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Past carbon dioxide concentrations compared to possible future emissions scenarios Jessica Tierney/University of Arizona Past carbon dioxide concentrations (left) compared to possible future emissions scenarios (right). The rate of current emissions is much faster – occurring over decades – unlike geological changes, which occur over millions of years. If emissions continue unabated, carbon dioxide levels by the year 2300 could meet or exceed values associated with past warm climates, such as the Cretaceous period 100 million years ago or the Eocene epoch 50 million years ago.