Historical Background and Scientific Foundations

Data that have been gathered from ice cores, growth rings of trees, sediment at the bottom of lakes, and other locations have revealed large changes in climatic factors such as precipitation and temperature.

Some of the climate shifts occurred over hundreds of years, but others occurred over only one or several decades, and so represent abrupt climate changes. Furthermore, some of the climate shifts affected the entire Northern or Southern Hemisphere. An abrupt climate change is considered to be a change that takes place within a period of time that is fast relative to the cause of the change. For example, the climate change caused by the eruptions of Krakatoa changed the global climate within a year. This was abrupt given that the change was only possible once the volcanic ash had circulated throughout the atmosphere.

The cause of other climate changes can be slower. For example, consider the southward migration of glaciers during the various ice ages that took tens of thousands of years. Relative to such a length of time, a climate change that took place over a century or more was abrupt.

The best-known abrupt climate change took place approximately 11,000 years ago, as the Northern Hemisphere was emerging from a glacial period to a warmer state. Over about one hundred years, temperatures returned to near glacial levels, where they remained for the next thousand years.

The climate of the Northern Hemisphere then quickly warmed. The century-long period of cooling is called the Younger Dryas. Measurements have indicated that in some regions, such as present day Greenland, the cooling to glacial temperatures occurred within 10 years rather than a century-a very abrupt climate change.

The reasons behind abrupt climate change are still not well understood.

The abrupt climate changes associated with ice ages likely involved fluctuations in atmospheric carbon dioxide (CO2). During the time when the land was covered by glacial ice, the level of CO2 in the atmosphere decreased, since less vegetation was present on land and the gas was mostly stored by the ocean water.

This lessened the trapping of sunlight by the atmosphere. With the return of vegetation, the atmospheric level of CO2 rose and the atmosphere warmed. In the twentieth century and particularly since the 1960s, the development of more sophisticated means of measurement, and the ability to obtain atmospheric data by means of weather balloons and, beginning in the 1960s, the use of orbiting satellites, has provided much more information on the change in the atmosphere over time.

 It is now clear that the global climate depends on the composition of the atmosphere and the circulation patterns in the atmosphere, and that these atmospheric aspects depend on the global circulation pattern of the ocean. In the ocean, the global cycling of water (known as the ocean conveyor and also as thermohaline circulation, among other terms) distributes the heat energy in the water to all regions of Earth and vertically through the ocean depths. If the ocean conveyor were disrupted, as has been proposed as a consequence of altered atmospheric circulation, the climate would change enormously. Modeling studies have indicated that the change would occur over years, not days.

Terms:

ATMOSPHERE: The air surrounding Earth, described as a series of shells or layers of different characteristics. The atmosphere-composed mainly of nitrogen and oxygen with traces of carbon dioxide, water vapor, and other gases-acts as a buffer between Earth and the sun. The layers-troposphere, stratosphere, mesosphere, thermosphere, and the exosphere-vary around the globe and in response to seasonal changes.

CLIMATE MODEL: A quantitative way of representing the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

ECOSYSTEMS: According to the Intergovernmental Panel on Climate Change (and as published in IPCC reports): A system of living organisms interacting with each other and their physical environment.

The boundaries of what could be called an ecosystem are somewhat arbitrary, depending on the focus of interest or study. Thus, the extent of an ecosystem may range from very small spatial scales to the entire planet Earth ultimately.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth’s surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth’s atmosphere, causing global warming and global climate change.

ICE AGE: Period of glacial advance.

ICE CORE: A cylindrical section of ice removed from a glacier or an ice sheet in order to study climate patterns of the past. By performing chemical analyses on the air trapped in the ice, scientists can estimate the percentage of carbon dioxide and other trace gases in the atmosphere at that time.

TREE RINGS: Marks left in the trunks of woody plants by the annual growth of a new coat or sheath of material. Tree rings provide a straightforward way of dating organic material stored in a tree trunk. Tree-ring thickness provides proxy data about climate conditions: most trees put on thicker rings in warm, wet conditions than in cool, dry conditions.

THERMOHALINE CIRCULATION: Large-scale circulation of the world ocean that exchanges warm, low-density surface waters with cooler, higher-density deep waters. Driven by differences in temperature and saltiness (halinity) as well as, to a lesser degree, winds and tides. Also termed meridional overturning circulation.

YOUNGER DRYAS: A relatively recent episode of abrupt climate change. About 12,900 years ago, conditions in the Northern Hemisphere cooled in about a decade (extremely rapidly), in some locations by 27 º F (15 º C). The cold period persisted for about 1,300 years and then reversed, also suddenly. The causes of the Younger Dryas are not well understood, but the event does show that Earth’s climate is capable of extremely rapid and dramatic shifts.