I am moving forward to Chapter 18. All notes were things I highlighted in my book.

Note: Last weeks group will present this week so the material in the presentation this week will not match this weeks chapter 18.

Chapter 18 Climate Changes Since 1850

Reconstructing Changes in Sea Level

• The average level of the ocean is a key source of information on global climatic trends over the last 150, but reconstructing changes in global sea level is complicated by lingering effects from ice sheets that melted thousands of years ago.
• Beginning as early as the late eighteenth century, seaport towns and cities installed tide gauges to measure changes in the level of the ocean caused by tides and large storms.
  
  • 100 to 200 years later, these records show short-term changes caused by tides and storms but also longer-term trends of change during the decades and centuries
• Deriving sea level trends from tide gauge records is difficult because tide gauge records in some regions indicate rapid sea level falls, while others show a slow sea level rise, and still others indicate faster rises.
• Some of these regional differences result from modern processes that vary regionally.
  • Areas of active tectonic uplift caused by mountain building or of active subsidence caused by recently added sediment loads have to be avoided.
  • adjustments must also be made for human effects such as subsidence caused by pumping of groundwater and impoundment of rainfall runoff in reservoirs behind dams.

18-1 Fading Memories of Melted Ice Sheets

• By far the greatest problem in reconstructing past sea levels is the fact that the land and ocean bedrock still retain a memory of the ice sheets from the most recent glaciation.
  
  • This bedrock “memory” causes different behaviors in the present movements of Earth’s land and seafloor surfaces. 
• One group of tide gauges shows rapid drops in relative sea level in recent centuries. These gauges are located in regions that were once directly beneath the ice sheets.
  • Relative sea level is now falling rapidly in these regions because the bedrock is still rebounding from the removal of the ice sheet load thousands of years ago.
    • A local rise in bedrock means a local fall in relative sea level.
• Bedrock at depths of 100 to 200 km has a slow viscous component of response, and it takes many thousands of years to recover fully from loads imposed upon it or removed from it.
• The enormous load of glacial ice thousands of years ago depressed bedrock surfaces beneath the central parts of the ice sheets by as much as 1 km, causing deep rock to flow slowly outward at great depths. When the ice melted, rock slowly flowed back into this region, gradually raising the depressed bedrock toward its former elevation
• The second major group of tide gauge responses shows a relatively fast rate of sea level rise.
  • This group is clustered in a halo pattern surrounding the former ice sheets but extending well beyond the ice margins.
• The rapid present rise of sea level in these regions is also caused by a memory of the glacial maximum ice sheets, even though these areas were not located directly beneath the ice loads.
  • During glacial times, the deep rock displaced from beneath the center of the ice sheet load had to go somewhere.
  • Flowing outward from the margins of the ice sheets, it caused an increase in the elevation of the land, called a peripheral forebulge.
•  After the ice sheets melted, the rock displaced beyond the ice margins gradually flowed back into the region where the ice sheet had formerly been.
  
  • Because the land in the region of the collapsing forebulges is still sinking, this local bedrock effect roughly doubles the rate of relative sea level rise in these regions.
•  The third group of tide gauge responses comes from coastlines located far from the northern hemisphere ice sheets.
  • Relative sea level in these regions is rising at rates slightly less than the global rate of sea level rise.
  • The return of glacial meltwater to the oceans has added an extra load on the bedrock beneath the ocean floor in these regions
•  At maximum size, the ice sheets extracted from the world ocean a layer of water some 110–125 m thick. With this water load removed, the average level of the crust in the ocean basins rose by over 30 m compared to the level of the nearby continents that were not directly affected by the weight of ice sheets or ocean water.
  • When this layer of water returned to the oceans during ice melting, it loaded down the ocean crust and caused it to sink. This slow sinking of the ocean crust is still going on today, and it counteracts a small part of the true rise of global sea level, producing a slightly reduced rise in relative sea level in regions far from the ice sheets.
•  Attempts to remove all these complications from the melted ice sheets indicate that sea level rose by about 17 cm during the 1900s.

