Continuing Unit 5 this week covering chapters 17. Notes below are the things I highlighted in my book.

Chapter 17 Climate Changes During the Last 1,000 Years

The little Ice Age

• As the last millennium began, scattered evidence  suggests a time of relatively warm climate known as the medieval warm period. (From 1000 to 1300). The subsequent cooling into the Little Ice Age (1400–1900)
• Sea ice appears to have been very rare until 1200 and infrequent until 1600. Sea ice increased in frequency and reached a maximum during the 1800s, but then all but disappeared from the coasts during the twentieth century. Because major ice sheets did not actually form, the Little Ice Age was not a true ice age
• In a few locations, small ice caps that persist at or near sea level are presently melting rapidly in the warmth of the modern climate.
  • Rock outcrops in these regions are covered by lichen, a primitive moss-like form of vegetation that can live on bare rock surfaces even under inhospitable conditions. Use lichen to indicate and date ice age
• The Little Ice Age is often used as the “type example” of a cooler Earth from the recent past, with the implicit assumption that the cooling that caused it was hemispheric or even global in extent. Might have been a local phenomenon restricted to this one region.
• Before the nineteenth century, people rarely kept records of temperature or precipitation.
  • As a result, climate scientists have to rely mainly on proxy records in archives like those used for earlier intervals of Earth’s history, such as lake sediments and ice cores.
  • In addition, sources of proxy records such as mountain glaciers, tree rings, and corals can also be used to reconstruct climate changes across Earth’s surface during the last few centuries.

17-1 Ice Cores from Mountain Glaciers

• Glaciers in mountain valleys and small ice caps covering mountain summits are excellent climate archives. Some ice dates back many thousands of years into the last glaciation, while others span only a few hundred years of climate history. Layers deposited at the surface usually contain annual signals, but ice flow may degrade the resolution deeper in the ice.
• Retrieving ice cores from mountains is a formidable task because of heavy equipment transport, mountain ice caps thickness, lack of oxygen, and ice core transport.
• Only a few mountain glaciers have been cored. By far the most extensive efforts have been those by the intrepid glacial geologist Lonnie Thompson.
  
  • Based on counting annual layers and matching volcanic ash layers with historically documented eruptions, he found that these records extend back to 1,500 years ago.
• Cores taken in the 1980s show annual-scale changes in δ18O values and dust concentrations that can be averaged over decadal intervals.
  • The variations in δ18O values reflect the same processes as those affecting continent-sized ice sheets: changes in source area, transport paths, amount of water vapor carried to the glacier, and particularly temperature in the atmosphere above the ice where the snow condenses. Higher dust concentrations indicate some combination of drier source areas and stronger winds
• Early cores from the Quelccaya ice cap registered a shift toward more positive δ18O values and less dust near the year 1900, implying that a change toward some combination of warmer temperatures, weaker winds, and different source areas occurred at that time.
  • This δ18O record also resembles the Little Ice Age pattern, with more positive values from 1000–1400 and then more negative values after 1500. In contrast, the dust record does not show the expected match before 1600
  • if anything, the lowest dust concentrations occurred within the early part of the Little Ice Age, and higher concentrations during the medieval warm period.
• During the 1970s and 1980s, previous coring expeditions at the top of the ice cap had found annual layering extending from the most recent ice all the way down to the deepest layers deposited 1,500 years ago.
• In the new cores, however, meltwater percolating down from the surface had begun to destroy the uppermost annual layer. Tropical ice cap that had been continuously recording intact annual layering for 1,500 years had suddenly begun to melt. In this location, the warming of the late 20th century was obviously unprecedented for the last millennium and a half.

