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Abrupt Change in Land Hydrology




Much of the research on the climate response to increased GHG concentrations, and most of the public’s understanding of that work, has been concerned with global warming.  Accompanying this projected globally uniform increase in temperature, however, are spatially heterogeneous changes in water exchange between atmosphere and the Earth’s surface that are expected to vary much like the current daily mean values of precipitation and evaporation (IPCC, 2007).  Although projected spatial patterns of hydroclimate change are complex, these projections suggest that many already wet areas are likely to get wetter and already dry areas are likely to get drier, while some intermediate regions on the poleward flanks of the current subtropical dry zones are likely to become increasingly arid. 
These anticipated changes will increase problems at both extremes of the water cycle, stressing water supplies in many arid and semi-arid regions while worsening flood hazards and erosion in many wet areas. Moreover, the instrumental, historical, and prehistorical record of hydrological variations indicates that transitions between extremes can occur rapidly relative to the time span under consideration.  Over the course of several decades, for example, transitions between wet conditions and dry conditions may occur within a year and can persist for several years.
Abrupt changes or shifts in climate that lead to drought have had major impacts on societies in the past. Paleoclimatic data document rapid shifts to dry conditions that coincided with downfall of advanced and complex societies.  The history of the rise and fall of several empires and societies in the Middle East between 7000 and 2000 B.C. have been linked to abrupt shifts to persistent drought conditions (Weiss and Bradley, 2001).  Severe drought leading to crop failure and famine in the mid-8th century has been suggested as cause for the decline and collapse of the Tang Dynasty (Yancheva et al., 2007) and the Classic Maya (Hodell et al.,1995).  A more recent example of the impact of severe and persistent drought on society is the 1930s Dust Bowl in the Central United States (Fig. 1.4), which led to a large-scale migration of farmers from the Great Plains to the Western United States.  Societies in many parts of the world today may now be more insulated to the impacts of abrupt climate shifts in the form of drought through managed water resources and reservoir systems.  Nevertheless, population growth and over-allocation of scarce water supplies in a number of regions have made societies even more vulnerable to the impacts of abrupt climate change involving drought.

                               Figure 1.4. Photograph showing a dust storm approaching Stratford, Texax, during
                                          the 1930s Dust Bowl.  (NOAA Photo Library, Historic NWS collection).

Variations in water supply, in general, and protracted droughts, in particular, are among the greatest natural hazards facing the United States and the globe today and in the foreseeable future.  According to the national Climatic Data Center, National Oceanic and Atmospheric Administration (NCDC, NOAA), over the period from 1980 to 2006 droughts and heat waves were the second most expensive natural disaster in the United States behind tropical storms.  The annual cost of drought to the United States is estimated to be in the billions of dollars.  Although there is much uncertainty in these figures, it is clear that drought leads to (1) crop losses, which result in a loss of farm income and an increase in Federal disaster relief funds and food prices, (2) disruption of recreation and tourism, (3) increased fire risk and loss of life and property, (4) reduced hydroelectric energy generation, and (5) enforced water conservation to preserve essential municipal water supplies and aquatic ecosystems (Changnon et al., 2000;  Pielke and Landsea, 1998;  Ross and Lott, 2003).
History of North American Drought
In Chapter 3 of this report, we examine North American drought and its courses from the perspective of the historical record and, based on paleoclimate records, the last 1,000 years and the last 10,000 years.  This longer temporal perspective relative to the historical record allows us to evaluate the natural range of drought variability under a diverse range of mean climatic conditions, including those similar to the present. 
Instrumental precipitation and temperature data and tree-ring analyses provide sufficient information to identify six serious multiyear droughts in western North America since 1856.  Of these, the most famous is the "Dust Bowl” drought that included most of the 1930s decade (Fig. 1.4). The other two in the 20th century are the severe drought in the Southwest from the late 1940s to the late 1950s and the drought that began in 1998 and is ongoing. Three droughts in the middle to late 19th century occurred (with approximate dates) from 1856 to 1865, from 1870 to 1876, and from 1890 to 1896.

