Evidence for global climate change

Evidence for global climate change

CAUSES & CONSEQUENCES OF RAPID CLIMATE CHANGE

This web page describes a talk I give for Carolina Speakers. Both the summary text and illustrations are examples of the information that I adapt for different audiences. Please contact me or Carolina Speakers at the addresses below if you are interested in this talk, or contact Carolina Speakers for talks by any of the approximately 90 speakers now available for presentations.

John J. W. Rogers, Dept. of Geological Sciences, UNC-CH, (919) 962-2581, fax: (919) 966-4519.

Sandra Roberts, Carolina Speakers, (919) 962-1993, fax: directly to UNC Speakers website

Global changes in the next century may be far larger than most people realise. As a geologist, I look at the history of climate and find evidence that there have been times when tempera- tures have increased 10 degrees F or more in one century, and sea levels have risen more than 10 feet during the same time. This talk discusses that history and the risks that we may face in the very near future.

Evidence for recent temperature change comes from numerous observations. In the past 40 years, worldwide atmospheric tem- peratures have increased about 1 degree F (1) and ocean tem- peratures about 0.11 degree F (2). The thickness and extent of ice covering the Arctic Ocean in the summer have both decreased dramatically in the same time (3,4), and some people predict that by 2050 ships can sail freely across the Arctic during the summers. Both marine and land animals and plants appear farther north in the northern hemisphere than they did 50 years ago, and plants bloom one or more weeks earlier than in the past.

These changes are taking place during an "interglacial", or warm, period that followed the end of the last glaciation about 15,000 years ago. During the glacial period, mountain valleys were filled by glaciers, and much of the northern hemisphere was covered by ice sheets.

The glacial period that ended about 15,000 years ago was the latest in a series of glacial and interglacial intervals that began more than one million years ago (modified from People and the Earth). Each of the most recent six to eight cycles consisted of a slow global cooling that lasted for about 100,000 years followed by a rapid warmup for 10,000 to 20,000 years. These alternations can be measured by a variety of methods, such as the isotopic composition of oceanic sediments, but we know more about the most recent four cycles because an ice core at the Russian station Vostok, in East Antarctica, has penetrated to ice as old as 420,000 years (5). Temperatures, concen- trations of greenhouse gases, and rise and fall of sea level all show similar curves of variation during this time.

The continental ice sheets took so much water out of the oceans that sea level fell about 400 feet below the present during maximum advance of ice sheets. On the eastern and Gulf coasts of North America, dry land extended 100 or more miles seaward from the present coastline. Sea level rose rapidly as the glaciers melted and reached its present position about 7000 years ago. This is my interpretation of sea level data (from 6). By about 12,000 years ago, rising sea level and melting ice cut the land connection between North America and Asia. This isolated the Americas from the rest of the world and left Native Americans to develop without any of the inventions developed in the "Old World". They include: the absence of wheels, possibly as a result of the lack of pack animals; lack of writing; lack of metals except as ornaments; and absence of any fuel except wood. The types of plants and animals in the Americas were also very different from those in the rest of the world with food such as corn, most varieties of beans, potatoes, tomatoes, and turkeys. In turn, the Americas received wheat and other grains, rice, most fruits and berries, cattle, sheep, pigs, and chickens.

Melting of ice appears to have occurred very rapidly at the end of each glacial period. The sea level curve (above) shows the possibility of a 50 foot rise in sea level in a few hundred years. North American glaciers melted so fast that the Mississippi River was 50 miles wide during spring floods.

Studies of ice cores in Greenland show that the last ice age ended very rapidly 14,670 years ago. Temperatures rose 15 degrees F in just 50 years. Within the next few hundred years, the dust that blew into the ice from barren plains in front of the glaciers was replaced by nitrogen compounds formed by burning of the new forests.

Rapid change can also occur during interglacial warm periods similar to the one that we are now in. At the end of the last interglacial about 120,000 years ago, sea level rose 15 to 20 feet and then dropped back within a few centuries (9). This rate of change is shown by wave-cut notches at this height on cliffs in the Bahamas and, more poorly preserved, on a sea cliff in the Orkney Islands. This rapid change was probably caused by sudden melting of the West Antarctic Icesheet (see below).

The climate change of the northern hemisphere is sum- marized in this diagram (from History and Environment of North Carolina's Piedmont), with changes in vegetation in North Carolina on the right (The term "Younger Dryas" is discussed below).

Rapid change can be caused by a number of processes, acting either singly or in combination. One is extremely large volcanic eruptions or a series of eruptions. Large eruptions may send enough dust into the atmosphere to cause global cooling for several years, but over a long period of time they put enough carbon dioxide into the atmosphere to cause long-term temperature increases.

CAUSES OF RAPID CLIMATE CHANGE & FLOODING

1. Cooling by volcanoes. Lasts a few years unless eruption is nearly continuous. 2. Warming by volcanoes. Could release enough carbon dioxide to trigger changes over hundreds or thousands of years. Might cause rapid warming at end of each ice age. 3. Release of methane hydrates from sea floor. Triggered by ocean warming (possibly related to volcanism) or by lowering of sea level. 4. Melting of West Antarctic Icecap, possibly related to volcanism in rift beneath the icecap.

Another cause of warming could be the rapid release of methane from sediments on the sea floor. Methane and water form a solid at low temperatures and high pressures, and any decrease in sea floor pressure by sea level lowering or increase in temperature could release the methane into the atmosphere. This has been regarded as a cause of the rapid warmup following each glacial period (10).

