Earth System Modeling Experiments
Earth System Models (ESM)
Earth System Models (also called climate models, or GCM's) simulate past and future climate using computer code to build a three dimensional map of the climate system that is divided up into grid boxes. A climate model can simulate the movement of energy (heat) and matter (such as water vapor) between these grid boxes. Although a climate model can simulate weather events, it is not intended to simulate the exact weather on a given day; instead a climate model is intended to accurately represent the long-term, average weather in a given region. If you want to read more about climate models, see the climate modeling website I made earlier in graduate school.
(Image credit: GFDL, http://www.gfdl.noaa.gov/earth-system-model)
Although we may not have perfect records of past sea-surface temperatures and rainfall, we can use Earth System Models that simulate variations in temperature, rainfall, ocean circulation, and winds (among many other variables in the climate system) to study climate variability, past, present, and future. We can use models to study if a region will become more dry or wet with climate change; to ask what may happen, theoretically, if we were to shut down the conveyor-belt like circulation of water in the North Atlantic; or to examine the effects of large volcanic eruptions on droughts in a warming world.
Climate Modeling Experiments
Atlantic Ocean and the "Hosing" Experiment:
Oceans store and transport massive quantities of energy around the globe. Specifically, the Atlantic Ocean moves large amounts of heat from the equator to higher latitudes in the northern hemisphere. This movement of energy (known as the Atlantic Meridional Overturning Circulation, or AMOC) plays an important role in global climate, and the strength of the AMOC has varied significantly through time. Paleoclimate data suggest that past rapid warming and melting of ice sheets in and around Greenland may have released fresh (less dense) water into the North Atlantic, slowing and severely weakening this ocean circulation and heat transport.
In my first year of graduate school, I helped study output from a climate modeling experiment (we used the NOAA/GFDL ESM2M) in which a simulated freshwater cap in the North Atlantic severely weakened the AMOC. Our work showed that a weakening or shutdown of the AMOC and associated Atlantic ocean heat transport can cause the tropical rainfall belts over South America to shift to the south, drying out Central American and northern Amazonia. Importantly, extended droughts in the tropics can cause vegetation die off, releasing large quantities of stored carbon back into the atmosphere.
Illustration of Atlantic Meridional Overturning in the North Atlantic Ocean (Image Credit: R. Curry, Woods Hole Oceanographic Institute)