AGW Observer

Observations of anthropogenic global warming

New research from last week 33/2011

Posted by Ari Jokimäki on August 22, 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:

Dune migration rate in Antarctica has increased with climate change

Is climate change affecting rates of dune migration in Antarctica? – Bristow et al. (2011) “Antarctica is the coldest, driest, and windiest continent on Earth, and contains sand dunes, like deserts elsewhere. The structure and age of the cold climate dunes found in the Victoria Valley, Antarctica, are described in the light of changing climate in the Ross Sea region of Antarctica during the late Holocene. Ground penetrating radar (GPR) was used to image sand dune stratigraphy, and optically stimulated luminescence (OSL) was used to determine when the sands were deposited. The timing of sand dune accretion identified from the GPR stratigraphy ranges from the present day to ca. 1.3 kyr B.P. The OSL ages were used to calculate end-point migration rates of 0.05–1.5 m/yr, lower than migration rates from photogrammetry and field surveys undertaken over the past 50 yr. The earliest recorded dune development, ca. 1.3 kyr B.P., was probably controlled by intensification of circumpolar westerlies at that time as well as by drier conditions and lower temperatures that promoted dune stabilization. The dune reactivation ca. 300 yr ago coincides with cooling ca. A.D. 1700–1850, and strengthening of southern circumpolar westerlies. The increase in rate of dune migration after this period and especially the past 200 yr may coincide with the modern rise in CO2 and the warmest temperatures in Antarctica during the past 800 kyr.” Charlie S. Bristow, Paul Augustinus, Ed J. Rhodes, Irene C. Wallis and Harry M. Jol, Geology, v. 39 no. 9 p. 831-834, doi: 10.1130/G32212.1. [Supplementary information]

Sea ice loss stays reversible says a model study

The reversibility of sea ice loss in a state-of-the-art climate model – Armour et al. (2011) “Rapid Arctic sea ice retreat has fueled speculation about the possibility of threshold (or ‘tipping point’) behavior and irreversible loss of the sea ice cover. We test sea ice reversibility within a state-of-the-art atmosphere–ocean global climate model by increasing atmospheric carbon dioxide until the Arctic Ocean becomes ice-free throughout the year and subsequently decreasing it until the initial ice cover returns. Evidence for irreversibility in the form of hysteresis outside the envelope of natural variability is explored for the loss of summer and winter ice in both hemispheres. We find no evidence of irreversibility or multiple ice-cover states over the full range of simulated sea ice conditions between the modern climate and that with an annually ice-free Arctic Ocean. Summer sea ice area recovers as hemispheric temperature cools along a trajectory that is indistinguishable from the trajectory of summer sea ice loss, while the recovery of winter ice area appears to be slowed due to the long response times of the ocean near the modern winter ice edge. The results are discussed in the context of previous studies that assess the plausibility of sea ice tipping points by other methods. The findings serve as evidence against the existence of threshold behavior in the summer or winter ice cover in either hemisphere.” Armour, K. C., I. Eisenman, E. Blanchard-Wrigglesworth, K. E. McCusker, and C. M. Bitz (2011), Geophys. Res. Lett., 38, L16705, doi:10.1029/2011GL048739. [Full text]

Thoroughly radiocarbon-dated 25000 year lake sediment record from East Africa

High-resolution 14C dating of a 25,000-year lake-sediment record from equatorial East Africa – Blaauw et al. (2011) “We dated a continuous, 22-m long sediment sequence from Lake Challa (Mt. Kilimanjaro area, Kenya/Tanzania) to produce a solid chronological framework for multi-proxy reconstructions of climate and environmental change in equatorial East Africa over the past 25,000 years. The age model is based on a total of 168 AMS 14C dates on bulk-organic matter, combined with a 210Pb chronology for recent sediments and corrected for a variable old-carbon age offset. This offset was estimated by i) pairing bulk-organic 14C dates with either 210Pb-derived time markers or 14C dates on grass charcoal, and ii) wiggle-matching high-density series of bulk-organic 14C dates. Variation in the old-carbon age offset through time is relatively modest, ranging from 450 yr during glacial and late glacial time to 200 yr during the early and mid-Holocene, and increasing again to 250 yr today. The screened and corrected 14C dates were calibrated sequentially, statistically constrained by their stratigraphical order. As a result their constrained calendar-age distributions are much narrower, and the calibrated dates more precise, than if each 14C date had been calibrated on its own. The smooth-spline age-depth model has 95% age uncertainty ranges of 50–230 yr during the Holocene and 250–550 yr in the glacial section of the record. The δ13C values of paired bulk-organic and grass-charcoal samples, and additional 14C dating on selected turbidite horizons, indicates that the old-carbon age offset in Lake Challa is caused by a variable contribution of old terrestrial organic matter eroded from soils, and controlled mainly by changes in vegetation cover within the crater basin.” Maarten Blaauw, Bas van Geel, Iris Kristen, Birgit Plessen, Anna Lyaruu, Daniel R. Engstrom, Johannes van der Plicht and Dirk Verschuren, Quaternary Science Reviews, doi:10.1016/j.quascirev.2011.07.014.