Other Instrumental Records

• The “modern” era of instrumental measurements of climate can be divided into two parts:
  • An earlier period in which measurements of climate were made using land-based instruments invented during the era of scientific exploration that began centuries ago
  • An era extending back about four decades during which satellites in space have sensed climatic changes remotely

18-2 Thermometers: Surface Temperature

• Thermometers have been used to measure air temperature at a few locations in Eurasia and North America for over 200 years.
  • During this time, the surface temperature of the ocean has been measured along heavily traveled shipping routes and at ocean islands, but with large gaps in coverage at middle and high latitudes of the Southern Hemisphere because of frequent storms and extensive sea ice. Only since the late 1800s have enough stations been recording temperature to permit reasonably reliable estimates of the surface temperature of the entire planet.
• Measurements both on land and at sea have been difficult.
  • Ocean temperatures were once measured by scooping up seawater in a canvas bucket and inserting a thermometer. If a few minutes elapsed between collecting the water and measuring its temperature, evaporation cooled the water by several tenths of a degree Celsius.
  • More reliable measurements came later from thermometers embedded in the outer parts of the intake valves that drew in seawater to cool the ship’s engines.
• On land, the largest complication has been the growth in populations around many land stations.
  • Near large towns and cities, the spread of asphalt surfaces and the loss of vegetation has led to increased absorption of solar radiation during the day and greater back radiation of heat at night.
  • These changes caused an extra warming effect at urban stations called the urban heat island effect.
    • Although this warming reflects real temperature changes at these stations, it is not characteristic of changes across much more extensive rural areas. (Bias)
  • Fortunately, stations in regions of little or no population growth provide a check on the heat island effect.
  • Adjusting for this bias has reduced initial estimates of warming trends during the last century
• Reconstructions of global temperature during the last century by different groups are very similar, mainly disagreeing in the early 1900s when station coverage was still sparse.
  • Temperatures have warmed by about 0.8C over the last century. Temperatures were considerably cooler before the early 1900s, rose quickly during the 1920s to early 1940s, stabilized or fell slightly from the late 1940s through the late 1970s, and have again risen abruptly since 1980.
  • The decade between 2000 and 2010 was warmer than the 1990s, which were warmer than the 1980s, which were warmer than the 1970s.

18-3 Subsurface Ocean Temperature

• The ocean has the capacity to store enormous amounts of heat for long intervals of time.
• Changes in surface climate do not easily penetrate below the wind-mixed upper 100 meters, but important information on deeper ocean trends during the last half-century can be extracted by examining millions of subsurface temperature profiles.
• These observations have provided a detailed picture of the slow penetration of heat from the atmosphere into the subsurface layers.
  • Slow downward molecule-by-molecule diffusion has transferred some of the surface heat to depths of a few hundreds of meters.
  • In addition, near-horizontal movement of water sinking at higher latitudes has transferred even more heat into the somewhat deeper subsurface ocean. Still slower penetration to depths of 1,000 m or more has also occurred in areas of deep overturning like the subpolar North Atlantic Ocean.
• Large increases in stored heat are evident in records spanning the last half-century.
  • This trend, expressed in joules is equivalent to a net ocean temperature increase of ~0.07C.
  • Although the ocean warming since the middle 1900s is much smaller than the rise in surface air temperature, the total amount of heat stored in the ocean exceeds that in the atmosphere by more than a factor of ten. This large storage of ocean heat is evidence of a marked change in the heat balance of the entire climate system compared to earlier decades.

