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Climate models, the way scientists predict climate change and its impact, explained!

A climate model is a computer simulation of the Earth’s climate system, including the atmosphere, ocean, land and ice.

By B. Mohita
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How do scientists predict climate change and its impact; Climate models, explained!

The Intergovernmental Panel for Climate Change in its 2021 report warned that “unless there are immediate, rapid and large-scale reductions in greenhouse gas emissions, limiting global warming to 1.5 °C above pre-industrial temperatures or even 2°C will be beyond reach.” It also investigated the impacts such warming would have on the earth's systems. Have you ever wondered, where do these findings really come from?

What is a climate model?

A climate model is a computer simulation of the Earth’s climate system, including the atmosphere, ocean, land and ice. As per the National Oceanic and Atmospheric Administration (NOAA), these models are based on well-documented physical processes to simulate the transfer of energy and materials through the climate system. 

The Geophysical Fluid Dynamics Laboratory of NOAA defines a global climate model (GCM) as a complex mathematical representation of the major climate system components (atmosphere, land surface, ocean, and sea ice), and their interactions.  The GCMs make use of the earth’s energy balance between the four components which are the key to long-term climate prediction.  

Steps in climate modelling

Building and running a climate model is a complex process. In simple terms, it includes- 

  • identifying and quantifying Earth system processes,
  • representing them with mathematical equations, 
  • setting variables to represent initial conditions and subsequent changes in climate forcing,
  • repeatedly solving the equations using powerful supercomputers.

Climate models calculate many different properties of the climate, including atmospheric temperature, pressure, wind, and humidity.  These properties are calculated for thousands and thousands of different points on a three-dimensional grid. These involve multitudes of mathematical equations. By solving the relevant mathematical equations, the climate model is able to calculate how the state of the atmosphere and ocean evolves over time.

How does a model go about calculating all these equations?

A climate model divides up the Earth into a series of boxes or “grid cells”. A global model can have dozens of layers across the height and depth of the atmosphere and oceans. The model then calculates the state of the climate system in each cell – factoring in temperature, air pressure, humidity and wind speed.

Illustration of grid cells used by climate models and the climatic processes that the model will calculate for each cell (bottom corner). Source: NOAA GFDL
3D representation of grid cells as used by climate models | Source: NOAA GFDL

The size of the grid cells in a model is known as its “spatial resolution”. Because the Earth is a sphere, the cells for a grid based on longitude and latitude are larger at the equator and smaller at the poles. The higher the resolution of the climate model, the smaller will be the boxes. This enables more specific climate information for a particular region. However, this comes at the cost of taking longer to run because the model has more calculations to make.

Climate models generate a nearly complete picture of the Earth’s climate, including thousands of different variables across hourly, daily and monthly timeframes. Here are some of the common variables that most climate models include - 

  • temperatures and humidity of different layers of the atmosphere from the surface to the upper stratosphere,
  • temperatures, salinity and acidity (pH) of the oceans from the surface down to the sea floor.
  • estimates of snowfall, rainfall, snow cover and the extent of glaciers, ice sheets and sea ice. 
  • wind speed, strength and direction, as well as climate features, such as the jet stream and ocean currents.

Significance of climate models

Climate models help scientists to test their understanding of our climate system, and to predict future changes to our climate. They do so by enabling scientists to improve the understanding and prediction of atmosphere, ocean, and climate behaviour. The models also help in determining the distinct influence of different climate features with experiments that cannot be performed on the actual earth.

GDFL acknowledges that although there is wide disagreement among different climate models.  However, it stipulates that all of them show rising global temperatures with amplified warming in the Arctic, enhancement of the hydrologic cycle (dry places becoming dryer and wet places becoming wetter), and rising sea levels. Additionally, the results of each experiment are extensively checked by a large community of modellers and researchers around the world (for example, as part of the IPCC), which reduces uncertainty.  The fact that the models have produced simulations of current and past large-scale climates that agree with observations, adds to their credibility. 

The output from these models drives forward climate science, helping scientists understand how human activity is affecting the Earth’s climate. It is owing to the climate models that climate policy decisions on national and international scales for the past five decades have been formulated.

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