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Landrum L.,U.S. National Center for Atmospheric Research | Otto-Bliesner B.L.,U.S. National Center for Atmospheric Research | Wahl E.R.,National Climatic Data Center | Conley A.,U.S. National Center for Atmospheric Research | And 3 more authors.
Journal of Climate

An overview of a simulation referred to as the "Last Millennium" (LM) simulation of the Community Climate System Model, version 4 (CCSM4), is presented. The CCSM4LMsimulation reproduces many largescale climate patterns suggested by historical and proxy-data records, with Northern Hemisphere (NH) and Southern Hemisphere (SH) surface temperatures cooling to the early 1800s Common Era by ;0.5°C (NH) and ;0.3°C (SH), followed by warming to the present. High latitudes of both hemispheres show polar amplification of the cooling from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA) associated with sea ice increases. The LM simulation does not reproduce La Niñ a-like cooling in the eastern Pacific Ocean during the MCA relative to the LIA, as has been suggested by proxy reconstructions. Still, dry medieval conditions over the southwestern and central United States are simulated in agreement with proxy indicators for these regions. Strong global cooling is associated with large volcanic eruptions, with indications of multidecadal colder climate in response to larger eruptions. The CCSM4's response to large volcanic eruptions captures some reconstructed patterns of temperature changes over Europe and North America, but not those of precipitation in the Asian monsoon region. The Atlantic multidecadal oscillation (AMO) has higher variance at centennial periods in theLMsimulation compared to the 1850 nontransient run, suggesting a long-term Atlantic Ocean response to natural forcings. The North Atlantic Oscillation (NAO), Pacific decadal oscillation (PDO), and El Niñ o-Southern Oscillation (ENSO) variability modes show little or no change. CCSM4 does not simulate a persistent positive NAO or a prolonged period of negative PDO during the MCA, as suggested by some proxy reconstructions. © 2013 American Meteorological Society. Source

Tang Q.,CAS Beijing Institute of Geographic Sciences and Nature Resources Research | Leng G.,University of Chinese Academy of Sciences | Groisman P.Y.,National Climatic Data Center
Journal of Climate

A pronounced summer warming is observed in Europe since the 1980s that has been accompanied by an increase in the occurrence of heat waves. Water deficit that strongly reduces surface latent cooling is a widely accepted explanation for the causes of hot summers. The authors show that the variance of European summer temperature is partly explained by changes in summer cloudiness. Using observation-based products of climate variables, satellite-derived cloud cover, and radiation products, the authors show that, during the 1984-2007 period, Europe has become less cloudy (except northeastern Europe) and the regions east of Europe have become cloudier in summer daytime. In response, the summer temperatures increased in the areas of total cloud cover decrease and stalled or declined in the areas of cloud cover increase. Trends in the surface shortwave radiation are generally positive (negative) in the regions with summer warming (cooling or stalled warming), whereas the signs of trends in top-of-atmosphere (TOA) reflected shortwave radiation are reversed. The authors' results suggest that total cloud cover is either the important local factor influencing the summer temperature changes in Europe or a major indicator of these changes. © 2012 American Meteorological Society. Source

Shi X.,University of Washington | Dery S.J.,University of Northern British Columbia | Groisman P.Y.,National Climatic Data Center | Lettenmaier D.P.,University of Washington
Journal of Climate

Using the Variable Infiltration Capacity (VIC) land surface model forced with gridded climatic observations, the authors reproduce spatial and temporal variations of snow cover extent (SCE) reported by the National Oceanic and Atmospheric Administration (NOAA) Northern Hemisphere weekly satellite SCE data. Both observed and modeled North American and Eurasian snow cover in the pan-Arctic have statistically significant negative trends from April through June over the period 1972-2006. To diagnose the causes of the pan-Arctic SCE recession, the authors identify the role of surface energy fluxes generated in VIC and assess the relationships between 15 hydroclimatic indicators and NOAA SCE observations over each snowcovered sensitivity zone (SCSZ) for both North America and Eurasia. The authors find that surface net radiation (SNR) provides the primary energy source and sensible heat (SH) plays a secondary role in observed changes of SCE. As compared with SNR and SH, latent heat has only a minor influence on snow cover changes. In addition, these changes in surface energy fluxes resulting in the pan-Arctic snow cover recession are mainly driven by statistically significant decreases in snow surface albedo and increased air temperatures (surface air temperature, daily maximum temperature, and daily minimum temperature), as well as statistically significant increased atmospheric water vapor pressure. Contributions of other hydroclimate variables that the authors analyzed (downward shortwave radiation, precipitation, diurnal temperature range, wind speed, and cloud cover) are not significant for observed SCE changes in either the North American or Eurasian SCSZs. © 2013 American Meteorological Society. Source

