Aren’t computer climate models unreliable?
Many models seem reliable because, starting with real observations, they successfully model climate change in the recent past. These different models also consistently predict rising temperatures in future.
The Earth’s climate system is highly complex with many interacting natural systems, together with changes that mankind is causing, that involve the Sun’s radiation, the atmosphere, the oceans, vegetation, ice and so on. There are also factors that can cause knock-on effects so that an increase in one component may cause a much larger increase in some other component of the system (so-called positive feedback mechanisms). Undoubtedly too there are parts of the system that we do not fully understand. All this means that climate is difficult to predict. Nevertheless, computer models are the best tool we have to make quantitative predictions of future climate. Further, projections of future climates rely on assumptions about the rate of greenhouse gas emissions and these assumptions may need to be altered to take into consideration changes in technology, economics, human population and human behaviour.
The validity of many climate models has been tested successfully against climate history; in other words, models that have been run forward from some time in the past, when conditions were reasonably well known, can successfully predict past changes in climate. This gives a degree of confidence in the reliability of the models in predicting future climate.