FUTURE CLIMATE CHANGE
Greenhouse gas concentrations in the atmosphere will increase during the next century unless greenhouse gas emissions8 decrease substantially from present levels. Increased greenhouse gas concentrations are likely to raise the Earth's average temperature, influence precipitation and some storm patterns as well as raise sea levels. The magnitude of these changes, however, is uncertain.
As noted in the Climate Change Technology Program Strategic Plan9 (PDF, 16 pp., 172 KB, About PDF10): "…stabilizing greenhouse gas concentrations, at any atmospheric concentration level, implies that global additions of greenhouse gases to the atmosphere and global withdrawals of greenhouse gases from the atmosphere must come into a net balance. This means that growth of net emissions of greenhouse gases would need to slow, eventually stop, and then reverse, so that, ultimately, net emissions would approach levels that are low or near zero."
The amount and speed of future climate change will ultimately depend on:
- Whether greenhouse gases and aerosol concentrations increase, stay the same or decrease.
- How strongly features of the climate (e.g. temperature, precipitation and sea level) respond to changes in greenhouse gas and aerosol concentrations.
- How much the climate varies as a result of natural influences (e.g. from volcanic activity and changes in the sun ’s intensity).
Virtually all published estimates of how the climate could change in the future are produced by computer models of the Earth’s climate system. These models are known as general circulation models (GCMs). According to the National Research Council (2001):
Because climate is uncontrollable (albeit influenceable by humans), the models are the only available experimental laboratory for climate. They also are the appropriate high-end tool for forecasting hypothetical climates in the years and centuries ahead. However, climate models are imperfect. Their simulation skill is limited by uncertainties in their formulation, the limited size of their calculations, and the difficulty of interpreting their answers that exhibit almost as much complexity as in nature.
Climate models are able to simulate many global scale features of the climate. Their skill diminishes, however, in reliably simulating regional changes. In fact, at regional scales, different climate models sometimes yield different results. Average results from a set of models often produce more accurate results than any single model.
It is important to recognize that projections of climate change in specific areas are not forecasts comparable to tomorrow’s weather forecast. Rather, they are hypothetical examples of how the climate might change and usually contain a range of possibilities as opposed to one specific high likelihood outcome.
The following pages provide a summary of the projected changes in the atmosphere and climate over the next century based on the current state of knowledge:
- National Research Council (NRC), 2001. Climate Change Science: An Analysis of Some Key Questions. National Academy Press, Washington, DC