Catalogue of abrupt shifts in Intergovernmental Panel on Climate Change climate models

Abstract

Abrupt transitions of regional climate in response to the gradual rise in atmospheric greenhouse gas concentrations are notoriously difficult to foresee. However, such events could be particularly challenging in view of the capacity required for society and ecosystems to adapt to them. We present, to our knowledge, the first systematic screening of the massive climate model ensemble informing the recent Intergovernmental Panel on Climate Change report, and reveal evidence of 37 forced regional abrupt changes in the ocean, sea ice, snow cover, permafrost, and terrestrial biosphere that arise after a certain global temperature increase. Eighteen out of 37 events occur for global warming levels of less than 2°, a threshold sometimes presented as a safe limit. Although most models predict one or more such events, any specific occurrence typically appears in only a few models. We find no compelling evidence for a general relation between the overall number of abrupt shifts and the level of global warming. However, we do note that abrupt changes in ocean circulation occur more often for moderate warming (less than 2°), whereas over land they occur more often for warming larger than 2°. Using a basic proportion test, however, we find that the number of abrupt shifts identified in Representative Concentration Pathway (RCP) 8.5 scenarios is significantly larger than in other scenarios of lower radiative forcing. This suggests the potential for a gradual trend of destabilization of the climate with respect to such shifts, due to increasing global mean temperature change.

Keywords: CMIP5; IPCC; abrupt climate change; climate change; critical transitions.

Fig. 1.

Geographical location of the abrupt climate change occurrences. All 30 model cases listed in Table 1 are depicted. Of the 41 abrupt shifts, when regarding similar events for different simulations by the same climate model, this reduces to 30 distinct model cases. Marker color indicates the lowest global warming level, at which the abrupt change occurs, and the shape indicates category.

Fig. 2.

Examples of different categories of abrupt climate change detected in the CMIP5 database. Evolutions of (A) (category I: internally generated switches between two different states, case b in Fig. 1) regional annual mean sea ice cover in the Southern Ocean in the preindustrial control run of bcc-csm1-1-m; (B) (category II: a forced transition to switches between two different states, case f in Fig. 1) regional annual mean sea ice cover in the Southern Ocean in the historical and rcp4.5 run of GISS-E2-H; (C) (category III: singular rapid abrupt change toward a new state, case t in Fig. 1) SST in the Labrador Sea in the historical and rcp4.5 run of GFDL-ESM2G; and (D) (category IV: gradual sequence of abrupt changes toward a new state, case E in Fig. 1) tree cover in the Arctic tundra in the historical and rcp8.5 run of HadGEM2-ES.

Fig. 3.

Time series of all abrupt events not shown in Fig 2. All cases display annual means. Type-2 sic_GISS-E2_R_rcp45 and Type-2 sic-GISS-E2-H_rcp45 are ensemble members r2i1p3; Type-4 sic_CanESM2_rcp85 is ensemble member r5i1p1; Type_10 snw_GISS-E2-R_rcp45 is ensemble member r2i1p2; Type_10 snw_GISS-E2-R_rcp85 is ensemble member r1i1p2; all other types display time series from ensemble member r1i1p1; uswr, upward shortwave radiation; mpp, marine primary production; smc, soil moisture content.

Fig. 4.

Abrupt shifts as a function of global temperature increase. Shown are the number of abrupt climate changes occurring in the CMIP5 database for different intervals of warming relative to the preindustrial climate.

Fig. 5.

The frequency of occurrence of forced abrupt shifts as a function of global temperature increase. This number depends on the width of the temperature interval. Nevertheless, its qualitative shape gives an indication of how forced abrupt changes are divided over the range of temperature increases relative to the preindustrial area as simulated by the CMIP5 models. The frequency of occurrence is displayed per 0.5° temperature interval; see also Methods. Forced abrupt shifts occur only once in a particular scenario.