Climate Change

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Climate Change Concepts

Difference between Climate and Weather

The difference between weather and climate is a measure of time. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere “behaves” over relatively long periods of time.

When we talk about climate change, we talk about changes in long-term averages of daily weather. If summers seem hotter lately, then the recent climate may have changed. An earlier spring, in most parts of the world, is indicative of a possible change in the climate.

In addition to long-term climate change, there are shorter term climate variations. This so-called climate variability can be represented by periodic or intermittent changes related to El Niño, La Niña, volcanic eruptions, or other changes in the Earth system.

 

Climate Forcing

A climate forcing can be defined as an imposed perturbation of Earth’s energy balance. Energy flows in from the sun, much of it in the visible wavelengths, and back out again as long-wave infrared (heat) radiation.

An increase in the luminosity of the sun, for example, is a positive forcing that tends to make Earth warmer. A very large volcanic eruption, on the other hand, can increase the aerosols (fine particles) in the lower stratosphere (altitudes of 10–15 miles) that reflect sunlight to space and thus reduce the solar energy delivered to Earth’s surface.

These examples are natural forcings. Human-made forcings result from, for example, the gases and aerosols produced by fossil fuel burning, and alterations of Earth’s surface from various changes in land use, such as the conversion of forests into agricultural land. Those gases that absorb infrared radiation, i.e., the “greenhouse” gases, tend to prevent this heat radiation from escaping to space, leading eventually to a warming of Earth’s surface. The observations of human-induced forcings underlie the current concerns about climate change.

The common unit of measure for climatic forcing agents is the energy perturbation that they introduce into the climate system, measured in units of watts per square meter (W/m2). The consequences from such forcings are often then expressed as the change in average global temperature, and the conversion factor from forcing to temperature change is the sensitivity of Earth’s climate system. Although some forcings—volcanic plumes, for example—are not global in nature and temperature change may also not be uniform, comparisons of the strengths of individual forcings, over comparable areas, are useful for estimating the relative importance of the various processes that may cause climate change.

 

Radiation Balance of Earth

Methods of transferring energy in the atmosphere include conduction, convection, latent heating, advection and radiation. The method of transferring energy through radiative processes is different from the other mechanisms in that the transfer process does not require molecules.

If we consider the planet as a whole, the Earth exchanges energy with its environment (the solar system) via radiation. The radiation balance of the planet is a fundamental parameter that determines our climate. This balance includes energy from the sun, or solar energy, which is an energy source for the planet. Any object that has a temperature emits radiation, we refer to this energy as terrestrial or infrared energy. The hotter the object, the greater the amount of energy emitted. Terrestrial energy can cool or warm an object depending on the object’s temperature, and the temperature of the surrounding environment.

 

Net Radiation Balance

The difference between the absorbed solar energy and the OLR is referred to as the net radiation. The annual variation in net radiative energy follows that of the solar declination due to the annual variation of the incoming solar energy being greater than the annual variation of the albedo.

In general, the absorbed solar radiation exceeds the outgoing longwave radiation in the tropical and subtropical regions, resulting in a net radiative heating of the planet, while in the middle to polar latitudes there is a net cooling. This equator-to-pole difference, or gradient, in radiative heating is the primary mechanism that drives the atmospheric and oceanic circulations. On an annual and long-term basis in which no energy storage and no change in the global mean temperature occurs, this radiative imbalance between the tropics and Polar Regions must be balanced by meridional heat transport by the atmosphere and oceans.

The measured outgoing longwave radiation and albedo also indicate regional forcing mechanisms. For example, in the tropics east-west variations can be as large as the north-south averages and are associated with east-west circulations. Tropical regions, in general, display a net radiative heating, the Sahara is often experiencing a net radiatively cooling. This is due to the high surface albedo, the warm surface temperatures and the dry and cloud free atmosphere. The radiative cooling is maintained by subsidence warming, which also has a drying effect and therefore helps maintain the desert.

The albedo, OLR and net radiation are closely related to surface type and the weather regime. For example, look at the Sahara Desert and the Amazon basin in the summer and winter. The incoming solar radiation is a function of latitude and time of year. The desert is approximate 20 degrees north latitude while the Amazon basin is approximately 20 degrees south of the equator. So the incoming solar radiation in the Amazon in January (Southern Hemisphere Summer) is nearly the same as the incoming solar radiation over the Sahara in July (Northern Hemisphere Summer).

The two regions also have very high albedos during their respective summers — but for two different reasons. The high albedos of the Amazon are the result of highly reflecting deep-convective cloud systems. Over the desert, there are few clouds, but the surface, which is mostly dry soil, is highly reflective. The OLR is very different for these two regions. The amount of terrestrial radiation is a function of temperature, the tops of the convective clouds are very cold, and so the outgoing energy is small. In contrast the desert surface is very warm, and so the OLR is large.

