AGW Observer

Observations of anthropogenic global warming

New research from last week 21/2011

Posted by Ari Jokimäki on May 30, 2011

Here is the new research published last week. I’m not including everything that was published but just some papers that got my attention. Those who follow my Facebook page (and/or Twitter) have already seen most of these, as I post these there as soon as they are published. Here, I’ll just put them out in one batch. Sometimes I might also point out to some other news as well, but the new research will be the focus here. Here’s the archive for the news of previous weeks. By the way, if this sort of thing interests you, be sure to check out A Few Things Illconsidered, they have a weekly posting containing lots of links to new research and other climate related news. Planet 3.0 also reports new research.

Published last week:

Deglacial CO2 rise might not be from the oceans

Deglacial radiocarbon history of tropical Atlantic thermocline waters: absence of CO2 reservoir purging signal – Cléroux et al. (2011) “A current scenario to explain much of the atmospheric CO2 increase during the Glacial to Holocene climate transition requires the outgassing of a deep, old oceanic CO2 reservoir thought to be located in the Southern Ocean. In this scenario, CO2-rich and 14C-depleted subsurface Antarctic-sourced water, ventilates the thermocline where it is purged to the atmosphere in the equatorial regions, a view that has been met with conflicting results. Using a novel approach (paired surface and deep-dwelling planktonic foraminifer radiocarbon analyses), we document that the equatorial Atlantic thermocline did not see old, 14C-depleted water, which would be characteristic of the proposed isolated deep ocean CO2 reservoir. Data from several studies concur that, during the deglaciation, Antarctic intermediate waters were contributing to Atlantic thermocline waters even more than today, therefore, our observations challenge the current purging hypothesis. Together with other studies, these results suggest that the mechanism responsible for the deglacial CO2 rise cannot invoke contemporary circulation modes and/or thermocline ventilation pathways.” Caroline Cléroux, Peter deMenocal and Thomas Guilderson, Quaternary Science Reviews, doi:10.1016/j.quascirev.2011.04.015.

New aerosol-cloud effect has significance to climate

The aerosol–Bénard cell effect on marine stratocumulus clouds and its contribution to glacial-interglacial cycles – Bar-Or et al. (2011) “Aerosol-cloud interactions, such as aerosol loading in convective clouds resulting in either precipitation suppression or cloud invigoration, in higher cloud tops, and in longer-lived clouds, are well known. Here we investigate a new aerosol-cloud interaction, the effect of aerosol loading on Bénard cells, on the stratocumulus cloud fraction, and ultimately on the climate over glacial-interglacial cycles, using a two-dimensional model running a million year continuous simulation. This radiative effect is observed only in marine boundary layer stratocumulus clouds that have a convective cellular structure. Recent research suggests that aerosols can switch the direction of convection in Bénard cells (from open cells to closed cells) by suppressing precipitation and therefore dramatically change the cloud fraction. The effect investigated in this work differs from previously known aerosol effects on convective clouds by its intensity and magnitude and has never been taken into account in past climate simulations. The results show that accounting for the aerosol–Bénard cell effect alone contributes a negative radiative forcing, affecting both the Northern Hemisphere mean annual surface temperature and ice volume. Adding the aerosol–Bénard cell effect to the direct radiative effect of dust and to the effect of dust on snow and ice albedo shows that the aerosol–Bénard cell effect plays a significant role in glacial-interglacial climate change, strengthening the earlier glacial cycles and creating a larger glacial-interglacial surface temperature amplitude while preserving the continental ice volume amplitude. Because of the model limitations, there are a number of uncertainties involved. However, the results serve to give a preliminary evaluation of the aerosol–Bénard cell effect at least qualitatively if not quantitatively.” Bar-Or, R. Z., H. Gildor, and C. Erlick (2011), J. Geophys. Res., 116, D10119, doi:10.1029/2010JD014470.

Are coasts uplifting worldwide?