Is the 1930s Dust Bowl drought the worst that can conceivably occur over North America?  The instrumental and historical data only go back about 130 years with an acceptable degree of spatial completeness over the United States, which does not provide us with enough time to characterize the full range of hydroclimatic variability that has happened in the past and could conceivably happen in the future independent of any added effects due to greenhouse warming.  To do so, we must look beyond the historical data to longer natural archives of past climate information to gain a better understanding of the past occurrence of drought and its natural range of variability.
Much of what we have learned about the history of North American drought over the past 1,000 years is based on annual ring-width patterns of long-lived trees that are used to reconstruct summer drought based on the Palmer Drought Severity Index (PDSI).  This information and other paleoclimate data have identified a period of elevated aridity during the "Medieval climate anomaly” (MCA) period (A.D. 900-1300) that included four particularly severe multidecadal megadroughts (Fig.  1.5) (Cook et al., 2004).  The range of annual drought variability during this period was not any larger than that seen after 1470, suggesting that the climate conditions responsible for these early droughts each year were apparently no more extreme than those conditions responsible for droughts during more recent times.  This can be appreciated by noting that only 1 year of drought during the MCA was marginally more severe than the 1934 Dust Bowl year.  This suggests that the 1934 event may be used as a worst-case scenario for how severe a given year of drought can get over the West.  What sets these MCA megadroughts apart from droughts of more modern times, however, is their duration, with droughts during the MCA lasting much longer than historic droughts in the Western United States.


                         Figure 1.5.   Percent area affected by drought (PDSI<–1) in the area defined as
                                 the West (see Chapter 3 of this report) (from Cook et al., 2004).  Annual data
                                 are in gray and a 60-year low-pass filtered version is indicated by the thick
                                 smooth curve. Dashed blue lines are 2-tailed 95% confidence limits based on
                                 bootstrap resampling. The modern (mostly 20th century) era is highlighted in
                                 yellow for comparison to an increase in aridity prior to about A.D. 1300.

The emphasis up to now has been on the semiarid to arid Western United States because that is where the late-20th century drought began and has largely persisted up to the present time. Yet, previous studies indicate that megadroughts have also occurred in the important crop-producing states in the Midwest and Great Plains as well (Stahle et al., 2007). In particular, a tree-ring PDSI reconstruction for the Great Plains shows the MCA period with even more persistent drought than the Southwest, but now on a centennial time scale.
Examination of drought history over the last 11,500 years (referred to as the Holocene Epoch) is motivated by noting that the projected changes in both the radiative forcing and the resulting climate of the 21st century far exceed those registered by either the instrumental records of the past century or by geologic archives that can be calibrated to derive climate (proxy records) of the past few millennia.  In other words, all of the variations in climate over the instrumental period and over the past millennia reviewed above have occurred in a climate system whose controls have not differed much from those of the 20th century. Consequently, a longer term perspective is required to describe the behavior of the climate system under controls as different from those at present as those of the 21st century will be, and to assess the potential for abrupt climate changes to occur in response to gradual changes in large-scale forcing.
It is important to emphasize that the controls of climate during the 21st century and during the Holocene differ from one another, and from those of the 20th century, in important ways. The major difference in controls of climate between the early 20th, late 20th and 21st century is in atmospheric composition (with an additional component of land-cover change).  In contrast, the major difference between the controls in the 20th and 21st centuries and those in the early to middle Holocene is in the latitudinal and seasonal distribution of solar radiation.  Accordingly, climatic variations during the Holocene should not be thought of either as analogs for future climates or as examples of what might be observable under present-day climate forcing if records were longer, but instead should be thought of as the result of a natural experiment within the climate system that features large perturbations of the controls of climate. 
The paleoclimatic record from North America indicates that drier conditions than present commenced in the mid-continent between 10 and 8 thousand years ago (ka) (Webb et al., 1993), and ended after 4 ka. The variety of paleoenvironmental indicators reflect the spatial extent and timing of these moisture variations, and in general suggest that the dry conditions increased in their intensity during the interval from 11 ka to 8 ka, and then gave way to increased moisture after 4 ka.  During the middle of this interval (around 6 ka) dry conditions were widespread.  Lake-status indicators at 6ka indicate lower-than-present levels (and hence drier-than-present conditions) across most of the continent, and quantitative interpretation of pollen data shows a similar patter of overall aridity, but again with some regional and local variability, such as moister-than-present conditions in the Southwestern United States (Williams et al., 2004).  Although the region of drier-than-present conditions extends into the Northeastern United States and eastern Canada, most of the evidence for mid-Holocene dryness is focused on the mid-continent, in particular the Great Plains and Midwest, where the evidence for aridity is particularly clear.


