Anthropogenic Climate Change

4.3 Anthropogenic Climate Change

There may not be any skeptical world governments left when it comes to the hard scien- tific evidence regarding anthropogenic causes of accelerated global warming. This may still not be true for some scientists who argue that the observed global rise in temperature is just part of the normal and well-documented, long-term Milankovitch cycles of climate change. It is very likely that the change of the official position of some governments, no- tably of Australia and the United States, was triggered by a series of reports produced by the IPCC and released during 2007. The reports are available for free download at the following Web site: http://www.ipcc.ch/ipccreports/assessments-reports.htm.

What has also undeniably helped in turning the attention of the general public in the United States to this scientific effort is the reality on the ground, as illustrated by the following excerpts from a report issued by the National Climatic Data Center in January of 2007 (NCDC, 2007b):

The 2006 average annual temperature for the contiguous U.S. was the warmest on record and nearly identical to the record set in 1998, according to scientists at NOAA’s National Climatic Data Center

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in Asheville, N.C. Seven months in 2006 were much warmer than average, including December, which ended as the fourth warmest December since records began in 1895. Based on preliminary data, the 2006 annual average temperature was 55 ◦

C) above the 20th century mean and 0.07 ◦

F or 2.2 ◦

F (1.2 ◦

C) warmer than 1998. These values were calculated using a network of more than 1,200 U.S. Historical Climatology Network stations. These data, primarily from rural stations, have been adjusted to remove artificial effects resulting from factors such as urbanization and station and instrument changes which occurred during the period of record.

F (0.04 ◦

U.S. and global annual temperatures are now approximately 1.0 ◦ F warmer than at the start of the 20th century, and the rate of warming has accelerated over the past 30 years, increasing globally since the mid-1970’s at a rate approximately three times faster than the century-scale trend. The past nine years have all been among the 25 warmest years on record for the contiguous U.S., a streak which is unprecedented in the historical record.

The IPCC (2007) shows that 11 of the last 12 years (1995 to 2006) rank among the

12 warmest years in the instrumental record of global surface temperature (since 1850). The temperature increase is widespread over the globe and is greater at higher northern latitudes. Land regions have warmed faster than the oceans.

Earth is naturally heated by the incoming energy from the sun. Over the long term, the amount of incoming solar radiation absorbed by the earth and atmosphere is balanced by the earth and atmosphere releasing the same amount of outgoing longwave (thermal) radiation. About half of the incoming solar radiation is absorbed by the earth’s surface. This energy is transferred to the atmosphere by warming the air in contact with the surface (thermals), by evapotranspiration, and by thermal radiation that is absorbed by clouds and greenhouse gases (GHGs). The atmosphere, in turn, radiates thermal energy back to earth as well as out to space (Kiehl and Trenberth, 1997).

Thermal (longwave) radiation that leaves the earth (is not radiated back to the land surface) allows earth to cool. The portion that is reabsorbed by water vapor, CO 2 , and other greenhouse in the atmosphere (called GHGs because of their heat-trapping capac- ity) and then re-radiated back toward the earth’s surface contributes to its heating. On the whole, these reabsorption and re-radiation processes are beneficial. If there were no GHGs or clouds in the atmosphere, the earth’s average surface temperature would be a very

chilly –18 ◦ C (0 F) instead of the comfortable 15 C (59

F) that it is today (Riebek, 2007). Changes in atmospheric concentrations of GHGs and aerosols, land cover, and solar radiation all can alter the energy balance of the climate system. Global atmospheric con- centrations of main GHGs, CO 2 , methane (CH 4 ), and nitrous oxide (N 2 O), have increased markedly as a result of human activities since 1750 and now far exceed preindustrial val- ues determined from ice cores spanning back 650,000 years. CO 2 is the most important anthropogenic GHG. Its annual emissions grew by about 80 percent between 1970 and 2004 (IPCC, 2007). The long-term trend of declining CO 2 emissions per unit of energy supplied reversed after 2000, causing an additional alarm as energy consumption is in- creasing in all countries. Global increases in CO 2 concentrations are due primarily to fossil fuel use, with land use change providing another significant but smaller contribu- tion. It is very likely that the observed increase in CH 4 concentration is predominantly due to agriculture and fossil fuel use. Methane growth rates have declined since the early 1990s, consistent with total emission (sum of anthropogenic and natural sources) being

nearly constant during this period. The increase in N 2 O concentration is primarily due to agriculture (IPCC, 2007). Observations of the CO 2 concentrations in the atmosphere and the related increasing temperature (see Fig. 4.1), as well as hundreds of scientific studies analyzed by the IPCC,

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univocally support the theory that GHGs are warming the world. Between 1906 and 2006, the average surface temperature of the earth rose 0.74 ◦

C. Average northern hemisphere temperatures during the second half of the twentieth century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years.

