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2006-2008 Literature Review Archives - Climate System Studies

Allan, R. and B.J. Soden. Atmospheric warming and the amplification of precipitation extremes. Science Vol 321, 12 September, pp1481-1484.
Projections of future changes in rainfall extremes due to global warming may be underestimated by climate models.
In a recent study, daily precipitation from satellite observations for the period 1988-2004 was compared to output from a number of climate models. The authors used these data to analyse the response of tropical precipitation events to naturally-driven changes in surface temperature and atmospheric moisture content, represented here by variability in the El Niño Southern Oscillation - ENSO. The results show that for current climate, there's a direct link between a warmer climate -warm El Nino events - and an increase in very heavy rain events (90th percentile) in both satellite observations and model simulations. However, while the models qualitatively reproduce the observed behaviour, the rate of amplification of extreme rainfall events to atmospheric warming is found to be weaker in the models compared to observations. The study also showed that while the frequency of light rainfall events (below 30th percentile) tended to be anti-correlated with the frequency of very heavy precipitation (1-5% heaviest) in the satellite data, the reverse was found for the models. To examine the implications of the differing precipitation responses in models and observations for climate prediction, the authors used the GFDL CM2.1 fully coupled climate model forced with the A1B emission scenario for the periods 2001-2005 and 2101-2105. They found that the simulated response (future climate) shows an increased frequency of very heavy and moderate precipitation at the expense of light and heavy precipitation. However, the response of the heaviest precipitation to warming is lower than that expected from a well known thermodynamic relationship (Clausius Clapeyron equation) and when compared to present day simulations. For the authors, this underestimation of model simulated response of the heaviest precipitation also implies that model projections of future changes in extreme precipitation events in response to global warming may also be underpredicted.

Archer, D, and V. Brovkin. (2008), The millennial atmospheric lifetime of anthropogenic CO2, Climatic Change, 90: 283-297.
Model studies show that a significant portion of CO2 remains in the atmosphere for millennia. This demonstrates that climate change impacts will persist far longer than many people think.
In this paper, Archer and Brovkin set out to dispel what they claim is a widely held belief in both the public and scientific communities - that anthropogenic CO2 won't remain in the atmosphere more than about 200 years, and that climate change impacts will only persist for a few centuries. The misconception may have its root in an oversimplification of the carbon cycle in climate models where CO2 uptake by the ocean follows a single exponential decay that yields complete uptake after 100 years. Archer and Brovkin review recently published studies using long-term carbon cycle models, where CO2 exchanges between the ocean (surface, deep layer and deep sea sediment) and atmosphere are considered over thousands of years. Despite the fact that the models used in the studies reviewed differed quite substantially from one another, the model results were quite consistent. CO2 concentrations were shown to peak then fade on a time scale of a few centuries to millennia. A significant fraction of CO2 released remains in the atmosphere for a thousand years or longer, varying from 20 to 60% among the models. The models agreed that a substantial fraction of emitted CO2 remained in the atmosphere for many thousands of years. That fraction is larger when CO2 emissions are greater, because the CO2 uptake capacity of the oceans diminishes with increasing atmospheric CO2. Taking these results into account and assuming a climate sensitivity of 3°C for doubling atmospheric CO2, 90% of the equilibrium warming at the time of the CO2 peak, and neglecting other anthropogenic greenhouse gases, the authors calculate that atmospheric CO2 concentration should be kept under 490 ppm to keep the global warming below 2°C. This corresponds to a maximum allowable total emission of 700 Gigatonnes (Gt; billion tonnes) of carbon, of which 300 Gt has already been released by humankind. They also note that the warming we have experienced so far is only about 60% of the equilibrium warming expected at today's atmospheric CO2 value. Considering the longevity of CO2 in the atmosphere, impacts on components of the climate system that respond more slowly, such as ice sheets and sea level, will be stronger over time.

Arzel, O., Fichefet, T. and Goosse, H. 2006. Sea ice evolution over the 20th and 21st centuries as simulated by current AOGCMs. Ocean Modelling 12 pp 401-415 doi:10.1016/j.ocemod.2005.08.002.
Sea ice results from a new set of simulations performed by over a dozen global climate models (including the Canadian model CGCM3.1) for the IPCC Fourth Assessment Report are presented in this paper. The multi-model average sea ice extent in both March (sea ice maximum) and September (sea ice minimum) agrees reasonably well with observations in both hemispheres although there are substantial differences among models. The multi-model average trend in sea ice for the NH (1981-2000 period) is also close to the observed trend (-2.15 x 105 km2 per decade vs -2.4 x 105 km2 per decade). In the SH, most models show a decrease in sea ice extent while observations indicate a slight increase over the same period. This difference is not yet understood, and clearly models are still having some difficulty in reproducing SH sea ice behaviour. The climate change projections (based on the IPCC SRES AIB scenario) indicate that sea ice decline will continue over the 21st century and half the models show an ice-free summer Arctic by the end of the century.

