AGW Observer

Observations of anthropogenic global warming

New research from last week 35/2011

Posted by Ari Jokimäki on September 5, 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. Note to Finnish readers: research of the last week is now available also in Finnish.

Published last week:

Reduction of Walker circulation over last six decades

Regional Patterns of Tropical Indo-Pacific Climate Change: Evidence of the Walker Circulation Weakening – Tokinaga et al. (2011) “Regional patterns of tropical Indo-Pacific climate change are investigated over the last six decades based on a synthesis of in situ observations and ocean model simulations, with a focus on physical consistency among sea surface temperature (SST), cloud, sea level pressure (SLP), surface wind, and subsurface ocean temperature. A newly-developed bias-corrected surface wind dataset displays westerly trends over the western tropical Pacific and easterly trends over the tropical Indian Ocean, indicative of a slow down of the Walker circulation. This pattern of wind change is consistent with that of observed SLP change showing positive trends over the Maritime Continent and negative trends over the central equatorial Pacific. Suppressed moisture convergence over the Maritime Continent is largely due to surface wind changes, contributing to observed decreases in marine cloudiness and land precipitation there. Furthermore, observed ocean mixed-layer temperatures indicate a reduction in zonal contrast in the tropical Indo-Pacific characterized by larger warming in the tropical eastern Pacific and western Indian Ocean than in the tropical western Pacific and eastern Indian Ocean. Similar changes are successfully simulated by an ocean general circulation model forced with the bias-corrected wind stress. Whereas results from major SST reconstructions show no significant change in zonal gradient in the tropical Indo-Pacific, both bucket-sampled SSTs and nighttime marine-air temperatures (NMAT) show a weakening of the zonal gradient consistent with the subsurface temperature changes. All these findings from independent observations provide robust evidence for ocean-atmosphere coupling associated with the reduction in the Walker circulation over the last six decades.” Hiroki Tokinaga, Shang-Ping Xie, Axel Timmermann, Shayne McGregor, Tomomichi Ogata, Hisayuki Kubota and Yuko M. Okumura, Journal of Climate 2011, doi: 10.1175/JCLI-D-11-00263.1.

Effect of melt ponds and aerosols on Arctic sea ice now included to climate model CCSM4

Improved sea ice shortwave radiation physics in CCSM4: The impact of melt ponds and aerosols on Arctic sea ice – Holland et al. (2011) “The Community Climate System Model, 4 has revisions across all components. For sea ice, the most notable improvements are the incorporation of a new shortwave radiative transfer scheme and the capabilities that this enables. This scheme uses inherent optical properties to define scattering and absorption characteristics of snow, ice and included shortwave absorbers and explicitly allows for melt ponds and aerosols. The deposition and cycling of aerosols in sea ice is now included and a new parameterization derives ponded water from the surface meltwater flux. Taken together, this provides a more sophisticated, accurate, and complete treatment of sea ice radiative transfer. In preindustrial CO2 simulations, the radiative impact of ponds and aerosols on Arctic sea ice is 1.1 W/m2 annually, with aerosols accounting for up to 8 W/m2 enhanced June shortwave absorption in the Barents/Kara Seas, and with ponds accounting for over 10 W/m2 in shelf regions in July. In 2XCO2 simulations with the same aerosol deposition, ponds have a larger effect whereas aerosol effects are reduced, thereby modifying the surface albedo feedback. While the direct forcing is modest, because aerosols and ponds influence the albedo, the response is amplified. In simulations with no ponds or aerosols in sea ice, the Arctic ice is over a meter thicker and retains more summer ice cover. Diagnosis of a 20th century simulation indicates an increased radiative forcing from aerosols and melt ponds, which could play a role in 20th century Arctic sea ice reductions. In contrast, ponds and aerosol deposition have little effect on Antarctic sea ice for all climates considered.” Marika M. Holland, David A. Bailey, Bruce P. Briegleb, Bonnie Light, Elizabeth Hunke, Journal of Climate 2011, doi: 10.1175/JCLI-D-11-00078.1. [Full text]

Even with the cold spells last two winters were unnaturally warm in NH

Recent warm and cold daily winter temperature extremes in the Northern Hemisphere – Guirguis et al. (2011) “The winters of 2009–2010 and 2010–2011 brought frigid temperatures to parts of Europe, Russia, and the U.S. We analyzed regional and Northern Hemispheric (NH) daily temperature extremes for these two consecutive winters in the historical context of the past 63 years. While some parts clearly experienced very cold temperatures, the NH was not anomalously cold. Extreme warm events were much more prevalent in both magnitude and spatial extent. Importantly, the persistent negative state of the North Atlantic Oscillation (NAO) explained the bulk of the observed cold anomalies, however the warm extremes were anomalous even accounting for the NAO and also considering the states of the Pacific Decadal Oscillation (PDO) and El Niño Southern Oscillation (ENSO). These winters’ widespread and intense warm extremes together with a continuing hemispheric decline in cold snap activity was a pattern fully consistent with a continuation of the warming trend observed in recent decades.” Guirguis, K., A. Gershunov, R. Schwartz, and S. Bennett (2011), Geophys. Res. Lett., 38, L17701, doi:10.1029/2011GL048762.

