Have you ever stopped to wonder how forecasters can predict the weather days in advance, or how scientists figure out how the climate might evolve under different policies?The Earth system is a vast web of intertwined processes, from microscopic chemical reactions to towering storms. Ocean currents circulating deep in the Atlantic, forests exchanging carbon with the atmosphere, and humans altering the composition of the air all have effects that ripple through the system. These processes are governed by physical laws, such as conservation of mass, energy and momentum.All of this plays out on such a large scale that no single human mind can truly grasp it in full. And yet, the system is so sensitive that a small perturbation, given enough time, can steer its trajectory in a dramatically different direction. This sensitivity is called “chaos,” also known as the “butterfly effect.” The planet is, at once, immense and delicate.AdvertisementAdvertisementDespite this complexity and scale, scientists are able to simulate and anticipate how the climate will change.How is this even possible? Behind the long-term climate projections that affect our lives sits one of the most remarkable scientific achievements of the modern era: climate models that run on supercomputers.I am a climate data scientist. My colleagues and I try to understand extreme weather and long-term climate risks by using virtual versions of Earth inside these machines.What a climate model really isHere is the simplest way to picture a climate model:Imagine dividing the entire planet into 3D boxes. At the surface, each box might represent an area 50 to 100 kilometers across. Then we stack boxes upward into the atmosphere and downward into the oceans to create a 3D grid wrapping around the globe.AdvertisementAdvertisementEach box contains numbers: temperature, wind speed, humidity, sea ice thickness, soil moisture and hundreds of other variables. The model contains mathematical expressions that describe how these variables influence one another: how heat moves, how air rises and sinks, how moisture condenses into clouds, how the ocean absorbs and redistributes energy.Climate models are systems of differential equations based on the basic laws of physics, fluid motion and chemistry. They divide the planet into a 3D grid, apply the equations and evaluate the results. Within these models, the atmosphere component, for example, calculates winds, heat transfer, radiation, relative humidity and surface hydrology. NOAAWe then let the model march forward in time, solving the math and updating every variable in every box. Then again. And again.More in ScienceNow scale that up. Millions of grid boxes. Hundreds of variables per box. Calculations carried out millions of times to simulate decades or even centuries.And because the system is chaotic, we do not run the model just once. We run it many times with slightly different initial conditions – what scientists call an ensemble – to make sure the result is in fact a true system response to the considered scenario, such as warming temperatures due to increased emissions, and not an effect of chaos.AdvertisementAdvertisementThe result is an astronomical number of calculations. Performing them requires com …
