The research aims to help us understand year-to-year variations in climate around the world. This includes the occurrence of floods and droughts, of heat waves and cold spells. The project examines the largest source of year-to-year climate variability on Earth, namely, El Niño. The El Niño is a warm ocean current that appears off the coast of NW South America every 3-5 years, and it is a result of a much larger scale phenomenon involving changes to the winds, rainfall, temperature and ocean currents across the whole of the tropical Pacific. The larger scale phenomenon is known as the El Niño Southern Oscillation, a name which reflects the fact that it involves a natural cycle in the circulation of both the atmosphere and the surface ocean and how they interact. Although we know that ENSO originates in the tropical Pacific, it has near world-wide impacts because of the way it affects the circulation of the atmosphere, and hence the winds and transport of moisture from the tropics to the extra-tropics. Floods and droughts and changed incidence of storminess from El Niño directly affect the lives and livelihoods of well over a billion people, and major El Niño events are associated with tens of thousands of human deaths, billions of pounds of damage, and devastation to some natural ecosystems such as coral reefs. Even Europe experiences changed weather patterns associated with ENSO! Although we now understand quite well the basic mechanisms behind the ENSO cycle, some major questions remain. In particular, we do not understand why some El Niño events are much stronger than others, why some decades show much stronger El Niño activity, or how ENSO will respond to climate change. To help answer some of these questions, we will reconstruct changes in ENSO over the past 5,000 years by analysing growth rings in the skeletons of old dead ('fossil') corals that lived in the Galápagos. The Galápagos Islands experience extreme changes in weather associated with El Niño (warmer and wetter during events), and these changes are recorded in the chemistry of the skeletons of corals living in the surrounding ocean. Some of these corals live for up to a hundred years, or longer, laying down layers of skeleton a bit like tree rings. We will collect cores through old dead corals, including some that lived thousands of years ago. Then, by analysing the chemistry of their growth bands we will be able to reconstruct the changes in climate, and ENSO, that the corals experienced during their life time. By combining the records from many such corals we will build up a picture of the natural variability in ENSO, helping us see how often major events occurred, and how much decade-to-decade variability in ENSO occurred. These coral records can let us reconstruct the history of past changes in ENSO, but on their own they do not help us to understand the causes of the changes. Were they due to changes in the sun's radiation? Or due to the cooling effects of major volcanic eruptions? Or were they simply random variations that we should expect without any sort of trigger? To answer these questions, we need to use climate models. The same models that we now use to predict future climate can be used to research changes in ENSO. In our work, we used the most up-to-date climate models to see if they can correctly replicate the observed changes in ENSO over the past few thousand years as defined by our coral records. We can also saw what the effects are of changing volcanic eruptions, solar radiation and greenhouse gases in these models. By comparing the model results with the coral records we gained a better understanding of the nature and causes of changes in ENSO, and the skill of the models at predicting this.