Scientists are also interested in global warming's impact on smaller regional and local levels. Because of readily available data, climatolo-gists often refer to data collected from 1600 through 1860 as a baseline to compare against current data. Human activity has drastically changed the composition of the atmosphere since 1600 and by comparing present data to the baseline data, scientists can determine how much it has changed. Before the Industrial Revolution, the biggest controller of climate change was solar variation and volcanic eruptions. Through modeling techniques, GISS scientists were able to determine that changes in incoming solar radiation have had a larger impact on regional climate than volcanic eruptions have. Solar variability cycles can last for many years, while the particles from volcanic eruptions that stay in the atmosphere and block incoming solar radiation typically affect regional climate for only a couple of years. By studying these older climate records, scientists can understand past conditions.
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