Causes of North American Drought
Empirical studies and climate model experiments show that droughts over North America and globally are significantly influenced by the state of tropical sea surface temperatures (SSTs), with cool, persistent La Niña-like SSTs in the eastern equatorial Pacific frequently causing development of droughts over the Southwestern United States and Northern Mexico. Climate models that have evaluated this linkage need only prescribe small changes in SSTs, no more than a fraction of a degree Celsius, to result in reductions in precipitation. It is the persistence of the SST anomalies and associated moisture deficits that creates serious drought conditions.  In the Pacific, the SST anomalies presumable arise naturally from dynamics similar to those associated with the El Niño Southern Oscillation (ENSO) on time scales of a year to a decade (Newman et al., 2003).  On long time scales, the dynamics that link tropical Pacific SST anomalies to North American hydroclimate appear as analogs of higher frequency phenomena associated with ENSO (Shin et al., 2006).  In general, the atmospheric response to La Niña-like conditions forces descent of air over western North America that suppresses precipitation.  In addition to the ocean influence, some modeling and observational estimates indicate that soil-moisture feedbacks also influence precipitation variability. 
The causes of the MCA megadroughts appear to have similar origin to the causes of modern droughts, which is consistent with the similar spatial patterns, expressed MCA and modern droughts (Herweijer et al., 2007).  In particular, modeling experiments indicate that these megadroughts may have occurred in response to cold tropical Pacific SSTs and warm subtropical North Atlantic SSTs externally forced by high irradiance and weak volcanic activity (Mann et al., 2005; Emile-Geay et al., 2007).  However, this result is tentative, and the exceptional duration of the droughts has not been adequately explained, nor whether they also involved forcing from SST changes in other ocean basins.
Over longer time spans, the paleoclimatic record indicates that even larger hydrological changes have taken place in response to past changes in the controls of climate that rival in magnitude those predicted for the next several decades and centuries. These changes were driven ultimately by variations in the Earth’s orbit that altered the seasonal and latitudinal distribution of incoming solar radiation.  The climate boundary conditions associated with those changes were quite different from those of the past millennium and today, but they show the additional range of natural variability and truly abrupt hydroclimatic change that can be expressed by the climate system. 

The paleoclimatic record reveals dramatic changes in North American hydroclimate over the last millennium that was not associated with changes in greenhouse gases and human- induced global warming. Accordingly, one important implication of these results is that because these megadroughts occurred under conditions not too unlike today’s, the United States still has the capacity to enter into a prolonged state of dryness even in the absence of increased greenhouse-gas forcing.
In response to increased concentration of GHGs, the semi-arid regions of the Southwest are projected to dry in the 21st century, with the model results suggesting, if they are correct, that the transition may already be underway (Seager et al., 2007). The drying in the Southwest is a matter of great concern because water resources in this region are already stretched, new development of resources will be extremely difficult, and the population (and thus demand for water) continues to grow rapidly.  Other subtropical regions of the world are also expected to dry in the near future, turning this feature of global hydroclimatic change into an international issue with potential impacts of migration and social stability.  The midcontinental U.S. Great Plains could also experience changes in water supply impacting agricultural practices, grain exports, and biofuel production.                                                  


There is no clear evidence to date of human-induced global climate change on North American precipitation amounts. However, since the IPCC AR4 report, further analysis of climate model scenarios of future hydroclimatic change over North America and the global subtropics indicates that subtropical aridity is likely to intensify and persist due to future greenhouse warming. This projected drying extends poleward into the United States Southwest, potentially increasing the likelihood of severe and persistent drought there in the future. If the model results are correct, then this drying may have already begun, but currently cannot be definitively identified amidst the considerable natural variability of hydroclimate in Southwestern North America.


NOTE:  The term "forcing” is used throughout this report to indicate any mechanism that causes the climate system to change, or respond. Examples of forcings discussed in this report include freshwater forcing of ocean circulation and changes in sea-surface temperatures and radiative forcing as a forcing of drought. As defined by the IPCC Third Assessment Report (Church et al., 2001), radiative forcing refers to a change in the net radiation at the top of the troposphere caused by a change in the solar radiation, the infrared radiation, or other changes that affect the radiation energy absorbed by the surface (e.g., changes in surface reflection properties), resulting in a radiation imbalance. A positive radiative forcing tends to warm the surface on average, whereas a negative radiative forcing tends to cool it. Changes in GHG concentrations represent a radiative forcing through their absorption and emission of infrared radiation.




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