Relative sea-level fall since the last interglacial stage: Are coasts uplifting worldwide? – Pedoja et al. (2011) “The growing interest in quantification of vertical ground motion stems from the need to understand in detail how the Earth’s crust behaves, for both scientific and social reasons. However, only recently has the refinement of dating techniques made possible the use of paleoshorelines as reliable tools for tectonic studies. Although there are many local studies of Quaternary vertical motions of coastlines, we know of no comprehensive worldwide synthesis. Here we provide a compilation of 890 records of paleoshoreline sequences, with particular emphasis on the last interglacial stage (Marine Isotopic Stage [MIS] 5e, ~ 122 ka). The quality of dating MIS 5e makes it a reliable marker to evaluate vertical ground motion rates during the late Quaternary on a global scale. The results show that most coastal segments have risen relative to sea-level with a mean uplift rate higher than 0.2 mm/yr, i.e. more than four times faster than the estimated eustatic drop in sea level. The results also reveal that the uplift rate is faster on average for active margins than for passive margins. Neither dynamic topography nor glacio-hydro-isostasy may explain sustained uplift of all continental margins, as revealed by the wide distribution of uplifted sequences of paleoshorelines. Instead, we suggest that only plate-tectonic processes reconcile all observations of Quaternary coastal uplift. We propose that long-term continental accretion has led to compression of continental plates and uplift of their margins. Therefore this study concludes that plate-tectonics processes impact all margins and emphasizes the fact that the notion of a stable platform is unrealistic. These results therefore seriously challenge the evaluation of past sea levels from the fossil shoreline record.” Kevin Pedoja, Laurent Husson, Vincent Regard, Peter Robert Cobbold, Emilie Ostanciaux, Markes E. Johnson, Stephen Kershaw, Marianne Saillard, Joseph Martinod, Lucille Furgerot, Pierre Weill and Bernard Delcaillau, Earth-Science Reviews, doi:10.1016/j.earscirev.2011.05.002.

Temperature annual cycle amplitude trends in China

Nonlinear trend in the amplitude of the temperature annual cycle in China and its implication for climate change research – Qian et al. (2011) “Climate change is not only reflected by the changes in annual means of climate variables, but also by the changes in their annual cycles (seasonality), especially in the regions outside the tropics. In this study, the ensemble empirical mode decomposition (EEMD) method is applied to investigate nonlinear trend in the amplitude of the annual cycle (which contributes 96% of total variance) of China’s daily mean surface air temperature for the period 1961–2007. The results show that variation and change in the amplitude are significant, with a peak-to-peak annual amplitude variation of 13% (1.8°C) of its mean amplitude and a significant linear decrease in amplitude by 4.6% (0.63°C) for this period. Also identified is a multidecadal change in amplitude from significant decreasing (−1.7%/decade or −0.23 °C/decade) to significant increasing (2.2%/decade or 0.29°C/decade) occurring around 1993 that overlaps the systematic linear trend. This multidecadal change can be attributed mainly to the change in surface solar radiation, from dimming to brightening, rather than to warming or an enhanced greenhouse effect. We further propose that the combined effect of the global dimming/brightening transition and a gradual increase in greenhouse warming has led to a perceived warming trend that is much larger in winter than in summer and to a perceived accelerated warming in the annual mean since the early 1990s in China. We also note that the deseasonalization method (considering either the conventional repetitive climatological annual cycle or the time-varying annual cycle) can also affect trend estimation.” Cheng Qian, Congbin Fu, Zhaohua Wu, Journal of Climate 2011, doi: 10.1175/JCLI-D-11-00006.1.

