Global Warming: the Physical Science Basis

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What Is The Greenhouse Effect?

When the sunlight (solar radiation) reaches Earth, some of it is reflected back into space by our atmosphere. However, most of the solar radiation, having a short wavelength, passes through the atmosphere. As the solar radiation hits earth’s surface some of it does reflect back through the atmosphere and into space. Yet, much of the solar radiation is absorbed by the land and water surfaces. When the solar radiation is absorbed by land and sea, it warms the surface and is converted into long wavelength, or infrared, radiation.

The infrared radiation is emitted by the earth’s surface and reaches the atmosphere. Some of the infrared radiation passes through the atmosphere into space. However, “greenhouse gases” in the atmosphere (mainly, water vapor and Carbon Dioxide, CO2) absorb some of this long wavelength radiant energy and the molecules generate heat. Some of the heat escapes above into space and some is emitted back toward Earth.

This atmospheric re-emitted infrared radiation supplements the earth-warming effect from the solar radiation, further warming the earth and the layer of air immediately above the earth, known as the troposphere. This radiant energy “trapped” by the atmosphere is known as the Greenhouse Effect.

Without the Greenhouse Effect, the earth would be a lot colder as the infrared radiation escapes back into space. (Think of the cold nights over a high desert with a thin atmosphere above.) The infrared radiation which is partially trapped by the atmosphere makes the average temperature of the earth warmer. This is a natural phenomenon. The global warming problem arises from changes in the concentration of the greenhouse gases in the atmosphere.

How Can Global Warming Be Traced to Carbon Dioxide (CO2)?

The gases that make up 99 percent of our atmosphere—nitrogen, oxygen, and argon—do not absorb visible or infrared light and thus let both forms of radiation pass through. The next most abundant gases—water vapor and CO2—do absorb a portion of the infrared heat radiated by the earth, thereby preventing it from reaching space.

While water vapor and CO2 make up a small percentage of the atmosphere, they share their absorbed heat with the other molecules when they bump against one another when mixing.

The heating effect of extra CO2, methane, nitrous oxide, and many other minor gases can be calculated with confidence based on properties that have been measured carefully in the lab.

Currently, the total heating produced by the increases of all such long-lived greenhouse gases since pre-industrial times is equal to about 1 percent of all solar radiation absorbed at the surface. The effect would be somewhat similar if the sun had started to shine 1 percent more brightly during the 20th century.

Small changes of the earth’s heat balance can lead to large climatic changes. Between ice ages and warm periods, the difference in global average temperatures has been only about 5 degrees Celsius in the tropics and 8 degrees C in polar regions.
Greenhouse Gas Effect

Why are climatologists so highly confident that human activities are dangerously warming the earth?

Atmospheric concentrations of carbon dioxide, methane, and nitrous oxide remained roughly stable for nearly 10,000 years, before the abrupt and rapidly accelerating increases of the past 200 years. Growth rates for concentrations of carbon dioxide have been faster in the past 10 years than over any 10- year period since continuous atmospheric monitoring began in the 1950s.

  • CO2 concentrations are now roughly 35 percent above pre-industrial levels (which can be determined from air bubbles trapped in ice cores).
  • Methane levels are roughly 250 percent of pre-industrial levels.
  • Nitrous oxide levels are around 20 percent higher.

How can we be sure that humans are responsible for these increases?

  1. Geographic differences in concentrations reveal that sources occur predominantly over land in the heavily populated Northern Hemisphere.
  2. Analysis of isotopes which can distinguish between sources of emissions demonstrates that the majority of the increases in CO2 come from combustion of fossil fuels (coal, oil, and natural gas). Methane and nitrous gas increases derive from agricultural practices and the burning of fossil fuels.

Scientists use a concept called radiative forcings. Radiative forcings (expressed as watts per square meter) is the change that is caused in the global energy balance of the earth relative to pre-industrial times. Positive radiative forcings warm the earth; negative radiative forcings cool the earth. Scientists can quantify the radiative forcings of long-lived greenhouse gases fairly precisely because we know their concentrations, their spatial distribution, and the physics of their interaction with radiation.

In addition to changes in industrial age greenhouse gas concentrations, additional human factors include human generated microscopic particles called aerosols, ozone, surface albedo (reflectivity), and aircraft contrails—although the influences of these mechanisms is much less certain. For example, some aerosols may brighten clouds, increase their reflectivity. To the extent this aerosol cloud albedo effect creates negative radiative forcings, it is thought to be minor.

Natural drivers also create radiative forcings. Among these are solar activity and large volcanic eruptions. Aerosols can also be from dust storms, forest fires, and volcanic eruptions. (The extent to which the dust storms and forest fires are natural or exaggerated by humans involves complex measurements and are generally assumed to be natural.)

The 2007 International Panel on Climate Change estimates that total human-induced positive radiative forcings is 10 times larger than the best estimate of the natural radiative forcings caused by changes in solar energy.
CO2 Concentrations and Effects

What is the observational data?

The warming trend is 0.74 +/- 0.18 degree C over the last 100 years. In the last 50 years alone, the warming trend is 0.65 +/- .15 degree C, emphasizing that the majority of the warming in the 100 years occurred in the last 50 years. The average number of frost days and cold days and nights have lessened, while heat waves and warm days and nights have increased. Spring snow cover is down. Permafrost temperatures are up.

Sea warming is up; the oceans have absorbed 80 percent of the heat added to the climate, contributing to sea level rise (3.1 +/- 0.7 mm per year) due to expansion and due to glacier melt. Artic sea ice is declining 2.7 +/- 0.6 percent per decade.

Hydrological cycles (evaporation and precipitation of water) are consistent with warming trends. Ocean salinity shows freshening in middle and higher latitudes (increases in rain patterns) and greater saltiness in lower latitudes (increases in drought patterns).
Radiative Forcings

Are human activities responsible for observed climate changes, or is it possible they result from some other cause, such as natural forcings or simply spontaneous variability within the climate system?

The 2007 IPCC concluded that it is very likely (90%) that most of the warming since the mid-20th century was attributable to humans (up from 66% likely five years earlier).

  • Increased observational data.
  • Changes in more aspects of the climate (atmospheric circulation, temperatures within the oceans) create a broadened picture of the effect of human activity.
  • Differences in satellite and balloon data have been resolved.

Climate models have improved and can represent the current climate and that of the recent past with considerable fidelity. Coordination of an ensemble of simulations from 18 modeling groups from around the world improves understanding. Using many models help quantify the effects of uncertainties (clouds, ocean eddies, vegetation transpiration).

In the multi-model simulations, natural forcings are unable to explain the observed global warming since the mid-20th century, whereas they can be explained when they include anthropogenic (human) factors in addition to the natural forcings.
This article is adapted from Scientific American, August 2007, The Physical Science Behind Climate Change: Why are climatologists so highly confident that human activities are dangerously warming the earth; by William Collins, Robert Colman, James Haywood, Martin R Manning, and Philip Mote. It also references the Intergovernmental Panel on Climate Change, 10th Session of Working Group 1; Climate Change 2007: The Physical Science Basis, Summary for Policy Makers.

Global Warming: the Physical Science Basis