Bond-Lamberty, B., S.D. Peckham, D.E. Ahl and S.T. Gower. Fire as the dominant driver of central Canadian boreal forest carbon balance. Nature Vol 450, 1 November 2007, pp89-93.
A large area of the Canadian boreal forest region is shown to have been a weak carbon source over the past 60 years. Changes in carbon balance over this period appear to be driven primarily by changes in fire disturbance.
An analysis of changes in carbon balance over the period 1948-2005 in the Boreal Ecosystem-Atmosphere Study (BOREAS) region of Canada was published recently in the journal Nature. The authors used the Biome-BGC process model which, the authors note, has already been extensively validated in earlier work. The simulation was begun under the assumption that the boreal landscape was a small C sink in 1948, an assumption that other work supports. The simulation experiment was designed to investigate the effects of climate, carbon dioxide concentrations and fire disturbance on net biome production, net primary production and vegetation dominance within the 100Mha region of study. The simulation indicated that the region lost on average 423gC per m2 (about 245Tg C for the entire region) over the 58 year period (about 2% of soil carbon) relative to what would have happened if mid-century conditions had remained unaltered. The effect of increasing atmospheric CO2 was positive while the disturbance and climate effects were negative. The carbon balance of this region was shown to be driven by changes in fire disturbance, with climate changes affecting the variability but not the mean landscape carbon balance (and with precipitation changes exerting more of an influence than temperature changes). They also showed a shift in dominance among vegetation types, with deciduous trees and mosses increasing production and conifers declining as older stands burned with increasing frequency. Overall, the authors conclude that to date, variations in the landscape carbon balance have been driven largely by increases in fire frequency and that direct ecophysiological effects of climate change are not yet noticeable.

Bony, Sandrine et al. (August 1, 2006) How Well Do We Understand and Evaluate Climate Change Feedback Processes? Journal of Climate, 19(15):3445-3482.
Bony et al. review the literature on four physical feedbacks identified as most important to the outputs of GCMs. This is done in the context of the uncertainty created by differences between GCMs with respect to the workings of these feedbacks, and focuses on literature published since the IPCC Third Assessment Report in 2001. The four feedbacks involve water vapour, cryospheric (ice and snow) albedo effects, tropospheric lapse rates and clouds. Of these, cloud feedbacks are said to show the largest range of estimates among GCMs. Combined water vapour-lapse rate feedbacks are said to be relatively stable between models, and therefore well-understood. Cryospheric feedbacks remain relatively uncertain. The authors make the generalization that recent studies have led to improvements in understandings of feedbacks and the reasons why they differ between GCMs. Furthermore, they state that further improvement will "[h]opefully…lead to progress in narrowing the range of climate sensitivity estimates in the future."

Dakos, V., M. Scheffer, E.H. van Nes, V. Brovkin, V. Petoukhov and H. Held. (2008), Slowing down as an early signal for abrupt climate change, PNAS, 105 (38), 14308-14312.
A possible early warning signal for abrupt climate change: study shows that abrupt transitions of the past were all preceded by a slowing down of climate fluctuations.
Abrupt transitions in climate are an important consideration in discussions of future climate change, as they are likely to be very disruptive. Abrupt climate changes have occurred many times in the past. For example the climate in North Africa suddenly shifted some 5000 years ago from a savanna-like state to a desert. While scientists are still unable to predict when abrupt changes could occur in the future, there is a theoretical basis for developing predictive capacity. In this study, the authors analysed the time series around eight ancient abrupt climate shifts reconstructed from geological records, to explore the theory that slowing down is a universal property of dynamical systems undergoing gradual changes and approaching a tipping point. Their results show that, indeed, all the abrupt climate shifts were preceded by a slowing down of climate fluctuations that started well before the actual shift. Since they were interested in seeing if such information could be used as an early warning signal for future climate changes, they used only the data from before the actual shift to look for slowing down. They found that while there were differences in the degree to which systems slowed down prior to a transition, since a slowdown was present in all cases, it was very unlikely a random behaviour. In a further step, the authors used their method to analyze simulation results from climate models that were slowly driven across known thresholds for three past abrupt climate transitions: transition to an icehouse Earth, collapse of the thermohaline circulation and desertification of North Africa. In all cases, they found an increase in the slowing down of the system, comparable to that found in the geological record. The authors offer these results as an independent line of evidence, separate from model-based approaches, which support the existence of tipping points in the climate system. They also suggest their results offer a way to predict the approach of a tipping point, but they emphasize that a slowing down of the system will occur only if the system is moving gradually toward a threshold. Systems experiencing sudden large disturbances would not exhibit this behaviour. The results could be used in principle, the authors state, to develop early warning systems for a variety of complex systems for which critical tipping points are suspected to exist.