Consistent with warming, global average sea level has risen since 1961 at an average rate of 1.8 (1.3 to 2.3) mm/year and since 1993 at 3.1 (2.4 to 3.8) mm/year, with contri- butions from thermal expansion, melting glaciers and ice caps, and the polar ice sheets. The world’s mountain glaciers and snow cover have receded in both hemispheres, and Arctic sea ice extent has been continuously shrinking by 2.7 percent per decade since 1978, with larger decreases observed during summers (average decrease of 7.4 [range 5.0 to 9.8] percent per decade; IPCC, 2007).

As the 2007 reports by the IPCC were being released, new data from satellites and ground observations indicated that the rate of ice melting is even faster than originally estimated. One of the mechanisms of particular concern is the accelerated formation of basal streams of melted water flowing at the base of continental glaciers (Fig. 4.12). Basal streams can cause accelerated melting and retreat of glaciers, as well as their increased movement and sliding into the oceans. This means that the worldwide observed sea level rise may continue at a more rapid rate than what various models of global warming have predicted. A faster shrinking of snow and ice cover will also produce faster global warming because less solar energy will be reflected back into the atmosphere.

F IGURE 4.12 Basal glacial stream flowing into the Gulf of Alaska near Seward, AK. Ice discharge can be accelerated by basal streams like this one. (Photograph courtesy of Jeff Manuszak.)

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The emerging evidence suggests that global warming is already influencing the weather. For example, the IPCC reports the following:

r From 1900 to 2005, precipitation increased significantly in eastern parts of North and South America, northern Europe, and northern and central Asia but declined

in the Saharan Africa, the Mediterranean, southern Africa, and parts of southern Asia. Globally, the area affected by drought has likely increased since the 1970s.

r It is very likely that over the past 50 years cold days, cold nights, and frosts have become less frequent over most land areas, and hot days and hot nights have

become more frequent. r It is likely that heat waves have become more frequent over most land areas, the frequency of heavy precipitation events has increased over most areas, and since 1975 the incidence of higher sea levels has increased worldwide.

r There is observational evidence of an increase in intense tropical cyclone activ- ity in the North Atlantic since about 1970, with limited evidence of increases

elsewhere. By using best estimates from various modeled scenarios, the IPCC projects that aver-

age surface temperatures could rise between 1.8 ◦ C and 4

C by the end of the twenty-first century. Taking into consideration modeling uncertainties, the likely range of these sce-

C (IPCC, 2007). The lower estimates come from best-case scenarios in which environmental-friendly technologies such as fuel cells and solar pan- els replace much of today’s fossil-fuel combustion.

narios is between 1.1 ◦ C and 6.4

As discussed by Riebek (2007), these numbers probably do not seem threatening at first glance. After all, temperatures typically change a few tens of degrees whenever a storm front moves through. Such temperature changes, however, represent day-to-day regional fluctuations. When surface temperatures are averaged over the entire globe for extended periods of time, it turns out that the average is remarkably stable. Not since the end of the last ice age 20,000 years ago, when earth warmed about 5 ◦

C, has

the average surface temperature changed as dramatically as the 1.1 to 6.4 ◦

C plausible change predicted for the twenty-first century.

Regional-scale changes projected by the IPCC models are as follows:

r Warming will be greatest over land and at most high northern latitudes and least over Southern ocean and parts of the North Atlantic ocean, continuing recent

observed trends in contraction of snow cover area, increases in thaw depth over most permafrost regions, and decrease in sea ice extent; in some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the twenty-first century.

A likely increase in the frequency of hot extremes, heat waves, and heavy pre- cipitation.

A likely increase in tropical cyclone intensity.

A poleward shift of extra-tropical storm tracks with subsequent changes in wind, precipitation, and temperature patterns. r Precipitation increase in high latitudes and likely decreases in most subtropical land regions, continuing observed recent trends.

