There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to preindustrial
levels. These pathways would require substantial emissions reductions over the next few
decades and near zero emissions of CO2 and other long-lived GHGs by the end of the century.
Implementing such reductions poses substantial technological, economic, social, and institutional
challenges, which increase with delays in additional mitigation and if key technologies are not
available. Limiting warming to lower or higher levels involves similar challenges, but on different
timescales. {3.4}