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2010 Literature Review Archives - Climate Trends and Variability

Brown, R., C. Derksen, L. Wang. 2010. A multi-data set analysis of variability and change in Arctic spring snow cover extent, 1967-2008. Journal of Geophysical Research, Vol 115, D16111, doi:10.1029/2010JD013975.

A new record of Arctic snow cover extent (SCE) derived from multiple datasets reveals significant declines in spring SCE (14% in May and 46% in June) over the period 1967-2008.  The declines in SCE are related to earlier snowmelt which can be attributed to increases in spring surface air temperature in the Arctic.

Scientists from Environment Canada have compiled a new record of Arctic (area north of 60oN excluding Greenland) snow cover extent from multiple datasets covering the period 1967-2008.  This new record combines snow cover data from visible and microwave satellite observations, surface snow depth observations, information derived from thaw dates and reconstructed snow cover based on daily temperature and precipitation.  Although each of these data sources records different values of snow cover at a particular time, the anomaly series are highly correlated.  The new multi-source SCE record is therefore more robust than records from any individual one of these data sources.  Over the 1967-2008 period, statistically significant decreases in May (14%) and June (46%) SCE are identified.  The declines are more linear than those detected in previous studies and correlate significantly with increasing spring surface air temperatures over the Arctic.  The sensitivity of Arctic SCE to temperature changes (based on linear regression against land temperatures north of 60oN) is given as -0.8 to -1.0 x 106 km2 per oC during the May-June melt period.  The trends in the spring SCE series from the Arctic region are corroborated by independent observations (for Canada and Alaska), and published accounts (for Eurasia), that reveal a decrease in snow cover duration across the Arctic over the last 30 years.  The decline in June SCE also corresponds with observed decreases of Arctic sea ice extent. 

Durack, P.J. and S.E. Wijffels. 2010. Fifty year trends in global ocean salinities and their relationship to broad-scale warming. J. of Climate, Vol 23(19), pp. 4342-4362, doi:10.1175/2010JCLI3377.1; Also, Helm, K.P., N.L. Bindoff, and J.A. Church. 2010. Changes in the global hydrological-cycle inferred from ocean salinity, Geophysical Research Letters, Vol 37, L18701, doi: 10.1029/2010GL044222.

Two recent papers link changes in ocean salinity observed over the past 50 years with an intensification of the global hydrological cycle consistent with broad-scale warming.

Climate projections suggest that anthropogenic climate change will lead to an enhancement of the global hydrological cycle as the troposphere warms.  Two recent papers explore observed trends in global ocean salinity (from historical data and Argo observations) and use them to make inferences about the global hydrological cycle.  Durack and Wijffels (2010) explore mulitdecadal linear trends in global ocean surface salinity patterns over the period 1950-2008.  Spatially coherent patterns of change are identified including salinity increases in evaporation-dominated regions and freshening in precipitation-dominated regions.  In the second paper Helm and colleagues calculated salinity changes along ocean-density surfaces (surfaces of equal density) over the period 1970-2005.  The salinity changes they report indicate a 3% decrease in precipitation minus evaporation (P-E) in the mid and low latitudes of both hemispheres, a 7% increase in the high latitudes of the northern hemisphere and a 16% increase in the southern Oceans.  The authors note that these changes (i.e., increased precipitation at high latitudes and decreased precipitation in low latitudes) are consistent with land-based records and the short satellite record.  Both studies conclude that the trends detected are evidence of an acceleration of the global hydrological cycle.

Lyman,J.M., S.A. Good, V.V. Gourestski, et al. 2010. Robust warming of the global upper ocean. Nature 465:334-337; Also, Trenberth, K.E. 2010. The ocean is warming, isn’t it? Nature 465:304.

A reappraisal of data on changes in upper ocean heat content confirms that there has been significant warming of the global upper ocean since 1993. This is a robust indicator that more energy is being retained within the Earth system than is leaving.

