How do climate models work?

Computer programs, known as climate models, are invaluable tools for researchers.(1) These models forecast long-term trends by simulating the interactions between the air, oceans, land surface, ice and sun.(2),(3) Universities and research institutions all over the world independently developed more than 60 advanced models.(4),(5) All agree that only human carbon dioxide (CO2) emissions can explain the observed temperature increases over the past few decades.(6),(7)

Advanced models, which account for changes in solar energy, cannot simulate the warming seen over the past few decades without human carbon dioxide.(8) Natural factors alone simply cannot recreate the trend. Climate models find that the level of human carbon dioxide emissions has a strong influence on future temperatures and natural disaster risks such as flooding, drought and wildfire.(9),(10),(11),(12)


Crunching the numbers
How are the projections that climate models produce calculated, and why do scientists have confidence in them? The models consider so many factors that they must be run on supercomputers to finish in a reasonable amount of time. NASA’s “Discover” supercomputer, which runs many of the agency’s climate models, has 15,000 processors (the typical home computer has one to four).(13) Some models use more than 50 billion satellite readings to simulate time scales up to decades and even centuries into the future.(14),(15)


How do climate models account for an uncertain future?
Climate model projections are typically reported in ranges because some parts of the climate system, such as clouds and ocean circulation, are not completely understood. Model developers also must predict how much pollution will be produced by cars, power plants and other human sources.(16)

To account for uncertainty, scenarios with different natural and human assumptions are fed into independent climate models.(17) The results from all the runs are then sorted to create a range of possible outcomes. Model results are typically presented in ranges to account for uncertain natural processes and different possibilities for future human activities. As scientists continue to improve their understanding of the Earth, their models will also become more accurate.

How Climate Models Evolve
Scientists continually upgrade their climate models as they understand more about the climate, and as new technology becomes available. The models have made significant progress in their forecasting ability since being created in the mid-1970s.(18),(19) Despite extensive changes to the models, the effect of increased carbon dioxide on raising average temperatures has remained the same.(20)

Models Have Improved Over Time
Climate models have become more sophisticated, incorporating more factors over time.

What does statistics tell us?
Statistics and climate model results can generate probabilities for different amounts of warming. MIT translated 800 climate simulations to a “temperature roulette wheel” that shows the likelihood of different amounts of future warming under different levels of greenhouse gases.

The “CO2 reduction policy wheel” compared to the “no CO2 policy wheel” shows that lowering carbon dioxide emissions can substantially reduce future risks. By the end of the century, temperatures are likely to rise between 3.6-5.4 degrees F° with a policy and 7.2-10.8 degrees F° without a policy.(21)


Predicting long-term trends with climate modeling
Although climate models are not as specific as our weather forecasts, scientists remain confident in their ability to predict long-term trends.(22),(23),(24) In fact, they accurately reconstructed historic conditions going back 21,000 years.(25)

“The models are far from perfect, but they have successfully captured fundamental aspects of air, ocean and sea-ice circulations and their variability.”
– Gavin A. Schmidt, Director of NASA’s Goddard Institute for Space Studies, 2007

NASA has a strong projection record
In 1991, Mount Pinatubo in the Philippines erupted with the second-greatest volcanic explosion of the 20thcentury. Mount Pinatubo released heavy smoke and about 20 million tons of sulfur dioxide that rose 12 miles into the sky. The pollution then spread across the planet, altering global temperatures for the next few years.

A NASA climate model, developed in the early 1990s, accurately projected the eruption’s impact on the Earth’s climate using satellite measurements of atmospheric sulfur dioxide. The eruption temporarily decreased warming because sulfate particles have a cooling effect by shielding the Earth from the sun’s energy.(26)


Identifying local climate impacts
Climate models, software tools that forecast climate decades into the future, are only designed to forecast high-level global changes. In order to translate climate risks to specific states and cities, planners must develop regional simulations.(27),(28)

“It’s eye-opening to see how much it will warm where you live. This data lays a foundation for really confronting this issue and I’m very optimistic we can confront and adapt to a changing climate.”
– Dr. Alex Hall, UCLA Professor of Atmospheric & Oceanic Sciences, 2012

The city of Los Angeles, for example, is working with UCLA researchers to understand the effects of climate change at the neighborhood level to inform policy decisions.(29) The results of the preliminary study are striking: Individual Los Angeles area neighborhoods will be impacted in very different ways.




  1. NASA: The Physics of Climate Modeling
  2. NOAA: Climate Modeling
  3. University Corporation for Atmospheric Research: Climate Model Components and Evolution
  4. Lawrence Livermore National Laboratory
  5. National Academy of Sciences
  6. NASA: Carbon Dioxide Controls the Earth’s Temperature
  7. NOAA: Global Temperature Projections
  8. NASA: Causes
  9. NOAA: Heavy Downpours More Intense, Frequent in a Warmer World
  10. NOAA: Global Sea Level Rise Scenarios
  11. Nature
  12. NOAA: With Rising Greenhouse Gases, US Heatwaves Become More Common & Longer-Lasting
  13. NASA: NASA Center for Climate Simulation: Data Supporting Science
  14. NASA: NASA Center for Climate Simulation: Data Supporting Science
  15. NASA: NASA Goddard Introduces the NASA Center for Climate Simulation
  16. University Corporation for Atmospheric Research: Climate Modeling
  17. University Corporation for Atmospheric Research: Climate Modeling
  18. University Corporation for Atmospheric Research: Climate Model Components and Evolution
  19. NOAA: Climate Modeling
  20. NASA: Causes
  21. Massachusetts Institute of Technology
  22. Department of Energy: Climate Models: An Assessment of Strengths and Limitations
  23. University Corporation for Atmospheric Research: Climate Models and Clouds
  24. NASA: NASA’s Role
  25. University Corporation for Atmospheric Research: New Cause for Past Global Warming Revealed by Massive Modeling Project
  26. NASA: Volcanoes and Climate Change
  27. University of Illinois
  28. Southwest Climate Change Network
  29. Climate Change in Los Angeles
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