Climate FoRCINGpoSitive And Negative Feedback

The Earth's heat budget is controlled by the amount of energy it receives from the Sun and subsequently returns to space. The Earth's global energy balance is maintained when the incoming energy from the Sun is balanced with the outgoing heat from the Earth. There are mechanisms that cause the Earth's energy budget to get out of balance. Then it can result in climate change.

There are several mechanisms that can change the Earth's energy balance in either a positive or negative way. Some of these mechanisms are natural and some are human-caused. One of the natural mechanisms that can affect the Earth's energy balance, and therefore climate, is fluctuations in the Earth's orbit. This includes the properties that were discussed in chapter 5: changes in the Earth's eccentricity, tilt, and precession. As the orbit becomes more eccentric, tilt becomes more pronounced, or precession shifts, the Earth's climate experiences changes. Changes in ocean circulation can also affect climate by moderating temperatures of ocean water and air masses transported around the globe. Volcanic eruptions are another natural mechanism that can have an effect on the energy balance. When a significant amount of particulates are added to the atmosphere from a powerful volcanic eruption, they can effectively block out incoming radiation and lower temperatures on Earth, cooling climate for a certain period of time. There is also a natural variability in solar radiation as sunlight goes through its natural cycles of energy fluctuation. The balance can be altered by reflection from clouds or gases, absorption by various gases or surfaces, and emission of heat by certain materials. Variations in the amount of active gases present also can change the energy balance.

However, in the past several decades, changes in the composition of the Earth's atmosphere that have seriously affected the energy balance have not been natural: they have been caused by human actions. In particular, they have been in the form of air pollution due to the emission of greenhouse gases. They end up forcing the climate to change. Scientists refer to these various natural and human-induced mechanisms as climate forcing mechanisms.

Scientists refer to climate forcing in both the positive and negative sense. According to Gavin A. Schmidt of the National Aeronautics and Space Administration (NASA), both the positive and negative forcings must be taken into account in order to build a reliable model to base future predictions on. A positive forcing is a forcing that warms

Excessive amounts of ash from volcanic eruptions can cause cooling because the ash and other particulates block out incoming solar radiation. This photo shows the eruption of Mount Pinatubo on June 12, 1991. It caused a noticeable worldwide cooling. (Dave Harlow, USGS)

the system. A negative forcing cools the system. When radiative forcing (change between the incoming radiation energy and the outgoing radiation energy in a climate system) occurs, it alters the equilibrium and causes a new balance to be reached and maintained.

It is important to understand radiative forcing because as these factors exert an influence on the Earth's energy balance, they directly affect climate and climate change. Scientists who study global warming pay specific attention to these various forcings when they build mathematical models in an attempt to better understand climate behavior. As shown in the figure on page 150 there are distinct forcings important to the study of global warming. Those elements that contribute a positive forcing (and increase global warming) include greenhouse gases, stratospheric water vapor, and black carbon on snow. Negative forcings—elements that counteract global warming—include stratospheric ozone and the albedo associated with certain types of land use.

The figure makes clear that the net anthropogenic component is quite significant, supporting the theory that it is largely human activity causing global warming.

There is nothing we can do about natural forcings, i.e., when the Sun's intensity changes. Humans, however, do have control over forcings caused by emissions of greenhouse gases from the burning of fossil fuels and methane released by production of animals for food, reduction in CO2 storage through deforestation, adding CO2 to the atmosphere through the burning of forests, and deliberate changes in land use that affect global warming. If humans choose wisely and curtail these detrimental activities, they can control much of the radiative forcings that currently act on the environment to accelerate global warming. When scientists talk about changes in global warming and the damage done to specific ecosystems, they often put it into context with climate forcing values because these tell scientists much about what has happened in an

Exxonmobil Fawley Cat Cracker

Industrial pollution is a human-caused positive forcing that serves to increase the atmospheric temperature by adding greenhouse gases.

(Nature's Images)

Radiative Forcing Components

Radiative Forcing Components

Die Alten Gypter Zeitleiste

This graphic illustrates the concept of radiative forcing. In order to understand global warming, it is necessary to understand the components in the atmosphere that control the overall warming and cooling on a short- and long-term basis. (modeled after NASA)

area, what will likely happen, and where focus needs to be directed to offset unnecessary negative impacts.

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