Lessons from early Holocene sea level rise

The early Holocene sea level rise – Smith et al. (2011) “The causes, anatomy and consequences of the early Holocene sea level rise (EHSLR) are reviewed. The rise, of ca 60m, took place over most of the Earth as the volume of the oceans increased during deglaciation and is dated at 11,650–7000 cal. BP. The EHSLR was largely driven by meltwater release from decaying ice masses and the break up of coastal ice streams. The patterns of ice sheet decay and the evidence for meltwater pulses are reviewed, and it is argued that the EHSLR was a factor in the ca 8470 BP flood from Lake Agassiz-Ojibway. Patterns of relative sea level changes are examined and it is argued that in addition to regional variations, temporal changes are indicated. The impact of the EHSLR on climate is reviewed and it is maintained that the event was a factor in the 8200 BP cooling event, as well as in changes in ocean current patterns and their resultant effects. The EHSLR may also have enhanced volcanic activity, but no clear evidence of a causal link with submarine sliding on continental slopes and shelves can yet be demonstrated. The rise probably influenced rates and patterns of human migrations and cultural changes. It is concluded that the EHSLR was a major event of global significance, knowledge of which is relevant to an understanding of the impacts of global climate change in the future.” D.E. Smith, S. Harrison, C.R. Firth and J.T. Jordan, Quaternary Science Reviews, doi:10.1016/j.quascirev.2011.04.019.

More evidence for tropical belt widening

Poleward Shift of Subtropical Jets Inferred from Satellite-observed Lower Stratospheric Temperatures – Fu & Lin (2011) “One pronounced feature in observed latitudinal dependence of lower-stratospheric temperature trends is the enhanced cooling near 30° latitudes in both hemispheres (Fu et al. 2006). The observed phenomenon has not, to date, been explained in the literature. This study shows that the enhanced cooling is a direct response of the lower-stratospheric temperature to the poleward shift of subtropical jets. Further, this enhanced lower-stratospheric cooling can be used to quantify the poleward shift of subtropical jets. Using the lower-stratospheric temperatures observed by satellite-borne microwave sounding units, it is shown that the subtropical jets have shifted poleward by 0.6 ± 0.1 and 1.0 ± 0.3 degrees latitude in the southern and northern hemispheres, respectively, in last 30 years since 1979, indicating a widening of tropical belt by 1.6 ± 0.4 degrees latitude.” Qiang Fu and Pu Lin, Journal of Climate 2011, doi: 10.1175/JCLI-D-11-00027.1.

Climate information from sediment sand grain size

Patterns of local and regional grain size distribution and their application to Holocene climate reconstruction in semi-arid Inner Mongolia, China – Yin et al. (2011) “The semi-arid temperate steppe in northern central China is one of the main areas influenced by frequent dust and sand storms, and is at the same time a primary source of dust from deteriorated grasslands; thus, the sediment grain size distribution of inland lakes in this region can be a particularly useful indicator of palaeoenvrionmental change. The local pattern of grain size suggests that aeolian activity is the most important agent for sedimentation in the lake center in this region, as strong northwesterly winds prevail for most of the year and the surface runoff is very weak. Meanwhile, the regional pattern of topsoil grain size and its close association with mean annual precipitation (MAP) allows the establishment of a statistical model for palaeo-moisture reconstruction from sediment grain size. In this study, we reconstructed a humidity time series based on the sediment grain size sequence from Anguli Nuur Lake on the southern Inner Mongolian Plateau in China and found that it coincides very closely with the C/N ratio (carbon to nitrogen ratio) and other humidity indices revealed in previous studies of this temperate steppe region and from the δ18O values of stalagmite calcite in southern, monsoon-dominated China. This close relationship suggests that climate change in the semi-arid areas of Asia is strongly influenced by the Pacific summer monsoon and that it is reasonable to use sediment grain size as an indicator of humidity variability in the semi-arid steppe region. The reconstructed humidity increased during the early Holocene, and generally humid conditions lasted from about 10,400 until 7,000 yr BP. The period from around 7,000 to 5,200 yr BP was a transition phase from humid to semi-arid conditions, and the monsoon intensity of that time may have been at the threshold for a semi-arid vegetation ecosystem. Finally, since approximately 5,200 yr BP to present, the climate has become more arid, with corresponding vegetation deterioration and strong aeolian activity.” Yi Yin, Hongyan Liu, Siyuan He, Fengjun Zhao, Jiangling Zhu, Hongya Wang, Guo Liu and Xiuchen Wu, Palaeogeography, Palaeoclimatology, Palaeoecology, doi:10.1016/j.palaeo.2011.05.011.

