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Space observations of inland water bodies show rapid surface warming since 1985
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Surface temperatures were extracted from nighttime thermal infrared imagery of 167 large inland water bodies distributed worldwide beginning in 1985 for the months July through September and January through March. Results indicate that the mean nighttime surface water temperature has been rapidly warming for the period 1985–2009 with an average rate of 0.045 ± 0.011°C yr−1 and rates as high as 0.10 ± 0.01°C yr−1. Worldwide the data show far greater warming in the mid‐ and high latitudes of the northern hemisphere than in low latitudes and the southern hemisphere. The analysis provides a new independent data source for assessing the impact of climate change throughout the world and indicates that water bodies in some regions warm faster than regional air temperature. The data have not been homogenized into a single unified inland water surface temperature dataset, instead the data from each satellite instrument have been treated separately and cross compared. Future work will focus on developing a single unified dataset which may improve uncertainties from any inter‐satellite biases.
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Quantifying the negative feedback of vegetation to greenhouse warming: A modeling approach
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Several climate models indicate that in a 2 × CO2 environment, temperature and precipitation would increase and runoff would increase faster than precipitation. These models, however, did not allow the vegetation to increase its leaf density as a response to the physiological effects of increased CO2 and consequent changes in climate. Other assessments included these interactions but did not account for the vegetation down‐regulation to reduce plant’s photosynthetic activity and as such resulted in a weak vegetation negative response. When we combine these interactions in climate simulations with 2 × CO2, the associated increase in precipitation contributes primarily to increase evapotranspiration rather than surface runoff, consistent with observations, and results in an additional cooling effect not fully accounted for in previous simulations with elevated CO2. By accelerating the water cycle, this feedback slows but does not alleviate the projected warming, reducing the land surface warming by 0.6°C. Compared to previous studies, these results imply that long term negative feedback from CO2‐induced increases in vegetation density could reduce temperature following a stabilization of CO2 concentration.
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Changes in winter precipitation extremes for the western United States under a warmer climate as simulated by regional climate models
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We find a consistent and statistically significant increase in the intensity of future extreme winter precipitation events over the western United States, as simulated by an ensemble of regional climate models (RCMs) driven by IPCC AR4 global climate models (GCMs). All eight simulations analyzed in this work consistently show an increase in the intensity of extreme winter precipitation with the multi-model mean projecting an area-averaged 12.6% increase in 20-year return period and 14.4% increase in 50-year return period daily precipitation. In contrast with extreme precipitation, the multi-model ensemble shows a decrease in mean winter precipitation of approximately 7.5% in the southwestern US, while the interior west shows less statistically robust increases.
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Climate commitment in an uncertain world
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Climate commitment—the warming that would still occur given no further human influence—is a fundamental metric for both science and policy. It informs us of the minimum climate change we face and, moreover, depends only on our knowledge of the natural climate system. Studies of the climate commitment due to CO2 find that global temperature would remain near current levels, or even decrease slightly, in the millennium following the cessation of emissions. However, this result overlooks the important role of the non‐CO2 greenhouse gases and aerosols. This paper shows that global energetics require an immediate and sig- nificant warming following the cessation of emissions as aerosols are quickly washed from the atmosphere, and the large uncertainty in current aerosol radiative forcing implies a large uncertainty in the climate commitment. Fundamental constraints preclude Earth returning to pre‐industrial temperatures for the indefinite future. These same constraints mean that observations are currently unable to eliminate the possibility that we are already beyond the point where the ultimate warming will exceed dangerous levels. Models produce a narrower range of climate commitment, but under- sample observed forcing constraints.
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Impact of reduced Arctic sea ice on Greenland ice sheet variability in a warmer than present climate
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A global climate model with interactive vegetation and a coupled ice sheet-shelf component is used to test the response of the Greenland ice sheet (GIS) to increased sea surface temperatures (SSTs) and reduced sea ice (SI) cover during the mid-Pliocene warm period (∼3 Ma) as reconstructed from proxy records. Seasonally open water in the Arctic and North Atlantic are shown to alter regional radiation budgets, storm tracks, and moisture and heat advection into the Greenland interior, with increases in temperature rather than precipitation dominating the ice sheets response. When applied to an initially glaciated Greenland, the presumed warm, ice-free Pliocene ocean conditions induce rapid melting of nearly the entire ice sheet and preclude a modern-like GIS from (re)growing, regardless of orbital forcing. The sensitivity of Greenland to imposed Pliocene ocean conditions may have serious implications for the future response of the ice sheet to continued warming in the Arctic basin.
