Loading in 2 Seconds...
Loading in 2 Seconds...
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Ocean StudiesIntroduction to OceanographyAmerican Meteorological Society Chapter 12 The Ocean and Climate Change
Case in Point • People and other animals living in the Arctic face an uncertain future because of a recent warming trend. • Inuit people (also called Eskimos) live around the Arctic Ocean, in Greenland, Canada, Alaska, and Siberia, one of Earth’s coldest regions and among the most remote and inhospitable for humans. • Still depend largely on hunting seals, caribou, and polar bears, herding reindeer, and gathering berries and other foods from the land • Should the warming trend continue, the Arctic Ocean could be nearly ice-free during summer as soon as the year 2013.
The Ocean and Climate Change • Driving Question: • How and why does climate change and how does the ocean participate in and respond to climate change?
The Ocean and Climate Change • In this chapter, we examine: • The instrument-derived and reconstructed climate record • How climate changes through time • What is known about the climate future particularly relating to the world ocean
The Climate Record • Reliable instrument-based record of past weather and climate is limited to not much more than a century or so. • For information on earlier fluctuations in climate, scientists rely on reconstructions of climate based on historical documents and longer-term geological and biological evidence such as bedrock type, fossil plants and animals, pollen, tree growth rings, glacial ice cores, and deep-sea sediment cores.
The Climate Record • MARINE SEDIMENTS AND CLIMATE • Much of what we know about the climate fluctuations of the Pleistocene Ice Age is based on analysis of the shell and skeletal remains of microscopic marine organisms that are found in deep sea sediment cores. • Identification of the environmental requirements of these organisms plus oxygen isotope analysis of their remains enables scientists to distinguish between cold and mild climatic episodes of the past. • Scientists use a special property of water to reconstruct large-scale climate fluctuations of the Pleistocene Ice Age. • A water molecule is composed of one of the two stable isotopes of oxygen, O16 or O18. • The lighter isotope (O16) is much more abundant than the heavier isotope (O18). • Small but significant variations occur in the amount of light oxygen compared to heavy oxygen circulating in the global water cycle.
The Climate Record Sediment cores extracted from beneath the ocean floor provide valuable information on the geologic and climatic past
The Climate Record • MARINE SEDIMENTS AND CLIMATE • Seawater has more O18 at subtropical latitudes where evaporation exceeds precipitation and less in middle latitudes where rainfall is greater. • Over time the proportion of light to heavy oxygen in ocean water decreases with increasing glacial ice cover. • Oxygen isotope analysis of deep-sea sediment cores indicates that the Pleistocene was punctuated by abrupt changes between numerous glacial and interglacial climatic episodes.
The Climate Record • OTHER PROXY CLIMATIC DATA SOURCES • Pollen is a valuable source of information on late Ice Age vegetation and climate, especially over the past 15,000 years. • Pollen is dispersed by the wind and accumulates on the bottom of lakes (and in other depositional environments) along with other organic and inorganic sediments. • Scientists use a corer to extract a sediment column (core) that chronicles past changes in pollen (and therefore, vegetation). • Changes in dominant pollen type in a core signal changes in nearby vegetation, likely in response to climate change.
The Climate Record • OTHER PROXY CLIMATIC DATA SOURCES • Tree growth rings can yield a year-to-year record of past climate variations stretching back many thousands of years. • Tree growth rings are especially sensitive to moisture stress and have been used to reconstruct lengthy chronologies of drought prior to the era of instrument based records. • Ice cores extracted from glaciers yield a record of past seasonal snowfall preserved as thin layers of ice. • Using the oxygen isotope technique scientists can distinguish between cold and mild episodes in the past and through chemical analysis of tiny air bubbles trapped in the ice.
The Climate Record This 6-m (20-ft) long ice core was extracted from the Greenland ice sheet and is a source of information on past variations in climate and atmospheric composition. The concentration of atmospheric carbon dioxide in parts per million by volume from about 425 million years ago to 2007 based on glacial ice core analysis from the Vostok station in Antarctica.
The Climate Record • GEOLOGIC TIME • The geologic past and its climate record is subdivided using the geologic time scale, a standard division of Earth history into eons, eras, periods, and epochs based on large-scale geological events. • Geologic evidence points to an interval of extreme climate fluctuations about 570 million years ago, corresponding to the transition between Proterozoic and Phanerozoic Eons. • The Mesozoic Era, from about 245 million to 70 million years ago, was characterized by a generally warm Earth free of large glacial ice sheets. • The Cenozoic Era was a time of great climatic fluctuations. • By 40 million years ago, however, Earth’s climate began shifting toward colder, drier, and more variable conditions setting the stage for the Pleistocene Ice Age.
