Picture Banner for SECC K12 Site

Vegetation body


 

Vegetation includes all plants from evergreen forests to grassy meadows and cropland.  All types of plants play a role in both the water cycle and the earth’s energy balance.  They affect weather and climate mostly through evapotranspiration and albedo.

Effects on Weather and Climate

With vegetation covering about 20% of our planet, it’s no surprise that plants affect climate.  However, it is surprising how much plants affect weather.  Plants process and release water vapor (necessary for cloud formation) and absorb and emit energy used to drive weather.  Plants also produce their own micro-weather by controlling the humidity and temperature immediately surrounding their leaves through transpiration.  Most plants and forest soils have a very low albedo, (about .03 to .20) and absorb a large amount of energy. However, plants don’t contribute to overall warming because the excess warmth is offset by evaporative cooling from transpiration.

A Pie Chart showing the percentage breakdown of Land Use over the entire US

Figure A

Since climate is basically an average of the weather over a long period of time, vegetation is important to climate.  In fact, the process of photosynthesis is responsible for building up atmospheric oxygen to the level we enjoy today (21% concentration).  Plants also help keep our climate stable over time by offsetting temperature and moisture fluctuations through transpiration.  Plants also use carbon dioxide during photosynthesis, which slightly offsets the amount of greenhouse gas being released in the atmosphere through the burning of fossil fuels.  Vegetation is necessary for normal weather and climate. 

Land Use

There are nearly 2 billion acres of land in the continental United States.  Land use classifies the types of vegetation on the land.  In the past thirty years, there has been an increase in developed land and a decrease in cultivated crop land.  An increase in developed land means more areas are covered by buildings, concrete and asphalt. 

It is interesting to note that historical land use changes have also affected the climate of the Southeast.  In Georgia, for example, around 1900 there were large areas of bare ground associated with cotton fields and other crops.  By the late 1900s, most of Georgia (over 70 percent) had reverted to pine and deciduous forests once cotton was no longer farmed due to the boll weevil (beetle which feeds on cotton buds and flowers) and soil degradation.  Some climatologists think that the slight cooling of the Southeast over the 1900s may have been due in part to the higher evapotranspiration and cooler conditions associated with forests compared to bare ground in cropland.

Pie Chart of the percentage breakdown of Land Use in the Southeast to be compared with Land Use in entire US (above).

Figure B

The more recent increase in developed land, especially around urban areas, causes a “heat island” effect, and large cities with no exposed ground or vegetation become very hot during the day.  The lack of vegetation means there is no transpiration to cool the air.  Also, asphalt has a very low albedo and absorbs a lot of sunlight, which adds to the excessive heat.  The concrete and asphalt release their heat slowly at night, too, leading to warmer conditions overnight as well as during the day.  Uncultivated crop land also stays warmer due to the lack of transpiration, but doesn’t get as hot as developed areas.   Figure A displays different types of land use for the entire United States in 2003.  In the past ten years there was a major slowdown in the amount of land being developed.

Figure B shows the distribution of land use in the Southeast (AL, FL, GA, NC, SC and VA).  On average, the Southeast has twice as much developed land as the rest of the United States, although this is offset by the fact that over half of the land in the Southeast is forest.  Forests are beneficial because they use copious amounts of carbon dioxide, as well as mitigating heat through transpiration.

Last modified date: Friday, May 17, 2013 - 1:33pm