Past studies into trends in the heat content of the upper 700 m of the world’s oceans have consistently suggested that upper oceans have been warming in recent decades.  However, there has been considerable discrepancy among the various estimates of the magnitude of such warming.  A new study by an international team of oceanographers concludes that this discrepancy is largely due to different methods for correcting biases in the portion of the ocean data collected by expendable bathythermographs.  When the researchers involved in the study account for these uncertainties, they obtain a change in global upper ocean heat content over the period 1993-2008 of 0.64 W/m2 (per unit area of the Earth’s surface) with a 90% confidence interval of 0.53 - 0.75 W/m2. This is slightly higher that the estimate for 1993-2003 of 0.5 ± 0.18 W/m2 reported in the Fourth IPCC assessment report.  In an accompanying commentary, NCAR’s Kevin Trenberth notes that an apparent slowdown in upper ocean heat uptake during the last five years of the new record may be due to increased penetration of heat into the deeper ocean.

Menne, M.J., C.N. Williams Jr., and M.A. Palecki. 2010. On the reliability of the U.S. surface temperature record. J. Geophys. Res. Vol 115, D11108, doi:10.1029/2009JD013094.

Poorly sited stations do not create a warm bias in the US temperature record, as skeptics imply with photos posted online.

Recently, skeptics have posted photographs on the internet of poorly sited weather gauges in the U.S and have used these to claim that the temperature data record from stations in the U.S. Historical Climatology Network (USHCN) is unreliable for climate trend analysis.  To answer this criticism, scientists from NOAA carried out a statistical analysis to determine the effect poorly located stations had on the data record.   They calculated maximum and minimum temperature trends for the last thirty years from well sited stations and compared these to those from poorly sited stations. They determined there were some minor artificial biases in the temperature record from poor siting of the instruments, but these were small in comparison to the artificial biases due to changes in the instruments themselves.  In fact, it was the instrument changes, which involved switching from glass thermometers to digital thermistors, which often led to instruments becoming poorly sited due to practical difficulties arising from the change in instruments (e.g.. situating the instrument appropriately while dealing with cable length limitations and barriers such as sidewalks). The authors determined that there was a small positive (warm) bias in unadjusted minimum annual temperatures, and a larger negative (cool) bias in unadjusted maximum annual temperatures. The climate community is well aware of the importance of ‘homogenization’ of climate data - a process that adjusts data appropriately to account for discontinuities in the data from non-climatic factors. Adjustments have already been made to climate data in the U.S. Historical Climatology Network (version 2) to account for siting and instrumentation changes.  Adjusted data from well-sited and poorly sited stations showed close agreement. Nonetheless, this study revealed a small residual cool bias in the adjusted maximum temperature series which warrants further investigation. Of note is that this cool bias is opposite to the warm bias implied by the photographic evidence of poorly situated stations. Therefore, the study found no evidence of artificial inflation of the U.S. temperature record.

Murton, J.B., M.D. Bateman, S.R. Dallimore, J.T. Teller and Z. Yang. 2010.  Identification of Younger Dryas outburst flood path from Lake Agassiz to the Arctic Ocean. Nature. doi:10.1038/nature08954. See also  Schiermeier, Q. 2010. River reveals chilling tracks of ancient flood. Nature. doi: 10.1038/464657a

New geological evidence supports the prevailing hypothesis that an outburst flood from
LakeAgassiztriggered the Younger Dryas cold period.