 

Factors influencing the radiation balance

The temperature of earth as a whole is determined by the balance between incoming and outgoing energy. Climate change arises largely from changes to the earth’s heat balance. Many factors can influence this—both natural processes and anthropogenic processes. It is the impacts from anthropogenic processes, through the enhanced greenhouse effect, which are likely to be causing contemporary climate change, which in turn could bring about considerable environmental, social and economic disruption if adequate mitigation and adaptation measures are not implemented.

The main factors influencing climate change are:

  1. Extraterrestrial factors, such as variations in the sun’s activity and slow changes in the earth’s orbit and tilt of its axis.
  2. The main influence within earth is volcanism. Slow drift of continents and mountain building also influence climate, but this occurs only over millions of years, so can be considered constant over time scales of decades to centuries.
  3. Factors operating on the earth’s surface include the reflectivity or Albedo of the surface; the amount of heat in the oceans and the atmosphere and the level of heat exchange between them; and the influence of land vegetation on the composition and heat balance of the atmosphere.
  4. Atmospheric factors include the composition of the atmosphere and its reflectivity, from the surface of the earth to the stratosphere.

Many of these factors are inter-related, and atmospheric, ocean and land interactions can involve complex feedback mechanisms can either enhance or dampen changes to the climate system.

The natural influences on the climate system have caused variations in the earth’s climate over hundreds of thousands of years, as well as on shorter timescales of decades. Many of these processes are unaffected by human activity, including the extraterrestrial factors and factors associated with the earth’s tectonic activity. Exchange of heat and gases between the earth’s atmosphere and its oceans and land vegetation are also natural influences on the climate system, but these processes are now being affected by human activities to various degrees.

For example, the presence and composition of the earth’s atmosphere produces a natural greenhouse effect. The atmosphere contains gases such as water vapour and carbon dioxide that are relatively transparent to the sun’s shortwave radiation, but absorb some of the longwave radiation that is re-radiated from the earth’s surface. This causes the average temperature on earth to be about 33°C warmer than it would be if there were no atmospheric ‘blanket’.

 

Climate Feedback mechanisms

Any change in the environment leading to additional and enhanced changes in that system is the result of a positive feedback mechanism. Alternatively, if a change in the environment leads to a compensating process that mitigates the change it is a negative feedback mechanism. In climate change discussions the focus is on the atmospheric radiation field as a forcing of the climate system (radiative forcing). Currently the discussion concentrate on the radiative forcing associated with the steadily increasing concentrations of different gases in the atmosphere – the so-called greenhouse gases: CO2, CH4, N2O, CFC-gases etc.

Other changes in the environment can also lead to changes in the radiative budget, like deforestation, changes in land use and air pollution (ozone, SO4-aerosols, CONTRAILS, É). Also non-anthropogenic changes are important in disturbing the radiative balance: fluctuations in the Solar output and volcanic activity.

Important positive feedback mechanisms include:

  1. The ice-albedo mechanism
  2. Lower tropospheric water vapor content
  3. Ocean warming.

Negative feedback mechanisms include

  1. Black body radiation
  2. Cloud mechanisms

Clouds are important factors in the climate system. They are known to have a negative impact on the surface temperatures in the present climate system. However, a changing climate may involve changes in the clouds with both positive and negative effects on the radiative balance. Currently it is not known whether the total effect of these changes will be a negative or positive feedback.

Ice-albedo feedback

Ice-albedo feedback (or snow-albedo feedback) is a positivefeedback climate process where a change in the area of snow-covered land, ice caps, glaciers or sea ice alters the albedo.

 

Polar amplification

The warming trend in the Arctic is almost twice as large as the global average in recent decades. This is known as Arctic amplification. Changes in cloud cover, increases in atmospheric water vapour, more atmospheric heat transport from lower latitudes and declining sea ice have all been suggested as contributing factors.

The vertical profile of Arctic warming (eg – how much warming occurs at different altitudes) gives us insight into the underlying cause. If atmospheric heat transported from lower latitudes was the major driver, more warming would be expected at greater heights. On the other hand, if retreating snow and sea ice cover was the major cause, maximum warming would be expected at the surface. Figure 1 shows the simulated warming expected in each season if declining sea ice was the major cause of warming.

Polar Amplification

The warming trend in the Arctic is almost twice as large as the global average in recent decades. This is known as Arctic amplification. Changes in cloud cover, increases in atmospheric water vapour, more atmospheric heat transport from lower latitudes and declining sea ice have all been suggested as contributing factors.

The vertical profile of Arctic warming (eg – how much warming occurs at different altitudes) gives us insight into the underlying cause. If atmospheric heat transported from lower latitudes was the major driver, more warming would be expected at greater heights.

On the other hand, if retreating snow and sea ice cover was the major cause, maximum warming would be expected at the surface. Figure 1 shows the simulated warming expected in each season if declining sea ice was the major cause of warming.

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