Forests absorb CO2 as trees and other plants grow. If we cut down or burn down forests, we reduce the amount of CO2 that they absorb, leaving more CO2 in the atmosphere. We can treat the additional CO2 as the equivalent of emissions. webDICE includes these emissions but they contribute only a limited amount overall. more
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The parameters you chose produce a result that is physically impossible. This can occur because climate change causes harms that are sufficiently bad that the economy is forced to operate below subsistence levels. It may also occur because you have specified emissions reductions that are greater than the emissions from the participating countries. webDICE does not contain the flexibility to predict what might happen as we approach these circumstances so it continues to produce the graphs under the parameters you chose.
Although the results are not physically possible, it does not mean that they are not meaningful. webDICE is telling you that under the parameter choices you made, something very extreme will happen: the population will have to shrink because the economy cannot support the full population, or the emissions reductions cannot be achieved with the participation rates. The particular results in the graphs, however, are not meaningful.
Click this dialogue to dismiss it.