AGW Observer

Observations of anthropogenic global warming

New research from last week 41/2011

Posted by Ari Jokimäki on October 17, 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.

New improved GHCN version shows same rate of warming as earlier versions

An overview of the Global Historical Climatology Network monthly mean temperature data set, version 3 – Lawrimore et al. (2011) “Since the early 1990s the Global Historical Climatology Network-Monthly (GHCN-M) data set has been an internationally recognized source of data for the study of observed variability and change in land surface temperature. It provides monthly mean temperature data for 7280 stations from 226 countries and territories, ongoing monthly updates of more than 2000 stations to support monitoring of current and evolving climate conditions, and homogeneity adjustments to remove non-climatic influences that can bias the observed temperature record. The release of version 3 monthly mean temperature data marks the first major revision to this data set in over ten years. It introduces a number of improvements and changes that include consolidating “duplicate” series, updating records from recent decades, and the use of new approaches to homogenization and quality assurance. Although the underlying structure of the data set is significantly different than version 2, conclusions regarding the rate of warming in global land surface temperature are largely unchanged.” Lawrimore, J. H., M. J. Menne, B. E. Gleason, C. N. Williams, D. B. Wuertz, R. S. Vose, and J. Rennie (2011), J. Geophys. Res., 116, D19121, doi:10.1029/2011JD016187.

Global analysis of vegetation changes shows both greening and browning

Trend changes in global greening and browning: contribution of short-term trends to longer-term change – de Jong et al. (2011) “Field observations and time series of vegetation greenness data from satellites provide evidence of changes in terrestrial vegetation activity over the past decades for several regions in the world. Changes in vegetation greenness over time may consist of an alternating sequence of greening and/or browning periods. This study examined this effect using detection of trend changes in Normalized Difference Vegetation Index (NDVI) satellite data between 1982 and 2008. Time series of 648 fortnightly images were analyzed using a trend breaks analysis (BFAST) procedure. Both abrupt and gradual changes were detected in large parts of the world, especially in (semi-arid) shrubland and grassland biomes where abrupt greening was often followed by gradual browning. Many abrupt changes were found around large-scale natural influences like the Mt Pinatubo eruption in 1991 and the strong 1997/98 El Niño event. The net global figure – considered over the full length of the time series – showed greening since the 1980s. This is in line with previous studies, but the change rates for individual short-term segments were found to be up to 5 times higher. Temporal analysis indicated that the area with browning trends increased over time while the area with greening trends decreased. The Southern Hemisphere showed the strongest evidence of browning. Here, periods of gradual browning were generally longer than periods of gradual greening. Net greening was detected in all biomes, most conspicuously in croplands and least conspicuously in needleleaf forests. For 15% of the global land area, trends were found to change between greening and browning within the analysis period. This demonstrates the importance of accounting for trend changes when analyzing long-term NDVI time series.” Rogier de Jong, Jan Verbesselt, Michael E. Schaepman, Sytze de Bruin, Global Change Biology, DOI: 10.1111/j.1365-2486.2011.02578.x.

Central Asia is facing a difficult challenge with water management

Will climate change exacerbate water stress in Central Asia? – Siegfried et al. (2011) “Millions of people in the geopolitically important region of Central Asia depend on water from snow- and glacier-melt driven international rivers, most of all the Syr Darya and Amu Darya. The riparian countries of these rivers have experienced recurring water allocation conflicts ever since the Soviet Union collapsed. Will climate change exacerbate water stress and thus conflicts? We have developed a coupled climate, land-ice and rainfall-runoff model for the Syr Darya to quantify impacts and show that climatic changes are likely to have consequences on runoff seasonality due to earlier snow-melt. This will increase water stress in unregulated catchments because less water will be available for irrigation in the summer months. Threats from geohazards, above all glacier lake outbursts, are likely to increase as well. The area at highest risk is the densely populated, agriculturally productive, and politically unstable Fergana Valley. Targeted infrastructural developments will be required in the region. If the current mismanagement of water and energy resources can be replaced with more effective resource allocation mechanisms through the strengthening of transboundary institutions, Central Asia will be able to successfully address these future climate-related challenges.” Tobias Siegfried, Thomas Bernauer, Renaud Guiennet, Scott Sellars, Andrew W. Robertson, Justin Mankin, Peter Bauer-Gottwein and Andrey Yakovlev, Climatic Change, DOI: 10.1007/s10584-011-0253-z.