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Particularly alarming is the possibility of some abrupt or irreversible changes, which cannot be simulated by various models or show less agreement between models but nevertheless cannot be excluded. According to IPCC, they are as follows:

r Partial loss of ice sheets on polar land could imply meters of sea-level rise, major changes in coastlines, and inundation of low-lying areas, with the greatest effects

in river deltas and low-lying islands. Such changes are currently projected to occur over millennial time scales, but more rapid sea-level rise on century time scales cannot be excluded.

r Contraction of the Greenland ice sheet is projected to continue to contribute to sea-level rise after 2100. Current models suggest virtually complete elimination

of the Greenland ice sheet and a resulting contribution to sea-level rise of about

7 m if global average warming were sustained for a millennium in excess of 1.9 to

4.6 ◦ C relative to preindustrial values. The future temperatures in Greenland will

be comparable to those for the last interglacial period 125,000 years ago, when paleoclimatic information suggests a reduction in polar land ice and a 4 to 6 m of sea-level rise. However, as in the case of the Antarctic, net loss of ice mass may happen faster if dynamic ice discharge dominates the ice sheet mass balance (see Fig. 4.12).

r There is medium confidence that approximately 20 to 30 percent of species as- sessed so far are likely to be at increased risk of extinction if increases in global

average warming exceed 1.5 ◦ C to 2.5

C (relative to 1980 to 1999). As global ◦ average temperature increase exceeds about 3.5

C, model projections suggest

significant extinctions (40 to 70 percent of species assessed) around the globe.

4.3.1 Impacts on Surface Water and Groundwater Resources

Based on global climate models, the IPCC (2007) has high confidence that by mid-century, annual river runoff and water availability are projected to increase at high latitudes (and in some tropical wet areas) and decrease in some dry regions in the mid-latitudes and tropics. There is also high confidence that many semiarid areas (e.g., Mediterranean basin, western United States, southern Africa, and northeast Brazil) will suffer a decrease in water resources due to climate change. For example, Fig. 4.13 shows how the warm- ing trend in western United States winters mirrors annual global temperature trends. Warmer-than-average winter temperatures have been evident across the region, particu- larly over the last 6 years. The western United States has also experienced drought over the last 6 years.

The most obvious consequence of increasing winter temperatures will be the reduc- tion in snowpack, which is the main source of river runoff in the West. In 2004, the USGS modeled the effects of climate change on three river basins draining the Sierra Nevada Mountains (Dettinger et al., 2004). Two basins experienced a dramatic decrease in snow- pack under a “business as usual” scenario (no cutbacks in carbon emissions). Figure 4.14 demonstrates these results graphically for all three basins. The American River Basin has the lowest average elevation and, therefore, the highest sensitivity to climate change (Dettinger et al., 2004). The USGS model predicts that the snow cover in the American River Basin will be negligible by the year 2100. Reduced snowpack occurs as temperature increases result in a higher percentage of precipitation falling as rainfall, as demonstrated in Fig. 4.15 for the Merced River Basin.

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Precipitation (in.)

Ending year

F IGURE 4.13 Western United States (11 states) October–March precipitation and temperature, 1895–2005. Bold line is the 11-year running mean. (From Edwards and Redmond, 2005; source: Western Regional Climate Center.)

The USGS notes that the most significant hydrologic change, due to global warming, will be higher streamflows in the late winter/early spring (March) due to more rainfall and earlier snowmelt. The earlier snowmelt of reduced magnitude will cause water shortages in the late spring and summer, when water demand is high (Dettinger et al., 2004).

According to IPCC (2007), there is high confidence that some hydrological systems have been already affected through increased runoff and earlier spring peak discharge in many glacier- and snow-fed rivers. Global warming has also affected the thermal structure and water quality of warming rivers and lakes.

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150 Historical run

(a) Merced River Business-as-usual s 100

imeter Cent 50

Historical run (b) Carson River Business-as-usual 150

imeters 100 Cent 50

30 Historical run

(c) American River

25 Business-as-usual s

F IGURE 4.14 Basin average snow water contents on April 1, in three river basins draining the Sierra Nevada Mountains in the western United States. (From Dettinger et al., 2004.)

IPCC projects that the mechanism of decreasing snowpack, receding glaciers, and increasing early spring runoff will affect other mountainous regions across the globe and cause shortages of surface water supplies in regions at lower elevations. Most notably, the water supply of some of the most populated regions of India and China depends on large rivers originating in the high mountain ranges of Himalayas. Figure 4.16 is a glimpse of the receding glaciers across the globe.