Broecker et al. (1989: Nature) hypothesized that the Younger Dryas cold period - a sudden return to cold conditions in the Northern Hemisphere ~13,000 years ago - was triggered by a massive outburst flood from Lake Agassiz which formed along the margins of the retreating Laurentide Ice Sheet.  They theorized that these fresh waters flowed primarily eastward along the St. Lawrence River Valley into the North Atlantic Ocean where they suppressed the Atlantic meridional overturning circulation which brings warm waters northwards, causing widespread and rapid cooling in the North Atlantic region.  However, geological evidence for the flood has never been found until now.  In a paper recently published in the journal Nature, Murton and colleagues present geological evidence of this flood from sediments and erosional surfaces in the Mackenzie River system in northern Canada.  The dated evidence, in conjunction with topographic modeling, indicates that an outburst flood did occur shortly after 13,000 years ago (coincident with the onset of the Younger Dryas) but that the flow route was northwestward into the Arctic Ocean rather than eastward into the Atlantic Ocean.  In a separate NatureNews article, Broecker indicates that these waters would ultimately have flowed into the North Atlantic where they would have disrupted the thermohaline circulation as originally proposed.

Polyak, L., R.B. Alley, J.T. Andrews et al. 2010. History of sea ice in the Arctic. Quaternary Science Reviews, Vol 29, pp 1757-1778, DOI: 10.1016/j.quascirev.2010.02.010.

A new study combines multiple lines of proxy evidence to explore variations in the extent of Arctic sea ice over past millennia.  The data indicate that the recent decline in Arctic sea-ice extent is unprecedented over at least the last few thousand years.

In a recent paper, Polyak and colleagues attempted to place the recent declines in Arctic sea-ice extent into a longer-term context.  To accomplish this, the authors examined and, for the first time, synthesized results from hundreds of previous and ongoing studies that document dated proxy indicators of the presence of sea ice at specific locations in the Arctic.  The records considered include: chemical and biological indicators from marine sediment records from the seafloor; coastal landforms; deposits and sediment records; terrestrial plant remains; ice core and pollen records; and historical records.  Combined, these records provide a comprehensive history of variations in Arctic sea-ice extent (ice thickness and therefore volume cannot be determined using these proxies) over millions of years. The evidence suggests that sea ice appeared in the Arctic ~47 million years ago, acquired its perennial nature around 13-14 million years ago and has been most widespread over the past 2-3 million years.  The data indicate that the history of Arctic sea-ice extent is linked with climate changes driven by changes in greenhouse gas concentrations and orbital variations.  The authors conclude that the abruptness, wide geographic distribution and magnitude of recent ice loss appears to be unmatched over the last few thousand years and unexplainable by the known natural causes of variability.

Screen, J.A. and I. Simmonds. 2010 The central role of diminishing sea ice in recent Arctic temperature amplification. Nature Vol 464, pp 1334-1337. doi: 10.1038/nature09051.

A new study investigates the causes of Arctic temperature amplification over the past two decades and identifies diminishing sea ice as the primary cause.

Temperature increases in the Arctic over recent decades have been almost double the global average.  It is widely accepted that this ‘Arctic amplification’ is related to the decrease in albedo that accompanies reduced snow and ice cover but studies using models and reanalysis data have also implicated changes in cloud cover, water vapour and atmospheric and/or oceanic circulation.  Screen and Simmonds (2010) use a new and improved dataset (the ERA-Interim reanalysis) to investigate the contribution of these factors to the observed amplification over the period 1989-2008 on a seasonal basis.  The authors identify clear Arctic amplification in the dataset with maximum warming at the near-surface of 1.6oC, 0.9 oC, 0.5 oC and 1.6 oC per decade in winter, spring, summer and fall respectively.  Lesser warming is found with height in all seasons except summer and the authors suggest that this vertical profile indicates that surface processes are the primary cause of the arctic amplification noting that, for example, circulation changes, such as fluctuations in poleward heat and moisture transport, would likely have a broader vertical extent.  They run empirical analyses that indicate that this vertical profile is related statistically to reductions in Arctic sea ice cover.  Further analyses reveal no evidence that cloud-cover changes have contributed to the recent near-surface warming.  Increases in atmospheric water vapour content in the summer and early fall may explain some of the warming in the lower atmosphere during these seasons but these increases in water vapour arise partly in response to sea ice reductions. 

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