Projected distributions of novel and disappearing climates by 2100 AD

Abstract

Key risks associated with projected climate trends for the 21st century include the prospects of future climate states with no current analog and the disappearance of some extant climates. Because climate is a primary control on species distributions and ecosystem processes, novel 21st-century climates may promote formation of novel species associations and other ecological surprises, whereas the disappearance of some extant climates increases risk of extinction for species with narrow geographic or climatic distributions and disruption of existing communities. Here we analyze multimodel ensembles for the A2 and B1 emission scenarios produced for the fourth assessment report of the Intergovernmental Panel on Climate Change, with the goal of identifying regions projected to experience (i) high magnitudes of local climate change, (ii) development of novel 21st-century climates, and/or (iii) the disappearance of extant climates. Novel climates are projected to develop primarily in the tropics and subtropics, whereas disappearing climates are concentrated in tropical montane regions and the poleward portions of continents. Under the high-end A2 scenario, 12-39% and 10-48% of the Earth's terrestrial surface may respectively experience novel and disappearing climates by 2100 AD. Corresponding projections for the low-end B1 scenario are 4-20% and 4-20%. Dispersal limitations increase the risk that species will experience the loss of extant climates or the occurrence of novel climates. There is a close correspondence between regions with globally disappearing climates and previously identified biodiversity hotspots; for these regions, standard conservation solutions (e.g., assisted migration and networked reserves) may be insufficient to preserve biodiversity.

Fig. 1.

Conceptual diagram showing the intersection among the fundamental niches for four species (Sp.; colored ellipses) and climatic envelopes for hypothetical 20th- and 21st-century climates (black-bordered ellipses) (8). Novel climates are the portions of the 21st-century envelope that do not overlap 20th-century climates, and disappearing climates are the portions of the 20th-century envelope that do not overlap 21st-century climates. Species cooccur only if their fundamental niches simultaneously intersect with each other and the current climatic space. Future climate change may cause a variety of ecological responses, including shifts in species distributions (species 1–3), community disaggregation (species 1 and 3), new communities forming (species 2 and 3), and extinction (species 4). This conceptual model assumes fixed niches, i.e., that climate change will outpace evolutionary adaptation (8).

Fig. 2.

Mapped indices of climate change risk for local climate change (A and B), novel 21st-century climates (C and D), and disappearing 20th-century climates (E and F). (A) Local climatic change for the A2 scenario, represented by the SED between the 20th- and 21st-century climate realizations for each gridpoint. The color bar is scaled so that SED→SED_t (see Materials and Methods) are yellow to red. (B) As in A but for the B1 scenario. Locally high values over the Sahara, Arabian Peninsula, and southwestern Asia are an artifact of zero precipitation and precipitation variance simulated by the MRI–CGCM2.3.2 and CCSM3 models. In other models, the SED scores for these locations are similar to those of neighboring gridpoints. (C) Maps of the SED_min between the 21st-century realization for each gridpoint and the set of 20th-century climate realizations (A2 scenario). High dissimilarities indicate risk of novel climates. (D) As in C but for the B1 scenario. (E) Maps of the SED_min between the 20th-century realization for each gridpoint and the set of 21st-century climate realizations (A2 scenario). High dissimilarities indicate risk of disappearing climates. (F) As in E but for the B1 scenario. (C–F) The pool of potential climatic analogs is global.

Fig. 3.

A–D correspond to C–F in Fig. 1, except, here, the pool of potential analogs is restricted to gridpoints within 500 km of each target gridpoint. (A) SED_min between the 21st-century realization for each gridpoint and the set of 20th-century climate realizations (A2 scenario). High dissimilarities indicate risk of regionally novel 21st-century climates. (B) As in A but for the B1 scenario. (C) SED_min between the 20th-century realization for each gridpoint and the set of 21st-century climate realizations (A2 scenario). High dissimilarities indicate risk of regionally disappearing 20th-century climates. (D) As in C but for the B1 scenario.

Fig. 4.

Plots showing the relationship between global mean annual warming and the fractional global area with novel and disappearing climates. Each point represents an individual model; triangles represent A2 simulations, and circles represent B1 simulations. Filled symbols and the solid regression line represent risk indices for novel climates (corresponding to Figs. 2 C and D and 3 A and B); open circles and the dashed line represent risk indices for disappearing climates (corresponding to Figs. 2 E and F and 3 C and D). A2 and B1 scenarios are pooled for the regression model. (A) Search for climatic analogs is global (no dispersal constraint). (B) Search for analogs restricted to within 500 km (500-km dispersal constraint).