Delworth, Thomas L. and Keith W. Dixon (2006) Have anthropogenic aerosols delayed a greenhouse gas-induced weakening of the North-Atlantic thermohaline circulation? GRL vol.33, L02606,doi:10.1029/2005GL024980,2006.
With the use of an ensemble of simulations using GFDL's newly developed coupled ocean-atmosphere climate model CM2.1, the authors explore the impact of various climate change forcing agents on the evolution of the thermohaline circulation (THC) during the 20th and 21st centuries. They conduct ensemble simulations with several subsets of forcing agents which included experiments that isolated the effects of greenhouse gas and aerosol emissions, to see what is the most important in the weakening of the THC. They found that there was no statistically significant decrease in THC, over the period 1860 to 2000. This is the result of the compensation between GHG forcing, which produces a decrease in THC when acting in isolation, and anthropogenic aerosol (sulfate + black and organic carbon) forcing which tends to enhance the THC. Thus, aerosols in the 20th century have delayed a GHG induced weakening of the THC by several decades (40 years with their model), but this could be a temporary situation. The authors also found a significant decrease of the THC several decades into the 21st century, but no evidence of a complete shut down. At that point, the effects of increasing GHG dominate aerosol effects.

Elsner, J.B., and T.H. Jagger. 2008. United States and Caribbean tropical cyclone activity related to the solar cycle. GRL Vol.35, LI8705, doi:10,1029/2008GL034431.
Although tropical cyclone activity may seem to be getting stronger due to rising surface temperatures, the variation in the ultraviolet radiation entering our atmosphere due to the natural cycle of sunspots may be influencing their intensity and frequency.
On average, Atlantic tropical cyclones, from tropical storms to hurricanes, seem to be getting stronger due to an increase in the surface temperatures of the oceans in the Caribbean region. At the same time, the potential intensity of tropical cyclones is also inversely related to the temperatures at the top of the convective clouds in the lower stratosphere. A cooler lower stratosphere /upper troposphere increases tropical storm potential intensity while a warmer stratosphere /upper troposphere does the opposite because warmer temperatures cap the convective activity. The authors in this study investigated the relationship between the solar cycle and the intensity of tropical cyclones over the Caribbean and the Gulf of Mexico with a seasonal model for basin-wide tropical cyclones. They found that during the height of the solar cycle, the lower stratosphere warms due to the absorption of more ultraviolet radiation in the making of ozone. This stratospheric warming would limit conditions for the development and intensification of tropical cyclones. In the case of hurricanes specifically, they identified a relationship with the solar activity that explains a significant portion of the interannual variability in hurricane frequency along the US coast after accounting for oceanic heat, shear, and steering. This result is important to understanding the natural variability of tropical cyclone activity in concert with global warming.

Gameda, S., Qian, B., Campbell, C.A. and Desjardins, R.L. 2007. Climatic trends associated with summer fallow in the Canadian Prairies. Agricultural and Forest Meteorology 141:170-185.
Agriculture Canada study indicates that changing land use practices in western Canada have directly affected local climates.
In the early 20th century, western Canadian farmers began to keep a significant portion of their cultivated lands under summer fallow (that is, without a crop cover) in order to conserve soil moisture for improved drought resistance in future production years. However, between 1975 and 2001, improved land management practices have reduced the total acreage of land under fallow by more than 50%. Regional climate studies for the same period indicate that daily maximum temperatures, the daily range in temperatures and the amount of incoming solar radiation have all decreased during the mid-June to July period. This is the seasonal period when crops undergo rapid foliage expansion and substantial transpiration. Authors suggest that the observed climate trends, which are contrary to those expected for warmer climates, are likely linked to the large scale conversion of fallowed lands to productive crop fields, which would affect both local albedo and evapotranspiration processes.

