We study the natural variations that occurred in Earth's climate in the pre- industrial age, with a focus on the tropics, the heat engine of the climate system. This provides a baseline against which more recent changes caused by human activities can be assessed. Because satellites, thermometers, rain gauges and and the like did not exist or were not widely used prior to the start of the Industrial Age about 150 years ago we must develop indirect means to reconstruct the climate of that time. We specialize in developing a relatively new class of climate proxies based on the hydrogen and carbon isotope ratios of molecular fossils left behind by microalgae and mangroves. We put much effort into testing and calibrating these new measures of rainfall and salinity by growing phytoplankton cultures and mangrove trees under controlled conditions in our lab so that we can estimate these parameters in the past with precision. A more recent focus of our research is to understand the effect of ocean acidification on coral reefs with continuous measurements of pH, oxygen, temperature and salinity on a pristine atoll in French Polynesia.