The straightforward online presentation quickly lets users get a feel for how carbon dioxide emissions can affect climate in the future. Users choose scenarios for future emissions, deciding when, and by how much, emissions are cut, or whether to allow them to increase according to a business as-usual projection. Using these inputs, the model then calculates the future atmospheric concentration of carbon dioxide and the increase in global average surface temperature. (Other greenhouse gases are also taken into account)
Users get to see how both the amount of carbon dioxide in the atmosphere and temperature vary depending on emissions. Model users can also see how temperature changes depend on how sensitive the climate is to greenhouse gases. For instance, with the business-as-usual emissions scenario employed by the model, the temperature could increase by more than five degrees Celsius, by the end of the century (for a climate sensitivity equal to 4.5 degrees per CO2 doubling). However, if temperature is less sensitive to greenhouse gases, cutting global emissions in half by the end of the century could let us avoid overshooting the so-called 2-degree target. (The “2-degree target” refers to the goal of keeping global average surface temperature from increasing more than 2 degrees above pre-industrial levels.)
Additional examples of questions that can be explored with the model:
- What temperature increase will we see by 2100, if we keep global emission levels at the current level, or if we cut emissions by 3% annually, starting now?
- What if we delay emission cuts for another 10, 20, or 30 years?
- How much does climate sensitivity matter to global warming?
- What if only developed nations cut emissions and developing nations don’t take any measures?
- How much and how quickly do developed and developing nations, need to cut emissions,
- respectively, to stay below the 2-degree limit?
- What role does deforestation play for atmospheric carbon dioxide levels and for temperature?
- How much will stopping deforestation matter to global warming?
In order to answer questions related to developed and developing nations, or questions related to deforestation, the 2-region version of the model must be used.
Climate sensitivity
How much the temperature will increase in the future depends on how much greenhouse gases we emit. But the amount of warming also depends on how sensitive climate is to increasing concentrations of greenhouse gases, e.g., carbon dioxide, in the atmosphere. The uncertainty in climate sensitivity is substantial. When the temperature starts rising, the air can hold more water vapor, which in turn leads to more warming because water vapor is a greenhouse gas. This positive feedback amplifies the greenhouse effect. There are both negative and positive feedbacks; major uncertainties in this area have to do with clouds. Major climate models compute how water vapor and clouds are affected by warming. These calculations are complicated and take a long time. The models estimate how sensitive climate is--or, to be more specific, how much the average surface temperature will increase, at equilibrium, if the amount of carbon dioxide in the atmosphere is doubled.
Different models produce different results, mainly because of the difficulties in modeling the impact of climate change on clouds. With the Chalmers Climate Calculator, the user gets to determine this parameter. This allows the Calculator to reproduce the results from a wide range of models. The user can see that a high climate sensitivity makes the problem significantly more serious; a low climate sensitivity makes it less serious. Different climate sensitivities require different emissions reductions in order for us to stay below the 2-degree limit.
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