Climate Models

n Complex computer models of Earth’s climate that are used to simulate climate changes due to increased greenhouse gas concentrations

n The models attempt to include all important feedback processes

n Not all feedbacks are well understood (e.g., modeling cloud effects, changes in clouds)

n Some feedbacks may not be included

n The models are not perfect and do not make exact predictions. Best we can hope for is that they give us realistic long-term averages and some idea of the variability about the average, i.e., frequency of extreme events

n Several test cases give us confidence that they have predictive skill

n Simulations that closely reproduce recent climate (20^th century)

n Simulations that reproduce features of historical climate changes (e.g., Ice Ages)

n Simulations that reproduce short-term climate fluctuations

n Volcanic eruptions

n El Nino events

n However, keep in mind that predictions of future change are much more difficult than simulating current or past climate where the answers are known

n Baseline test – doubled CO₂ (Simulated by all climate models)

n (2-4.5)˚ C range in the predicted warming of global average surface temperature from various models (relatively high confidence from scientific community)

n Regional changes (for smaller areas) in temperature/precipitation show less agreement among models, as this more difficult to predict than global average (lower confidence from scientific community). Uncertainty gets larger as the spatial scale becomes smaller.

n Most recent projections (also depends on future greenhouse gas emissions)

n Global average surface temperature will increase anywhere from 2.0 - 11.5° F between 1990 and 2100. Considers a range of different models and different emissions.

n Details of predictions for regional changes need to be improved.

n The models continue to evolve and improve due to increases in computing power, improved understanding of climate system, and improved observations of climate.

n Because the climate system is so complex, there is the possibility that all models will turn out to be wrong. Perhaps the changes will be less severe than predicted; or perhaps we will be surprised and climate changes will occur suddenly and be quite severe.

Impact Studies

n Study of how climate change will affect humans and natural ecosystems

n Ultimate questions that must be answered

n Regional changes most important for ecosystem health

n Global concerns for human well-being, sea level rise, fresh water and food resources

n Difficult problem with uncertain answers

n Regional climate changes relatively uncertain (from GCMs)

n Even if climate changes could be predicted exactly, it is still difficult to answer the question: How sensitive and adaptable are systems to the changes?