Hale, R.C., K.P. Gallo, and T.R. Loveland. 2008. Influences of specific land use/land cover conversions on climatological normals of near-surface temperatures. J. of Geophys. Res. 113, D14113, doi:10.1029/2007JD009548.
Land-use and land cover changes are shown to explain half the change in minimum temperatures, but had little effect on maximum temperatures at a subset of U.S. stations for which information on proximate changes in land use/land cover was available. The rest of the changes in temperature trends are associated with other climatological factors.
Changes in near surface temperatures over time can have more then one cause, one of which has been speculated to be changes in the characteristics of the land use/land cover (LULC). Changing LULC, for example from crop use to urban use, can have an affect on surrounding temperatures, as there would be associated changes in how solar radiation is captured or reflected at the surface. An American study, focused on areas of the US that have been analyzed in the Land Cover Trends Project, tried to quantify the amount of change in temperature trends that is due to changes in LULC. They chose a subset of stations from the NCEP-NCAR 50-year Reanalysis dataset that were in close proximity to areas with changes in LULC. The authors calculated the trends in temperature associated with 13 different types of LULC changes. The results show the majority of the changes in maximum temperatures trends were not due to LULC changes. On the other hand, about half of the changes in minimum temperatures were attributable to LULC changes. Interestingly, clear cutting of trees did not seem to have a significant impact on temperatures trends in the areas in close proximity, as one might expect. This study supports the conclusion that the majority of temperature trends at the stations are explained by other climatological factors, including warming due to GHGs.

Hegerl, G.C., T.J. Crowley, W.T. He, and D.J. Frame, 2006. Climate sensitivity constrained by temperature reconstructions over the past seven centuries. Nature, 440, 1029-1032.
Hegerl and colleagues used multiple lines of evidence to assign some probabilities to the range of climate sensitivity values (i.e. the equilibrium mean temperature change in response to CO2 doubling from pre-industrial levels). They demonstrate that observational estimates of climate sensitivity can be tightened if reconstructions of Northern Hemisphere temperature over the past several centuries are considered. The authors found that the 5-95% probability range for climate sensitivity is 1.5° to 6.2°C. This compares with the IPCC TAR range of 1.5-4.5°C. The authors also report that there is a less than 3% chance of the temperature increasing more than 7°C in response to CO2 doubling. These results have not adjusted the likely minimum climate sensitivity but have helped to further elucidate a likely upper limit.

Joughin, I., S.B. Das, M.A. King, B.E. Smith, I.M. Howat and T. Moon, 2008. Seasonal Speedup Along the Western Flank of the Greenland Ice Sheet. Science Express, 17 April 2009. Science. 1153288; S.B. Das, I. Joughin, M.D. Behn, I.M. Howat, M.A. King, D. Lizarralde and M.P. Bhatia, 2008. Fracture Propagation to the Base of the Greenland Ice Sheet During Supraglacial Lake Drainage. ScienceExpress, 17 April 2008, Science. 1153360.
Direct observations confirm that meltwater from the surface of Greenland's ice sheet does indeed reach the bottom of the ice sheet, lubricating and speeding up the flow of ice inland. However, this process is shown to have a smaller effect than was feared.
The relatively recent discovery that meltwater on the Greenland ice sheet was percolating down through the ice sheet, lubricating its base and accelerating ice flow in the vicinity, worried scientists. With further global warming, they speculated that increased melting might have a catastrophic effect on the ice sheet; lubricating its base sufficiently and over a large enough area to cause rapid ice flow and ice discharge into the ocean. To better determine the influence of seasonal surface melting on ice sheet flow, scientists launched a two-pronged study on the western margin of the Greenland Ice Sheet. In one study, the investigators focused on the process of how meltwater lakes drained through to the ice sheet bed. In the second study, the investigators used satellite-borne radar observations and GPS to evaluate the impact of the seasonal melt on ice sheet flow. Monitoring two large meltwater lakes on the ice sheet's western margin, over the period July 2006 to July 2007, the scientists were able to observe the process of lake drainage. In one such event, a 5.6 km2 lake, with a volume of water of 0.044 ± 0.01 km3, drained completely within 2 hours, through a water-driven fracture in the ice sheet. The meltwater reached the bottom (980 meters lower), then dispersed subglacially. However, measurements with a GPS showed that the horizontal displacement of the ice sheet in response to the lake drainage was only a small extra distance of 0.5m compared to the daily average displacement of 0.25m. To get a broader view of summer ice movement, a comprehensive set of observations from satellite-borne radar, covering the period September 2004 to August 2007, were used to track ice-sheet movement along the western flank of the Greenland ice sheet and for fast-moving outlet-glaciers. For the ice-sheet, the results are consistent but somewhat larger than earlier observations, with the warmer summers of 2006 and 2007 showing ice flow speedups of 50-100% compared to annual mean speeds. However, the relative speedup of the fast-flowing outlet glaciers was far smaller (< 15%). These results indicate that flowing outlet glaciers are relatively insensitive to surface melt-enhanced basal lubrication and that other effects are much more important in causing accelerated flow (e.g. reduced back-stress from calving front retreat). In view of their results, the authors conclude that surface-melt induced speedup may influence large regions of the ice sheet in a warming climate, but that given the relative insensitivity of the outlet glaciers to this process, the consequences for ice sheet mass balance are less catastrophic than feared.