Thermospheric cooling may be from ozone depletion

Is thermospheric long-term cooling due to CO2 or O3? – Walsh & Oliver (2011) “While greenhouse gases trap heat emanating from the Earth and thereby heat the surface atmosphere, they act as emitters in the high atmosphere and cool the air there. In 1989 Roble and Dickinson (1989) estimated the cooling that would occur in the thermosphere (250–500 km altitude) due to a doubling of greenhouse gas densities. Ever since, long-term data bases have been scoured for evidence of this thermospheric “global cooling.” Here we show evidence that the thermosphere did indeed cool over the period 1966–1987, but the data suggest that the cooling accelerated at a “breakpoint year” around 1979 to a rate far larger than may be attributed to greenhouse cooling. This 1979 breakpoint year appears to coincide with a breakpoint year in ozone (O3) column density. Further, the cooling was confined largely to the daytime thermosphere while the nighttime showed only a small trend. These results suggest, first, that the greenhouse cooling of the thermosphere may well not be detectable with current data sets and, second, that the long-term cooling that is clearly seen may be due largely to O3 depletion.” Walsh, P. L. and Oliver, W. L., Ann. Geophys., 29, 1779-1782, doi:10.5194/angeo-29-1779-2011, 2011. [Full text]

Greatest solar radiation is detected when sky is partly cloudy

Influence of the extent and genera of cloud cover on solar radiation intensity – Matuszko (2011) “This paper aims to describe the influence of cloud cover, its extent and cloud genus, on solar radiation intensity measured at the Earth’s surface. Solar radiation intensity values at varying degrees of cloudiness, based on observations and measurements performed between 2003 and 2007 in Krakow (Poland), are analysed in this paper. Analyses show that cloud cover impacts solar radiation intensity in two ways: usually weakening it, but intensifying in certain weather conditions. The greatest solar radiation intensity can be detected not when the sky is cloudless, but when it is partly cloudy (3/8-6/8), with convective clouds present.” Dorota Matuszko, International Journal of Climatology, DOI: 10.1002/joc.2432.

Trees not migrating when they should

Failure to migrate: lack of tree range expansion in response to climate change – Zhu et al. (2011) “Tree species are expected to track warming climate by shifting their ranges to higher latitudes or elevations, but current evidence of latitudinal range shifts for suites of species is largely indirect. In response to global warming, offspring of trees are predicted to have ranges extend beyond adults at leading edges and the opposite relationship at trailing edges. Large-scale forest inventory data provides an opportunity to compare present latitudes of seedlings and adult trees at their range limits. Using the USDA Forest Service’s Forest Inventory and Analysis (FIA) data, we directly compared seedling and tree 5th and 95th percentile latitudes for 92 species in 30 longitudinal bands for 43,334 plots across the eastern U.S. We further compared these latitudes with 20th century temperature and precipitation change and functional traits, including seed size and seed spread rate. Results suggest that 58.7% of the tree species examined show the pattern expected for a population undergoing range contraction, rather than expansion, at both northern and southern boundaries. Fewer species show a pattern consistent with a northward shift (20.7%) and fewer still with a southward shift (16.3%). Only 4.3% are consistent with expansion at both range limits. When compared with the 20th century climate changes that have occurred at the range boundaries themselves, there is no consistent evidence that population spread is greatest in areas where climate has changed most; nor are patterns related to seed size or dispersal characteristics. The fact that the majority of seedling extreme latitudes are less than those for adult trees may emphasize the lack of evidence for climate-mediated migration, and should increase concerns for the risks posed by climate change.” Kai Zhu, Christopher W. Woodall, James S. Clark, Global Change Biology, DOI: 10.1111/j.1365-2486.2011.02571.x.