The principal cause of sea level rise and coastal flooding may be melting of the ice sheet that covers West Antarctica. Antarctica is divided into East Antarctica, which faces the Indian Ocean, and West Antarctica, which faces the Pacific and Atlantic Oceans. East Antarctica is covered by an ice sheet that rests on dry land and has elevations higher than 12,000 feet.

West Antarctica is lower and is covered by a thinner ice- sheet that rests on land mostly below sea level. It is under- lain by a rift that is volcanically active. The heat from this volcanism apparently melts the lowest layer of ice and causes ice to stream toward the oceans at speeds up to half a mile a year.

The ice streams would enter the ocean directly if the floating Ross Ice Shelf disappeared. Satellite images show icebergs as large as the state of Delaware breaking away from the shelf at the present time.

The West Antarctic Ice Sheet has completely disap- peared at least once, probably more often, in the last few hundred thousand years, causing sudden sea level rise of about 20 feet (11). This sudden melting probably was responsible for the sea level rise that caused the wave-cut notches in the Bahamas and Orkneys (see above).

CONSEQUENCES OF RAPID CLIMATE CHANGE & FLOODING

1. Flooding. Sea level rise of 20 feet would displace more than one billion people. 2. Moving climate zones to higher labreastudes. Northern Canada and Siberia might become the major agricul- tural areas of the world. 3. Increasing aridity, particularly in such areas as the midwestern United States. 4. Cooling of northern Europe below temperatures that permit significant agricultural activity.

Predicting future climate changes is tricky under the best of circumstances, but it is particularly difficult now because the atmosphere contains more greenhouse gases than at any known time in the past. The concentrations of carbon dioxide and methane rise during interglacials and fall during glacial periods (see discussion of glacial cycles above). Now, largely because of consumption of fossil fuels, carbon dioxide is 40% higher than ever before. Mostly because of the increase in domestic livestock, methane is 250% higher. Industrial chemicals that never occurred in nature have become major contributors to atmospheric warming.

The major dangers from global warming are coastal flooding and the movement of climate zones toward higher labreastudes. Rise in sea level could make the world's ports useless and displace a billion or more people inland. Zones of major grain production in the northern hemisphere could move from their present 30 to 50 degree labreastude range into northern Canada and Siberia.

Paradoxically, global warming could make Europe colder. This diagram shows how currents moving northeastward from the Florida area warm northern Europe as warm water is drawn northward by descent of cold Arctic water to the bottom of the Atlantic Ocean. If that cold water does not leave the Arctic, then the ocean could freeze farther south and shut off the flow of warm water.

Regardless of what happens, it is clear that global change in the 21st century could be large enough to affect the lives of everyone. This talk emphasizes the responsibility for everyone, not just those immediately affected, to take part in planning for this eventuality.

General Information:

Chapter 8 of People and the Earth by John J. W. Rogers and P. Geoffrey Feiss, published in 1998 by Cambridge University Press; a website containing new information is maintained at the College of William and Mary. History and Environment of North Carolina's Piedmont - Evolution of a Value-Added Society, self-published by John J. W. Rogers in 1999; this book is not copyrighted and can be obtained by:

1. free download. This is a PDF file requiring Acrobat Reader. 2. Purchase for $6 at the Bulls Head Bookshop on the UNC-CH campus (I receive no royalties). 3. Order from the Bulls Head Bookshop by phone The Bookshop will add $2.50 for shipping and handling, making a total price of $8.50. 4. Photocopy of any copies that are available.

1. A wealth of climate information is available from the continuing publications of the Intergovernment Panel on Climate Change (IPCC). 2. Levitus, S., Antonov, J.L., Boyer, T.P. and Stephens, C., 2000, Warming of the world ocean. Science, v. 287, p. 2225-2229. 3. McPhee, M.G., Stanton, T.P., Morison, J.H. and Martinson, D.G., 1998, Freshening of the upper ocean in the Arctic: Is perennial sea ice disappearing? Geophysical Research Letters, v. 25, p. 1729-1732. 4. Maslanik, J.A. and Serreze, M.C., 1999, On the record of reduction in 1998 western Arctic sea-ice cover. Geophys- ical Research Letters, v. 26, p. 1905-1908. 5. Pebreast, J.R. and 18 others, 1999, Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature, v. 399, p. 429-436. 6. Fleming, K. Johnston, P., Zwartz, W., Yokoyama, Y., Lambeck, K. and Chappell, J., 1998, Refining the eustatic sea-level curve since the last glacial maximum using far- and intermediate-field sites. Earth and Planetary Science Letters, v. 163, p. 327-342. 7. Severinghaus, J.P. and Brook, E.J., 1999, Abrupt climate change at the end of the last glacial period inferred from trapped air in polar ice. Science, v. 286, p. 930-934. 8. Legrand, M. and Mayewski, P., 1997, Glaciochemistry of polar ice cores: A review. Reviews of Geophysics, v. 35, p. 219-243. 9. Neumann, A.C. and Hearty, P.J., 1996, Rapid sea level changes at the close of the last interglacial (substage 5e) recorded in Bahamian Island geology. Geology, v. 24, p. 775-778. 10. Paull, C.K., Ussler, III, W. and Dillon, W.P., 1991, Is the extent of glaciation limited by marine gas-hydrates? Geophysical Research Letters, v. 18, p. 432-434. 11. Scherer, R.P., Aldahan, A., Tulaczyk, S., Possnert, G., Engelhardt, H. and Kamb, B., 1998, Pleistocene collapse of the West Antarctic Ice Sheet. Science, v. 281, p. 82-85. . . --