Marinov, I., A. Gnanadesikan, J.R. Toggweiler and J.L. Sarmiento. 2006. The Southern Ocean biogeochemical divide. Nature, 22 June 2006, vol. 441.
Different studies have shown that the Southern Ocean plays a crucial role in the uptake and storage of anthropogenic CO2 and in controlling global biological production. This theoretical research tries to elucidate the complex mechanisms governing Southern Ocean carbon sequestration. To do this, they deplete surface nutrients over nine Southern Ocean regions in the Princeton GCM: surface nutrients depletion is done by increasing nutrient uptake and converting nutrients to export biological production. This reduces atmospheric carbon dioxide partial pressure (CO2) in surface water, driving CO2 from the atmosphere into the ocean. Their results show that nutrient depletion (thus carbon sequestration) is more efficient in the southernmost regions than in the northern regions, suggesting the existence of a "biogeochemical divide" in the Southern Ocean. This divide corresponds with the presence, at the Antarctic polar front - APF (30ºS) - of a physical separation between two circulations at the ocean surface, which have water from different origins: deep water formation south of the APF and intermediate water formation north of it. For Marinov et al., mechanisms that attempt to explain the lower atmospheric CO2 during the Last Glacial Maximum should take into account differences in carbon sequestration efficiency between the Antarctic and Subantarctic.

Piao, S., Friedlinstein, P., Ciais, P. et al. 2006. Effect of climate and CO2 changes on the greening of the Northern Hemisphere over the past two decades. GRL 33, L23402, doi:10.1029/2006GL028205, 2006.
Recent increases in Northern Hemisphere biomass productivity attributed to rising CO2 concentrations and changing climates.
Past analyses of satellite data indicate that leaf area index (LAI), and hence biomass productivity of vegetation, has increased significantly over much of the Northern hemispheric land areas over the past two decades. In a new study published in Geophysical Research Letters, a team of French and American researchers use a dynamic global vegetation model to explore why. They conclude that almost half of the increased growth can be attributed to the fertilization effects of higher CO2 concentrations. In the temperate regions of North America, much of the remainder appears to be linked to increased precipitation, although this effect is highly variable from region to region. Most of the boreal forest, on the other hand, has responded strongly to warmer temperatures. When the model was used to project vegetation responses to continued increases in CO2 and future warming scenarios, the results indicate that the enhanced growth weakens significantly and even disappears in some regions. These results are broadly consistent with other studies that suggest the global terrestrial uptake of atmosphere CO2, while currently large, may saturate and even reverse as the world warms. The authors caution that these results are simplistic, since they have not considered other factors, such as nitrogen fertilization or damage from surface ozone exposure, and they have not used the high resolution climate data needed to fully explore the complex regional patterns of response.

Ping, C-L, G.J. Michaelson, M.T. Jorgenson et al., 2008. High stocks of soil organic carbon in the North American Arctic region. Nature Geoscience, 24 August 2008; doi:10.1038/ngeo284.
Two recent papers both come to similar conclusions: that the size of the carbon pool in Arctic soils has been underestimated. The first paper focuses on the North American Arctic region, whereas the second paper takes a circumpolar perspective. In both cases, it is accounting for carbon stored deeper in the permafrost that results in higher estimates than in previous studies.
In recent decades, permafrost in the Northern Hemisphere has started to thaw and global climate model projections indicate that degradation of permafrost will continue and may accelerate during the 21st century. Of concern is the potential for increased emissions of carbon from thawing permafrost that could lead to a positive climate forcing (warming feedback). In the Arctic, studies have been done to assess the soil organic carbon (SOC) pool, but these have mainly looked at a few sites and have been limited to the top layer (0-40 cm) of the soil). A new study published recently gives a more detailed picture of the soil carbon content in the treeless zone of the North American Arctic. The authors looked at the soil carbon content from 139 locations across a range of landscapes types. Furthermore, they analysed soil samples to depths of one metre or more, thereby extending down through the active layer into the permafrost. The results show that Arctic soils hold more carbon than previously estimated, with mean values of 34.8 kg of SOC per square meter, compared to earlier values between 20 and 29 kg of SOC per square meter). When extrapolated to the whole of the North American Arctic, the authors estimate the soil organic carbon pool to be 98.2 Gigatons (Gt).. For the Canadian Arctic only, the estimation is 76 GT. Previously, SOC in the Canadian Arctic had been estimated at only 43 Gt. According to the authors the size and mix of landscapes in northern Europe are about the same as North America and probably contain a comparable amount of carbon-dioxide producing matter. Considering the importance of thawing permafrost to the release of carbon dioxide to the atmosphere, the results of this study provide reason to better delineate the SOC stocks of the entire circumarctic region.

