Fact Sheet – Climate Change

The term climate change refers to the variation in the Earth's global climate or regional climates over time. It describes changes in the variability or average state of the atmosphere - or average weather - over time scales ranging from decades to millions of years. These changes can come from internal processes, be driven by external forces or, most recently, be caused by human activities.

In recent usage, especially in the context of environmental policy, the term "climate change" is often used to refer only to the ongoing changes in modern climate, including the average rise in surface temperature known as global warming. In some cases, the term is also used with a presumption of human causation, including in the United Nations Framework Convention on Climate Change. The UNFCCC uses "climate variability" for non-human caused variations.

Climate change factors

Climate changes reflect variations within the Earth's environment, natural processes going on around it, and the impact of humans. The external factors which can shape climate are often called climate forcings and include such processes as variations in solar radiation, the Earth's orbit, and greenhouse gas concentrations.

Variations within the Earth's climate

Weather, in and of itself, is a chaotic non-linear dynamical system, but in many cases, it is observed that the climate (i.e. the average state of weather) is fairly stable and predictable.

Glaciation

On moderate time scales, glaciers grow and collapse contributing both to natural variability and greatly amplifying external forces. The most important climate processes of the last several million years are the glacial and interglacial cycles of the present ice age. Though shaped by orbital variations, the internal responses involving continental ice sheets and 130m sea level change certainly played a key role is deciding what climate response would be observed in most regions. Other changes, including Heinrich events, Dansgaard-Oeschger events and the Younger Dryas show the potential for glacial variations to influence climate even in the absence of specific orbital changes.

Ocean variability

On the scale of mere decades, climate changes can also result from changes within the ocean/atmosphere systems. Many climate states, including the Pacific decadal oscillation, the North Atlantic oscillation, and the Arctic oscillation, have been recognized as modes within the climate system, owing their existence at least in part, to different ways that heat can be stored in the oceans and move between different reservoirs. On longer time scales, ocean processes such as thermohaline circulation play a key role in redistributing heat, and could if changed, dramatically impact climate.

The memory of climate

More generally, most forms of internal variability in the climate system can be recognized as a form of hysteresis meaning that the current state of climate reflects not only the inputs, but also the history of how it got there. In short, climate change can be a self-perpetuating process because different aspects of the environment respond at different rates and in different ways to the fluctuations that inevitably occur.

Non-climate factors driving climate

Plate tectonics

On the longest time scales, plate tectonics will reposition continents, shape oceans, build and tear down mountains and generally serve to define the stage upon which climate exists.

Solar variation

The sun, as the ultimate source of nearly all energy in the climate system, is an integral part of shaping Earth's climate. Early in Earth's history it is thought to have been too cold to support liquid water at the Earth's surface, leading to what is known as the faint young sun paradox.

On more modern time scales, there are also a variety of forms of solar variation, including the 11-year solar cycle, and longer term modulations. These variations are considered to be influential in triggering the Little Ice Age and for much of the warming observed from 1900 to 1950.

Orbital variations

In their impact on climate, orbital variations are in some sense an extension of solar variability, because slight variations in the Earth's orbit lead to changes in the distribution and abundance of sunlight reaching the Earth surface. Subtler variations are also present, such as the repeated advance and retreat of the Sahara desert in response to orbital precession.

Magnetic Field Change

Volcanism

A single eruption of the kind that occurs several times per century can impact climate, causing cooling for a period of a few years. Huge eruptions, known as large igneous provinces, occur only a few times every hundred million years, but can reshape climate for millions of years and cause mass extinctions.

Greenhouse gases

Carbon dioxide variations during the last 500 million years

Greenhouse gases are one of the key forces in the global warming discussion, they also have been important to the understanding of Earth's climate history. The greenhouse effect, which is the warming produced as greenhouse gases trap heat, plays a key and necessary role in regulating Earth's temperature.

Over the last 600 million years, carbon dioxide concentrations have varied from perhaps >5000 ppm to less than 200 ppm, due primarily to the impact of geological processes and biological innovations. Curiously, it has been argued (Veizer et al. 1999) that variations in greenhouse gas concentrations have not been well correlated to climate change, with perhaps plate tectonics playing a more dominant role. However there are several examples of rapid changes in the concentrations of greenhouse gases in the Earth's atmosphere that do appear to have led to strong warming, including the Paleocene-Eocene thermal maximum, the Permian-Triassic extinction event, and the end of the Varangian snowball earth event.

During the modern era, rising carbon dioxide levels are implicated as the primary cause to global warming since 1950.

Human influences

Anthropogenic factors are acts by humans that change the environment and influence climate. The biggest factor of present concern is increases in CO2 levels due to emission from fossil fuel combustion, but other factors, including land use, ozone depletion, and deforestation also impact climate.

Fossil fuels

Carbon dioxide variations over the last 400,000 years, showing a rise since the industrial revolution. Along with rising methane levels, these changes are anticipated to cause an increase of 1.4-5.6 °C between 1990 and 2100.

Prior to widespread fossil fuel use, humanity's largest impact on local climate is likely to have resulted from land use. Irrigation, deforestation, and agriculture fundamentally change the environment. A controversial hypothesis by William Ruddiman suggests that the rise of agriculture and the accompanying deforestation led to the increases in carbon dioxide and methane during the period 5000-8000 years ago.

Interplay of factors

If a certain forcing (for example, solar variation) acts to change the climate, then there may be mechanisms which act to amplify or reduce the effects. These are called positive and negative feedbacks. As far as is known, the climate system is generally stable with respect to these feedbacks: positive feedbacks do not "runaway". Part of the reason for this is the existence of a powerful negative feedback between temperature and emitted radiation, which increases as the fourth power of absolute temperature.

The glacial and interglacial cycles of the present ice age provide an important example. It is believed that orbital variations provide the timing for the growth and retreat of ice sheets. However, the ice sheets themselves reflect sunlight back into space and hence promote cooling and their own growth, known as the ice-albedo feedback. Further, falling sea levels and expanding ice decrease plant growth and indirectly lead to declines in carbon dioxide and methane. Similarly, rising temperatures caused, for example, by anthropogenic emissions of greenhouse gases could lead to retreating snow lines, revealing darker ground underneath, and consequently absorbing more sunlight.

Water vapour, methane, and carbon dioxide can also act as significant positive feedbacks, their levels rising in response to a warming trend, thereby accelerating that trend. More complex feedbacks involve the possibility of changing circulation patterns in the ocean or atmosphere. For example, a significant concern in the modern case is that melting glacial ice from Greenland will interfere with sinking waters in the North Atlantic and inhibit thermohaline circulation. Other potential feedbacks are not well understood and may either inhibit or promote warming. Similarly, increasing temperatures, may lead to either more or less cloud cover. Since on the balance cloud cover has a strong cooling effect, any change to the abundance of clouds also impacts climate.

Examples of climate change

Climate change has continued of through out the entire history of Earth. In addition to modern observations of climate, the field of paleoclimatology has provided information of climate change in the ancient past. Obviously, most of these changes are solely the result natural factors.

This fact sheet is based on an edited version of the Wikipedia entry on Climate Change.