Calcium stable isotopes have been used to study inputs and outputs of dissolved calcium in marine environments. For example, one study found that calcium levels have decreased between 25 and 50 percent over a 40 million year timespan, suggesting that dissolved Caoutputs have exceeded its inputs. The isotope Calcium-44 can help to indicate variations in calcium carbonate over long timespans and help explain variants in global temperature. Declines in the isotope Calcium-44 usually correlate with periods of cooling, as dissolution of calcium carbonate typically signifies a decrease in temperature. Thus, Calcium isotopes correlate with Earth's climate over long periods of time.