PerspectiveClimate

Running Out of Climate Space

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Science  04 Nov 2011:
Vol. 334, Issue 6056, pp. 613-614
DOI: 10.1126/science.1214215

Current biodiversity patterns across the globe are the result of the interplay between past and current climates and the degree to which species respond to these conditions (1). Climate affects both evolutionary processes, such as how fast species diversify, and ecological processes such as range shifts and species interactions. Understanding the spatial distribution of climatic conditions over time is thus crucial for understanding current and likely future species distributions and biodiversity patterns. Two reports in this issue, by Burrows et al. on page 652 (2) and Sandel et al. on page 660 (3), investigate past and future shifts in climate space, with the aim to explain and predict biogeographical patterns.

Climate space, or climate envelope, refers to the multidimensional climatic conditions of an area (see the figure). Climatic conditions change over time and these changes can be mapped in space. The extent, speed, and direction of such spatiotemporal shifts in climate space can help to explain and predict biotic responses to such changes.

Burrows et al. analyzed the speed with which climate has changed locally and across geographic space over the past 50 years. The authors use the simple yet appealing measure of climate change velocity, which quantifies how fast isotherms have shifted (4). In the study, high-velocity areas are those from which a large distance needs to be covered to reach areas that, by 2009, have the same temperature as the original area did in 1960. Low-velocity areas are those where in 2009, 1960-analogous conditions are nearby. At most latitudes, climate change velocity is similar in the ocean and on land; however, in sub-Arctic regions and around the equator, marine velocity is two to seven times higher than on land. Seasonal shifts are also generally faster in the sea than on land, especially in the Arctic and at northern mid-latitudes, where spring timing advanced more rapidly at sea than on land (2).

Lost and new climates.

Two-dimensional climate space (solid outlines) of a region at two time periods. Under changing climatic conditions, species will have to adapt to disappearing and novel climatic conditions at the edges of the region's original climate space. Two reports in this issue (2, 3) investigate the biogeography of such shifts.

Under warming climate conditions, climate change velocity varies across the globe, with generally low velocity in mountainous regions where isotherms are tightly packed, and high velocity in lowland areas (2). Loarie et al. have predicted very similar patterns of global climate velocity for the next 100 years (4). Thus, simple poleward shifts of species in response to warming climate conditions are highly oversimplified, especially in heterogeneous landscapes. It is the exception rather than the rule that isotherms show clear northward shifts. The spatial configuration and heterogeneity of shifting isotherms and associated climate space are crucial for understanding species' responses to these shifts over space and time.

When applying climate space analyses to ecological questions, three main things matter to species: the extent of areas with particular climate conditions, how far away these areas are, and whether there are any obstacles in the direction of these areas. Over time, the extent of climatic niche space will expand or shrink; some climates will disappear while other, novel combinations of climatic factors will emerge (see the figure). Patterns of novel and disappearing climates under future climate change have been modeled at the global (5), continental (6), and regional scale (7). The spatial extent of particular climatic conditions will affect the geographic distribution of species occupying this niche space. For instance, there is evidence that rare, small-range species are mostly found in currently rare, spatially restricted climates (8).

Sandel et al. also report strong evidence that not only current but also past conditions influence biodiversity patterns of endemic species. In their study, the climate velocity of an area is inversely related to endemic species richness: The faster isotherms have shifted in an area since the last glaciation, the fewer endemic species are found there. This relationship is weakest for highly mobile birds and strongest for much less mobile amphibians, indicating that dispersal ability may drive this relationship. Areas of low climate velocity where analogous climates were within easy reach since the last glaciation are safe havens for many species with currently small ranges (3).

These results provide support for the idea of assessing the conservation status not only of species but also of climates. Two studies recently provided such an assessment for Californian climate space under future climate change (7, 9); both studies show that assessing climatic heterogeneity and the extent of disappearing and novel climatic conditions within and outside protected areas can add useful information to biodiversity conservation planning.

Climate space analyses come with caveats. First, they are only as good as the climate data on which they are based. Circular argumentation is a real possibility, because gridded climate data are often filled in using spatial interpolation, particularly in regions with few weather stations. Climate is then a function of geographic distance (when the gridded climate data are filled in) and, in the climate space analysis, geographic distance is a function of climate (when velocity is calculated as the distance between isotherms). However, many climate models do not rely on spatial interpolation, and increased climate observation coverage will lead to more fine-resolution climate data sets, reducing the problem of circularity.

Second, shifts in climate space should be used with caution to infer shifts in species or ecosystems currently found in this space; species may or may not be able to track their suitable climate space, depending on their migration and dispersal capacity. Combining climate space analyses with recent advances in community (10) and individual and population modeling (11, 12) will help to address this issue, advancing our understanding of large-scale biotic responses to changing environmental conditions.

Finally, under changing climatic conditions and over time, adaptation and altered interactions between species will affect whether species remain limited to the conditions in which they currently occur or whether they can occupy new climate space (13, 14). The degree to which a taxonomic group tends to retain similar ecological traits and to occupy similar niches over time (niche conservatism) is the key process linking evolution and ecology (15). Combining climate space analyses with phylogeographic and biodiversity data can shed light on the degree, tempo, and spatial patterns of niche conservatism versus niche evolution.

Despite these caveats, climate space analyses such as those by Burrows et al. and Sandel et al. broaden our understanding of how environments change in space and time and how this in turn affects patterns of disappearing, persisting, and novel climates, species, and ecosystems.

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