Although the effects of agriculture itself is bad, the deforestation that takes place in order to clear this land in the first place can have even more noticeable results. First of all take a look at the image below, now compare that to the one underneath it.
Average Monthly Temperature and Rainfall in Brazil from 1900-1930
Average Monthly Temperature and Rainfall in Brazil from 1990-2009
As you may have noticed when comparing the two charts, that in the first one, the 1900 to 1930 one, that rainfall was generally more even in the dry months than in the second chart above. And when comparing the temperature ranges in the two charts you will notice that the temperature range was much more ? back in the early 20th century than in the later 20th/early 21th century.
Cattle Ranching in Brazil,
As seen in the diagram on the right, cattle ranching has been the main cause for deforestation over the years, with 80% of deforested land being the cause of cattle ranching. Although the cattle ranching industry has boosted Brazilian economy significantly over the years, this does come with a cost, cattle, like other Ungulates (hoofed mammals) have hard hooves which causes the fragile topsoil to be removed. Erosion of the topsoil is a significant concern because without the topsoil, little plant life is possible, decreasing the yield of crops. The erosion of the topsoil then becomes sediment in the rivers, this then eventually build-ups, causing blockages that have to be removed, in short the land begins to produce less, so less money, and increases the maintenance cost of drainage systems. Not good. It should be noted as well that the topsoil takes a long to regenerate, about 500 years.
Referring to the diagram at the top of this page, we can see that agriculture comes up second when it comes to causes of deforestation of the Brazilian Amazon. Agriculture, and more specifically, soybean farming was the main cause of deforestation in the Brazilian Amazon for a while at least, til a moratorium was put in place back in July 24, 2006 where soy trader were not allowed to buy soybeans from lands that have been deforested in the Amazon biome, originally the mortium was to only last for a year but has been in effect for 8 years now and will end on December 31 2014, this has allowed the government time to put proper surveillance in place as well as permanent protection to ensure that illegal cutting of trees and farming do not go unknown.