Groundwater depletion is also a factor in sea level rise

Contribution of global groundwater depletion since 1900 to sea-level rise – Konikow (2011) “Removal of water from terrestrial subsurface storage is a natural consequence of groundwater withdrawals, but global depletion is not well characterized. Cumulative groundwater depletion represents a transfer of mass from land to the oceans that contributes to sea-level rise. Depletion is directly calculated using calibrated groundwater models, analytical approaches, or volumetric budget analyses for multiple aquifer systems. Estimated global groundwater depletion during 1900–2008 totals ∼4,500 km3, equivalent to a sea-level rise of 12.6 mm (>6% of the total). Furthermore, the rate of groundwater depletion has increased markedly since about 1950, with maximum rates occurring during the most recent period (2000–2008), when it averaged ∼145 km3/yr (equivalent to 0.40 mm/yr of sea-level rise, or 13% of the reported rate of 3.1 mm/yr during this recent period).” Konikow, L. F. (2011), Geophys. Res. Lett., 38, L17401, doi:10.1029/2011GL048604.

In Europe biggest change towards drier and wetter happens in south and north

The future of dry and wet spells in Europe: A comprehensive study based on the ENSEMBLES regional climate models – Heinrich & Gobiet (2011) “Under the aspect of future climate change, it is important for decision makers to know how drought conditions might change on the regional scale in order to map out adequate adaptation and mitigation strategies. Recent RCM simulations provided by the ENSEMBLES project are used to analyse changes in dry and wet conditions in Europe by the mid of the 21st century under the A1B emission scenario. Eight RCMs are selected to capture the uncertainties of the projected changes. An empirical statistical error correction approach is applied to modelled daily mean air temperature and precipitation amount to account for RCM errors, and commonly used drought indices—the Standardized Precipitation Index (SPI), the self calibrated Palmer Z-Index (scZI) and Palmer Drought Severity Index (scPDSI)—are calculated. Changes in the mean, in interannual variability, and in frequency, length, distance, magnitude, and area of dry and wet events are investigated. The statistical significance of the projected multi-model mean changes and the according uncertainties are analysed for nine European subregions. Furthermore, distributional changes of the dry and wet spell characteristics are assessed. Changes in the mean, and in dry and wet event characteristics show the most pronounced changes towards drier and wetter conditions in the southern- and northernmost European subregions, respectively. Here, the changes are highly significant and confident, while the projected changes are more dissonant for the other subregions. Severe changes in the extremes of event length, distance, magnitude, and area particularly arise in the southern- and northernmost European subregions. The projected changes in interannual variability are less significant and confident. However, significantly increasing interannual variability is projected in regions with pronounced changes in the mean towards wetter as well as towards drier conditions.” Georg Heinrich, Andreas Gobiet, International Journal of Climatology, DOI: 10.1002/joc.2421.

New climate sensitivity estimate close to previous ones

Sea surface and high-latitude temperature sensitivity to radiative forcing of climate over several glacial cycles – Rohling et al. (2011) “We compile global sea-surface temperature (SST) records that span around one glacial cycle or more. We compare these with changes in the Earth’s radiative balance over the last 520,000 years, as determined from greenhouse gas concentrations, albedo changes related to ice-sheet area and atmospheric dust fluctuations, and insolation changes. A first scenario uses global mean values for the radiative changes, and a second scenario uses zonal means for 10° latitude bands for a more regionally specific perspective. On the orbital timescales studied here, we find a smooth increase of SST response from the equator to high latitudes when comparison is made to global mean radiative forcing, but a sharply ‘stepped’ increase at 20–30° latitude when comparing with the more regionally specific forcings. The mean global SST sensitivities to radiative change are within similar limits for both scenarios, around 0.8 ±0.4 °C/Wm−2. Combined with previous estimates of 1.3 to 1.5 times stronger temperature sensitivity over land, this yields an estimate for global climate sensitivity of 0.85 (−0.4/+0.5) °C/Wm−2, close to previous estimates. Had we considered aerosol (dust) feedback as a fast feedback, then the estimated central value for SST sensitivity would change to ~0.95 °C/Wm−2 and that for global climate sensitivity to ~1.05 °C/Wm−2. Our zonal-mean scenario allows an assessment of (long-term) ‘normalised amplification’ for Greenland and Antarctic temperature sensitivities, which is the ratio of temperature sensitivity for those sites relative to the global mean sensitivity, normalised per Wm−2 of radiative change. This ratio is found to be 0.9 (−0.2/+0.6) and 1.4 (−0.4/+1.1) for Greenland and Antarctica, respectively. Given its value close to 1 for Greenland, but that we cannot exclude larger Arctic amplification on shorter timescales due to fast sea-ice albedo processes, we suggest that current high Arctic sensitivity is mainly due to sea-ice albedo feedback processes and may decrease considerably if and when the Arctic sea-ice cover has been eliminated. The normalised amplification value of 1.4 for Antarctica supports previous reconstructions of polar amplification in that region. We propose that this amplified response resulted from ~3-fold glacial-interglacial changes in the area of sea-ice cover around Antarctica.” Rohling, E. J., Medina-Elizalde, M., Shepherd, J. G., Siddall, M., Stanford, J. D., Journal of Climate 2011, doi: 10.1175/2011JCLI4078.1.