A review of sea level measurements and causes of change

Sea level and climate: measurements and causes of changes – Cazenave & Remy (2011) “We review present-day observations of sea level change and variability at global and regional scales, focusing on the altimetry era starting in the early 1990s. Over the past ∼18-years, the rate of global mean sea level rise has reached 3.3 ± 0.4 mm/year, nearly twice that of the previous decades, although the observed larger sea level rise rate may be influenced by decadal or longer variations in the ocean. Moreover, sea level rates are not geographically uniform; in some regions like the tropical western Pacific, rates are up to 3–4 times higher than the global mean rate. We next discuss the climate-related components of the global mean sea level rise. Over the last ∼18-years, ocean thermal expansion contributes about one third to the observed rise while total land ice (glacier melting plus ice sheet mass loss) contribute the other two third. The spatial trend patterns evidenced over the altimetry period mostly result from nonuniform steric sea level changes (effects of ocean temperature and salinity), largely caused by wind-driven ocean circulation changes. Such patterns are not stationary but oscillate through time on decadal/multidecadal time scale, in response to natural modes of the coupled ocean-atmosphere system. We close up this review by briefly discussing future (21st century) sea level rise. Current limited knowledge of the future evolution of the mass balance of the Greenland and Antarctica ice sheets leads to high uncertainty on the global mean sea level rise expected for the next 50–100 years.” Anny Cazenave, Frédérique Remy, Wiley Interdisciplinary Reviews: Climate Change, DOI: 10.1002/wcc.139.

New analysis of the recycling rate of atmospheric moisture

The recycling rate of atmospheric moisture over the past two decades (1988–2009) – Li et al. (2011) “Numerical models predict that the recycling rate of atmospheric moisture decreases with time at the global scale, in response to global warming. A recent observational study (Wentz et al 2007 Science 317 233–5) did not agree with the results from numerical models. Here, we examine the recycling rate by using the latest data sets for precipitation and water vapor, and suggest a consistent view of the global recycling rate of atmospheric moisture between numerical models and observations. Our analyses show that the recycling rate of atmospheric moisture has also decreased over the global oceans during the past two decades. In addition, we find different temporal variations of the recycling rate in different regions when exploring the spatial pattern of the recycling rate. In particular, the recycling rate has increased in the high-precipitation region around the equator (i.e., the intertropical convergence zone) and decreased in the low-precipitation region located either side of the equator over the past two decades. Further exploration suggests that the temporal variation of precipitation is stronger than that of water vapor, which results in the positive trend of the recycling rate in the high-precipitation region and the negative trend of the recycling rate in the low-precipitation region.” Liming Li et al 2011 Environ. Res. Lett. 6 034018 doi: 10.1088/1748-9326/6/3/034018. [Full text]

No clear cyclicity between Holocene cold and warm periods

Structure and origin of Holocene cold events – Wanner et al. (2011) “The present interglacial, the Holocene, spans the period of the last 11,700 years. It has sustained the growth and development of modern society. The millennial-scale decreasing solar insolation in the Northern Hemisphere summer lead to Northern Hemisphere cooling, a southern shift of the Intertropical Convergence Zone (ITCZ) and a weakening of the Northern Hemisphere summer monsoon systems. On the multidecadal to multicentury-scale, periods of more stable and warmer climate were interrupted by several cold relapses, at least in the Northern Hemisphere extra-tropical area. Based on carefully selected 10,000-year-long time series of temperature and humidity/precipitation, as well as reconstructions of glacier advances, the spatiotemporal pattern of six cold relapses during the last 10,000 years was analysed and presented in form of a Holocene Climate Atlas (HOCLAT; see http://www.oeschger.unibe.ch/research/projects/holocene_atlas/). A clear cyclicity was not found, and the spatiotemporal variability of temperature and humidity/precipitation during the six specific cold events (8200, 6300, 4700, 2700, 1550 and 550 years BP) was very high. Different dynamical processes such as meltwater flux into the North Atlantic, low solar activity, explosive volcanic eruptions, and fluctuations of the thermohaline circulation likely played a major role. In addition, internal dynamics in the North Atlantic and Pacific area (including their complex interaction) were likely involved.” Heinz Wanner, Olga Solomina, Martin Grosjean, Stefan P. Ritz and Markéta Jetel, Quaternary Science Reviews, doi:10.1016/j.quascirev.2011.07.010.