Reichler, T., and J. Kim. 2008. How do coupled models simulate today's climate? BAMS DOI:10.1175/BAMS-89-3-303, March 2008, pp 303-311.
Coupled climate models continue to improve with time, better representing present climate.
Not surprising to most, but reassuring to all, climate models are getting better, at least according to two American researchers. The authors created a multivariable index to evaluate just how well coupled climate models represent certain aspects of the observed climate system. This index is applied to the output from three coupled global climate model intercomparison projects (CMIP-1, CMIP-2, CMIP-3, amounting to a comparison of 57 models) to examine how the models have improved over time. The index is calculated by comparing fourteen observed variables with the respective simulated variables for the present climate (the period used to represent the present climate is 1979-99, which covers the period from good satellite coverage through the period covered in the CMIP-1 study). Some of the variables examined are: air temperature; precipitation; sea level pressure; and snow fraction. The variables are normalized for each grid point, averaged globally, and then the index is derived from the global means of all the climate variables. The results show that the first generation of coupled models generally under performed, in other words, the models tended to under estimate the present climate conditions. There was also a large spread of the index results for the CMIP-1 models, indicating little agreement among the different model results. The most recent models shows a much more even balance of under and over performing, showing there is less bias in the model results, with a much smaller spread in the index results, indicating much greater agreement from model to model. The authors attribute the improvements to better representation of the physical environment in the models, as well as greater computer power, allowing for finer resolution. Also of note, most of the models in the first CMIP study, used some flux correction, where as most present day models do not need to include this adjustment, reducing the overall model bias. Although this study shows that there have been quantifiable improvements in global climate models, adding to the confidence in model results, further development of the models are still required to improve the usefulness of the results.

Robock, Alan and Li, Haibin. 2006. Solar dimming and CO2 effects on soil moisture trends. Geophysical Research Letters 33, L20708, doi: 10.1029/2006GL027585.
Summer soil moisture increased significantly from 1958 to the mid 1990s in Ukraine and Russia, a trend that cannot be explained by changes in precipitation and temperature alone. The authors investigate the potential contribution of solar dimming and increasing carbon dioxide to this trend using a state-of-the-art land surface model (a modified version of the Community Land Model 3.0). When a slow dimming was applied to the model experiment, using a 0.5% per year decrease in solar insolation between 1961 and 1980 (and 1960 level concentrations of atmospheric CO2), results projected a 5% reduction in evapotranspiration for the Ukraine, and 9% reduction for Russia. This increased to 16% and 20%, respectively, when the rate of dimming was increased to -1% per year. Repeating these experiments using rising CO2 concentrations as observed since 1960 produced results that only slightly reduced evapotranspiration relative to that for constant CO2 levels. Comparison with observed changes in soil moisture suggest that the best fit occurs for experiments with enhanced solar dimming. While the CO2 fertilization effects seems to have been very small for this region, authors note that it could be much more significant in regions where evapotranspiration is composed of primarily transpiration (e.g., Amazon rainforest).

Santer, B.D., Wigley, T.L., Gleckler, P.J. et al. 2006. Forced and unforced ocean temperature changes in Atlantic and Pacific tropical cyclogenesis regions. PNAS 103 (38): 13905-13910.
More than a dozen authors combined efforts in a recent PNAS article aimed at identifying the potential causes of SST changes, believed to be the main driver for recent increases in hurricane intensity. Researchers used climate models to determine the relative contribution of anthropogenic and natural factors to changes in SSTs in the Atlantic and Pacific tropical cyclogenesis regions (ACR and PCR) over the last 20-100 years. Results were compared to 2 observed SST datasets from NOAA and the Hadley Centre. The null hypothesis that SST trends could be explained by natural internal variability alone was rejected in 29 of 32 cases studied. Subsequent analysis revealed an 84% chance that at least 67% of the observed SST increases in the ACR and PCR were a result of external forcing. Finally, a single-forcing experiment identified increases in mixed greenhouse gases as the main driver for these ACR and PCR SST increases over the course of the last century.

