolution of
|
From page 178... ...
Improve knowledge of decadal- to century scale natural climate patterns, their distributions in time and space, opti mal characterization, mechanistic controls, feedbacks, and sensitivities, including their interactions with, and responses to, anthropogenic climate change. • Paleorecord.
|
From page 179... ...
consistent monitoring of the most fundamental state variables (e.g., atmospheric temperature and moisture profiles, ocean surface temperature and salinity values) , and monitoring of those variables specifically relevant to climate system components to initialize (including via assimilation)
|
From page 180... ...
Each of the state variables must be monitored, including their vertical distributions through the troposphere and lower stratosphere, to evaluate the nature of anthropogenic and natural changes. One of the most hotly debated topics in modern climatology is how atmospheric moisture distribution will change in response to the addition of greenhouse gases and therefore whether, or by how much, this moisture response will moderate the temperature response.
|
From page 181... ...
temperature, salinity, oxygen, and tracer sections; velocity profile surveys and repeat sections (starting with World Ocean Circulation Experiment sections) ; and higher-frequency time series stations (starting with past and present weather ship stations)
|
From page 182... ...
Cryosphere Observations Critical cryosphere-related observations for climate patterns on decadal to centennial timescales include long-term monitoring of surface salinity along with SST, since salinity represents the dominant control on the density of seawater in high-latitude regions. Also, measurements of the sea ice fields themselves, including motion fields and ice thickness, are required to determine the freshwater transports and buoyancy fluxes associated with the ice fields.
|
From page 183... ...
These changes alter not only the distribution of surface reservoirs and the surface-atmosphere exchange of radiatively active gases but also albedo and even surface stress and evapotranspiration efficiency -- and the last two both influence the hydrological cycle. This serves as an external forcing to the planet that cannot be predicted and must be introduced into the models as they occur to properly maintain the models' surface forcing conditions.
|
From page 184... ...
; if longer-term change is to be detected using relatively short time series, these measures require considerably higher resolution (and precision and accuracy)
|
From page 185... ...
CHANGES IN THE CLIMATE SYSTEM ON DECADAL TO CENTURY TIMESCALES 185 NOTES 1. Buckland et al.
|
From page 186... ...
1989. The influence of the North Pacific atmospheric circulation and streamflow in the West.
|
From page 187... ...
1996. On the nature of decadal anomalies in North Atlantic sea surface temperature.
|
From page 188... ...
1996. Joint spatio-temporal modes of surface temperature and sea level pressure variability in the northern hemisphere during the last century.
|
From page 189... ...
1993. Characteristic variations of sea surface temperature with multiple timescales in the North Pacific.
|
From page 190... ...
1996. Climate variations: Cycling around the south pole.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.