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The Most Important Models in the World

They require massive supercomputers, they simulate the Earth's future, they're poorly understood—and they're analyzed above the Seinfeld diner in New York.

by Brian Merchant
May 13 2015, 4:05pm

Some of the most important simulations in the world are among the most poorly understood. That's probably not surprising; that they're being analyzed in a building most famous for its affiliation with the biggest sitcom of the 90s might be.

We know the climate is warming now—observational data, satellite records, and paleoclimate analysis have established that the planet has become over 1˚F hotter since humans began emitting fossil fuels into the atmosphere over a century ago. But to get an idea of how much warmer the future will be, scientists turn to models: complex simulations that use lengthy mathematical equations, processed by supercomputers, to produce sophisticated forecasts of the physical and chemical processes that will govern tomorrow's climate.

"To 'run' a model, scientists divide the planet into a 3-dimensional grid, apply the basic equations, and evaluate the results," the National Oceanographic and Atmospheric Administration (NOAA) explains. "Atmospheric models calculate winds, heat transfer, radiation, relative humidity, and surface hydrology within each grid and evaluate interactions with neighboring points."

These models are some of the most useful tools scientists have for understanding the planetary system. They allow climatologists to simulate what will happen when the atmosphere becomes saturated with carbon dioxide and other greenhouse gases, for instance, or to examine the aftermath of a future volcanic eruption—or to evaluate what would happen if humanity finally acted to reduce its pollution output.

Unfortunately, and undeservingly, climate models remain somewhat controversial, especially amongst the vociferous skeptics and denialists who discount even the more rudimentary elements of climatology. The models were in the headlines just last April, when Republicans demanded that the Environmental Protection Agency explain the models to them at a Congressional hearing. And just about any time there's an unexpected weather event, you can count on a commentator to seize the opportunity to pronounce any and all such predictive models faulty.

That's a pretty common refrain, too. Unfortunately, it's fundamentally wrong. For one thing, weather forecasts are entirely different than climate models; weather predictions take environmental conditions in the present and work forward in time, while climate models, as EarthLabs puts it, "are based purely on the physics and chemistry of the Earth system." And they predict climate averages, not individual regional events.

There are some things the climate models can't do well, and some things they can, Gavin Schmidt, the director of NASA's Goddard Institute for Space Studies, told me recently. "So for instance, if I tried to predict how many tornadoes there are going to be using one of these global scale models, I'm going to fail miserably, because they don't have the physics or the small-scale aspects that would allow them to reproduce tornadoes."

I had dropped by Schmidt's office at GISS, which is located a few floors above Tom's Restaurant, the diner most famous for playing the Seinfeld coffee shop on TV. I wanted to get a sense of the current state of NASA's climate models, which are only getting smarter and more accurate.

The first climate model is over a century old

"There are lots of things they do do well, and global mean temperature change is one of those things," Schmidt told me. "The patterns of rainfall change they do well, the changes in the dynamics, the changes in sea level they do well. And we can evaluate that, we can test that, by comparing the results of the model in cases where we've had big changes in the past, and where we have data to say what happened."

These models are actually quite accurate in predicting those averages—but it took a long time for them to get there.

A climate model, via NOAA

The first climate model is over a century old. The New York Times explains: "In the first paper on global warming, published in 1896, the Swedish scientist Svante Arrhenius constructed a simple set of equations predicting that the earth would warm from the carbon dioxide humans were pumping into the atmosphere; it took 80 years to be sure he was right, but he was."

The first modern climate model was built much later, in the 60s, by NOAA scientists. Here's NOAA on this important part of its history: "In the late 1960s, NOAA's Geophysical Fluid Dynamics Laboratory... developed the first-of-its-kind general circulation climate model that combined both oceanic and atmospheric processes. Scientists were now able to understand how the ocean and atmosphere interacted with each other to influence climate. The model also predicted how changes in the natural factors that control climate such as ocean and atmospheric currents and temperature could lead to climate change."

It seems crude now, but it "still stands today as a breakthrough of enormous importance for climate science and weather forecasting." And it provided the foundation for the evolution of more sophisticated climate models in subsequent decades.

NASA's climate models run between 500,000 and 700,00 lines of code

Here, courtesy of Skeptical Science, a cohort of scientists and advocates who work to debunk anti-climate science claims made by politicians and the media, is just about the most basic climate model in existence:

T = [(1-α)S/(4εσ)]1/4

T is temperature, α is the albedo (a measure of the reflectivity of the Earth's surface), S is the incoming solar radiation, ε is the emissivity, and σ is the Stefan-Boltzmann constant (which measures the total intensity radiated over all wavelengths increases as the temperature increases).

"It's one line long, and is at the heart of every computer model of global warming," the climate brain trust at Skeptical Science explains. "Using basic thermodynamics, it calculates the temperature of the Earth based on incoming sunlight and the reflectivity of the surface. The model is zero-dimensional, treating the Earth as a point mass at a fixed time. It doesn't consider the greenhouse effect, ocean currents, nutrient cycles, volcanoes, or pollution."

So, naturally, all of those variables need to be filled in, too. As scientists learn more about the chemical and physical systems that govern the planet's climate, they expand the code; this has happened many times over the years. NASA's climate models, for instance, run between 500,000 and 700,000 lines of code; some of which is decades old and pretty simple, some of which is extraordinarily sophisticated. Today, they are powered by a massive supercomputer at the Goddard Space Flight Center in Maryland.

The National Center for Atmospheric Research, the UK's Met Office, NOAA, and many other research institutions run models, too. They all use different inputs and data, and they all arrive at slightly different results. Climate modelers load the equations with data based on the most recent science, and run them for the most accurate results. They change the variables—whether, for instance, world leaders suddenly woke up and transitioned away from fossil fuels, or we keep using the sky as an open sewer—and examine a number of different outcomes.

That's why you'll see the terms like "worst case" or "status quo" emissions scenarios in writing about climate change: the models help us understand what will happen if we pump more carbon into the atmosphere. The more CO2 we emit, the hotter it gets, and the higher the seas rise.

To render the sizable, wide-ranging battery of projections useful to the policymakers who are (ostensibly) concerned with preparing society for a changing climate, the UN-led Intergovernmental Panel for Climate Change (IPCC) synthesizes the findings. In its latest synthesis report, for example, the IPCC lays out four separate possible futures for the climate, called "Representative Concentration Pathways," depending on how much carbon pollution we end up adding to the atmosphere. These range from optimistic—if world leaders unite and act to reduce coal, oil, and gas pollution, and transition to clean energy sources—to dire—if we don't do anything, and continue burning fossil fuels with reckless abandon.

RCP 8.5 is the scenario where fossil fuel burning continues unabated. RCP3-PD is the scenario in which we get our act together.

The models provide a crucial input for both scientists and lawmakers grappling with global warming—and a crucial look at what our world will look like after it has been transformed by fossil fuels. These are some of the most important models in the world—they have been decades in the making, with some of the best scientific minds contributing to their complexity, ingenuity, and accuracy. We ignore them at our peril.

"The progress we've made in simulating the emergent properties of the climate system has been a huge intellectual achievement," Schmidt told me, "but to know this is impacting some of the most important decisions people and societies are making is daunting."

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