8.5 Model Simulations of Extremes

Society’s perception of climate variability and climate change is largely formed by the frequency and the severity of extremes. This is especially true if the extreme events have large and negative impacts on lives and property. As climate models’ resolution and the treatment of physical processes have improved, the simulation of extremes has also improved. Mainly because of increased data availability (e.g., daily data, various indices, etc.), the modelling community has now examined the model simulations in greater detail and presented a comprehensive description of extreme events in the coupled models used for climate change projections.

Some extreme events, by their very nature of being smaller in scale and shorter in duration, are manifestations of either a rapid amplification, or an equilibration at a higher amplitude, of naturally occurring local instabilities. Large-scale and long-duration extreme events are generally due to persistence of weather patterns associated with air-sea and air-land interactions. A reasonable hypothesis might be that the coarse-resolution AOGCMs might not be able to simulate local short-duration extreme events, but that is not the case. Our assessment of the recent scientific literature shows, perhaps surprisingly, that the global statistics of the extreme events in the current climate, especially temperature, are generally well simulated by the current models (see Section 8.5.1). These models have been more successful in simulating temperature extremes than precipitation extremes.

The assessment of extremes, especially for temperature, has been done by examining the amplitude, frequency and persistence of the following quantities: daily maximum and minimum temperature (e.g., hot days, cold days, frost days), daily precipitation intensity and frequency, seasonal mean temperature and precipitation and frequency and tracks of tropical cyclones. For precipitation, the assessment has been done either in terms of return values or extremely high rates of precipitation.