Box 2b: Overshooting of 2° C Temperature limits

Model results show that temperature trends could overshoot and then drop again below temperature limits as a result of natural “sinks” acting to gradually reduce the atmospheric burden of the greenhouse gases over time. However, since this process occurs slowly, it is expected that once temperatures overshoot a target, they will drop below the target only on the time-scale of a century (Lowe et al 2009). This process could be accelerated, if negative CO2 emissions were achieved as discussed earlier (Azar et al 2006, Azar et al 2010).

Overshoot pathways often arise in three different contexts: (1) deliberate policy choice to minimise mitigation costs; (2) failure to meet certain emission targets or goals; or (3) late participation by all major emitters in global mitigation efforts (Clarke et al. 2009, van Vlietet al. 2009). While deliberate overshoot may minimise mitigation costs over time, it does run the risk of lock-in of further fossil fuel use and thereby limiting the rate at which emissions can decline in subsequent years.

In the assessed IAM pathway set, four pathways have a temporary temperature overshoot before dropping below 2° C again35. All of these pathways have global negative CO2 emissions to help achieve the target. In these pathways the constraint on 2020 emissions is relaxed slightly, and the peak is postponed to 2020 and beyond.

Delayed action may have economic benefits (as noted above), but also has risks associated with the higher, albeit temporary, temperatures. These include higher mitigation costs over the long term and later and larger damages from climate change impacts. Huntingford and Lowe (2007) argue that there are significant risks from exceeding temperature limits during overshoot scenarios, due to uncertainty about so-called tipping points. An additional risk of overshooting temperature limits is that positive feedbacks, not known in advance, might result in a larger temperature increase than anticipated.

  35    In addition to the 27 of the 126 IAM pathways that are able to meet the 2° C limit during the twenty-first century without a temperature overshoot and with a probability higher than 50 per cent.