Schuur, E.A.G., et al. Vulnerability of Permafrost Carbon to Climate Change: Implications for the Global Carbon Cycle. BioScience, September 2008 / Vol. 58 No. 8.
An overview article based on collaborative work progressing under a U.S. led International Polar Year project discusses the responses of permafrost and ecosystem processes to thawing of previously frozen organic carbon. The authors maintain that the potential for increased microbial decomposition of the global permafrost carbon pool constitutes a significant terrestrial feedback to the atmosphere in a changing climate. By including soils to depths of 3 metres or more below the surface, Schuur et al estimate the total soil carbon pool in the northern circumpolar permafrost zone to be at least 1672 Gigatonnes and comprised of mineral soil, peatlands, deep sediment, and alluvial accumulations. This estimate is more than twice the size of previous estimates. With regard to the fate of this carbon from thawing permafrost, evidence from the release of carbon from microbial activity over six wetland types shows climate forcing due to carbon dioxide rather than methane release on a century time-scale. By 2100 losses of carbon to the atmosphere are similar in size to those projected from tropical land-use change. Researchers have projected that the tree line will move north and this increased biomass could potentially offset carbon losses from permafrost. The authors calculated the projected offsets in new boreal biomass in Alaska and found that they would be only one tenth of the projected carbon released from thawing permafrost (net loss of 35 kg carbon/m2). Ecosystem responses to permafrost thawing and loss are difficult to model. In general global circulation modeling is only beginning to include permafrost dynamics while coupling physical permafrost dynamics to hydrology and biogeochemistry remains undeveloped.

Shukla, J., T. DelSole, M. Fennessy, J. Kinter and D. Paolino. 2006. Climate model fidelity and projections of climate change. GRL vol. 33, L07702, 4 pages.
This study measures 13 coupled global climate models' fidelity in simulating the present climate (surface air temperature during the past 100 years). Here, the fidelity of a model is measured in terms of 'relative entropy', which is a mathematical measure of the closeness of two probability distributions (in this case, simulated and observed climate). This measure was calculated for seasonal and annual cycles. In a second step, the study also computes the sensitivity to changing greenhouse gases for the 13 models, by taking the difference between two emission scenarios: A1B for 2XCO2 (relative to current concentrations) and an "observed" time series of CO2 for the past 100 years. Looking at the results for the two parts of the study, the authors find that models that have higher fidelity in simulating the present climate are more sensitive and produce higher estimates of global warming for a doubling of CO2 than models with lower fidelity. The authors conclude from these results that actual warming over the next century is likely to be closer to the high end of the projected range from the current generation of GCMs.

Sitch, S., P.M. Cox, W.J. Collins and C. Huntingford. 2007. Indirect radiative forcing of climate change through ozone effects on the land-carbon sink. Nature. Published on line, 25 July, 2007, 5 pages.
A recent paper finds that tropospheric (near surface level) ozone pollution reduces the ability of plants to absorb atmospheric carbon dioxide, increasing the importance of changes in atmospheric chemistry as a driver of 21st century climate change.
A new study looked at how rising tropospheric ozone (O3) pollution could affect the ability of plants to absorb CO2 from the atmosphere over the coming century. The study used, for the first time, a global land carbon cycle model modified to include the effect of ozone deposition on photosynthesis and to account for interactions between tropospheric ozone (O3) and carbon dioxide (CO2). Growing plants are vital carbon sinks and it is estimated that they currently sequester around 25% of the CO2 emitted in the atmosphere. Many studies have demonstrated that increased concentrations of CO2 boost plant growth and productivity. On the other hand, studies have found that high ozone levels (≥ 40 parts per billion) damage plants, thus reducing CO2 fixation. Using the IPCC SRES A2 scenario (a "Business as Usual" scenario), the authors find that ozone concentrations by 2100 are projected to be above 40 ppb over almost all regions, and to exceed 70 ppb in many. Depending on plant sensitivities to ozone, this could lead to a reduction of between 17% and 31% in the projected land uptake of CO2 that would have occurred in response to CO2 increases alone. Suppression of this carbon sink results in additional CO2 emissions accumulating in the atmosphere and can therefore be considered as an indirect radiative forcing of climate change by O3. As such, these results suggest that ozone effects on vegetation could double the effective radiative forcing due to increases in O3, significantly increasing the importance of changes in atmospheric chemistry as a driver of 21st century climate change.

Sjoukje, P. and G.J. van Oldenborgh. 2006. Shifts in ENSO coupling processes under global warming. GRL Vol. 33, L11704.
Most models that describe ENSO reasonably well in the current climate show only small changes in its characteristics (period, pattern and amplitude) under a doubled CO2 climate, despite large changes in the mean states of the Pacific Ocean. In this paper, the authors try to explain why the changes in ENSO are so small, by investigating the different couplings and feedback loops between the ENSO characteristics and the mean states of sea surface temperature (SST), zonal wind stress, thermocline depth and mixed layer depth (MLD) of the Pacific ocean. Using six AR4-GCMs shown to have the most realistic description of the mechanisms of ENSO, they simulate the current climate and a warmer climate in the period 2200-2300. They find that the most important mechanisms that affect El-Niño are the SST response to thermocline and wind variabilities, but also to damping (cloud feedback), and the wind response to SST perturbations. The most important result of the study is that feedback loops between SST, wind stress and thermocline do show changes, in the same direction, in a warmer climate, following changes in the mean state. On the other hand, the higher mean SST provides higher damping through cloud feedback. All these changes do have large impacts on the ENSO characteristics, but because they have opposing signs, the residual change is almost zero.