17-2 Tree Rings

• The use of tree rings to reconstruct climate change over the last several hundred years or more is called dendroclimatology. 
• In regions of large seasonal changes, trees produce annual rings of varying distinctness depending on species. The rings shift from lighter, low-density “early wood” of spring and early summer to darker, denser bands of “late wood” at the end of the growing season.
• Warmth and abundant rainfall during the growing season are favorable to tree growth, while cold and drought inhibit growth.
• The basic strategy for tree ring studies is to search out regions where trees are most sensitive to climatic stress, usually at the limit of their natural temperature or precipitation ranges. Changes between favorable and unfavorable growth years produce distinctive variations in the widths and other properties of tree rings
• An integrated signal developed from trees distributed over the entire circumarctic region shows cool conditions in the late 17th and early 18th centuries, some warming in the middle and late 18th century, a deeper cooling in the early to mid-19th century, a slow but substantial warming after 1850, a brief cooling between 1950 and 1970, and a small warming since 1980.
• The 320-year trend covers the middle and end of the Little Ice Age and the instrumentally recorded warming of the twentieth century.
  • Estimated regional temperature variations larger than 1C occurred around the Arctic within the Little Ice Age, which was clearly not a time of unrelieved cold. The warming of the Arctic in the mid-twentieth century reached values unique for the 320 years of record, but temperatures in the late twentieth century decreased to values similar to those in the 1700s.
• Central Asia important region for tree ring studies because Asia is the largest continent, its climate is less subject to the moderating effects of ocean water than other regions.
• Far fewer data are available from trees at middle to high latitudes of the Southern Hemisphere, which is mainly ocean. Records from pines on the island of Tasmania, south of Australia, extend back more than 2,000 years, and the warmth of the last several decades matches any level reached during that interval.
• The interval of the Little Ice Age after 1500 is cooler than the late twentieth century but does not stand out distinctively.

17-3 Corals and Tropical Ocean Temperatures

• Observations of climate changes at annual or decadal resolution are generally not available from the deep ocean because of slow deposition and mixing of sediments by burrowing organisms.
• Climate scientists have exploited corals as climate archives, using annual bands in their MgCO3 or CaCO3 structures.
  • Most corals grow in warm tropical or subtropical oceans, the information they provide complements ice core and tree ring studies from higher latitudes and altitudes.
• The most widely used climatic index in corals is δ18O measurements at seasonal or better resolution. The two major controls on δ18O variations are temperature and salinity. 
• Longer-term δ18O trends in the Pacific Ocean are overprinted by large year-to-year fluctuations in the El Niño and ENSO systems.
  • These annual δ18O variations largely reflect temperature changes in the eastern tropical Pacific Ocean and combined temperature-salinity changes in the central and western Pacific Ocean
• Modern corals deposited on the coast of the Galápagos Islands in the eastern Pacific record seasonal temperature changes similar to those measured directly by thermometers in surface waters.
  • The match between the δ18O record and ocean temperature is not perfect because salinity changes also affect the δ18O values. Prominent El Niño years appear as δ18O minima that indicate warm temperatures
• A regional average signal created by combining records from the Pacific and Indian Oceans shows that the late twentieth century is the warmest period for at least the last 400 years. An interval of heavier δ18O values during the 1800s was preceded by one of greater warmth during the 1700s, although the earliest part of the record is based on just a handful of sites.

17-4 Other Historical Observations

• As human civilizations developed, people in some regions began to keep records of climatic phenomena for reasons unique to the cultures in which they lived.
• Climate scientists who attempt to use these early historical records to reconstruct past climate have to weigh their reliability carefully.
• The phenomena recorded also vary widely from place to place. They include: the frequency and timing of first and last frosts and of droughts and floods, the timing of autumn lake or river freeze-up and spring ice breakup, the dates of the first flowering of shrubs and trees, and the dates of harvests
• Immediately after conquering the Inca empire, the Spanish began to make environmental observations along the coast of Peru.
  • Ships’ logs are the major source of information from this region, supplemented by records kept by missionaries and others.
• These observations include responses now understood to result from El Niño events, including sea-surface temperatures warmer than normal, reduced catches of anchovy and other fish, departure of sea birds from coasts and islands, unusually heavy rains and floods, and outbreaks of cholera and malaria.
• The records start in 1525 and continue through most of the twentieth century.
• El Niño events during this interval have been ranked by historians on a qualitative scale ranging from none to very severe.
  • With 115 events in 465 years, the time between successive El Niño events averages 4 years, but the timing varies widely around this number.El Niño events can cluster within certain intervals but be rare in others.
• For a number of reasons, histories of climatic phenomena such as the El Niño record and the Icelandic Sea ice record are difficult to use in comparing large-scale climate changes.
  • The records come from widely scattered locations that do not even provide regional, much less global, coverage. Also, different indices are sensitive to climate changes during different seasons of the year. As a result, these indices record changes in parts of the climate system with widely varying response times.
  • For this reason, historical observations recorded before the era of weather instruments give us only anecdotal information about climate changes during preceding centuries.

