Climate forcings shape our planet's temperature. Natural forcings like solar cycles and volcanoes cause short-term changes. Human-made forcings, especially greenhouse gases, have become the main driver of long-term climate change since the Industrial Revolution.

Radiative forcing measures how factors affect Earth's energy balance. Greenhouse gases trap heat, warming the planet. Aerosols can cool or warm. Land-use changes alter surface properties. Solar and volcanic influences play smaller roles in recent climate change compared to human activities.

Natural and Anthropogenic Climate Forcings

Natural vs anthropogenic climate forcings

• Natural climate forcings originate from natural processes and phenomena such as solar variability, volcanic eruptions, and changes in Earth's orbit
• Natural forcings typically cause short-term or cyclical changes in climate (sunspot cycles, volcanic cooling)
• Anthropogenic climate forcings result from human activities including greenhouse gas emissions, aerosol emissions, and land-use changes (deforestation, urbanization)
• Anthropogenic forcings have become the dominant driver of climate change since the Industrial Revolution leading to long-term, persistent changes in the climate system

Radiative forcing of climate factors

• Greenhouse gases (GHGs) absorb and re-emit infrared radiation trapping heat in the atmosphere
  • GHGs cause positive radiative forcing leading to warming
  • Main anthropogenic GHGs include carbon dioxide ($CO_2$), methane ($CH_4$), and nitrous oxide ($N_2O$)
• Aerosols are tiny particles suspended in the atmosphere that can have both cooling (negative radiative forcing) and warming (positive radiative forcing) effects
  • Sulfate aerosols reflect sunlight causing cooling
  • Black carbon (soot) absorbs sunlight causing warming
• Land-use changes are alterations in land surface properties due to human activities that can affect surface albedo, evapotranspiration, and carbon storage
  • Deforestation reduces carbon storage and increases $CO_2$ in the atmosphere
  • Urbanization increases surface albedo and heat island effect

Solar and volcanic climate influences

• Solar variability involves changes in solar irradiance (energy output) over various timescales
  1. 11-year sunspot cycle causes small, short-term fluctuations in global temperature
  2. Long-term changes in solar activity (Maunder Minimum) can influence climate on centennial timescales
• Overall, solar variability has had a minor impact on recent climate change compared to anthropogenic forcings
• Volcanic eruptions inject sulfur dioxide ($SO_2$) into the stratosphere forming sulfate aerosols
  • Sulfate aerosols reflect sunlight causing temporary cooling (negative radiative forcing) typically lasting 1-3 years depending on the size of the eruption (Mount Pinatubo, Krakatoa)

Evidence for anthropogenic climate change

• Observed warming of ~1.0℃ since the mid-20th century is unprecedented in the context of the past 2,000 years
• Attribution studies use climate models to simulate the response to different forcings showing that anthropogenic forcings (mainly GHGs) are necessary to explain the observed warming while natural forcings alone cannot account for the observed changes
• Fingerprints of anthropogenic influence include:
  1. Warming of the troposphere and cooling of the stratosphere
  2. Increased downward longwave radiation at the surface
  3. Rising sea levels due to thermal expansion and melting of land ice
• Scientific consensus shows that an overwhelming majority (97%+) of climate scientists agree that recent climate change is primarily caused by human activities