Background information

This infographic is based on the Cross Chapter Box 8 of the IPCC Special Report on Global Warming of 1.5°C that was completed in 2018. It illustrates three storylines of these possible warmer worlds based on different mitigation options as outlined in Cross-Chapter Box 8, Table 2 located in Chapter 3, which draws knowledge across chapters such as Chapter 2’s mitigation scenarios, Chapter 3’s assessments of changes in regional climate and accounting for different characteristics of internal climate variability in the next decade, and other elements from Chapters 4 and 5 of the report.

All three storylines illustrate how differing actions to mitigate and adapt to climate change, as well as how the Earth’s internal climate variability, can result in very different future warmer worlds. The three different storylines show: a path among a best-case scenario where there is a collective, early move to mitigation; an intermediate, mid-case scenario where delayed action occurs only after a warm decade in the 2020s; and a worst case scenario where there is uncoordinated action that occurs only late in 21st century. In the intermediate and worst-case storylines, internal climate variability plays a role in determining climate action (realisation vs overlooking of urgency).

Glossary

(definitions are adapted from the SR15, SROCC and SRCCL Glossaries)

Mitigation scenarios and pathways

Mitigation refers to human intervention to reduce emissions or enhance the sinks of greenhouse gases. A mitigation scenario is a plausible description of the future that describes how the (studied) system responds to the implementation of mitigation policies and measures. A mitigation pathway is a temporal evolution of a set of mitigation scenario features, such as greenhouse gas emissions and socio-economic development.

Climate variability

Climate variability refers to deviations of climate variables from a given mean state (including the occurrence of extremes, etc.) at all spatial and temporal scales beyond that of individual weather events. Variability may be intrinsic, due to fluctuations of processes internal to the climate system (internal variability), or extrinsic, due to variations in natural or anthropogenic external forcing (forced variability).

Human activities are estimated to have caused approximately 1.0°C of global warming above pre-industrial levels. Estimated anthropogenic global warming is currently increasing at 0.2°C (likely between 0.1°C and 0.3°C) per decade due to past and ongoing emissions. Superimposed internal climate variability can modulate decadal temperature variations and other climate characteristics at the global and regional scales.

These three storylines explore contrasted characteristics of internal variability in the next decade (2020s). The orange world’s very warm conditions in the 2020s is the result of a scenario that follows higher transient warming in that decade as a result of internal climate variability, in addition to human-induced trends. In contrast, the red world portrays a scenario in which the mean warming in the 2020s is slightly lower than that expected from human-induced trends due to natural climate variability. In both scenarios, internal variability is back to normal after 2030.

Negative emissions

Removal of greenhouse gases from the atmosphere by deliberate human activities, i.e., in addition to the removal that would occur via natural carbon cycle processes.

Ecosystem services

Ecological processes or functions having monetary or non-monetary value to individuals or society at large. These are frequently classified as (1) supporting services such as productivity or biodiversity maintenance, (2) provisioning services such as food or fibre, (3) regulating services such as climate regulation or carbon sequestration, and (4) cultural services such as tourism or spiritual and aesthetic appreciation.

Carbon sink

Any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere.

Overshoot pathways

Pathways that exceed the stabilization level (concentration, forcing, or temperature) before the end of a time horizon of interest (e.g., before 2100) and then decline towards that level by that time. Once the target level is exceeded, removal by sinks of greenhouse gases is required.