Rates of change

Causes of climate change often trigger additional changes (feedbacks) within the climate system that can amplify or subdue the climate's initial response to them. For instance, if changes in the Earth's orbit trigger an interglacial (warm) period, increasing CO2 may amplify the warming by enhancing the greenhouse effect. When temperatures get cooler, CO2 enters the oceans, and the atmosphere becomes cooler. According to the Intergovernmental Panel on Climate Change (IPCC) in 2007, during the past 650,000 years, the CO2 levels have tended to track the glacial cycles. In other words, during warm interglacial periods, CO2 levels have been high and during cool glacial periods, CO2 levels have been low.

Sometimes the Earth's climate seems to be quite stable; other times it seems to have periods of rapid change. According to the U.S. Environmental Protection Agency, interglacial climates (such as the climate today) tend to be more stable than cooler, glacial climates. Abrupt, or rapid, climate changes often occur between glacial and interglacial periods.

There are many components in a climate system, such as the atmosphere, the Earth's surface, the ocean surface, vegetation, sea ice, mountain glaciers, deep ocean, and ice sheets. All of these components affect, and are affected by, the climate. They all have different response times, however. Some are fast, others slow, as shown in the following table.

Climate System Components and Response Times

CLIMATE SYSTEM COMPONENT

RESPONSE TIME

EXAMPLE

Fast Responses

Land surface

Hours to months

Heating of the Earth's surface

Ocean surface

Days to months

Afternoon heating of the water's surface

Atmosphere

Hours to weeks

Daily heating; winter inversions

Sea ice

Weeks to years

Early summer breakup

Vegetation

Hours to centuries

Growth of trees in a rain forest

Slow Responses

Ice sheets

100-10,000 years

Advances of ice sheets over Greenland

Mountain glaciers

10-100 years

Loss of glaciers in Glacier National Park

Deep ocean

100-1,500 years

Deep-water replacement

Because the components of the climate system are diverse in location, function, and size, the way they respond can be diverse as well. The amount of change applied and the innate ability to respond determine what the climate actually ends up doing. For instance, if there is a slow climate change, but the system component reacts quickly, then the response will be visible. If the climate change is rapid, but then reverts back to its previous condition and the component's response time is naturally slow, then there will be no response. If the climate change alternates from one extreme to another at a rate that the components can keep up with, these changes will be seen as visible adaptations. It is these types of rates of change that are most enlightening for clima-tologists because it allows them to more efficiently model all the subtle components of the climate system.

Planetary and Global Motions in the Atmosphere That Affect Climate

Because the Earth is in motion rotating on its axis and revolving around the Sun, climatologists must also take this into account when they study climate and global warming. In addition, there are global motions in the atmosphere—large-scale features—that have a significant effect on the Earth's local, regional, and global climate. This chapter discusses those planetary motions—eccentricity, tilt, and precession—and how they have been tied to climate change, as well as the role of the Earth's rotation and its influence on the movement of the global atmospheric system. The chapter also addresses the Coriolis force, the trade winds, Hadley cells, monsoon systems, and the El Niño phenomenon.

Astronomers have known for centuries that characteristics of the Earth's orbit are not constant; they vary in a cyclic manner. The orbit

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