Natural forest disturbances have an effect on climate

Radiative forcing of natural forest disturbances – O’Halloran et al. (2011) “Forest disturbances are major sources of carbon dioxide to the atmosphere, and therefore impact global climate. Biogeophysical attributes, such as surface albedo (reflectivity), further control the climate-regulating properties of forests. Using both tower-based and remotely-sensed data sets, we show that natural disturbances from wildfire, beetle outbreaks, and hurricane wind throw can significantly alter surface albedo, and the associated radiative forcing either offsets or enhances the CO2 forcing caused by reducing ecosystem carbon sequestration over multiple years. In the examined cases, the radiative forcing from albedo change is on the same order of magnitude as the CO2 forcing. The net radiative forcing resulting from these two factors leads to a local heating effect in a hurricane-damaged mangrove forest in the sub-tropics, and a cooling effect following wildfire and mountain pine beetle attack in boreal forests with winter snow. Although natural forest disturbances currently represent less than half of gross forest cover loss, that area will likely increase in the future under climate change, making it imperative to represent these processes accurately in global climate models.” Thomas L. O’Halloran, Beverly E. Law, Michael L. Goulden, Zhuosen Wang, Jordan G. Barr, Crystal Schaaf, Mathew Brown, José D. Fuentes, Mathias Göckede, Andrew Black, Vic Engel, Global Change Biology, DOI: 10.1111/j.1365-2486.2011.02577.x.

Arctic warming has negative effect on lichen diversity

Arctic warming on two continents has consistent negative effects on lichen diversity and mixed effects on bryophyte diversity – Lang et al. (2011) “Little is known about the impact of changing temperature regimes on composition and diversity of cryptogam communities in the Arctic and Subarctic, despite the well-known importance of lichens and bryophytes to the functioning and climate feedbacks of northern ecosystems. We investigated changes in diversity and abundance of lichens and bryophytes within long-term (9 -16 yrs) warming experiments and along natural climatic gradients, ranging from Swedish subarctic birch forest and subarctic/subalpine tundra to Alaskan arctic tussock tundra. In both Sweden and Alaska, lichen diversity responded negatively to experimental warming (with the exception of a birch forest) and to higher temperatures along climatic gradients. Bryophytes were less sensitive to experimental warming than lichens, but depending on the length of the gradient, bryophyte diversity decreased both with increasing temperatures and at extremely low temperatures. Among bryophytes, Sphagnum mosses were particularly resistant to experimental warming in terms of both abundance and diversity. Temperature, on both continents, was the main driver of species composition within experiments and along gradients, with the exception of the Swedish subarctic birch forest where amount of litter constituted the best explanatory variable. In a warming experiment in moist acidic tussock tundra in Alaska, temperature together with soil ammonium availability were the most important factors influencing species composition. Overall, dwarf shrub abundance (deciduous and evergreen) was positively related to warming but so were the bryophytes Sphagnum girgensohnii, Hylocomium splendens and Pleurozium schreberi; the majority of other cryptogams showed a negative relationship to warming. This unique combination of intercontinental comparison, natural gradient studies and experimental studies shows that cryptogam diversity and abundance, especially within lichens, is likely to decrease under arctic climate warming. Given the many ecosystem processes affected by cryptogams in high latitudes (e.g. carbon sequestration, N2-fixation, trophic interactions), these changes will have important feedback consequences for ecosystem functions and climate.” Simone I. Lang, Johannes H. C. Cornelissen, Gaius R. Shaver, Matthias Ahrens, Terry V. Callaghan, Ulf Molau, Cajo J. F. ter Braak, Adam Hölzer, Rien Aerts, Global Change Biology, DOI: 10.1111/j.1365-2486.2011.02570.x.

