Envisioning Future Philippine Climate Conditions: An Analysis of General Circulation Models based on the Special Report on Emissions Scenarios

Visualization of the extent of climate change in the Philippines is important in vulnerability and adaptation studies. Data will have to be downscaled from the result of General Circulation Models (GCMs) modelled by the developed countries. Three different models were studied and compared to determine which model is best suited for application for studies in the Philippines. These are models developed by the Hadley Centre for Climate Prediction and Research (HadCM3), Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Canadian Center for Climate Modelling and Analysis (CGCM2ma). These are based on the Special Reports on Emissions Scenarios (SRES) scenarios, which cover a wide range of key “future” characteristics such as demographic change, economic development, and technological change developed by the
Intergovernmental Panel on Climate Change to incorporate recent findings and developments on the understanding of the driving forces of emissions and methodologies.

The study aims to resolve a range of probable temperature and precipitation change derived from SRES-based GCMs, to serve as input for vulnerability, adaptation, and mitigation studies. These two climate variables are considered the primary climate variables with which other secondary climate variables such as vapor pressure, potential temperature, relative humidity, and solar radiation are derived. These conditions and consequent changes determine the effects on the different sectors (agriculture, water resources, health, and marine and coastal areas). However, due to limitations inherent in any model, conditions and projections stated in this paper are merely guides for adaptation and vulnerability assessments that must not be taken as absolute. To determine the model most closely suited for application to Philippine conditions, a comparison of the baselines of the SRES and the internationally established DDC (Data Distribution Centre) baseline was done. Correlation between the SRES models and the DDC baseline is important in understanding the above-mentioned limitations of the models when downscaled to a regional level. This is especially important when the focus of the study is a country as small as the Philippines.

With these considerations in mind, the HadCM model seems to be most closely suited for application to Philippine conditions in terms of precipitation, while the CSIRO model seems to be best suited for temperature. This model shows the closest and most consistent correlation with the DDC baseline, and its vertical orientation also suggests that this was the only model wherein topography was a primary consideration. However, the choice of the HadCM as the model most applicable to the Philippines does not completely discount the results of the other models. Results of the different model may provide an additional range of climate condition for each scenario.

Based on all the models, an increase of up to 30 percent in rainfall during the rainy season (June, July, August), and a decrease of up to 42 percent in rainfall during the dry season (December, January, February) is projected. However, changes are not uniform throughout the country. Temperature, on the other hand, is projected to continuously increase for the 21st century. A range of 0.5 to 2.4 degrees Celsius is the projected increase for temperature.