Earth system model

An Earth system model (ESM) is a numerical representation of the Earth's climate system that couples the atmosphere, ocean, land surface, cryosphere, and biogeochemical cycles, particularly the carbon cycle. Unlike general circulation models (GCMs) that focus on physical climate, ESMs include interactive representations of vegetation, soil, and ocean biology, allowing them to simulate feedbacks between climate and the biosphere.¹ ESMs are used to study past climates (paleoclimate), current variability, and future projections under different greenhouse gas emission scenarios. They are essential tools for the IPCC assessments, providing quantitative evidence for climate change attribution and future warming estimates.²

The lineage of ESMs traces back to the first numerical weather prediction models of the 1950s, which evolved into atmospheric general circulation models by the 1960s. By the 1990s, models began to incorporate ocean and sea-ice components, and later, terrestrial and marine biogeochemistry. The first true ESMs emerged in the early 2000s, integrating carbon cycle feedbacks. The Coupled Model Intercomparison Project (CMIP) has driven systematic model development, with CMIP6 (2020s) featuring models that include interactive chemistry and improved land-use representation.³ Notable milestones include the development of the Hadley Centre's HadCM3 and the NCAR Community Earth System Model (CESM), which have been widely used for research and policy.

An ESM typically comprises component models for the atmosphere, ocean, land surface, sea ice, and ice sheets, each with its own grid and physics. These components are coupled via a flux coupler that exchanges energy, momentum, water, and carbon between them. For example, the ocean component simulates circulation and marine biogeochemistry, while the land component simulates vegetation dynamics and soil carbon. Coupling introduces complexities such as conservation of mass and energy, and requires careful handling of spatial and temporal scales. Modern ESMs also include interactive aerosols and atmospheric chemistry, which affect radiation and cloud formation.⁴

ESMs are used for a wide range of applications, from seasonal forecasting to long-term climate projections. They are central to the IPCC's Special Report on Global Warming of 1.5°C and the Sixth Assessment Report, providing scenarios such as SSP1-1.9 and SSP5-8.5. ESMs also help attribute extreme events to anthropogenic climate change and assess the impacts of geoengineering proposals. However, ESMs have limitations: they operate on coarse grids (typically 100 km), which cannot resolve clouds, storms, or local topography, leading to biases in regional precipitation and temperature. They also struggle to represent tipping points, such as ice-sheet collapse or Amazon dieback, due to incomplete process understanding.⁵

Beyond the headline capabilities, ESMs have several niche aspects. For instance, some ESMs include a representation of the Earth's orbital variations (Milankovitch cycles) to simulate paleoclimate, and they have been used to test the 'Snowball Earth' hypothesis. Others incorporate dynamic vegetation that can shift biomes in response to climate change, affecting carbon storage. A lesser-known fact is that ESMs are also used for 'geoengineering' experiments, such as simulating stratospheric aerosol injection, to evaluate their potential effectiveness and side effects. Additionally, ESMs are increasingly coupled with economic models to create integrated assessment models (IAMs) that explore climate policy pathways. The computational demands of ESMs have driven advances in high-performance computing, with some models running on millions of cores.

General circulation model (GCM)

A model that simulates atmospheric and oceanic circulation, often the physical core of an ESM.

Carbon cycle

The exchange of carbon among the atmosphere, oceans, land, and living organisms.

Coupled Model Intercomparison Project (CMIP)

A framework for coordinating climate model experiments to compare and evaluate model performance.

Biogeochemistry

The study of chemical, physical, geological, and biological processes that govern the composition of the natural environment.