Land Surfaces

1
Land Surfaces

• Lecture 7

2
Land Surfaces

• Where people live, grow food, build home and
  collect drinking water
• Land surface vital for survival
• Exploitation of large areas of continents for
  societal needs causes massive land disturbances
• Exact nature of state of land and direction of
  changes not easy to determine

3
Surface Energy Budget

• QS SWD SWULWD-LWU-LH-SE
• Where
• QS Net Energy Budget at the surface (W/m2)
• SWD Shortwave down at the surface (bottom)
• SWU Is the reflected SW at the surface, SWU
  AsSWD
• LWD Longwave down at the surface
• LWU Longwave up at the surface, LWU ess Ts4
• As Surface albedo
• Ts Surface temperature
• - es Surface emissivity

4
Surface Conditions

• 70 energy absorbed in climate system is at
  Earths surface
• Surface energy budget is sensitive to conditions
  at the land surface, in particular the vegetation
  cover
• 50 of world continental surface has already been
  altered by people
• Forest replaced by fields
• Valleys flooded for reservoirs
• Roads and cities sprawling across coastal regions
  
• People activities cause or contribute to
  desertification, deforestation, soil erosion
  salinisation, reforestation, irrigation and
  landscape development

5
Land surface changes

• Many relationship between land surface and global
  environment are very complex
• Potential contributions of land surface changes
  to global environment
• Greenhouse gas emissions
• Creation of aerosols (particulates and droplets)
• Surface reflectivity (albedo) and emissivity
  disturbances
• Impacts on surface roughness and hydrology
• Land clearance and land use now recognized as
  having the potential to impact and be impacted by
  local, regional and global climate

6
Land Changes impact on climate

• Replacing forests with grassland (deforestation)
  and stripping scrubs from arid regions
  (desertification) and urbanization have three
  primary changes
• Higher albedo and changes in emissivity
• Less evaporation
• Lower roughness

7
Desertification

• Affects millions of people
• Sparse vegetation of arid and semi-arid areas can
  be easily removed or destroyed by overgrazing or
  poor agricultural practices or as a result of
  minor changes in climate
• Removal of vegetation and exposure of bare soils
• decreased soil water storage capacity
• Increased runoff
• Increased albedo
• Less moisture availablegtdecreased latent heat
  flux gt increased temperature
• Higher temperaturegtincreased LW up and surface
  radiative loss and large scale subsidencegt less
  clouds and precipitation gt POSITIVE FEEDBACK

8
Deforestation

• Changes in surface hydrological characteristics
  (less evapotranspiration)
• Less moisture recycling gt less cloud and
  precipitation
• Not clear what the impact on temperature is
• Impacts can be felt at a distance from the
  deforestation due to teleconnection

9
Biosphere

• Very important to climate
• Transfers moisture into atmosphere
• Modifies albedo which changes the amount of
  radiation absorbed by climate system
• Responsible for carbon and other chemical
  exchanges
• Modifies surface roughness which affects
  exchanges of momentum (friction by wind)

10
Vegetation Reflectance
R SWBUP/SWBDOWN
11
Cryosphere

• Regions of the world covered by ice
• Greenland, Antarctic, large mountainous regions
  (e.g., Himalaya) or high latitude regions
  (Siberia)
• Major changes over time (over long time scales)
• Amount of snow and ice controls variations in
  global sea level
• Regions where climate change is expected to be
  first felt (temperature increase, snow/ice
  melting)
• Observable changes due to snow/ice albedo feedback

12
Snow/Albedo Feedback

• Greenhouse gases increase gt warming of surface
• When snow and ice are present on the warming
  surface, it will melt
• Melting induces reduction in surface albedo
  (mostly by reduction of extend of snow/ice
  regions)
• As albedo decreases, more SW radiation is
  absorbed
• Surface warms up from SW absorption and its
  temperature increases

13
Snow Reflectance
Reflectance
0.25
1.00
Wavelength (microns)