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Revised draft (BK Dec 5, 2016 that time, GHGs and temperature moved in lock step controlled by earth's orbit around the sun. However, orbital changes are not behind today's global warming —our current orbit indicates we should be cooling not warming! Atmospheric carbon dioxide (CO2) has increased approximately 40% since the 1700s, from 280 ppm (parts per million) to today's 400 ppm. The optimum CO2 level is 350 ppm or less. Ocean acidification is evil companion of climate change. The ocean absorbs atmospheric CO2, leading to its increased acidification, 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 corals to construct reefs and mollusks to calcify their shells. Reef building is particularly vulnerable to climate change and current loss a sign of that change. If current trends continue, all coral reefs will cease to grow and start dissolving over the next 50 years. Over the last 30 years Australia's Great Barrier Reef has declined by 50 %. Models Scientists use models to test hypotheses, make predictions, and frame new questions. Models are tested by replicating observed data. When Global Circulation Models (GCMs) do a good job of replicating past and current observations, scientists have increased confidence that models can forecast the future. GCMs are able replicate the more rapid warming seen since 1950 as well as short -term cooling after recent volcanic eruptions (Mount Pinatubo, 1991). GCMs also predict many of the extreme events we are experiencing, such as major flooding, and extended heat waves and drought. GCMs tell us about the drivers of large, sustained changes in global temperature observed over the last century. Models using only natural forcings (solar variation and volcanic activity) show essentially no temperature increase over the last 100 yrs. But when anthropogenic effects are included the GCM fit with observed data is very good. To successfully mimic climate behavior over the past century the models must include CO2- driven warming. Part 2 will discuss how climate change in the geologic past informs us about climate change today.