Climate Change in the 21st Century
Prediction of climate change over the next 100 to 150 years is based solely on climate model simulations. Understandably, the vast majority of modelling has concentrated on the effects of continued man-made pollution of the atmosphere by greenhouse gases, and to a lesser extent, atmospheric aerosols. The main concern, at present, is to determine how much the Earth will warm in the near future. This fact sheet reviews projected climate change in the 21st century.
Climate Model Simulations
During the first part of the last decade, a number of complex climate models attempted to simulate future man-made climate change. Significant results indicated:
The ability to model the time-dependent nature of the climate more adequately has allowed scientists to investigate the delaying effects of the oceans on climate change. Because the response time of the oceans, in particular the deep ocean, is much longer than for the atmosphere, they have a regulating or damping effect on the warming associated with the enhanced greenhouse effect, slowing down the rate of global warming. In addition, the more complex models have allowed increased attention to focus on the critical role of climate feedback processes in determining the climate's overall response to the initial enhancement of the greenhouse effect.
Early climate models estimated the temperature change associated with a doubling of atmospheric CO2 concentrations in the absence of feedback processes to be approximately 1.2°C (the theoretical temperature change associated with a doubling of pre-industrial atmospheric carbon dioxide concentrations). When feedbacks were included within the model, the CO2 doubling temperature change is estimated to be between 1.5°C to 4.5°C. Notice the large range of uncertainty in the model outputs. The additional rise in temperature in the models can be attributed to climatic feedbacks. Three of the most important climatic feedbacks to the enhanced greenhouse effect include the water vapour feedback, the cloud feedback and the ice-albedo feedback.
Water Vapour Feedback
The importance of water vapour feedback in climate change has long been recognised. Water vapour is itself a greenhouse gas. The increase in temperature resulting from higher levels of carbon dioxide, methane and nitrous oxide causes an increase in evaporation and hence the amount of atmospheric water vapour. This in turn enhances the greenhouse effect still further leading to further warming, further evaporation and more water vapour, and so on. This feedback is called positive feedback.
Cloud feedback is the term used to encompass effects of changes in cloud and their associated properties with a change of climate. Although clouds further enhance the greenhouse warming of the climate by absorbing more outgoing infra-red radiation (positive feedback), they can also produce a cooling through the reflection and reduction in absorption of solar radiation (negative feedback).
The conventional explanation of the amplification of global warming by snow and ice feedback is that a warmer Earth will have less snow cover, resulting in a lower global albedo (reflectivity) and further absorption of more sunlight, which in turn causes a further warming of the climate. Most climate models have simulated this positive surface albedo feedback. The greatest changes in temperature projected to occur in the 21st century are at the high (snow-covered) latitudes.
Future Climate Change
By using simpler climate models, it is possible to estimate future changes in the global average surface air temperature resulting from the man-made enhancement of the greenhouse effect. Future enhancement of the greenhouse effect is determined by policy scenarios which estimate the time evolution of greenhouse gas concentrations in response to various greenhouse gas emission scenarios.
The most recent models estimate that global average surface temperature will rise by between 1.4 and 5.8°C over the next hundred years, with a best guess of about 3°C. The significant margin of uncertainty is due to the uncertainty over the precise role of the greenhouse feedbacks and the uncertainty surrounding the precise growth in greenhouse gas emissions during the 21st century.
Projected Global Warming during 21st Century
According to recent climate models that simulate the effects of increasing greenhouse gas concentrations, global average surface temperature may rise by between 1.4 and 5.8°C by 2100, depending on the rate of global development. This rate of climate change is faster than at any time during Earth history. If nations fail to respond, the world may experience numerous adverse impacts as a result of global warming in the decades ahead.