Some scientists believe that if the man-made portion of greenhouse gases were cut back, the increases in atmospheric concentration of greenhouse gases would cease. But this is like saying that nature can tell the difference between CO2 made by man and that arising naturally. In other words, it assumes that nature sorts out the 157 billion tonnes a year rising into the atmosphere and somehow says these are caused naturally, let's send them back to Earth. Accurate observations of the precise amounts of gas involved in these exchanges are difficult to make, but it turns out that when the total amount of CO2 emanations from Earth's surface drops, a smaller proportion is returned. In other words, a cutback in man-made gas emissions would lower the atmospheric CO2 by less than the amount of the cutback.
More gas is retained in warmer years and less in cooler years, regardless of emission levels.
Some scientists suggest this means something other than greenhouse gases causes temperature variations. Possibly the existence of a warmer atmosphere merely necessitates that more greenhouse gases be retained in the atmosphere in order to obtain equilibrium. This is certainly a plausible explanation for earlier climate changes. Another theory was developed by reviewing past temperatures, chiefly since 1850 or sometimes 1900, and assuming that all temperature change since is due to increases in these gases. By ignoring all other potential causes for recent climate history, this method projects the average annual temperature to rise by 2 degreesC by the year 2100, assuming modest economic growth in developed countries and with the developing countries gradually reaching our standard of living, driving more cars, and using air conditioning in their better and bigger homes, over and above basic industrial use of energy in their economies.