CAUSES AND 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.
email: jrogers@email.unc.edu

Sandra Roberts, Carolina Speakers, (919) 962-1993, fax: (919) 962-9335.
email: sandy_roberts@unc.edu; or go 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 temperatures 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 temperatures have increased about 1 degree F (1) and ocean temperatures 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 toe of the Rhone Glacier has progressively melted back from the valley that it once filled

The valley of the Rhone River in Switzerland

Beartooth Mountains in Montana, showing lakes in depressions left by a melting glacier

Rocks cascading into a lake left by a glacier in the Canadian Rockies

Area in the Orkney Islands smoothed by ice sheets moving out of northern Europe toward the Atlantic Ocean

Ponds and lakes in depressions, such as this one in North Dakota, are formed where retreating glaciers left irregular piles of debris on the ground

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, concentrations 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.

Norwegian fjord created as rising sea level filled a glaciated valley.

Islands near Finland that are the former tops of hills, isolated by rising water.

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.

Modern meltwater creates these falls in Iceland.

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.

Greenland icecap (top of picture) and fjord, showing how far a glacier has melted back. White specks in fjord are icebergs calved from glacier front.

Summary of data in the Greenland icecap for the period 14,700 to 14,000 years ago (interpreted from 7, 8)

Barren outwash plain in front of glacier in Alaska.

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).

Wave-cut notch on Little Sale Cay in the Bahamas
(photo by C. Neumann)

Cliff on Hoy, Orkney Islands, showing how extreme modern erosion has removed most evidence of previous sea level changes (note "beach" consisting mostly of boulders the size of people's heads).

After the end of the last ice age, climate changes have been smaller and have proceeded more slowly.

The climate change of the northern hemisphere is summarized 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).

At the peak of glaciation, the forests of North Carolina may have looked like this spruce forest in British Columbia.

While glaciers were retreating, North Carolina may have had vegetation like this area in northern Sweden.

Present North Carolina Blue Ridge during leaf season.

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 AND 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.

Mt. Ruapehu, in New Zealand, is not large enough to affect climate, but I include it here because I like the sign at the entrance to the ski area. When I was there it said "Skiers are advised to leave the slopes during eruptions."

The term Younger Dryas comes from the dryas plant, which looks like a cotton ball on a twig and occurs in very cold climates, such as this area in Iceland. The Younger Dryas interval was a time when the dryas plant grew further south in Europe and North America.

The 2000 year period of cold known as the Younger Dryas may have been caused by almost continual eruption of Mt. Hekla, in Iceland.

Long-term warming, including the rapid end of the last glacial period, may have been caused by volcanoes in Kamchatka and Alaska. Here is a picture of Mt. Avachinski near the city of Petropavlovsk, an example of Soviet-style architecture (photo by Jonathan Lees)

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 icesheet that rests on land mostly below sea level. It is underlain 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 disappeared 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 AND FLOODING

1. Flooding. Sea level rise of 20 feet would displace more than one billion people.
2. Moving climate zones to higher latitudes. Northern Canada and Siberia might become the major agricultural 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 latitudes. 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 latitude range into northern Canada and Siberia.

Much of the present grain belt in the U.S. could become arid, like this desert in New Mexico.

Much of the world's currently temperate areas might look like the arid continent of Australia.

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.