Climate system models

Question 1

What is a model?

Answer 1

Models help us to work through complicated problems and understand complex systems. They also allow us to test theories and solutions. From models as simple as toy cars and kitchens to complex representations such as flight simulators and virtual globes, we use models throughout our lives to explore and understand how things work.

Question 2

What are climate models?

Answer 2

Climate models are based on well-documented physical processes to simulate the transfer of energy and materials through the climate system. Climate models, also known as general circulation models or GCMs, use mathematical equations to characterize how energy and matter interact in different parts of the ocean, atmosphere, land. Building and running a climate model is complex process of identifying and quantifying Earth system processes, representing them with mathematical equations, setting variables to represent initial conditions and subsequent changes in climate forcing, and repeatedly solving the equations using powerful supercomputers.

Question 3

How does climate model resolution work?

Answer 3

Climate models separate Earth’s surface into a three-dimensional grid of cells. The results of processes modeled in each cell are passed to neighboring cells to model the exchange of matter and energy over time. Grid cell size defines the resolution of the model: the smaller the size of the grid cells, the higher the level of detail in the model. More detailed models have more grid cells, so they need more computing power.

Question 4

What is a time step?

Answer 4

Climate models also include the element of time, called a time step. Time steps can be in minutes, hours, days, or years. Like grid cell size, the smaller the time step, the more detailed the results will be. However, this higher temporal resolution requires additional computing power.

Question 5

What is hind-casting?

Answer 5

Once a climate model is set up, it can be tested via a process known as “hind-casting.” This process runs the model from the present time backwards into the past. The model results are then compared with observed climate and weather conditions to see how well they match. This testing allows scientists to check the accuracy of the models and, if needed, revise its equations. Science teams around the world test and compare their model outputs to observations and results from other models.

Question 6

How do climate models predict future climate changes?

Answer 6

Once a climate model can perform well in hind-casting tests, its results for simulating future climate are also assumed to be valid. To project climate into the future, the climate forcing is set to change according to a possible future scenario. Scenarios are possible stories about how quickly human population will grow, how land will be used, how economies will evolve, and the atmospheric conditions (and therefore, climate forcing) that would result for each storyline.

Question 7

How are Climate Models Different from Weather Prediction Models?

Answer 7

Unlike weather forecasts, which describe a detailed picture of the expected daily sequence of conditions starting from the present, climate models are probabilistic, indicating areas with higher chances to be warmer or cooler and wetter or drier than usual. Climate models are based on global patterns in the ocean and atmosphere, and records of the types of weather that occurred under similar patterns in the past.

Question 8

What are 4 key points about climate models?

Answer 8

1. Climate models are our attempt to represent Earth’s climate system, so that we can better understand how it works, since we can’t conduct whole-earth experiments.
2. Climate models are grounded on physics, chemistry, and biology.
3. Climate models are constrained by observations in the real world, and can also help inform further observational efforts.
4. Even fairly simple climate models, like energy balance models, can help us understand and represent important processes in Earth’s climate system.

Question 9

What does GCM stand for?

Answer 9

General Circulation Models

Question 10

What does EBM stand for?

Answer 10

Energy Balance Model

Question 11

What does hierarchy indicate in climate modeling?

Answer 11

The range of models ordered with respect to complexity.

Question 12

List different types of climate models

Answer 12

When creating climate models, scientists use one of three common types of simple climate models: energy balance models, intermediate complexity models, and general circulation models.

From less complex to more complex:

1. zero-dimensional (0D) Energy Balance Model
2. 1D EBM still vertically averaged but it includes energy exchange between latitudinal bands.
3. Intermediate complexity models or 2D EBM vertically averaged but include zonal transport of energy and moisture.
4. three-dimensional GCM, which include sea ice and land surface processes such as snow cover, soil moisture and runoff of water through river drainage basins into the oceans.

Question 13

What is a parameterization?

Answer 13

A mathematical description of the process that depends on the resolved variables.

Question 14

How are climate models evaluated?

Answer 14

by comparing their output to observations.

Question 15

What are the main applications of climate models?

Answer 15

Paleoclimate studies, detection and attribution studies, and future projections.

Question 16

When were the first climate models developed?

Answer 16

In the 1960s: they were 1D EBMs and simple GCMs of the atmosphere and ocean at very coarse resolution.

Question 17

Which components of the climate are currently included and excluded in most climate models?

Answer 17

Most state-of-the-science coupled, three-dimensional CGMs include sea ice and land surface processes such as snow cover, soil moisture and runoff of water through river drainage basins into the ocean. Many models also include dynamic vegetation with separate plant functional types such as trees and grasses.

However they do not include interactive ice sheet components. This is because ice sheets have long equilibration (response) times of tens of thousands of years and therefore they need to be run for a much longer time than the other climate system components.

Question 18

What is an Earth System Model?

Answer 18

Models that include biogeochemistry, such as the carbon cycle and/or ice sheets. These calculate atmospheric CO2 concentrations interactively based on changes in land and ocean carbon stocks. They can be forced directly with emissions of anthropogenic carbon, whereas models without carbon cycles need to be forced with prescribed atmospheric CO2 concentrations.

Question 19

What are some inputs (boundary conditions) required for a climate model simulation?

Answer 19

Incident solar radiation at the top of the atmosphere or concentrations of greenhouse gases are usually required as input to the calculation. These are also called radiative forcings.