In February, polar climate researchers gathered at the National Center for Atmospheric Research in Boulder, Colorado to discuss the newest updates to models of the Earth’s climate system. The researchers are working together to create better models of the Arctic and Antarctic climates, which will feed into larger models of the whole Earth that help scientists understand how climate will change in the future. What goes into a climate model, and what can scientists learn from models that they cannot learn from observations?
What’s in a model?
Computer climate models are based on scientists’ understanding of Earth’s climate. The models use mathematical relationships to try to quantify the relationships between parts of the climate system. If you tweak one factor in climate, how does the simulated climate system respond? Models that bring many factors together help scientists learn how the climate system works, and let them run simulations on Earth’s climate. They also allow scientists to assess how climate may be affected by present and future changes in greenhouse gases and solar forcing, and how much of a role natural variability plays.
In climate, researchers know that at least one factor is changing—the amount of greenhouse gases in the atmosphere has increased by 40 percent since the Industrial Revolution. Basic physics tells us that increasing greenhouse gases will increase temperatures on average, but where, when, and how exactly the climate will change depends on a lot of other variables such as atmospheric conditions, ocean circulation, ice loss, and plant growth. NSIDC scientist Julienne Stroeve studies sea ice and climate. She said, “Climate models can tell us about how these components of our planet interact and influence each other, and to understand what happens when we perturb one of these components.”
To test their models and their current understanding of how climate works, climate scientists also run their models through past and current climate scenarios, to determine how well they can replicate the observed climate variability and trends. Stroeve said, “Because the real world provides just one realization of both natural variability and forced climate change, we cannot separate out natural climate change from say greenhouse gases in observational data.” Models, she said, allow researchers to run a test many times, and answer questions such as how much added greenhouse gases are changing climate, compared to natural variability.
Experimenting with the climate
Today’s global climate models are made up of many sub-models that replicate a specific system such as the atmosphere, the ocean, or land. They include factors like algae in the ocean and dust on sea ice, which can both be affected by climate and affect climate themselves. Adding in detailed factors like shrub growth make the models more complicated, but they also help scientists better understand climate. NCAR scientists David Lawrence said. “The best models can take this information and very accurately reproduce the Earth’s climate and weather, with major precipitation zones and circulation patterns and modes of variability such as the El Niño Southern Oscillation (ENSO) matching very reasonably with observations.”
For example, Lawrence has been researching how changing plant life in the Arctic could affect climate. On the Arctic tundra, the number of shrubs is increasing as the air and land get warmer. But the shrubs themselves have additional effects on the climate. Lawrence said, “Shrubs are darker than the surrounding tundra so they tend to absorb more solar radiation, thereby warming the nearby air.” But at the same time, shrubs tend to shade the ground below them, making the soil cooler. In a recently published study, Lawrence used a climate model to study how an expected increase in shrubs will alter permafrost vulnerability to climate change. He found that more shrubs are likely to increase permafrost vulnerability to climate change, in contrast to previous studies that suggested that shrubs would have the opposite effect.
Lawrence, D. M. and S. Swenson. 2011. Permafrost response to increasing Arctic shrub abundance depends on the relative influence of shrubs on local soil cooling versus large-scale climate warming. Environmental Research Letters 6 045504. doi:10.1088/1748-9326/6/4/045504
Stroeve, J. C., A. P. Barrett. Assessment of Arctic Sea Ice in the CMIP5 Climate Models. American Geophysical Union 2011 Fall Meeting. C21D-04
Community Earth System Model Project Web site: http://www.cesm.ucar.edu/