The Climate Record • PAST TWO MILLION YEARS • Over the past two million years mountain ranges, continents, and ocean basins were essentially as they are today. • The climate was unusual in favoring the development of huge glacial ice sheets. • During the Pleistocene Ice Age the climate shifted numerous times between glacial climates and interglacial climates. • A glacial climate favors the thickening and expansion of glaciers. • An interglacial climate favors the thinning and retreat of existing glaciers or no glaciers at all. • During major glacial climatic episodes of the Pleistocene, the Laurentide ice sheet developed over central Canada and spread westward to the Rocky Mountains, eastward to the Atlantic Ocean and southward over the northern tier states of the United States.
The Climate Record Extent of glacial ice cover over North America about 18,000 to 20,000 years ago, the time of the last glacial maximum.
The Climate Record • PAST TWO MILLION YEARS • An increase in the magnitude of a climatic change with increasing latitude is known as polar amplification, indicating that polar areas are subject to greater changes in climate. • A general warming trend followed the last glacial maximum, punctuated by relatively brief returns to glacial climatic episodes. • Relatively cold interval from about 11,000 to 10,000 years ago known as the Younger Dryas. • The present interglacial is known as the Holocene.
Reconstructed records of (A) the variation in global glacial ice volume over the past 600,000 years based on analysis of the oxygen isotope ratio of shells in deep-sea sediment cores, and (B) temperature variation over the past 160,000 years derived from oxygen isotope analysis of an ice core extracted from the Antarctic ice sheet at Vostok and expressed as a departure in Celsius degrees from the 1900 global mean temperature. Reconstructed temperature variation over the past 18,000 years based on a variety of proxy climatic indicators and expressed as a departure in Celsius degrees from the 1900 global mean temperature.
The Climate Record Reconstructed temperature variation over the past 1000 years based on analysis of historical documents and expressed as a departure in Celsius degrees from the 1900 global mean temperature
The Climate Record • INSTRUMENT-BASED TEMPERATURE TRENDS • The most reliable temperature records date from the late 1800s with the birth of national weather services including those of the U.S. and Canada, along with the predecessor to today’s World Meteorological Organization (WMO). • The trend in global mean temperature is generally upward from 1880 until about 1940, downward or steady from 1940 to about 1970, and upward again through the 1990s and early 2000s. • In spring 2000, NOAA’s NCDC reported that global warming accelerated during the final quarter of the 20th century.
Instrument-derived trends in mean annual global (land plus ocean), sea-surface, and land temperatures; expressed as departures in degrees Celsius and degrees Fahrenheit from the 125-year period average.
The Climate Record • INSTRUMENT-BASED TEMPERATURE TRENDS • A general consensus in the scientific community holds that a global-scale warming trend has prevailed since the end of the Little Ice Age. • The simplest scientific explanation for the observed warming trend is the steady build-up of carbon dioxide in the atmosphere and the consequent enhancement of the natural greenhouse effect.
Lessons of the Climate Past • Climate is inherently variable over a broad spectrum of time scales ranging from years to decades, to centuries, to millennia. • Variations in climate are geographically non-uniform in both sign (warming or cooling) and magnitude. • Climate change may consist of a long-term trend in various elements of climate and/or a change in the frequency of extreme weather events. • Climate change tends to be more abrupt than gradual. • Only a few cyclical variations can be discerned from the long-term climate record. • Climate change impacts society.
Factors Contributing to ClimateChange • CLIMATE AND PLATE TECTONICS • Plate movements are so slow compared to the span of human existence that we can consider topography and the geographical distribution of the ocean and continents as essentially fixed controls of climate. • Over the vast expanse of geologic time plate tectonics was a major player in large-scale climate change. • Changes in the location of continents (continental drift) altered the local and regional radiation budget and the response of air temperature. • Opening and closing of ocean basins changed the course of heat-transporting ocean currents and altered the thermohaline circulation.
Factors Contributing to ClimateChange • CLIMATE AND SOLAR VARIABILITY • Fluctuations in the sun’s energy output, sunspots, or regular variations in Earth’s orbital parameters are external factors that can alter Earth’s climate. • Satellite monitoring reveals that the sun’s energy output varies directly with sunspot number. • More sunspots may contribute to a warmer global climate and fewer sunspots may translate into a colder global climate.