18-4 Mountain Glaciers

• Today, mountain glaciers cover 680 km2 of Earth’s land surface and represent about 4% of the total surface area of land ice on Earth today.
  • Mountain glaciers at middle and high latitudes respond to local climate, primarily changes in summer temperature, and also to variations in winter snowfall. At lower latitudes, solar radiation and precipitation are also important.
  • Because of these differences in sensitivity, mountain glaciers in different regions show varying behavior. Individual glaciers may respond to climate over intervals that range from about a decade to as much as several hundred years. The response times of most mountain glaciers fall in the range of 10 to 40 years.
• Despite this wide range of possible responses, historical observations of the lower limits of glaciers on mountain sides between 1860 and 1900 show that 35 of the 36 glaciers examined were already in retreat.
  • Between 1900 and 1980, 142 of the 144 glaciers analyzed were retreating. The average retreat of all glaciers between 1850 and 2000 was ,1,750 meters, or just over a mile. In some cases, studies of glacier lengths have been supplemented by analyses of ice thickness that permit calculations of full glacier volume.
  • The energy used to melt these ice sheets absorbed part of the excess heat generated during the industrial era, but far less than was stored in the deep ocean.
• Rare exceptions to this general pattern of retreat exist.
  • Some glaciers in the mountains of Norway advanced during the 1960s and 1970s during an interval of cooling in the Norwegian Sea, but the prevailing trend during the twentieth century has been one of melting. In recent decades, the rate of melting has accelerated for many glaciers. All tropical mountain glaciers studied are in retreat, and some have disappeared entirely.
• This pervasive, near-global retreat cannot be the result of reduced snowfall. That explanation would require an average drop in precipitation of 25% in many sites across the globe.
• Instrumental evidence indicates that precipitation changes in most regions are much smaller than 25%, with increases in some glacier areas and reductions in others.
• In contrast, the temperature increases observed during the late 1800s and the 1900s have been both global in scale and of the right magnitude to explain the glacial melting

18-5 Ground Temperature

• Heat probes inserted into soils or bedrock boreholes reaching hundreds of meters below the surface measure past changes in temperature that have slowly penetrated from the atmosphere into subsurface layers.
  • Profiles are sensitive to longer term temperature changes at the surface, but smooth out the shorter decadal-scale variations.
  • Subsurface temperature records have been taken at hundreds of stations in both hemispheres, and most profiles show warmer temperatures in the near-surface layers than a few tens of meters below.
  • The measurements indicate that a warming has occurred at the surface in the last century or two and that it is in the process of penetrating to deeper layers. This warming of the subsurface across the continents has also absorbed a small fraction of the excess heat generated during the industrial era.
• Models that simulate the penetration of the temperature anomalies beneath the surface indicate that the warming during the last two centuries lies at or slightly above the upper end of the range of surface-temperature reconstructions based on climatic proxies.
  • This mismatch reflects the fact that both the ground temperature profiles and the proxy reconstructions showing larger variations tend to be based on high-latitude and/or high-altitude sites, where temperature responses are larger than the global average.
  • Other complications include the depth of snow, which may shield the ground from extreme temperature changes in winter, and clearance of forests, which can cool local temperatures because of the higher reflectivity of open land surfaces.

Satellites Observations

• The range of measurements of Earth’s climate increased markedly with the advent of satellite sensors in the late 1900s. Different satellite sensors have come on line at different times, including a few within the last decade.

