Climate feedback

Negative climate feedback helps preserve the Earth’s climate in its existing state, whereas positive feedback accelerates changes, creating potential tipping points beyond which global warming or cooling becomes extremely difficult to reverse. Understanding both types of feedback is necessary to develop adequate climate policies.

Background

Like all planets, the Earth is bombarded by radiation from the Sun, stars, and other space-based sources of energy. Eventually, that energy is returned to space. Incoming and outgoing radiation must ultimately balance out, in conformity with the first law of thermodynamics, the law of conservation of energy. In the case of the Earth, which has a significant atmosphere and varied surface geography, the pathways by which radiation reaches the surface and is radiated back out to space are somewhat convoluted. Some incoming radiation is reflected back to space by clouds and particles in the atmosphere before it reaches the ground, some is reflected back toward space at ground level, and some is absorbed and then reradiated away in the form of long-wave, or infrared, radiation. Some of this reradiated infrared radiation is bounced back toward the ground by atmospheric gases or water vapor before it eventually makes its way back into space. Still, over time, the total Earth-atmosphere system is said to be in radiative balance.

Factors that can alter Earth’s radiative balance are called “climate forcings” and “climate feedbacks.” Understanding climate forcings and feedbacks is at the heart of understanding how greenhouse gases emitted into the atmosphere might affect future temperature trends. Climate feedbacks are particularly important to understand because computer models projecting future warming incorporate assumptions about such feedbacks (especially water vapor) that significantly elevate predicted temperature increases due to greenhouse gas emissions.

Climate Forcing

One cannot understand climate feedbacks without understanding climate forcings. A climate forcing (technically a “radiative forcing”) is something that exerts a direct effect on the radiative balance of the Earth’s atmosphere—that is, something that changes the balance of incoming versus outgoing radiation either permanently or transiently. Forcings include incoming solar radiation, the heat-retaining ability of greenhouse gases present naturally in the atmosphere, human greenhouse gas emissions and conventional air pollutants, changes in land use that might alter the reflectivity of the Earth’s surface, and more. The identifies nine major components, some of which are considered well understood, and others less so. Forcings identified by the IPCC include the greenhouse gases, ozone, stratospheric water vapor, surface albedo, aerosols, contrails, and solar irradiance.

Climate Feedback

Climate feedbacks are secondary changes to the radiative balance of the climate stemming from the influence of one or another climate forcing. Such climate feedbacks may be either positive or negative; a positive feedback would amplify the effect of a change in a given climate forcing, while a negative feedback would damp down the effect of a change in a given climate forcing. Thus, a change in the atmosphere’s water vapor content could cause greater cloudiness, which could, depending on the type of clouds, constitute a positive or negative feedback.

According to the National Research Council (part of the U.S. National Academies of Science), climate feedbacks that primarily affect the magnitude of climate change include clouds; atmospheric water vapor; the lapse rate of the atmosphere (defined as the change in temperature with altitude); the reflectivity, or albedo, of ice masses; biological, geological, and chemical cycles; and the carbon cycle. Feedbacks that primarily affect temporary responses of the climate include ocean heat uptake and circulation feedbacks. Finally, feedbacks that mostly influence the spatial distribution of climate change include land hydrology and vegetation feedbacks, as well as natural climate system variability.

When used in projecting future temperatures stemming from greenhouse gas emissions, estimates of some climate feedbacks are incorporated into computerized models of the climate system. These climate feedbacks—clouds, water vapor, surface albedo, and the lapse rate—are expected to contribute as much (or more) warming to the atmosphere as changes in the greenhouse gases do by themselves.

Context

The extent to which climate feedbacks might increase or decrease the heat-trapping effects of humanity’s greenhouse gas emissions is an important factor in public policy development. If computer models understate the extent of positive feedbacks, future warming could be worse than projected, and actions undertaken to combat climate change might be insufficient to the challenge. By contrast, if computer models overstate positive feedbacks, or underestimate negative feedbacks, projected future warming scenarios could be too high. In this case, massive resources spent on controlling greenhouse gas emissions could be wasted, leaving society less able to deal with other challenges, environmental or otherwise.

Key Concepts

• aerosols: tiny particles or liquid droplets suspended in the atmosphere; some, such as sea salt, are natural, while others, such as soot from power plants, are of human origin
• albedo: the extent to which an object reflects radiation; the reflectivity of objects with regard to incoming solar radiation
• climate forcing: factors that alter the radiative balance of the atmosphere (the ratio of incoming to outgoing radiation)
• greenhouse gases (GHGs): gases (or vapors) that trap heat in the atmosphere by preventing or delaying the outward passage of long-wavelength infrared radiation from the Earth’s surface out to space
• infrared radiation: radiation with wavelengths longer than those of visible light, felt by humans and animals as heat
• lapse rate: the change in a variable with height, often used to discuss changes in temperature with altitude in a context of climate change
• radiative balance: the balance between incoming and outgoing radiation of a body in space, such as Earth

Bibliography

"Assessing the U.S. and Global Climate in September 2024." National Centers for Environmental Information, 24 Oct. 2024, www.ncei.noaa.gov/news/assessing-us-and-global-climate-september-2024#:~:text=According%20to%20NCEI's%20Global%20Annual,in%20the%20130%2Dyear%20record. Accessed 21 Dec. 2024.

Houghton, John. Global Warming: The Complete Briefing. New York: Cambridge University Press, 2009.

Intergovernmental Panel on Climate Change. Climate Change, 2007—The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007.

National Research Council. Understanding Climate Change Feedbacks. Washington, D.C.: National Academies Press, 2003.

North, Gerald, and Tatiana Erukhimova. Atmospheric Thermodynamics: Elementary Physics and Chemistry. New York: Cambridge University Press, 2009.

Spencer, Roy. Climate Confusion: How Global Warming Hysteria Leads to Bad Science, Pandering Politicians, and Misguided Policies That Hurt the Poor. New York: Encounter Books, 2006.