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Revised draft (BK/PQ Oct 7, 2016 around the sun. However, orbital changes are not behind today's global warming —our current orbit dictates we should be cooling not warming! CO2 has increased by about 40% since the 1700s from 280 ppm (parts per million) to today's 400 ppm. Ocean acidification is the lesser known result of climate change. The ocean absorbs atmospheric CO2— leading to an increased acidification of the ocean, which is currently 30% more acid than 300 years ago (a drop in pH from 8.2 to 8.1). The ocean's ecosystem will begin crashing when pH falls below 7.8. As acidification worsens, it's more difficult for coral to construct reefs and mollusks to calcify their shells. Reef building is particularly vulnerable to climate change —with current loss a sign of that change. If current trends continue all coral reefs will cease to grow, and start dissolving, within 50 years. Australia's Great Barrier Reef has already declined by 50% over the last 30 years. Models Scientists use models to test hypotheses, make predictions, and frame new questions. They test them by replicating observed data. When Global Circulation Models (GCMs) do a good job of replicating past and current observations, we have increased confidence they can forecast the future. GCMs have been able to do just this — including more rapid warming seen since 1950, and short -term cooling after recent volcanic eruptions (Mount Pinatubo, 1991). GCMs also predict many of the extreme events we currently experience, such as major flooding, and extended heat waves and drought. GCMs tell us about the drivers of significant changes in global temperature observed over the last century. Models using only natural forcings (solar variation, volcanic activity) show essentially no temperature increase over the last 100 yrs. When including anthropogenic effects the GCM fit with observed data is very good. Despite skeptics' claims, nobody has created a general circulation model that includes anthropogenic effects explaining climate behavior over the past century without CO2- driven warming.