The reduced winter snowpack will also have consequences for groundwater recharge of intermountain basins; less and shorter snowmelt means that the surface flow will last

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ion 60 Historical run

F IGURE 4.15 Rainfall as a percentage of precipitation in the Merced River Basin. (From Dettinger et al., 2004.)

for shorter periods of time, which directly translates into less groundwater recharge at basin margins and via bedrock. A greater percentage of precipitation falling on moun- tain ranges and at basin margins will be in the form of rainfall immediately becoming surface water runoff. Similarly unfavorable for groundwater recharge conditions is the IPCC’s projection that an increase in rainfall in some regions will come in the form of bigger, wetter storms, rather than in the form of more rainy days. In between those

F IGURE 4.16 Receding alpine glaciers in the Nutzotin Mountains, eastern Alaska Range. Note the series of terminal moraines in the foreground. These glaciers feed a network of streams that will

become ephemeral once the glaciers have melted completely. (Photograph courtesy of Jeff Manuszak.)

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larger storms will be longer periods of light or no rain, so the frequency of drought will increase.

Rising temperatures and heat waves will result in an increased demand for water and, together with droughts, will put additional pressure on both surface water and ground- water resources. Warming of reservoirs and surface streams will result in water quality problems such as algal blooms. Higher temperatures will cause higher water losses from surface water reservoirs due to increased evapotranspiration. Increased evapotranspira- tion will also affect shallow subsurface soil, causing higher soil moisture deficits. A larger fraction of the infiltrated precipitation will therefore be spent on satisfying this soil mois- ture deficit before deep circulation and actual groundwater recharge can take place. In areas with irrigated agriculture, higher temperatures, heat waves, and droughts will put additional demand on water resources because more water will have to be diverted from streams and reservoirs or pumped from aquifers to offset higher evapotranspiration and higher soil moisture deficit.

More frequent episodes of heavy precipitation will cause deterioration of water qual- ity in surface streams due to increased turbidity and runoff from nonpoint sources of contamination. A related increased incidence of floods will put pressure on water sup- ply infrastructure and increase the risks of water-borne diseases.

A sea-level rise will cause salinization of irrigation water in coastal areas, estuar- ies, and freshwater systems, and saltwater intrusion in shallow aquifers underlying low coastal areas. In addition, population relocation from the semiarid and arid regions severely affected by prolonged droughts and from the low-lying coastal areas affected by the sea-level rise will put pressure on water resources in other regions that otherwise may not be significantly impacted by global warming.

Some of the projected continental-scale impacts of global warming are given below (IPCC, 2007).

Africa

r By 2020, between 75 and 250 million people are projected to be exposed to in-

creased water stress due to climate change. r By 2020, in some countries, yields from rain-fed agriculture could be reduced by up to 50 percent. Agricultural production, including access to food, in many

African countries is projected to be severely compromised. This would further adversely affect the predictability of food production and exacerbate malnutri- tion.

r Toward the end of the twenty-first century, the projected sea-level rise will affect low-lying coastal areas with large populations. The cost of adaptation could

amount to at least 5 to 10 percent of gross domestic product (GDP). r By 2080, arid and semiarid land in Africa is projected to increase by 5 to 8 percent

under a range of climate scenarios. Asia

r By the 2050s, freshwater availability in central, south, east, and southeast Asia,

particularly in large river basins, is projected to decrease. r Coastal areas, especially heavily populated megadelta regions in south, east, and southeast Asia, will be at greatest risk due to increased flooding from the sea and,

in some megadeltas, flooding from the rivers.

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r Climate change is projected to compound pressures on natural resources and the environment, along with rapid urbanization, industrialization, and economic

development. r Endemic morbidity and mortality due to diarrheal disease primarily associated

with floods and droughts are expected to rise in east, south, and southeast Asia due to projected changes in the hydrological cycle.

Australia and New Zealand r By 2020, a significant loss of biodiversity is projected to occur in some ecologically

rich sites including the Great Barrier Reef and Queensland Wet Tropics. r By 2030, water availability problems are projected to intensify in southern and eastern Australia and, in New Zealand, in Northland and some eastern regions. r By 2030, production from agriculture and forestry is projected to decline over

much of southern and eastern Australia, and over parts of eastern New Zealand, due to increased drought and fire. However, in New Zealand, initial benefits are projected for some other regions.

r By 2050, ongoing coastal development and population growth in some areas of Australia and New Zealand are projected to exacerbate risks from sea-level rise

and increases in the severity and frequency of storms and coastal flooding. Europe

r Climate change is expected to magnify regional differences in Europe’s natural resources and assets. Negative impacts will include an increased risk of inland

flash floods, more frequent coastal flooding, and increased erosion (due to an increase in storms and sea-level rise).