Factors Contributing to ClimateChange Variation in mean annual sunspot number since the early 17th century.
CLIMATE AND EARTH’S ORBIT Milankovitch cycles are regular variations in the precession and tilt of Earth’s rotational axis and the eccentricity of its orbit about the sun. Caused by gravitational influences exerted on Earth by other large planets, the moon, and the sun Drive climate fluctuations operating over tens of thousands to hundreds of thousands of years Factors Contributing to ClimateChange
Factors Contributing to ClimateChange • CLIMATE AND EARTH’S ORBIT • The tilt of Earth’s spin axis changes from 22.1 degrees to 24.5 degrees and then back to 22.1 degrees over a period of about 41,000 years. • As the axial tilt increases, winters become colder and summers become warmer in both hemispheres. • The shape of Earth’s orbit about the sun changes from elliptical to nearly circular in an irregular cycle of 90,000 to 100,000 years. • Changes the amount of solar radiation received by the planet • Milankovitch cycles do not alter appreciably the total amount of solar energy received by the Earth-atmosphere-ocean system annually, but they do change significantly the latitudinal and seasonal distribution of incoming solar radiation.
Factors Contributing to ClimateChange • CLIMATE AND VOLCANOES • Only explosive volcanic eruptions rich in sulfur dioxide are likely to impact global or hemispheric climate and then only for a few years at most. • A violent volcanic eruption can send sulfur dioxide high into the stratosphere. • Sulfur dioxide then combines with water vapor to form tiny droplets of sulfuric acid and sulfate particles, collectively called sulfurous aerosols. • Can remain suspended in the stratosphere for many months to perhaps a year or longer before they cycle to Earth’s surface • Absorb both incoming solar radiation and outgoing infrared radiation warming the lower stratosphere • Also reflect solar radiation to space • A violent sulfur-rich volcanic eruption is unlikely to lower the mean hemispheric or global temperature by more than about 1°C (1.8°F).
Factors Contributing to ClimateChange Large-scale cooling often followed massive volcanic eruptions that emitted sulfur dioxide into the stratosphere.
Factors Contributing to ClimateChange • CLIMATE AND EARTH’S SURFACE PROPERTIES • Any change in the physical properties of Earth’s water or land surfaces or in the relative distribution of ocean, land, and ice may affect Earth’s radiation budget and climate. • Variations in mean regional snow cover may contribute to climate change because an extensive snow cover has a chilling effect on the atmosphere. • Changes in Earth’s sea ice or glacial ice coverage are likely to have longer-lasting effects on climate. • Changes in ocean circulation and sea-surface temperatures (SST) contribute to large-scale climate change.
Factors Contributing to ClimateChange • CLIMATE AND HUMAN ACTIVITY • In 2007, the Intergovernmental Panel on Climate Change (IPCC) concluded that global warming since the mid-20th century very likely (estimated probability of greater than 90%) was caused mostly by human activities. • Many human activities affect climate over broad ranges of spatial and temporal scales: • Modification of the landscape • Clear-cutting of forests • Urbanization (the urban heat island effect) • Combustion of fossil fuels • Alters concentrations of certain key gaseous and aerosol components of the atmosphere
Factors Contributing to ClimateChange Trend in atmospheric carbon dioxide concentration since 1957, based on measurements made at the Mauna Loa Observatory in Hawaii.
Factors Contributing to ClimateChange • CLIMATE AND HUMAN ACTIVITY • Humankind’s contribution to the buildup of atmospheric CO2 began thousands of years ago with the clearing of land for agriculture and settlement. • By the mid-19th century, growing dependency on coal burning associated with the beginnings of the Industrial Revolution triggered a more rapid rise in CO2 concentration. • The concentration of atmospheric CO2 is now about 35% higher than it was in the pre-industrial era. • The ocean takes up 56.2% of the carbon dioxide of anthropogenic origin (via photosynthesis and cold surface waters absorbing CO2 and sinking) while terrestrial biomass is a sink for 13.7%.
Factors Contributing to ClimateChange • CLIMATE AND HUMAN ACTIVITY • Oceanic uptake of carbon dioxide is likely to decline in the future as the ocean becomes more stratified in response to higher global temperatures. • Rising levels of other infrared-absorbing gases will also likely enhance the greenhouse effect. • Methane, nitrous oxide, and halocarbons
The Climate Future • GLOBAL CLIMATE MODELS • A global climate model is a simulation of Earth’s climate system. • Differs from numerical models used for weather forecasting in that it predicts broad regions of expected positive and negative temperature and precipitation anomaliesand the mean location of circulation features such as jet streams and principal storm tracks over much longer time scales • Used to predict the potential impacts on climate of rising levels of atmospheric carbon dioxide and other greenhouse gases • Using current boundary conditions, a global climate model simulates the present climate. Then, holding all other variables constant, the concentration of carbon dioxide (or another greenhouse gas) is elevated and the model is run to a new equilibrium state. • Scientists take the ensemble approach to climate simulation.
