Box 1a: Understanding temperature limits

A warming limit of 2° C or 1.5° C refers to the increase in global annual average near surface temperature compared with pre-industrial times. This temperature is intended to be an indicator for local changes in a wide range of observable quantities, such as precipitation. It is important to note that a 2° C global average rise can translate into much larger (or smaller) temperature changes in different latitudes and elevations. Moreover, undesirable impacts will generally be driven by local climate changes (e.g. changes in rainfall patterns) and often by changes in extremes in different seasons rather than by annual average temperature values.

There are significant uncertainties in the relationship between temperature, emission pathways, cumulative emissions, and atmospheric concentrations. Therefore, in this assessment, each emission pathway is associated with probabilities of staying within a range of temperature limits. These probabilities reflect the uncertainties in the carbon cycle as well as many other aspects of the climate system. To illustrate, an emission pathway that has a 50 per cent chance of limiting warming to under 2° C may also have a 5 per cent probability that warming will exceed 3° C and, say, a 10 per cent probability of staying below 1.5° C. If we then consider an emission pathway that has a 66 per cent chance of being under 2° C, it may also have a probability of less than 3 per cent that warming will exceed 3° C, and, say, a 20 per cent probability of staying below 1.5° C.

Therefore, it is not possible to guarantee that a particular emission pathway will achieve a temperature limit of 2° C or 1.5° C, and probabilities of achievement are used instead. In this assessment we focus on two temperature limits, 2° C and 1.5° C; and two probabilities of meeting them – a “likely” chance (probability greater than 66 per cent) and a “medium” chance (probability between 50-66 per cent).

emission pathways     The trajectory of annual global greenhouse gas emissions over time.
cumulative emissions     Sum of annual global greenhouse gas emissions over a period of time. Because many greenhouse gases persist in the atmosphere for a long time, cumulative emissions greatly influence concentrations and therefore temperature.
temperature limits     Targets for maximum global average temperature increase above pre-industrial levels.