r Mountainous areas will experience glacier retreat, reduced snow cover and win- ter tourism, and extensive species losses (in some areas up to 60 percent by 2080).

r In southern Europe, climate change is projected to worsen conditions (high tem- peratures and drought) in a region already vulnerable to climate variability and

to reduce water availability, hydropower potential, summer tourism, and, in general, crop productivity.

r Climate change is also projected to increase the health risks due to heat waves and the frequency of wildfires. Latin America

r By mid-century, increases in temperature and associated decreases in soil water are projected to lead to gradual replacement of tropical forest by savanna in

eastern Amazonia. Semiarid vegetation will be slowly replaced by arid-land vegetation.

r There is a risk of significant biodiversity loss through species extinction in many areas of tropical Latin America. r Productivity of some important crops is projected to decrease and livestock pro- ductivity to decline, with adverse consequences for food security. In temperate

zones, soybean yields are projected to increase. Overall, the number of people at risk of hunger is projected to increase.

r Changes in precipitation patterns and the disappearance of glaciers are projected to significantly affect water availability for human consumption, agriculture, and

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North America

r Warming in the mountains of the American west is projected to cause decreased snowpack, more winter flooding, and reduced summer flows, exacerbating com-

petition for over-allocated water resources. r In the early decades of the century, moderate climate change is projected to increase aggregate yields of rain-fed agriculture by 5 to 20 percent, but with important variability among regions. Major challenges are projected for crops

that are near the warm end of their suitable range or that depend on highly utilized water resources.

r During the course of this century, cities that currently experience heat waves are expected to be further challenged by increased number, intensity, and duration

of heat waves during the course of the century, with the potential for adverse health impacts.

r Coastal communities and habitats will be increasingly stressed by the changing

climate. Polar regions

r The main projected biophysical effect is a reduction in thickness and extent of glaciers and ice sheets and sea ice, and changes in natural ecosystems with detri-

mental effects on many organisms including migratory birds and mammals. r For human communities in the Arctic, impacts, particularly those resulting from

changing snow and ice conditions, are projected to be mixed. r Detrimental impacts would include those on infrastructure and traditional in-

digenous ways of life. r In both polar regions, specific ecosystems and habitats are projected to be vul-

nerable, as climatic barriers to species invasions are lowered. Small islands

r Sea-level rise is expected to exacerbate inundation, storm surge, erosion, and other coastal hazards, thus threatening vital infrastructure, settlements, and fa-

cilities that support the livelihood of island communities. r By mid-century, climate change is expected to reduce water resources in many small islands, e.g., in the Caribbean and Pacific, to the point where they become

insufficient to meet demand during low-rainfall periods.

Finally, the author includes the following related discussion by the National Climatic Data Center of NOAA (NCDC, 2007c):

Climate change refers to the changes in average weather conditions that generally occur over long periods of time, usually centuries or longer. Occasionally, these changes can occur more rapidly, in periods as short as decades. Such climate changes are often characterized as “abrupt”.

Until recently, many scientists studying changes and variations in the climate thought that the climate system was slow to change, and that it took many thousands if not millions of years for ice ages and other major events to occur. Scientists are just beginning to formulate and test hypotheses regarding the causes of abrupt climate change, but only a handful of attempts have been made to model abrupt change using computer models. These efforts are focused not only on past events, but also on abrupt events that might occur in the future as greenhouse gases increase in the atmosphere

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Central Greenland Climate -25

-30 °C) -35

-40 Temperature ( -45

The Younger Dryas

-50 -55

20 15 10 5 0 Age—Thousands of Years Before the Present

F IGURE 4.17 As the earth’s climate emerged from the most recent ice age, the warming that began 15,000 years ago was interrupted by a cold period known as the Younger Dryas, which in turn ended with abrupt warming. (From NCDC, 2007c; source: Cuffey and Clow, 1997; Alley, 2000.)

One of the best studied examples of abrupt change occurred as the Earth’s climate was changing from a cold glacial to a warmer interglacial state. During a brief period lasting about a century, temperatures in most of the northern hemisphere rapidly returned to near-glacial conditions, stayed there for over 1,000 years in a time called the Younger Dryas (named after a small Arctic flower) then about 11,500 years ago quickly warmed again. In some places, the abrupt changes may have been as large as 10 ◦

C, and may have occurred over a decade (Fig. 4.17). Changes this large would have a huge impact on modern human society, and there is a pressing need to develop an improved understanding and ability to predict abrupt climate change events. Indeed, this is the goal of several national and international scientific initiatives.

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CHAPTER 5

Groundwater Quality