The greenhouse effect is not controversial, nor is it synonymous with global warming. Evidence from the ice ages shows that the carbon dioxide greenhouse has always played a crucial role in shaping the climate, but only as one element in a set of phenomena to which few pay attention. Owing to a large natural variability of the climate system, it has not been possible to ascribe the observed warming to changing greenhouse concentrations in a statistically rigorous manner. One recent analysis has concluded that there is no danger at present of a buildup of carbon dioxide in the atmosphere sufficient to cause a significant heating of the Earth. There will be a compensatory response by the planetary system. It is possible that the increase in the number of dust particles in the atmosphere will reflect back more of the sun's rays and produce a compensating reduction in the heating effect. Or global warming might have been caused by solar activity that will decrease next century and so a slight cooling will offset some of the greenhouse effect. Some scientists believe that as the planet warms, increased evaporation from the sea will produce more thick clouds, which will cool the Earth. There is also evidence that global warming is due to the warming cycle of natural rhythms of glaciation.
Two scientists at the University of Alabama-Huntsville, in a 1998 report, produced evidence from high altitude satellite measurement of atmospheric temperatures, that the planet actually cooled slightly in 1997. This evidence supports scientists who claim we still do not know enough about the earth's highly complex climate to draw conclusions of global warming.
Computer simulations claim that the greenhouse effect should have raised global temperature by about one degree Celsius. However actual temperature records only show a rise of 0.5 degrees over the century peaking before 1940 and then declining until the 1970's since it has risen modestly by 0.2 degrees. As 80% of carbon dioxide emissions have occurred since 1940, then the rise of 0.5 degrees before 1940 must natural. Based on the evidence of sunspots who Galileo began observing in 1610 suggests that computer models overlook the influence of sunspots on temperatures. Sunspots are cooler darker areas of strong magnetic fields. These sunspots peak and change direction every 11 years or so creating 22 year cycles. The sun is brightest at peak sunspot periods thus is brightest during short cycles. Changes in the length of the cycle and in northern hemisphere land temperatures are closely correlated over three centuries. If this is correct then changes in the sunspot cycle would explain the average temperature change of about one degree Celsius over the last 100 years.