Ice discharge from Greenland and Antarctic glaciers continues to increase

Accelerating ice loss from the fastest Greenland and Antarctic glaciers – Thomas et al. (2011) “Ice discharge from the fastest glaciers draining the Greenland and Antarctic ice sheets – Jakobshavn Isbrae (JI) and Pine Island Glacier (PIG)– continues to increase, and is now more than double that needed to balance snowfall in their catchment basins. Velocity increase probably resulted from decreased buttressing from thinning (and, for JI, breakup) of their floating ice tongues, and from reduced basal drag as grounding lines on both glaciers retreat. JI flows directly into the ocean as it becomes afloat, and here creep rates are proportional to the cube of bed depth. Rapid thinning of the PIG ice shelf increases the likelihood of its breakup, and subsequent rapid increase in discharge velocity. Results from a simple model indicate that JI velocities should almost double to >20 km a−1 by 2015, with velocities on PIG increasing to >10 km a−1 after breakup of its ice shelf. These high velocities would probably be sustained over many decades as the glaciers retreat within their long, very deep troughs. Resulting sea-level rise would average about 1.5 mm a−1.” Thomas, R., E. Frederick, J. Li, W. Krabill, S. Manizade, J. Paden, J. Sonntag, R. Swift, and J. Yungel (2011), Geophys. Res. Lett., 38, L10502, doi:10.1029/2011GL047304.

Global warming affects Earth’s rotation too

GRACE era secular trends in Earth rotation parameters: A global scale impact of the global warming process? – Roy & Peltier (2011) “Recent trends in the two primary anomalies in the rotational state of the planet are analyzed in detail, namely those associated with the speed and direction of polar wander and with the non-tidal acceleration of the rate of axial rotation (via the measurement of the changing oblateness of the Earth’s shape). It is demonstrated that a significant change in the secular trends in both of these independent parameters became evident subsequent to approximately 1992. It is suggested that both parameters might have come to be substantially influenced by mass loss from both the great polar ice sheets, and from the very large number of small ice-sheets and glaciers that are also being influenced by the global warming phenomenon. The modern values for the secular drifts in those parameters that we estimate are appropriate to the period during which measurements have been made by the satellites of the Gravity Recovery and Climate Experiment (GRACE). These changes in secular rates might greatly assist in understanding why the GRACE-inferred values of the time derivatives of the degree two and order one Stokes coefficients differ so significantly from those associated with Late Quaternary ice-age influence.” Roy, K., and W. R. Peltier (2011), Geophys. Res. Lett., 38, L10306, doi:10.1029/2011GL047282.

Snowline altitude changes in Andes mountains of Peru

Landsat TM and ETM+ derived snowline altitudes in the Cordillera Huayhuash and Cordillera Raura, Peru, 1986–2005 – McFadden et al. (2011) “The Cordilleras Huayhuash and Raura are remote glacierized ranges in the Andes Mountains of Peru. A robust assessment of modern glacier change is important for understanding how regional change affects Andean communities, and for placing paleo-glaciers in a context relative to modern glaciation and climate. Snowline altitudes (SLAs) derived from satellite imagery are used as a proxy for modern (1986–2005) local climate change in a key transition zone in the Andes. Clear sky, dry season Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM+) satellite images from 1986–2005 were used to identify snowline positions, and their altitude ranges were extracted from an Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) digital elevation model (DEM). Based on satellite records from 31 glaciers, average snowline altitudes (SLAs), an approximation for the equilibrium line altitude (ELA), for the Cordillera Huayhuash (13 glaciers) and Cordillera Raura (18 glaciers) from 1986–2005 were 5051 m a.s.l. from 1986–2005 and 5006 m a.s.l. from 1986–2002, respectively. During the same time period, the Cordillera Huayhuash SLA experienced no significant change while the Cordillera Raura SLA rose significantly from 4947 m a.s.l. to 5044 m a.s.l.” McFadden, E. M., Ramage, J., and Rodbell, D. T., The Cryosphere, 5, 419-430, doi:10.5194/tc-5-419-2011, 2011. [Full text]


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