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Observed relation between evapotranspiration and soil moisture in the North American monsoon region
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Soil moisture control on evapotranspiration is poorly understood in ecosystems experiencing seasonal greening. In this study, we utilize a set of multi-year observations at four eddy covariance sites along a latitudinal gradient in vegetation greening to infer the ET-q relation during the North American monsoon. Results reveal significant seasonal, interannual and ecosystem variations in the observed ET-q relation directly linked to vegetation greening. In particular, monsoon-dominated ecosystems adjust their ET-q relation, through changes in unstressed ET and plant stress threshold, to cope with differences in water availability. Comparisons of the observed relations to the North American Regional Reanalysis dataset reveal large biases that increase where vegetation greening is more significant. The analysis presented here can be used to guide improvements in land surface model parameterization in water-limited ecosystems.
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Declining annual streamflow distributions in the Pacific Northwest United States, 1948–2006
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Much of the discussion on climate change and water in the western United States centers on decreased snowpack and earlier spring runoff. Although increasing variability in annual flows has been noted, the nature of those changes is largely unexplored. We tested for trends in the distribution of annual runoff using quantile regression at 43 gages in the Pacific Northwest. Seventy-two percent of the stations showed significant (a = 0.10) declines in the 25th percentile annual flow, with half of the stations exceeding a 29% decline and a maximum decline of 47% between 1948 and 2006. Fewer stations showed statistically significant declines in either median or mean annual flow, and only five had a significant change in the 75th percentile, demonstrating that increases in variance result primarily from a trend of increasing dryness in dry years. The asymmetric trends in streamflow distributions have implications for water management and ecology well beyond those of shifted timing alone, affect both rain and snow-dominated watersheds, and contribute to earlier timing trends in high- elevation watersheds.
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Rising temperature depletes soil moisture and exacerbates severe drought conditions across southeast Australia
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Over the past decade the southern catchments of the Murray Darling Basin (MDB), responsible for much of Australia’s agricultural output, have experienced a severe drought (termed the ‘‘Big Dry’’) with record high temperatures and record low inflow. We find that during the Big Dry the sensitivity of soil moisture to rainfall decline is over 80% higher than during the World War II drought from 1937 – 1945. A relationship exists between soil moisture and temperature independent of rainfall, particularly in austral spring and summer. Annually, a rise of 1°C leads to a 9% reduction in soil moisture over the southern MDB, contributing to the recent high sensitivity. Since 1950, the impact from rising temperature contributes to 45% of the total soil moisture reduction. In a warming climate, as the same process also leads to an inflow reduction, the reduced water availability can only be mitigated by increased rainfall. Other implications for future climate change are discussed.
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Reduction of spring warming over East Asia associated with vegetation feedback
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Over East Asia, surface air temperature displays a significant increasing trend particularly in early months of the year for the period of 1982 – 2000. Warming per decade is strongest in late winter, 1.5°C in February and 1.1°C in March, but is significantly reduced in spring, 0.4°C in April and 0.1°C in May. During the analysis period, the reduced temperature increase from late winter to spring is found to be in contrast with the increased vegetation greenness derived from the satellite-measured leaf area index over the domain. We examined this inverse relationship using two climate model experiments— coupled with and without a dynamic vegetation model. In both experiments, strong warming in winter is relatively well reproduced, but weak warming in spring is observed only in the coupled experiment. Analysis of the surface energy budget indicates that weaker spring warming results from an evaporative cooling effect due to the increased vegetation greenness. Over East Asia, the vegetation-evaporation feedback, therefore, may produce seasonal asymmetry in the warming trend.
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How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006
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To distinguish between simultaneous natural and anthropogenic impacts on surface temperature, regionally as well as globally, we perform a robust multivariate analysis using the best available estimates of each together with the observed surface temperature record from 1889 to 2006. The results enable us to compare, for the first time from observations, the geographical distributions of responses to individual influences consistent with their global impacts. We find a response to solar forcing quite different from that reported in several papers published recently in this journal, and zonally averaged responses to both natural and anthropogenic forcings that differ distinctly from those indicated by the Intergovernmental Panel on Climate Change, whose conclusions depended on model simulations. Anthropogenic warming estimated directly from the historical observations is more pronounced between 45°S and 50°N than at higher latitudes whereas the model-simulated trends have minimum values in the tropics and increase steadily from 30 to 70°N.
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