18-6 Disagreement Between Satellite and Ground Stations Resolved

• As recently as the early 2000s, measurements of temperature from satellites did not appear to agree with the warming trend measured in stations at Earth’s surface.
  • The satellite data suggested no warming had occurred since 1980 and perhaps even a small cooling, in contradiction to the warming shown by surface stations.
• Satellite estimates of the temperature of the lower atmosphere are based on measurements of the brightness of vibrating molecules of oxygen (O2)
• This brightness parameter correlates with the temperature of the oxygen molecules in the air, and of the air itself. The satellite sensors integrate energy emissions from oxygen molecules across the entire lower atmosphere (0–10 km, the troposphere) as well as the lower part of the stratosphere. As Earth’s troposphere warms, the stratosphere cools.
  • Changes in the temperature of the lower troposphere from satellite measurements cannot be estimated without first removing the cooling trend in the lower stratosphere.
• These apparent mismatches have been resolved in recent years in favor of the ground station records. Subsequent reexamination found that incorrect adjustments had been used to remove the changes in stratospheric temperatures in order to isolate the temperature changes in the troposphere.
  • In effect, the cooling effect in the stratosphere had been overestimated, which resulted in too large a correction to derive the tropospheric signal. Additional problems were caused by the effect of friction, which caused a gradual drift of the satellite orbits toward lower altitudes and a slightly later arrival over particular locations on Earth’s surface.
  • With all these complications taken into account, the satellite data came into basic alignment with the surface station temperature trend.
• The problem with the radiosonde data turned out to be similar: an overcorrection for solar heating of the metal shields. Data from radiosondes sent aloft at night agreed with the evidence from ground stations, but data from daytime releases did not show the warming recorded at the surface.
  • Because radiosondes are made of metal, their measurements of air temperature have to be corrected for heating of their metal shields by the Sun.
  • The overcorrection for this solar heating effect imposed a false cooling signal on the measurements. A more accurate correction for the Sun’s heating showed temperatures that agreed well with ground stations.
• Satellite and radiosonde data now confirm that a major warming has occurred in the last century or more.

18-7 Circumarctic Warming

• The most dramatic climatic responses during recent decades are those in the Arctic Ocean and over nearby continents. This warming trend has been accompanied by large changes in a number of climatically sensitive indices.
• Satellites in use since 1979 map the extent of Arctic sea ice by distinguishing between brighter sea ice and darker open ocean water.
  • Each year, the minimum annual extent of sea ice occurs in early September at the end of the summer melt season. In the decades since the 1970s, the extent of September Sea ice has decreased by almost 40%, with a very rapid loss since 2006.
•  Ice in the central Arctic survives into subsequent years and can attain thicknesses of 3–4 m before being transported out of the Arctic Ocean into the Atlantic.
• Measurements made by upward looking sonar soundings from submarines during the middle 1900s and more recently from downward looking radar surveys from satellites show that the average thickness of all ice in the Arctic has shrunk by more than half since the middle to late 1900s.
• The combination of rapid retreat of marginal first-year ice, along with thinning of multi-year ice in the central Arctic, has produced an even more dramatic loss of total ice volume during all seasons of the year, ranging from about 40% in March to more than 70% in September.
• Satellite measurements from the higher latitudes of the Northern Hemisphere show other trends that are consistent with major regional warming.
  • The extent of northern hemisphere snow cover is decreasing, with earlier melting of snow in spring and later initiation of snow cover in autumn.
• An additional indication of warming at middle and high northern latitudes comes from satellite and surface station measurements of the length of the growing season.
  • Surface measurements in central Alaska indicate an irregular increase in the growing season by two weeks over the past 50 years.
  • Satellite sensing of the chlorophyll produced by vegetation north of 45N has also shown that by the mid-1990s the growing season was beginning a week earlier in spring than it had in the early 1980s and that it was ending half a week later in autumn.
• In contrast to the Arctic, the much larger amount of sea ice surrounding Antarctica shows no major trend in recent decades. The unusually complex interactions between the atmosphere, sea ice, and underlying ocean water in the Southern Ocean may be masking climatic trends.

