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Comparisons of climate model hindcasts with independent proxy data are essential for assessing model performance in non-analogue situations. However, standardized paleoclimate datasets for assessing the spatial pattern of past climatic change across continents are lacking for some of the most dynamic episodes of Earth's recent past. Here we present a new chironomid-based paleotemperature dataset designed to assess climate model hindcasts of regional summer temperature change in Europe during the late-glacial and early Holocene. Latitudinal and longitudinal patterns of inferred temperature change are in excellent agreement with simulations by the ECHAM-4 model, implying that atmospheric general circulation models like ECHAM-4 can successfully predict regionally diverging temperature trends in Europe, even when conditions differ significantly from present. However, ECHAM-4 infers larger amplitudes of change and higher temperatures during warm phases than our paleotemperature estimates, suggesting that this and similar models may overestimate past and potentially also future summer temperature changes in Europe.

A principle response of C3 plants to increasing concentrations of atmospheric CO 2 (CO 2 ) is to reduce transpirational water loss by decreasing stomatal conductance (g s ) and simultaneously increase assimilation rates. Via this adaptation, vegetation has the ability to alter hydrology and climate. Therefore, it is important to determine the adaptation of vegetation to the expected anthropogenic rise in CO 2 . Short-term stomatal opening-closing responses of vegetation to increasing CO 2 are described by free-air carbon enrichments growth experiments, and evolutionary adaptations are known from the geological record. However, to date the effects of decadal to centennial CO 2 perturbations on stomatal conductance are still largely unknown. Here we reconstruct a 34% (±12%) reduction in maximum stomatal conductance (g smax ) per 100 ppm CO 2 increase as a result of the adaptation in stomatal density (D) and pore size at maximal stomatal opening (a max ) of nine common species from Florida over the past 150 y. The species-specific g smax values are determined by different evolutionary development, whereby the angiosperms sampled generally have numerous small stomata and high g smax , and the conifers and fern have few large stomata and lower g smax . Although angiosperms and conifers use different D and a max adaptation strategies, our data show a coherent response in g smax to CO 2 rise of the past century. Understanding these adaptations of C3 plants to rising CO 2 after decadal to centennial environmental changes is essential for quantification of plant physiological forcing at timescales relevant for global warming, and they are likely to continue until the limits of their phenotypic plasticity are reached. cuticular analysis | subtropical vegetation L and plants play a crucial role in regulating our planet's hydrological and energy balance by transpiring water through the stomatal pores on their leaf surfaces. A fundamental response of C3 plants to increasing atmospheric CO 2 concentration (CO 2 ) is to minimize transpirational water loss by reducing diffusive stomatal conductance (g s ) and simultaneously increasing assimilation rates (1). The resulting increased intrinsic water-use efficiency (iWUE: the ratio of assimilation to g s ) improves the vegetation's drought resistance and reduces the cost associated with the leaf's water transport system like leaf venation (2, 3). On a regional to global scale, decreasing rates of transpiration concurrently affect climate through reduced cloud formation and precipitation (4) and with this exert a physiological feedback on climate and hydrology on top of the radiative forcing of increasing CO 2 (5-7). In the light of continuing anthropogenic climate change, it is therefore imperative to determine how plants adapt to rising atmospheric CO 2 .During their 400 million year history, land plants have been exposed to large variations in environmental conditions that prompted genetic adaptations toward mechanisms that optimize individual fitness. Over this period, plant ad...

Plant physiological adaptation to the global rise in atmospheric CO 2 concentration (CO 2 ) is identified as a crucial climatic forcing. To optimize functioning under rising CO 2 , plants reduce the diffusive stomatal conductance of their leaves (g s ) dynamically by closing stomata and structurally by growing leaves with altered stomatal densities and pore sizes. The structural adaptations reduce maximal stomatal conductance (g smax ) and constrain the dynamic responses of g s . Here, we develop and validate models that simulate structural stomatal adaptations based on diffusion of CO 2 and water vapor through stomata, photosynthesis, and optimization of carbon gain under the constraint of a plant physiological cost of water loss. We propose that the ongoing optimization of g smax is eventually limited by species-specific limits to phenotypic plasticity. Our model reproduces observed structural stomatal adaptations and predicts that adaptation will continue beyond double CO 2 . Owing to their distinct stomatal dimensions, angiosperms reach their phenotypic response limits on average at 740 ppm and conifers on average at 1,250 ppm CO 2 . Further, our simulations predict that doubling today's CO 2 will decrease the annual transpiration flux of subtropical vegetation in Florida by ≈60 W·m −2 . We conclude that plant adaptation to rising CO 2 is altering the freshwater cycle and climate and will continue to do so throughout this century.climate change | physiological forcing | plant evolution P lants respond to the complex of environmental signals they perceive by plastic changes in their phenotype to increase individual fitness (1). The most apparent environmental change that induces phenotypic adaptations in plants is the global increase in atmospheric CO 2 concentration (CO 2 ) (2). In response to this rise in CO 2 , plants reduce the diffusive stomatal conductance of their leaves [g s (mol·m −2 ·s −1 )] to increase drought resistance (3) and reduce physiological costs associated with water transport (4). Plants can reduce g s by dynamically closing their stomata within minutes (5, 6), and structurally within the lifespan of an individual by growing leaves with altered stomatal density [D (number of stomata·m −2 )] and pore size at maximal stomatal opening [a max (m 2 )] (7, 8). Structural adaptations thereby reduce maximal stomatal conductance [g smax (mol·m −2 · s −1 )], which critically reduces actual g s , especially when stomata are fully open during times with ample light and water (9).Reduction of g s via structural adaptation of g smax has the potential to reduce transpiration fluxes and, thus, cause land surface warming in addition to changes in the global hydrological cycle with rising CO 2 (10). This climatic effect is termed the physiological forcing of CO 2 , which acts beside and independent of its radiative forcing. Despite advances to quantify this physiological forcing with global climate models (11, 12), these models rely on semiempirical relations to simulate g s from environmental variables (...