3.2 Greenhouse effect and radiative forcing

The Greenhouse Effect and Earth's Temperature

The greenhouse effect is a natural process that keeps Earth warm enough to support life. Without it, Earth's average surface temperature would sit around -18°C (0°F) instead of the roughly 15°C (59°F) we experience today. That 33°C difference is entirely due to greenhouse gases in the atmosphere trapping outgoing heat.

Here's how it works: Earth's surface absorbs incoming solar radiation and then re-emits that energy as infrared (longwave) radiation. Greenhouse gases in the atmosphere absorb some of this outgoing infrared radiation and re-emit it in all directions, including back toward the surface. This keeps heat circulating in the lower atmosphere rather than escaping directly to space.

The enhanced greenhouse effect is what happens when human activities increase the concentration of greenhouse gases beyond natural levels. Burning fossil fuels, clearing forests, and expanding agriculture have all added greenhouse gases to the atmosphere, amplifying the natural warming process and driving additional temperature increases.

Main greenhouse gases and sources

Carbon dioxide ($CO_2$) is the most significant human-produced greenhouse gas. Its major sources include:

• Burning fossil fuels (coal, oil, natural gas) for energy and transportation
• Deforestation and land-use changes, which reduce the natural carbon sinks that would otherwise absorb $CO_2$
• Cement production, where the calcination of limestone releases $CO_2$ as a byproduct

Methane ($CH_4$) is less abundant than $CO_2$ but far more potent as a heat-trapping gas over shorter timescales (roughly 80 times more warming potential than $CO_2$ over 20 years). Sources include:

• Agriculture, particularly livestock digestion (enteric fermentation) and rice cultivation in flooded paddies
• Landfills, where organic waste decomposes without oxygen (anaerobically) and produces methane
• Fossil fuel extraction and distribution, where methane leaks during the processing and transport of natural gas and oil

Nitrous oxide ($N_2O$) comes primarily from:

• Agricultural practices, especially the use of nitrogen-based fertilizers, which stimulate soil bacteria to produce $N_2O$
• Industrial processes like nylon and nitric acid production
• Biomass burning, including wildfires and agricultural waste burning

Water vapor ($H_2O$) is actually the most abundant greenhouse gas, but humans don't directly control its concentration. Instead, it acts as a feedback mechanism: as other greenhouse gases warm the atmosphere, more water evaporates from oceans, lakes, and rivers, which adds more water vapor, which traps more heat. This positive feedback loop amplifies the warming caused by $CO_2$ and other gases.

Tropospheric ozone ($O_3$) forms near Earth's surface through photochemical reactions involving nitrogen oxides and volatile organic compounds (often from vehicle exhaust and industrial emissions). It acts as a secondary greenhouse gas in the lower atmosphere.

Radiative Forcing and Climate Change

Radiative forcing in climate systems

Radiative forcing measures the change in Earth's energy balance caused by some external factor. Specifically, it's the change in net energy (incoming minus outgoing) at the top of the troposphere, measured in watts per square meter (W/m²).

The sign tells you the direction of the effect:

• Positive radiative forcing means more energy is being retained than lost, which leads to warming
• Negative radiative forcing means more energy is leaving than arriving, which leads to cooling

Greenhouse gases impose positive radiative forcing because they absorb and re-emit more infrared radiation back toward the surface, reducing the amount of energy that escapes to space.

Other factors also contribute to radiative forcing:

• Solar irradiance changes: Variations in the sun's energy output can slightly increase or decrease forcing, but these changes have been small compared to greenhouse gas effects over the past century
• Volcanic eruptions: Large eruptions inject reflective sulfate aerosols into the stratosphere, temporarily increasing Earth's albedo and exerting short-term negative forcing (cooling)
• Land-use changes: Converting forests to cropland, for example, changes surface albedo (reflectivity), which alters how much solar energy the surface absorbs

The value of radiative forcing is that it lets you quantify and compare how much different factors push the climate toward warming or cooling. Over recent decades, anthropogenic factors (especially greenhouse gas emissions) have dominated the forcing trend.

Greenhouse gases and global temperatures

The link between greenhouse gas concentrations and temperature is well established. As concentrations rise, radiative forcing increases, and temperatures follow.

• Atmospheric $CO_2$ has risen from pre-industrial levels of roughly 280 ppm to over 420 ppm today, driven primarily by fossil fuel combustion and land-use changes
• Methane and nitrous oxide concentrations have also increased significantly since pre-industrial times

Earth's average surface temperature has risen by approximately 1.1°C since the pre-industrial era. The rate of warming has accelerated in recent decades, with the warmest years on record all occurring since 2000. This warming is not evenly distributed: higher latitudes and land areas warm faster than the global average, a pattern known as Arctic amplification (for the polar regions).

Climate models project further warming as greenhouse gas concentrations continue to rise. The amount of future warming depends on emission trajectories:

• Limiting warming to 1.5°C or 2°C above pre-industrial levels requires reaching net-zero emissions by mid-to-late century
• Continued high emissions lead to progressively more severe impacts, including accelerated sea level rise, ecosystem disruption, and more frequent extreme weather events