Other random fluctuations may also be occurring. Results from simplified climate models, which have been calibrated to comprehensive general circulation models used to study the climate, have been used to analyze which factors have been influencing global average temperatures. The increases in the concentrations of carbon dioxide and other greenhouse gases have created a strong warming influence. The somewhat chaotic nature of climate fluctuations prevents perfect agreement between model results and observations.
Present understanding seems to be capturing most of the major changes.
However, these influences, which are not yet included in climate models, do not appear to be the dominant factors in long-term climate change. The geopotential height is a measure of the build-up of colder and warmer air masses in different regions of the atmosphere. The contour interval is 150 m. These results indicate that, if the sea-surface temperatures can be predicted accurately using ocean models, the present versions of the atmospheric models have the ability to accurately predict seasonal mean climate anomalies over North America and the adjoining areas, thereby allowing predictions of dry and wet and of cold and warm conditions, at least under some climatic conditions. Reconstructions of the history of temperature and of the atmospheric CO 2 concentration going back 150,000 years have been made from the air bubbles trapped in the ice of Antarctica.