18-8 Ice Sheets

• Accurate measurements of the volume of ice sheets first became possible in the early 2000s when satellites began to measure the elevation of ice surfaces with sufficient accuracy. Multiple passes across the ice sheets, combined with computer analysis of the radar images received, made it possible to obtain accurate measurements across the entire surfaces of the ice sheets.
• Because the elevation of bedrock under the ice can also change through time, ice thickness and ice volume cannot be determined solely from measurements of surface ice elevation.
  • Until recently, the elevation of bedrock beneath the ice was measured by labor-intensive efforts in which recording stations were moved across the ice sheets, sending out radar waves that bounced off the bedrock and returned to the station. 
  • More recently, radar measurements of bedrock elevation have been made quickly and inexpensively from satellites, providing full coverage of these continent-sized masses of ice.
• In addition, surface stations installed in critical locations with global positioning receivers linked to satellites now measure both the elevation of the ice and the gravity field.
  • Because rock is almost three times denser than ice, the gravity field at each station primarily measures changes in the elevation of the underlying bedrock in response to ice melting or growth.
•  Based on a decade of measurements of the Greenland ice sheet, the highest and coldest part of the ice at elevations above 2 km have been slowly gaining ice as new snow falls, as typically occurs for ice sheets that are at equilibrium. Ice sheets that are stable accumulate snow at high altitudes and transport it as ice flow to lower altitudes.
•  But very recent trends have disproven the idea that the Greenland ice sheet is at a stable overall mass balance.
  • The upper limit of the line dividing net ice gain at higher elevations from ice loss at lower elevations has moved higher on the ice sheet in recent years, leaving a smaller area of net ice accumulation.
  • Overall, the annual loss of ice on Greenland has accelerated slightly since 2002. Years of additional observations will be needed to place these changes in a longer-term context.
•  Changes in Antarctic ice have been less dramatic and more variable from region to region. The narrow Antarctic Peninsula that protrudes north toward the southern tip of South America has been rapidly shedding ice during the last decade because that area has warmed very rapidly, much faster than the global average. In contrast, the huge mass of ice in the East Antarctic ice sheet shows both thickening and thinning in different areas, and the year-to-year variability in temperatures and ice budgets is large.
  • As a whole, the ice sheets in Antarctica appear to have lost a small amount of ice during the last two decades, but many more years of observation are needed to resolve the long-term trend.
  • Part of this trend may depend on narrow ice streams that drain ice from the interior of the East Antarctic ice sheet at rates as much as 100 times faster than the slow flow across the rest of the ice sheet.

Sources of the Recent Rise in Sea Level

• The 17-cm rise in the average level of the ocean was primarily resulted from three factors (Ocean thermal expansion, mountain glacier, and Greenland and Antarctic ice).
• Because water expands slightly when heated above 4C, warming of the ocean causes sea level to rise. The subsurface ocean warming trend along with less complete data from earlier decades, indicate that thermal expansion can explain about 4 cm of sea level rise since 1960.
• In addition, even though mountain glaciers and ice caps only account for about 1% of the total amount of ice on land, 99% of them have been melting and retreating and adding water to the ocean since the middle 1800s.
  • These glaciers account for at least another 5 cm of the rise in sea level during the last century. Current estimates are that about 2 cm of sea level rise during the 1900s came from melting of the Greenland and Antarctic ice sheets, although these estimates are not well constrained for earlier decades.
• The combination of thermal expansion, melting of mountain glaciers, and melting of ice on Greenland and Antarctica can account for 11 cm of the estimated sea level rise of 17 cm during the twentieth century. This calculation leaves a substantial gap unexplained, but uncertainties in all of the contributing factors are large prior to recent decades.
  • Some scientists infer that most of the remaining imbalance came from greater-than-expected melting of the Greenland ice sheet. Other scientists regard this discrepancy as an important “enigma” and have concluded that the 17-cm estimate may be too high because of errors in correcting tide gauge records for delayed responses to past ice sheets.
• Measurements from a range of satellites indicate that the rate of annual sea level rise has increased from the ,1.7 mm average during the 1900s to 2.5– 3.0 mm per year since the middle 1990s. Even for this more highly instrumented interval, large uncertainties remain for several contributing factors, although some estimates suggest that no “gap” is present.
  • At this point, mountain glaciers and thermal expansion remain the largest sources of meltwater, but in the future most mountain glaciers will disappear and accelerated melting of the Greenland ice sheet will become more important.
• In addition to satellites that measure the height of the sea surface directly, another technological innovation targets the portion of sea level change caused by warmer ocean temperatures and thermal expansion. The velocity of sound waves moving through the ocean is dependent on water temperature: the velocity averages about 1,500 meters per second but increases by 4.6 meters per second for each 1C of warming of the water.
• The SOFAR (Sound Fixing And Ranging) channel at a water depth near 1 km is particularly favorable for transmission of sound waves.
  • Sound waves moving through the overlying and underlying ocean layers are gradually bent into this channel from above and below because its temperature and density make the waves move slightly faster than in the surrounding layers. Scientists can use these far-traveled sound waves to measure the average sound velocity across the paths the waves follow, and therefore the average temperature, across large stretches of the subsurface ocean.
  • In addition, thousands of ARGO floats that have been deployed in a multinational effort covering most the world’s oceans will deliver greatly improved resolution of changes in ocean temperature and salinity that cause sea level changes.

