Evolution of Climate Models

In this series on Climate Change, we briefly discussed climate models such as General Climate Models (GCMs) and Earth System Models (ESMs) in earlier articles. There is, in fact, a hierarchy of such models depending on their complexity and their coverage. In this two-part article, we will look at a variety of climate models. In Part 1, we will examine the evolution of climate models and introduce basic climate models.

Climate Models: An Introduction

There are various climate models that range from simple Energy Balance Models to complex Earth System Models. While simple models take a slice of the whole picture and describe a part of the climate system, complex models such as Earth System Models take the entire climate system into account, including the oceans, atmosphere, land, ice sheets and the carbon cycle.

More generally, a climate model is a representation of the climate system, which consists of the atmosphere, oceans, land (forests and vegetation) and ice sheets. These are expressed in mathematical equations based on the laws of physics, such as the conservation of energy, momentum and mass, the laws of thermodynamics, and Newton’s second law of motion. These climate models, consisting of simultaneous equations, are run as iterative computer programs and help us make climate projections.

Most climate models divide the Earth’s surface into grids or boxes of about 100 km by 100 km. Energy, mass and momentum flow across these boxes, as do water and air. Variables such as temperature, humidity and wind velocity (both horizontal and vertical) are calculated for each box for one “time step” or time period, which typically ranges from a few seconds to 30 minutes. The equations are solved one time step at a time, and the results are then used for the next time step. This produces a series of values for variables such as temperature, humidity and wind.

The earliest modern climate model was proposed by Norman Phillips in 1956, when he undertook a simulation of the general atmosphere. Around the same time, John von Neumann initiated a conference in 1955 to discuss General Circulation Models (GCMs), which led to important institutional developments in climate modelling. The Geophysical Fluid Dynamics Laboratory (GFDL) was set up in 1955 by Smagorinsky and Manabe. The UCLA Meteorology Laboratory was established in the late 1950s by Yale Mintz and Akio Arakawa, and the National Center for Atmospheric Research (NCAR) was established in Boulder, Colorado, in the early 1960s.

In addition, the UK Meteorological Office proposed the Numerical Weather Prediction model in 1965 and a General Climate Model in 1973. The first ocean GCMs were developed at GFDL by Bryan and Cox in the 1960s, and these were later coupled with atmospheric models by Manabe and Bryan in the 1970s. Syukuro Manabe of GFDL was awarded the Nobel Prize in Physics in 2021 for his work on modelling the effects of carbon dioxide on global warming.

The earliest GCMs were produced at GFDL in the 1960s by Bryan and Cox.

Hierarchy of Climate Models

The various climate models range from the simplest Energy Balance Models to the more complex General Circulation Models (GCMs) and Earth System Models (ESMs). Let us discuss these briefly.

The simplest models are the Energy Balance Models (EBMs) with zero dimensions. These treat the Earth as a single unit and examine the incoming and outgoing radiation of the Earth without taking into account variations across latitudes and longitudes. The global mean temperature is determined by the net balance between incoming and outgoing radiation.

If latitudes are introduced into the basic zero-dimension EBM, we obtain a one-dimensional EBM, and the energy budget equation is solved separately for each latitude, with the atmosphere and oceans being treated as a single box. If vertical energy transfer is included in one-dimensional EBMs, they are referred to as radiative–convective models.

If energy and moisture exchange across zones is included, we arrive at models of intermediate complexity. These range from two-dimensional EBMs with energy and moisture transport to one-dimensional EBMs coupled with either ocean or atmospheric models.

General Circulation Models (GCMs) are at the top of the hierarchy of climate models, along with Earth System Models (ESMs). GCMs take into account all the physical processes that occur in the oceans and atmosphere and model them based on the laws of physics.

We will continue this discussion and provide more details on GCMs, ESMs and other developments in climate modelling in the next part.