ENSO and AMO important factors in past droughts in Mesoamerica

Pacific and Atlantic influences on Mesoamerican climate over the past millennium – Stahle et al. (2011) “A new tree-ring reconstruction of the Palmer Drought Severity Index (PDSI) for Mesoamerica from AD 771 to 2008 identifies megadroughts more severe and sustained than any witnessed during the twentieth century. Correlation analyses indicate strong forcing of instrumental and reconstructed June PDSI over Mesoamerica from the El Niño/Southern Oscillation (ENSO). Spectral analyses of the 1,238-year reconstruction indicate significant concentrations of variance at ENSO, sub-decadal, bi-decadal, and multidecadal timescales. Instrumental and model-based analyses indicate that the Atlantic Multidecadal Oscillation is important to warm season climate variability over Mexico. Ocean-atmospheric variability in the Atlantic is not strongly correlated with the June PDSI reconstruction during the instrumental era, but may be responsible for the strong multidecadal variance detected in the reconstruction episodically over the past millennium. June drought indices in Mesoamerica are negatively correlated with gridded June PDSI over the United States from 1950 to 2005, based on both instrumental and reconstructed data. Interannual variability in this latitudinal moisture gradient is due in part to ENSO forcing, where warm events favor wet June PDSI conditions over the southern US and northern Mexico, but dryness over central and southern Mexico (Mesoamerica). Strong anti-phasing between multidecadal regimes of tree-ring reconstructed June PDSI over Mesoamerica and reconstructed summer (JJA) PDSI over the Southwest has also been detected episodically over the past millennium, including the 1950–1960s when La Niña and warm Atlantic SSTs prevailed, and the 1980–1990s when El Niño and cold Atlantic SSTs prevailed. Several Mesoamerican megadroughts are reconstructed when wetness prevailed over the Southwest, including the early tenth century Terminal Classic Drought, implicating El Niño and Atlantic SSTs in this intense and widespread drought that may have contributed to social changes in ancient Mexico.” D. W. Stahle, D. J. Burnette, J. Villanueva Diaz, R. R. Heim, F. K. Fye, J. Cerano Paredes, R. Acuna Soto and M. K. Cleaveland, Climate Dynamics, DOI: 10.1007/s00382-011-1205-z.

Recent solar low was strong driver for NH weather but not for global temperature

Solar forcing of winter climate variability in the Northern Hemisphere – Ineson et al. (2011) “An influence of solar irradiance variations on Earth’s surface climate has been repeatedly suggested, based on correlations between solar variability and meteorological variables. Specifically, weaker westerly winds have been observed in winters with a less active sun, for example at the minimum phase of the 11-year sunspot cycle. With some possible exceptions, it has proved difficult for climate models to consistently reproduce this signal. Spectral Irradiance Monitor satellite measurements indicate that variations in solar ultraviolet irradiance may be larger than previously thought. Here we drive an ocean–atmosphere climate model with ultraviolet irradiance variations based on these observations. We find that the model responds to the solar minimum with patterns in surface pressure and temperature that resemble the negative phase of the North Atlantic or Arctic Oscillation, of similar magnitude to observations. In our model, the anomalies descend through the depth of the extratropical winter atmosphere. If the updated measurements of solar ultraviolet irradiance are correct, low solar activity, as observed during recent years, drives cold winters in northern Europe and the United States, and mild winters over southern Europe and Canada, with little direct change in globally averaged temperature. Given the quasiregularity of the 11-year solar cycle, our findings may help improve decadal climate predictions for highly populated extratropical regions.” Sarah Ineson, Adam A. Scaife, Jeff R. Knight, James C. Manners, Nick J. Dunstone, Lesley J. Gray & Joanna D. Haigh, Nature Geoscience, 2011, DOI: doi:10.1038/ngeo1282.


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