Mauna Loa temperature trends are consistent with GHG warming

Temperature trends at the Mauna Loa observatory, Hawaii – Malamud et al. (2011) “Observations at the Mauna Loa Observatory, Hawaii, established the systematic increase of anthropogenic CO2 in the atmosphere. For the same reasons that this site provides excellent globally averaged CO2 data, it may provide temperature data with global significance. Here, we examine hourly temperature records, averaged annually for 1977–2006, to determine linear trends as a function of time of day. For night-time data (22:00 to 06:00 LST (local standard time)) there is a near-uniform warming of 0.040 °C yr−1. During the day, the linear trend shows a slight cooling of −0.014 °C yr−1 at 12:00 LST (noon). Overall, at Mauna Loa Observatory, there is a mean warming trend of 0.021 °C yr−1. The dominance of night-time warming results in a relatively large annual decrease in the diurnal temperature range (DTR) of −0.050 °C yr−1 over the period 1977–2006. These trends are consistent with the observed increases in the concentrations of CO2 and its role as a greenhouse gas (demonstrated here by first-order radiative forcing calculations), and indicate the possible relevance of the Mauna Loa temperature measurements to global warming.” Malamud, B. D., Turcotte, D. L., and Grimmond, C. S. B.: Clim. Past, 7, 975-983, doi:10.5194/cp-7-975-2011, 2011. [Full text]

Using gas instead of coal doesn’t cool climate

Coal to gas: the influence of methane leakage – Wigley (2011) “Carbon dioxide (CO2) emissions from fossil fuel combustion may be reduced by using natural gas rather than coal to produce energy. Gas produces approximately half the amount of CO2 per unit of primary energy compared with coal. Here we consider a scenario where a fraction of coal usage is replaced by natural gas (i.e., methane, CH4) over a given time period, and where a percentage of the gas production is assumed to leak into the atmosphere. The additional CH4 from leakage adds to the radiative forcing of the climate system, offsetting the reduction in CO2 forcing that accompanies the transition from coal to gas. We also consider the effects of: methane leakage from coal mining; changes in radiative forcing due to changes in the emissions of sulfur dioxide and carbonaceous aerosols; and differences in the efficiency of electricity production between coal- and gas-fired power generation. On balance, these factors more than offset the reduction in warming due to reduced CO2 emissions. When gas replaces coal there is additional warming out to 2,050 with an assumed leakage rate of 0%, and out to 2,140 if the leakage rate is as high as 10%. The overall effects on global-mean temperature over the 21st century, however, are small.” Tom M. L. Wigley, Climatic Change, DOI: 10.1007/s10584-011-0217-3.

Wood harvesting is not a carbon-neutral activity

Harvesting in boreal forests and the biofuel carbon debt – Holtsmark (2011) “Owing to the extensive critique of food-crop-based biofuels, attention has turned toward second-generation wood-based biofuels. A question is therefore whether timber taken from the vast boreal forests on an increasing scale should serve as a source of wood-based biofuels and whether this will be effective climate policy. In a typical boreal forest, it takes 70–120 years before a stand of trees is mature. When this time lag and the dynamics of boreal forests more generally are taken into account, it follows that a high level of harvest means that the carbon stock in the forest stabilizes at a lower level. Therefore, wood harvesting is not a carbon-neutral activity. Through model simulations, it is estimated that an increased harvest of a boreal forest will create a biofuel carbon debt that takes 190–340 years to repay. The length of the payback time is sensitive to the type of fossil fuels that wood energy replaces.” Bjart Holtsmark, Climatic Change, DOI: 10.1007/s10584-011-0222-6.

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