Internal Oscillations

• Oscillations in climate that persist for a few years to as much as a few decades can affect distinct regions on Earth’s surface. These fluctuations arise from natural variability within the climate system, but in a form that is distinct from short-term noise. Efforts to assess longer-term climatic trends need to consider the contributions from these shorter-term oscillations
• The large-scale ENSO fluctuations in the tropical Pacific Ocean are the most prominent short-term oscillation in the climate system.
  • During El Niño years, sea level pressure falls and ocean temperatures warm across the east-central tropical Pacific Ocean, while sea level pressure rises, and precipitation decreases in the far western tropical Pacific Ocean and over New Guinea and northern Australia .
  • El Niño events recur irregularly within a broad band of 2 to 7 years, and each fluctuation lasts for about a year. In addition to the ENSO changes, other oscillations affect smaller regions. These oscillations appear as changes in surface pressure, temperature, and winds that may persist for many years.
• The Pacific Decadal Oscillation (PDO) has a spatial distribution similar to ENSO.
  • During “positive” PDO phases, sea-surface temperatures are warmer in the tropics and along the Pacific coast of western North America and cooler in the west-central North Pacific . “Negative” PDO phases have the opposite pattern.
• Based on a little more than a century of high-resolution instrumental observations, PDO patterns can persist for intervals ranging from several years to decades.
  • A prominent change from a negative to a positive PDO pattern occurred in 1976, near the time that the global temperature trend shifted from a few decades of stable or cooling climate to the rapid warming that continues today.
  • But the positive PDO pattern has been less persistent during the last 20–25 years, and yet global warming has continued. The century-length record is too short to determine whether the PDO has a cyclical signature of multi-decadal or century length.
• The North Atlantic Oscillation (NAO) is a fluctuation in atmospheric pressure between a subpolar El Niño and the Southern Oscillation Temperatures are warmer off western South America and across the eastern tropical Pacific during El Niño years. Warm El Niño years occur on average every 2 to 7 years. low-pressure center near Iceland and a high-pressure center in the Azores-Gibraltar region.
• The NAO is best developed in winter. The “positive” NAO mode features lower pressure over Iceland, higher pressure over the Azores, and a strengthening of westerly winds across the intervening latitudes of the Atlantic Ocean between the two pressure centers .
  • As a result, the subtropical Atlantic Ocean is warmer in a large region extending from the mid-Atlantic and southeast coast of the United States eastward to the Azores Islands.
  • The warm, moisture bearing winds arriving from this part of the Atlantic make Europe warmer and wetter than during years of negative NAO. Cooler temperatures occur off the west coast of Africa, where strong trade winds also send extra amounts of dust out across the ocean toward the Caribbean Sea.
• The NAO has varied in strength over multi-year time scales, with more frequent positive years in the early 1900s, many negative years between 1940 and 1975, and a return to more positive years during the 1980s and early 1990s. Some scientists have claimed that the rapid retreat of sea ice during the 1990s was affected by the NAO.
  • However, the NAO weakened during the late 1990s, yet the retreat of sea ice has not only continued but even intensified.
• Still another possibility is that recent climate may have been influenced by solar forcing at the relatively weak and short-term cycles of 440 years or less that were identified by differences between tree ages derived by counting rings and by 14C dating.
• Some high-resolution records show small tendencies toward cyclic behavior at these periods, but many of them also show similar tendencies at other periods unconnected to solar changes.
  • In addition, many other records show no evidence of short-term solar forcing. At this point, the possibility of short-term cycles in response to solar forcing has not been settled.