Impacts, uncertainties and non-linearities of extreme events (heavy precipitation and floods) in a changing climate Luis J. Mata Center for Development Research University of Bonn. Germany. Climate Change is a setback with a typical character. It is global, long term, and involves complex interactions between climatic, environmental, economic, political, institutional, social and technological processes. Developing a response to climate change is characterized by decision-making under uncertainty and risk, including the possibility of non-linear and irreversible changes (Metz et al., 2001). Therefore, the complexities of this problem should not be neglected. Uncertainties have so penetrated the issue of climate change that is still impossible to eliminate even weak let alone catastrophic outcomes. Even one of the most credible scientific international assessment body, the Intergovernmental Panel on Climate Change (IPCC), has not yet attempted subjective probabilistic estimates of future temperatures. This has forced people to ask, despite the scientific data supporting the global warming issue: will the temperature in 2100 increase by 1.4°C or by 5.8°C? What types of extreme events have important impacts on society and environmental systems? Extreme events such as floods are extremely important to analyze due to its tremendously impact on natural and human system. Recently, floods have occurred all over the world causing severe damage. Climate change may be the trigging mechanism by which the frequency and magnitude of extreme events such as floods are occurring more often and stronger, but there are still some uncertainties that need much more investigation. How do we assess the impacts of these events? Challenging for impacts assessment is to be able to estimate the damage of such events in the future, it is important to realize that a linear increase in the mean implies a non-linear increase in the probability of some extremes, and increase in variability implies an increase in the probability of extremes. [For example, if the threshold temperatures for crops damage are given, a change for instance of a factor of 2 in the variance, may greatly increase the numbers of days above the threshold, represented by the areas under the tail of the curve (red areas), i.e., more hot weather and more cold weather with a new climate] As an example, a summary of recent studies that used GCMs output directly to construct scenarios of extremes are indicated in the table below How can model projections analysis assist in developing adaptation strategies? Assessment of vulnerability (including adaptation) requires an estimate of the impacts of climate change, which is based on model projections of the future climate under different scenarios. Vulnerability is a multidimensional variable that can be used as the crossing point (interface) between diverse system elements or sectors. It is suggested as the link concept between projected changes and impact studies. V = F [E, S, Ac]. It is important to notice that both exposure (E) and sensitivity (S) are also function of Ac. Adaptive capacity is a function of multiple elements of the system Ac= G [Di]. Projected changes during the 21 st century in extreme climate phenomena with a very likely (90-99%) chance to occur for the case of simple extreme events are (a) higher maximum temperatures: more hot days and heat waves in nearly all land areas, (b) higher (increasing) minimum temperatures: fewer cold days, frost days, and cold waves, and (c) more intense precipitation events over many areas. These will lead to, among others, an increased incidence of death and serious illness in older age groups and urban groups, and increasing numbers of floods with landslides and mudslide damage (IPCC, 2001). Importantly, future assessment needs to consider adaptation as a broad process, rather than just a set of technical measures (de la Vega-Leinert and R. Nicholls, 2001) and as an endogenous variable. It is also important to point out that adaptive capacity concept needs to be transferred to an operational tool. In addition information and model projections should assist in adaptation (e.g., short-term forecasting, seasonal forecasting) as part of a general disaster management strategy. It is important to analyze how well this information is developing worldwide. However, if the world were to warm by almost 6°C (i.e., with three times the level of carbon dioxide in the atmosphere compared to the pre-industrial era) –what will be the impact on the environment? The whole climate system will change dramatically, and, among others, precipitation patterns and amounts will become entirely different. Storms tracks will be unrecognizable from what they are today. Of course, it might take some years before these changes occur, which would leave time to plan for the changes, but the changes would be massive.However, there is no doubt that it is precisely because the responsible scientific community cannot rule out adverse or dangerous effects of climate change and catastrophic outcomes, that climate mitigation and adaptation policies should be seriously considered. Thus, in terms of Article 2 of the United Framework Climate Convection (UNFCCC), the question of what constitutes a dangerous climate change is still very much in debate. This issue stills an open question. Contact: Luis Jose Mata -- Presented at: International Symposium on Climate Change (ISCC), Beijing, THEORY OBSERVATION