Forcings and Feedbacks

Radiative Forcing (RF)

  • Radiative Forcing (RF) describes how a factor has changed the climate since 1750.
    • RF="…perturbationtoenergybalanceofEarth−atmos…(inW/m2)…"RF = "…perturbation to energy balance of Earth-atmos…(in W/m^2)…"
    • The RF for a GHG is equal to (How much GHG concentration has increased) × GWP.
    • Positive RF causes warming; negative RF causes cooling.
    • RF is expressed as a global, annual mean change since 1750 (pre-Industrial Revolution).
  • Long-lived GHGs directly cause warming by absorbing outgoing IR.
  • Some GHGs have indirect effects by creating or destroying other GHGs.
    • Halocarbons (CFC + HCFC + HFC) destroy stratospheric O3O_3, leading to negative RF (cooling).
  • Effective Radiative Forcing (1750 to 2019) includes both direct and indirect effects.

Which GHGs Cause the Most Warming?

  • Figure TS.15 from IPCC TS, 6th Assessment Report (2021) illustrates this.

Role of Aerosols in Climate Change

  • Aerosol-cloud interactions: Humans have increased particulate matter (PM) amounts, leading to brighter clouds (↑ albedo).
  • Radiative forcings:
    • Aerosol-radiation interactions:
      • Particles scatter or absorb sunlight.
      • Most scatter sunlight back to space, causing cooling (negative RF).
      • Black carbon (soot) absorbs sunlight, causing warming.
  • Aerosol = (toxic) suspension of solid or liquid particles (not gases) in the air.
    • Emitted directly (e.g., diesel engines) & formed in the atmosphere from gases (e.g., SO2SO_2).
    • Aerosols are also called “particulate matter” or PM or just particles.

Forcings Summary

  • Total anthropogenic RF (1750-2019) is +2.7 W/m2W/m^2.
    • RFs: GHG ~+4 W/m2W/m^2 and aerosols ~ –1.3 W/m2W/m^2.
    • Aerosols cover up a fraction of GHG warming.
    • Since 1750, other forcings (e.g., solar changes) have been minor.
    • Particles kill ~ 4M people/yr, leading to efforts to reduce them.
  • Order of GHGs in terms of:
    • Concentration increase:
      • CO2CO_2 (130 ppm)
      • CH4CH_4 (methane) (1.1 ppm)
      • N2ON_2O (nitrous oxide) (0.075 ppm)
      • Halocarbons (CFCs, etc.) (Low & variable)
    • Warming efficiency:
      • Halocarbons (100s-1000s)
      • N2ON_2O (260)
      • CH4CH_4 (32)
      • CO2CO_2 (1)
    • Warming caused: need to multiply concentration increase by warming efficiency.

Feedbacks: Overview

  • Feedback = secondary change in climate in response to an initial perturbation.
  • Feedbacks can significantly impact climate.
  • Positive feedback: secondary change amplifies the initial perturbation.
  • Negative feedback: secondary change dampens the initial perturbation.
  • Forcings cause an initial perturbation, while feedbacks follow.
    • e.g., human emissions of CO2CO_2 are a forcing; the effect of melting sea ice is a feedback.
  • Some agents can be both forcings and feedbacks.
    • e.g., carbon-cycle.

Fast Feedbacks

  • Fast = feedback occurs on a timescale of years or shorter.
  • Water vapor (H2O(g)H_2O(g)) feedback (largest feedback).
    • As T↑, the amount of H2O(g)H_2O(g) in the atmosphere increases.
    • This increases T.
    • Positive feedback.
  • Ice albedo feedback (for sea ice and glaciers).
    • ↑T → ↓ amount of ice → ↑ albedo → ↑ T → …
    • Positive feedback.
  • Lapse-rate feedback.
    • As T↑, EoutE_{out} of Earth increases.
    • Warmer planet radiates more energy, causing cooling.
    • Negative feedback.
  • Cloud feedback: very uncertain but potentially large.
    • Clouds reflect incoming sunlight (cooling) & absorb outgoing IR (warming).
    • Clouds currently have a net cooling effect.
    • As the world warms, clouds may reflect less sun & absorb more IR.
    • The feedback could be positive or negative.

Slow Feedbacks

  • Slow = feedback occurs on a timescale of decades or longer.
  • Continental ice sheet albedo feedback (Greenland and Antarctica).
    • Similar to sea ice/glacier feedback, but much slower.
  • Carbon-cycle feedbacks.
    • An increase in T causes changes that can release GHGs to the atmosphere.
    • e.g., CO2CO_2 from permafrost.
    • Positive feedback.
  • Vegetation feedbacks.
    • Vegetation distribution is largely controlled by climate (T, precipitation…).
    • Vegetation can also impact climate: produces aerosols, affects albedo, etc.

How Important are Feedbacks?

  • Imagine an initial T perturbation of +1 °C from GHGs.
  • The figure shows the final T change (Final ΔT = Initial perturbation (+1 °C) + ΔT from feedbacks) for three scenarios:
    1. Ice-albedo feedback: example of a positive feedback.
    2. Lapse-rate feedback: example of a negative feedback.
    3. All Earth feedbacks.
  • Feedbacks are important for climate change.