Terando A.,Pennsylvania State University | Terando A.,North Carolina State University | Easterling W.E.,Pennsylvania State University | Keller K.,Pennsylvania State University | Easterling D.R.,National Climatic Data Center
Journal of Climate

The authors examine recent changes in three agro-climate indices (frost days, thermal time, and heat stress index) in North America (centered around the continental United States) using observations from a historical climate network and an ensemble of 17 global climate models (GCMs) from the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4). Agro-climate indices provide the basis for analyzing agricultural time series that are unbiased by long-term technological intervention. Observations from the last 60 years (1951-2010) confirm conclusions of previous studies showing continuing declines in the number of frost days and increases in thermal time. Increases in heat stress are largely confined to the western half of the continent. The authors do not observe accelerating agro-climate warming trends in the most recent decade of observations. The spatial variability of the temporal trends in GCMs is lower compared to the observed patterns, which still show some regional cooling trends. GCM skill, defined as the ability to reproduce observed patterns (i.e., correlation and error) and variability, is highest for frost days and lowest for heat stress patterns. Individual GCM skill is incorporated into two model weighting schemes to gauge their ability to reduce predictive uncertainty for agro-climate indices. The two weighted GCM ensembles do not substantially improve results compared to the unweighted ensemble mean. The lack of agreement between simulated and observed heat stress is relatively robust with respect to how the heuristic is defined and appears to reflect a weakness in the ability of this last generation of GCMs to reproduce this impact-relevant aspect of the climate system. However, it remains a question for future work as to whether the discrepancies between observed and simulated trends primarily reflect fundamental errors in model physics or an incomplete treatment of relevant regional climate forcings. © 2012 American Meteorological Society. Source

Crawled News Article
Site: http://news.yahoo.com/green/

With the midnight sun sinking lower in the sky each day, now is typically the time of year when the annual summer sea ice melt slows to a crawl in the Arctic. But 2016 is not your typical year in that part of the world. In fact, no year is "typical" anymore for a region that is warming at about twice as fast as the rest of the globe. Right now, broken ice and open waters are inching closer to the geographic North Pole. This is extremely rare, but likely not unprecedented, said Mark Serreze, the director of the National Climatic Data Center, in an interview. SEE ALSO: In diplomatic milestone, the US and China formally join Paris Climate Agreement The state of the sea ice pack at the top of the world is a sign of the rapid pace of warming taking place there. This year has been record warm across the Arctic, and has seen several unseasonably powerful storms swirl across the Arctic Ocean, churning the sea ice. The ice pack after these storms was more vulnerable to melting, since it was split into smaller chunks in greater contact with comparatively mild seawater. In addition, a late season warm spell has propelled 2016 to run close to 2007 for the title of the second-lowest sea ice minimum on record. The area of open water and broken ice chunks, which could be navigable to ships, is "quite near the Pole," Serreze said, nearing 87 degrees North. Satellite imagery indicates the broken ice and open water may even extend to 88 degrees North in some places. (Some of the areas of broken ice are known as "polynyas," spots of  open water surrounded by ice.) "[It's] At a very high latitude now," he said. "It's creeping closer [to the Pole] every year." A polar buoy network that had been operating in the region could have confirmed the presence of open water at or near the Pole, but funding cuts leaves satellite imagery as the best source of data on how the melt season is progressing. There are still at least a couple of weeks left in the melt season, meaning that the broken ice pack and open water could make it to the Pole itself, although weather conditions will have the final say in making that happen. Serreze said this season will end as either the second or third-lowest sea ice extent on record. "It's gonna be a race to the finish," he said, calling 2016 "another year in the new normal of the Arctic."

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