17-5 Orbital Forcing

• Orbital forcing contributed to the gradual cooling that occurred in higher northern latitudes between 1000 and 1850. Over the last 6,000 years, portions of the circumarctic have cooled by 1–2C because of decreasing summer insolation at both the tilt and precession cycles.
• If the rate of cooling were uniform over the entire 6,000 years, the fraction of the cooling that would have occurred between 1000 and 1850 would have amounted to between 0.15 and 0.3C.
  
  • The slow background cooling trend between 1000 and 1850 is consistent with gradual forcing from orbital changes.

17-6 The Millennia Bipolar Seesaw

• Millennial oscillations have been proposed as another explanation for the northern hemisphere cooling during the last millennium, but climatic trends during the last 8,000 years have been highly irregular and local in extent, with no evidence of widespread oscillations.
• If the north polar cooling into the Little Ice Age was part of a small millennial oscillation, it should have been accompanied by a very small warming in the Antarctic region, based on the pattern typical of the large glacial-age oscillations.
• The proxy coverage of the last 1,000 years in the Southern Hemisphere does not suggest such a pattern, but current coverage is insufficient to test this idea fully.

17-7 Solar Variability

• The 14C and 10Be evidence suggested that solar variability does not cause changes in climate over millennial time scales, but there is a chance that changes in solar output could still play a significant role in changes at decadal and century scales
• One possible solar-terrestrial link for the last several centuries is tied to the 11-year sunspot cycle.
  • Continuous and accurate observations of the number of dark sunspots visible on the Sun began with the invention of the telescope more than 400 years ago. 
• Intuitively, it would seem likely that the presence of cooler dark spots on a hot bright surface would reduce the total amount of solar radiation emitted, but in fact the actual relationship is just the reverse.
  • During years when sunspots are abundant, the amount of radiation emitted in solar flares is at a maximum because mechanisms operating within the Sun simultaneously regulate both sunspots and net solar emissions. The amount of solar radiation arriving at Earth during sunspot maxima is at a maximum rather than a minimum. Based on satellite measurements in the past two decades, the range of variation in solar irradiance is about 0.11%.
• A longer-term trend is also apparent in the sunspot record assembled from telescope observations, which show larger sunspot numbers in the last three centuries compared to the middle to late 1600s.
  • An interval between 1645 and 1715 that had almost no sunspots is called the Maunder sunspot minimum. A similar interval between 1460 and 1550 is called the Sporer sunspot minimum.
• Some scientists initially proposed that variations in solar output during the last several centuries were considerably larger than the small range measured during the satellite era. They suggested that solar output might have been as much as 0.25–0.4% weaker than today during the Maunder and Sporer minima, with a gradual rise in irradiance during the last few centuries. Because these intervals of low sunspot activity persisted for decades, the slow-responding parts of the climate system would have had time to respond to them more fully compared to the brief 11-year cycle.
• Later astronomical observations have failed to support this claim. Archival images of Sun like stars from several observatories failed to show variations comparable to those proposed for the Sun.
• The Sun also sends out a plasma or ionized gas called “the solar wind,” which interacts with Earth’s stratosphere as it is deflected by Earth’s magnetic field.
• One possibility under consideration is that the solar wind affects the formation of ozone, which in turn alters the formation of clouds in the troposphere and thereby affects climate at Earth’s surface. Other possible links via the stratosphere continue to be explored.
• The similarity of 14C and 10Be trends during the last 5,000 years seems to point to a common origin from solar changes.
• Scientists who have looked for a correlation between these isotopic trends and climatic proxies during recent millennia have met with mixed success.
  • Temperature-sensitive changes in δ18O within the last millennium show a substantial correlation with 10Be variations, implying a solar role. The fact that no such relationship is evident in previous millennia of the Holocene weakens this case. The possibility also remains that the similar changes in 14C and 10Be are both responses to changes occurring within the climate system.