In Europe last decade was warmer than any time during MWP

A robust spatial reconstruction of April to September temperature in Europe: Comparisons between the Medieval period and the recent warming with a focus on extreme values – Guiot (2011) “This report presents a study of the variability in a recently published April to September gridded reconstruction temperatures in Europe and an analysis of their extremes. This reconstruction was based on tree rings, historical documents, pollen assemblages and ice cores. The methodology used, an original spectral analogue method, preserves long-term variations and the variability of temperature series, which guarantees the pertinence of the analysis of the extremes over a period as long as the last millennium. The analysis of the extremes was performed with the peak-over-threshold (POT) method over two warm periods (A.D. 1000-1350 and 1880-2007) and a cold period (A.D. 1350-1950). We found that (1) according to the long-term variations shown in this reconstruction, the growing season temperature during the last decade has exceeded all of those observed during the Medieval period; (2) the return period of the maximum event of the Medieval period has been reduced by at least a factor of 5; and (3) all decades before AD 1350 were warm on average but relatively heterogeneous, while the last decade was homogeneously warmer. A new result of this study concerning Europe is that this anthropogenic change is characterized by spatial homogeneity, with similar changes in both average temperatures and in the distribution of extreme events, while natural climate forcings induce warm periods with heterogeneous spatial patterns and less frequent extreme events.” Joël Guiot, Global and Planetary Change, doi:10.1016/j.gloplacha.2011.07.007.

Greenland glacier retreat now is more widespread than in early 20th century

Multi-decadal retreat of Greenland’s marine-terminating glaciers – Howat & Eddy (2011) “Many marine-terminating glaciers draining the Greenland ice sheet have retreated over the past decade, yet the extent and magnitude of retreat relative to past variability is unknown. We measure changes in front positions of 210 marine-terminating glaciers using Landsat imagery spanning nearly four decades and compare decadal-scale rates of change with earlier observations. We find that 90% of the observed glaciers retreated between 2000 and 2010, approaching 100% in the northwest, with rapid retreat observed in all sectors of the ice sheet. The current retreat is accelerating and likely began between 1992 and 2000, coincident with the onset of warming, following glacier stability and minor advance during a mid-century cooling period. While it is clear an extensive retreat occurred in the early 20th century, a period of increasing air temperatures, a comparison of our results with historical observations provides evidence that the current retreat is more widespread. The onset of rapid retreat with warming relative to the slow and lagged advance with cooling suggests an asymmetry in the response of marine fronts to external forcing.” Howat, Ian M., Eddy, Alex, Journal of Glaciology, Volume 57, Number 203, August 2011 , pp. 389-396(8). [Full text]

UHI not a major player in USA temperature trends

Climatic trends in major U.S. urban areas, 1950–2009 – Mishra & Lettenmaier (2011) “We evaluate changes in climatic indices for the 100 largest U.S. urban areas and paired surrounding non-urban areas. During the period 1950–2009, we find that there were statistically significant changes in as many as half of the urban areas in temperature-related indices, such as heating and cooling degree-days and number of warm and cool nights, almost all of which are reflective of a general warming. Similarly, statistically significant changes (mostly increases) in indices related to extreme precipitation, such as daily maximum intensities and number of days with heavy precipitation, were detected in as many of 30% of the urban areas. A paired analysis of urban and surrounding non-urban areas suggests that most temperature-related trends are attributable to regional climate change, rather than to local effects of urbanization, although the picture is more mixed for precipitation.” Mishra, V., and D. P. Lettenmaier (2011), Geophys. Res. Lett., 38, L16401, doi:10.1029/2011GL048255.

Global effect of Arctic sea ice-albedo feedback

Estimating the global radiative impact of the sea ice–albedo feedback in the Arctic – Hudson (2011) “A simple method for estimating the global radiative forcing caused by the sea ice–albedo feedback in the Arctic is presented. It is based on observations of cloud cover, sea ice concentration, and top-of-atmosphere broadband albedo. The method does not rely on any sort of climate model, making the assumptions and approximations clearly visible and understandable and allowing them to be easily changed. Results show that the globally and annually averaged radiative forcing caused by the observed loss of sea ice in the Arctic between 1979 and 2007 is approximately 0.1 W m−2; a complete removal of Arctic sea ice results in a forcing of about 0.7 W m−2, while a more realistic ice-free summer scenario (no ice for 1 month and decreased ice at all other times of the year) results in a forcing of about 0.3 W m−2, similar to present-day anthropogenic forcing caused by halocarbons. The potential for changes in cloud cover as a result of the changes in sea ice makes the evaluation of the actual forcing that may be realized quite uncertain since such changes could overwhelm the forcing caused by the sea ice loss itself, if the cloudiness increases in the summertime.” Hudson, S. R. (2011), J. Geophys. Res., 116, D16102, doi:10.1029/2011JD015804.

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