The Climate Future • ENHANCED GREENHOUSE EFFECT AND GLOBAL WARMING • According to the 2007 IPCC Assessment Report, the climate models predict that over the next 20 years, the global mean annual temperature will increase at an average rate of about 0.2°C per decade. • Enhancement of the greenhouse effect could cause a climate change that would be greater in magnitude than any previous climate change over the past 10,000 years.
Impact of Climate Change on theOcean • SEA LEVEL FLUCTUATIONS • During glacial climatic episodes, glaciers on land thicken and expand, and the volume of water in the ocean basins decreases. • Conversely, during interglacial climatic episodes, glaciers on land thin and retreat, and the volume of water in the ocean basins increases. • Seawater always contracts when its temperature drops and expands when its temperature rises. • Persistence of the global warming trend appears likely to cause sea level to rise in response to melting of land-based polar ice sheets and mountain glaciers, coupled with thermal expansion of seawater.
Impact of Climate Change on theOcean • SEA LEVEL FLUCTUATIONS • Waxing and waning of glaciers plus ocean temperature fluctuations are two factors that govern eustasy, the global variation in sea level brought about by a change in the volume of water occupying the ocean basin Annually averaged sea level variation based on TOPEX/Poseidon measurements for 1993-2000
Impact of Climate Change on theOcean • SEA LEVEL FLUCTUATIONS • Climate models predict that global warming will cause a rise in mean sea level in the range of 30 to 40 cm (12 to 16 in.) during the 21st century. • Thermal expansion of ocean waters would account for more than 60% of the rise with the balance due to melting glaciers. • Rising sea level would disrupt coastal ecosystems and could threaten historical, cultural, and recreational resources.
Map of the U.S. Geological Survey’s Coastal Vulnerability Index (CVI) for Cape Cod National Seashore, MA showing the vulnerability of the coast to changes in sea level. The CVI is based on tidal range, wave height, coastal slope, historic shoreline change rates, geomorphology, and historic rates of relative sea level change due to eustatic sea level rise and tectonic uplift or subsidence.
Impact of Climate Change on theOcean • ARCTIC SEA ICE COVER • Shrinkage of Arctic sea ice is likely to trigger an ice-albedo feedback mechanism that would accelerate melting of sea ice and amplify warming. • As sea ice cover shrinks, the greater area of ice-free ocean waters absorbs more solar radiation, sea surface temperatures rise, and more ice melts.
Impact of Climate Change on theOcean Positive ice-albedo feedback in the Arctic is likely to accelerate warming of surface waters and shrinkage of sea ice cover.
ARCTIC SEA ICE COVER Less sea ice cover on the Arctic Ocean is likely to increase the humidity of the overlying air leading to more cloudiness. Shrinkage of the Arctic sea ice may be the direct consequence of higher air temperatures or indirectly the result of changes in ocean circulation. Impact of Climate Change on theOcean Arctic sea ice has been shrinking at least since the time satellite monitoring began in 1979, reaching a new end-of-season record low in 2007.
Impact of Climate Change on theOcean • MARINE LIFE • Climate change could alter the physical and chemical conditions in the ocean, perhaps exceeding the tolerance limits of organisms. • Shrinkage of Arctic sea ice may have serious implications for organisms living on, within, or under the ice. • Global warming could raise the sea-surface temperature sufficiently to threaten coral reefs. • Organisms are particularly vulnerable to environmental change that affects a limiting factor. • A limiting factor is an essential resource that is in lowest supply compared to what is required by the organism.
Mette Nielson and Rolf Gradinger are part of an international team studying the effects of diminishing Arctic ice cover on the marine community living on, within, and below the ice. Here, they are extracting ice cores for later analysis.
Conclusions • For information on prior climate regimes, scientists must rely on a variety of documentary, geological, and biological proxy climate indicators. • Although the climate record loses detail, continuity, and reliability with increasing time before present, it is evident that climate changes over a broad spectrum of time scales. • The interaction of many factors is responsible for the variability of climate.