17-8 Volcanic Explosions

• Explosive eruptions of volcanoes cool climate over intervals of a few years. Volcanoes emit sulfur dioxide gas, which mixes with water vapor in the air and forms droplets and particles of sulfuric acid called sulfate aerosols.
  • Eruptions can reach 20 to 30 km into the stratosphere, where the aerosols block some incoming solar radiation and keep it from reaching the ground. 
• The latitude of eruption determines the geographic extent of their climatic impact.
  • Volcanoes that erupt poleward of about 25° produce particles that mostly stay within the hemisphere in which the eruption occurs, limiting the cooling impact to that hemisphere. Explosions that occur in the tropics are redistributed by Earth’s atmosphere to both hemispheres and can have a global impact on climate
• Ocean-island volcanoes with iron- and magnesium rich compositions tend not to cause explosive eruptions but instead emit lava that flows across the land.
  • With so brief a time in the atmosphere, these particles cannot be widely enough distributed around the planet to produce large-scale effects on climate.
• In contrast, volcanoes located along converging plate margins are fed by magmas richer in silica and other elements found in continental crust.
  • Their eruptions are more explosive because the natural resistance of this kind of molten magma to internal flow causes internal pressures to build up to the point where highly explosive eruptions send volcanic particles up into the stratosphere, well above the level where precipitation can wash them out.
• Sulfate aerosol concentrations in the stratosphere caused by explosive eruptions reach their maximum distribution within months and then begin to decrease as gravity gradually removes the particles.
  • This decrease follows an exponential trend: each year about half of the remaining particles settle out, and within two to three years aerosol concentrations are much reduced. The effect of these aerosols on temperature follows the same trend, with a maximum initial cooling that soon fades away. If several explosions follow within an interval of a few years, their impact on climate may be sustained for a decade or more.
• Reconstructing the effects of older volcanic explosions on climate is difficult.
  • As a result, it is difficult to estimate the potential climatic effects of ancient eruptions reliably based on this kind of evidence.
• Sulfate layers preserved in well-dated ice cores offer another way to assess past explosions.
  • Comparison of these volcanic signals with the northern hemisphere temperature trends indicates that sequences of large eruptions played a role in decadal-scale cooling intervals, for example during the 1200s and near 1450.
  • Precise assessments of the temperature contributions from volcanic eruptions is difficult because of complications such as varying distances from the eruptive source to the ice sheets.

17-9 Greenhouse-Gas Effect on Climate

• Changes in carbon dioxide concentrations in the atmosphere are another potential factor in the cooling of the last millennium prior to 1850. CO2 concentrations were relatively high (284ppm) near 1,100 to 1,200 but fell as low as 274 ppm by 1600.
  • Although this declining CO2 trend broadly matches the reconstructed northern hemisphere cooling, the cooling was more gradual, while much of the CO2 decrease was concentrated between 1525 and 1600. This short drop in CO2 has generally been attributed to natural factors, but humans may also have played a role.
• The natural explanation relies mainly on the fact that CO2 solubility in seawater depends mainly on temperature, with lower temperatures increasing the solubility and allowing the ocean to hold larger amounts of CO2.
  • But the northern hemisphere cooling estimated for the 1525–1600 interval in most reconstructions was too small to explain the CO2 drop recorded in the ice cores.
  • Models that simulate interactions between the carbon cycle and the rest of the climate system indicate that the 1525–1600 cooling explains less than 2 ppm of the observed 10-ppm CO2 decrease.
• The other proposed explanation is that this CO2 decrease was anthropogenic in origin. The early anthropogenic hypothesis is based on the premise that deforestation by humans was the primary cause of the anomalous increase in atmospheric CO2 during the last 7,000 years.
• The drop in CO2 values between 1525 and 1600 correlates with a terrible pandemic in the Americas, when a host of diseases carried by arriving Europeans killed tens of millions of Native Americans between 1492 and 1700, or 85–90% of the previous population.
  • When the outbreaks of diseases occurred, most Native Americans had been farming the higher levees of river valleys or clearings in the forests. After the pandemics decimated the populations, the forests grew back over the untended fields, and the carbon stored in the trees was taken from the CO2 in the atmosphere.
  • One model simulation of past land use supports the possibility of an anthropogenic link by showing large net carbon sequestration on a global basis between 1525 and 1600.
• A major loss of population in China in the early 1600s may also have contributed to the CO2 decrease at that time. In addition, other smaller CO2 decreases match historically documented intervals of population decrease caused by pandemics and civil strife.
• Humans may also have played a role in the cooling of the last millennium by changing the surface cover of the land they cleared.
  • At high latitudes, replacement of darker woodlands by lighter pastures and croplands increases the amount of incoming solar radiation reflected back to space.
  • The first major clearance of evergreen forests anywhere on Earth occurred in the area of the eastern Baltic and Russia during the cooling between 1000 and 1850.
• Before reliable cause-and-effect conclusions can be drawn, greater geographic coverage of proxy sites is needed to define the global climatic response, especially in the Southern Hemisphere. In contrast, no such ambiguity exists about the large, rapid, and global warming since 1850.