**4. Climate change: A challenging**

Since 1988, agricultural scientists have been studying climate change mainly according to the rules of IPCC (Intergovernmental Panel on Climate Change). In recent years global studies have shown consistent changes in air temperature and rainfall in many places of the world [17, 18]. The Earth's average temperature can rise between 1.8ºC and 4.0ºC in the next 100 years [19] with more significant increases for minimum than maximum air temperature. However, in the case of South America temperature trends may not be consistent [20]. Contemporary scientific studies have warned of anomalies in temperature and precipitation patterns indicating the occurrence of global change, with direct consequences on human activities, especially those related to agricultural production [18].

Globally, agriculture accounts for 23% of all emissions of greenhouse gases (GHG) that come from human activity. Parts of these (15%) are derived from agricultural practices and the other part (8%) is from changes in land use [21]. However, in Brazil a greater proportion of agricul‐ ture's contribution to GHG comes from agricultural practices [22]. Soil is also an important source of carbon emission and sequestration [23]. Soil management practices affect to what degree these processes go on. Conventional tillage operations tend to increase carbon release to the atmosphere, whereas conservation tillage and crop rotation with their increased contribution of crop organic matter to the soil tend to increase carbon sequestration [24]. These practices also improve the physical, chemical, and biological balances in the soil. Current agriculture through its production of fiber, bioenergy and food reduces pollution and mitigates GHG emissions [25].

A good example are the results of a research carried out at Embrapa Soja (Londrina, PR, Brazil) for levels of Carbon (C) and Nitrogen (N) in soil under no-tillage (NT) and conventional sowing (CT). [26] found that the biggest difference in results between NT and CT systems occurs in the first 30 cm of the soil profile with a 29% increase in total C content of soil in NT vs. CT. Still, disputing claims of U.S. researchers [27], the survey confirmed significant increases in carbon sequestration in the 0-60cm soil layer. They determined that in NT vs. CT there was 18% and 16% increase in the C and N contents, respectively, of the soil organic matter. Within the same 0-60 cm profile, C and N content of the microbial mass increased 35% and 23%, respectively. Over the 20 years of the research, C and N rates of accumulation within the 0-60cm layer of soil were 800 kg C ha-1 year-1 and 70 kg N ha-1 year-1, respectively, in NT vs. CT. According to the Agricultural Census [28], in 2006 the total no-tillage area for crops in Brazil was 15.6 million hectares.

Because of the rising temperatures associated with global warming, the adaptability of certain crops to an area may change. In particular, drought problems may become worse and yield potential reduced. Because of this, agricultural scientists have been developing strategies to avoid potential adverse climatic changes, especially those related to drought stress.

Problems with drought and temperature stress have to be resolved with research for adapta‐ tion like developing new soybean genotypes and cultivars with heat and water loss tolerance.

In Brazil, the southeast region, which accounts for 40% of soybean production, has suffered severe losses due to the occurrence of droughts. Growing seasons in the years 2003 and 2004 for example, showed yield losses due to drought close to 24% and 44%, respectively. In 2004/05 growing season, Rio Grande do Sul state alone, lost an average 70% of its production. In the last 10 seasons, the direct losses can be estimated by more than \$18 billion due to the occurrence of drought [29].

The search for commercial cultivars more drought tolerant through classical breeding are relatively difficult because of the complex mechanisms developed by plants to ameliorate this stress. Biotechnology is also an important ally in the breeding for drought tolerant cultivars. With the sequencing of the soybean genome it becomes possible to understand the function of a specific gene and how it interacts with other genes. This allows for breeding new cultivars with greater resistance to environmental stresses.
