The analysis of historical trends in annual temperature and precipitation was based on monthly data observed for the period 1961–2004 from four meteorological stations close to the studied area (13º07'- 13º28'S; 86º10'- 86º23'W) at altitudes between 490 and 900 m.a.s.l. To provide a more robust analysis for the baseline, we also compared the observations with Climate Research Unit TS3 dataset (CRU TS3) for the corresponding grid and period (University of East Anglia Climate Research Unit 2008). The long-term historical temperature and rainfall in general show few clear trends for many parts of Central America (Magrin et al. 2007).
For the studied area, the annual mean temperatures increased significantly, by about 0.4°C/decade between the 1960s and the 2000s (Fig A.1.1a). The average annual rainfall showed an insignificant decline, although one of the rainfall stations experienced an abrupt decline in rainfall in the late 1990s that could have influenced the trend observed (Fig A.1.1b).
Extreme weather events and associated natural hazards have been particularly connected with El Niño and La Niña cycles (NOAA, 2010). The El Niño phases were associated with warmer regional temperatures and strong rainfall anomalies (Figure A.1.1a and b), i.e., low rainfall with severe droughts (1972, 1976, 1987, 1990–91, 1994, 2004) and peaks in rainfall (1966, 1969, 1998), causing floods and landslides. The La Niña phases were associated with floods, and occurred mostly in years with peak rainfall (e.g. 1968, 1970, 1998) while occasionally in years with low total annual rainfall (1962).
Fig. A.1.1a. Trends in annual mean temperature from 1961 to 2005 (the data have been standardized to facilitate comparison).
Fig. A.1.1b. Trends in annual total rainfall from 1959 to 2005 (the data have been standardized to facilitate comparison).
Future Projections and Potential Impacts
The future climate scenario for this study was developed from the two Special Reports on Emissions Scenarios A1B-projections for the 21st century with 1980–99 as a baseline. First, annual mean temperature and rainfall for the 2050s builds on Ruosteenoja et al. (2003). The annual changes were calculated as the mean changes for the dry and wet season, respectively. Second, seasonal changes for 2080–99 build on scenarios for Central America developed by Christensen et al. (2007). The projected differences in minimum, maximum, median, 25%, and 75% quartiles between the baseline and 2080–99 periods were used to modify the distribution of the observed baseline. The two original sources included seven and 21 General Circulation Models, respectively.
The resulting scenarios indicate a change in annual mean temperature by 0.8°C for 2050. Minimum temperatures may increase by 1.4–2.0°C and maximum temperatures by as much as 4.6–5.5°C. In terms of seasonal changes, the scenario indicates that winters may have more strong increases in both minimum and maximum temperatures, leading to frequent heat waves and dry spells. The change in annual mean precipitation may be more irregular and may range from -13.5% to +4% by 2050. This translates to a range of about -110 to +30 mm as minimum rainfall decrease by 45%–57%, whereas maximum rainfall increases by up to 24%. These scenarios further indicate that the frequency of dry seasons may increase by 15%–25%. In contrast, the seasonal scenarios, project winters with higher intensity rainfall and stronger and/or more frequent tropical storms. Figure A.1.2 shows the ranges of change in projections of average total annual rainfall for 2090. To illustrate the likely increase in extreme events (both floods and droughts), the maximum May rainfall between 1980–99 was 565 mm, whereas by the 2090s it may reach 650 mm. However, the median rainfall was only 80 mm and may decline further to 67 mm.
Fig. A.1.2. Range of past (1980–99) and projected (by 2090s) monthly rainfall.
The most likely immediate impact expected in the region is a more intense and recurrent drought risk in both seasons. Moreover, flood and wind-risk damages are expected in winter under the scenario studied. Modest shifts in the seasonality can lead to remarkable ecosystem changes. Seasonally, Central American dry forests are considered severely threatened by global warming (Halpin et al. 1995, Intergovernmental Panel on Climate Change 2007). Drought recurrence can also alter grassland/shrubland boundaries (Intergovernmental Panel on Climate Change 1998), species composition (Sala et al. 2000), reduce plant physiological functions due to heat stress (Battisti and Naylor 2008) and decrease water content in topsoil and soil erosion by wind (Magrin. et al. 2007). Grain-crop yield losses are expected to increase under extreme climatic events (Lobell et al. 2008). In particular, according to studies undertaken in Latin America’s dry regions (Giménez 2006), higher minimum temperatures, combined with water limitation in the fall, may shorten the growing season. Other studies show that in medium-altitude semiarid regions of Central America, maize yields may decrease between 6% and 17% by 2055, and other staple crops, e.g., beans, may be badly affected as a consequence of drought and other extreme climatic events (Jones and Thornton 2003). Species characterized by high reproduction rates are generally favored by temperature increases, leading to a potential rise in the distribution and occurrence of pest infestation and pathogens (Magrin et al. 2007). Consequently, increasingly fluctuating weather patterns could have a strong negative economic impact on agriculture, by increasing labor time and production costs (Intergovernmental Panel on Climate Change 1998). Similar studies in dry regions show that the expected key impacts of change in rainfall and drought recurrence and intensity on livelihoods (particularly smallholders) are: food insecurity, through diminished crop production and increased food prices (Hertel et al. 2010), declining survival rates of livestock (Richardson et al. 2007) and increasing spread of diseases, e.g., dengue/dengue hemorrhagic fever (Rosenzweig et al. 2001, Patz et al. 2005). Human migration from drought-affected areas and tenser social relations due to scarcity of land and natural resources (Barnett and Adger 2007) may also be expected in the study area.