6) Other Anthropogenic Impacts on Climate

Effects of deforestation

• Reduced carbon uptake → higher atmospheric CO₂

• Changes in surface albedo (forests = dark, absorb more; crops = light, reflect more)

• Reduced evapotranspiration, affecting regional humidity and rainfall

Effects of afforestation

• Lower albedo (dark surfaces absorb more solar radiation) → local warming

• But increases carbon sequestration → long-term cooling

• Net impact depends on latitude:

  • High latitudes: albedo effect dominates → net warming from afforestation

  • Tropics: carbon uptake dominates → net cooling from afforestation

Effects of urbanisation

• Urban areas create urban heat islands (UHI):

  • Artificial surfaces (e.g. tarmac, concrete) retain heat

  • Less vegetation → reduced evaporative cooling

• Increases local temperatures, but minor global effect

Effects of agriculture

• Methane (CH₄) from:

  • Enteric fermentation in livestock (especially cattle)

  • Rice paddies (anaerobic respiration of microbes)

• Nitrous oxide (N₂O) from:

  • Overuse of fertilisers (denitrification process in soils)

  • Manure management

• Agriculture contributes to:

  • ~10–12% of global GHG emissions (direct)

  • ~24% when including LUC

Effects of biomass burning

• Combustion of forests or crop residues releases:

  • CO₂, CH₄, N₂O, and black carbon

• Black carbon (soot) deposits on snow/ice:

  • Lowers albedo → increases warming

• Biomass burning also produces aerosols → short-term cooling effect

Effects of aviation

• Emissions occur at high altitudes, which:

  • Increases the radiative efficiency of GHGs like CO₂

  • Creates contrails and cirrus cloud enhancement → traps longwave radiation → warming

• Non-CO₂ impacts (e.g. NOx) significantly increase aviation's climate impact

Effects of shipping

• Emits SO₂ aerosols → increase cloud reflectivity (ship tracks)

• Cooling effect, but contributes to acid rain

• Also emits black carbon and GHGs

Direct effects of aerosol radiative forcing

• Scattering aerosols (e.g., sulphate, organic carbon) → increase albedo → cooling.

• Absorbing aerosols (e.g., black carbon, brown carbon) → absorb radiation → warming.

Indirect effects of aerosol radiative forcing

• Cloud Albedo Effect (First Indirect Effect): More aerosols → Smaller cloud droplets → More reflection → Cooling.

• Cloud Lifetime Effect (Second Indirect Effect): More aerosols → More cloud cover → Delayed precipitation → Cooling.

Main anthropogenic aerosol sources

• Fossil fuel burning (sulphate, black carbon).

• Biomass burning (organic carbon, black carbon).

• Industrial emissions (sulphates, nitrates).

Mechanisms for ozone radiative forcing

• Absorbs Incoming UV Radiation (Stratosphere)

  • Leads to cooling in the stratosphere but has little impact on the surface.

• Absorbs Outgoing Infrared Radiation (Troposphere)

  • Causes warming (~+0.35 W/m²).

  • More important for climate change than stratospheric ozone depletion.