Slott, J.M., A.B. Murray, A.D. Ashton and T.J. Crowley. 2006. Coastline responses to changing storm patterns. GRL vol 33, L18404, doi:10.1029/2006GL027445, 2006.
Most research on the coastal impacts of climate change looks at how sea level rise and potentially intensified storms may affect shorelines. Many studies assume that shorelines will retreat in a roughly alongshore-uniform manner in response to climate change. However, a new study in Geophysical Research Letters evaluates the heterogeneous nature of shoreline retreat related to changing storm patterns. Slott et al. use a numerical model to explore the effects of changes in wave climate (the amounts of wave energy approaching a shore from different directions) using Cape Hatteras as an illustrative case study. The authors conclude that their initial results suggest that coastal management strategies should not be based on the common assumption that climate change impacts will be alongshore uniform and that the cumulative effects of changing storm patterns could be as important as the impact of sea level rise.

Solomon, A. (2006). Impact of latent heat release on polar climate, Geophys. Res. Lett., 33, L07716, doi:10.1029/2005GL025607.
Global climate models consistently project amplified warming in polar regions in response to increases in GHGs. This effect has been primarily attributed to ice-albedo feedbacks. In this study, the authors assess to what extent the projected warming of the Polar Regions is due to changes in dynamical heat transport, which is forced by latent heat release through increased moisture availability. Outputs from 10 coupled climate models used in the upcoming IPCC Fourth Assessment indicate a systematic relationship between increased precipitation over extra-tropical oceans and warming in the polar regions. Further, AGCM experiments demonstrate that in a warmer climate, increased moisture availability will cause an increase in poleward heat transport by transient eddies over oceans by 30%. These changes are significantly larger than those observed in an ENSO event. The resultant impact due this mechanism is a warming of the poles by approximately 2°C in both hemispheres.

Stott, P.A., J.F.B. Mitchell, M.R. Allen, T.L. Delworth, J.M. Gregory, G.A. Meehl and B.D. Santer (2006), Observational constraints on past attributable warming and predictions of future global warming. J. of Climate, vol.19, pp.3055-3069.
Using three coupled global climate models (CGCM) with different sensitivities run with a range of natural and anthropogenic forcings, the authors investigate the impact of aerosol (sulphate) forcing uncertainty on the robustness of estimates of the 20th century warming attributable to anthropogenic greenhouse gas (GHG) emissions. Applying an optimal detection analysis, they also investigate whether there is sufficient information in observations of past near-surface temperature change to constrain predictions, whichever model is used, and, they investigate which indices of large scale temperature are responsible for discriminating the climate response between models.
Their results show that all three models have good simulations of global mean temperature changes during the 20th century, when they include both anthropogenic and natural forcings. Solar and volcanic forcings make a larger contribution relative to the total forcing in the early 20th century warming, whereas anthropogenic forcing is largely responsible for the warming observed in the last three decades. They note that the models differ much more in the projections of future warming rates than in their simulations of past temperature change. They find that the observationally constrained predictions for the three models are in much better agreement than raw model predictions and much less model dependent. The features that constrain the likely temperature response to anthropogenic and natural forcings are the temporal and spatial structure of the observed global mean temperature changes over the 20th century. Distinctive temporal structures in differential warming rates between the hemispheres, between land and ocean and between mid and low latitudes help to discriminate between models and determine the relative roles of greenhouse warming and sulfate cooling.

Szeto, K.K. 2008. On the extreme variability and change of cold-season temperatures in northwest Canada. J. Climate Vol 21, Issue 2, pp 94-113.
Mackenzie Basin's larger than average winter warming is due to both topography and changes in Pacific circulation.
If one were to have a quick look at the linear temperature trends across Canada during the winter months, the Mackenzie River Basin (MRB) would stand out, with a trend of about 4.7°C over the last 60 years, versus the national average of 2.3°C. In an effort to understand this large increase in cold season temperatures over the MRB, an Environment Canada scientist assessed the region's atmospheric heat budget using the NCEP-NCAR reanalysis dataset. The author concluded that the dominant reason for the temperature increase was the topographic characteristics of the basin. As warm moist air systems are pushed in from the north Pacific, they are forced over the Western Cordillera, condensing the moisture into precipitation, releasing latent heat. This latent heat is added to the warm dry air mass upon descent on the lee of the mountain, amplifying the initial warmth. There was a well documented change in the circulation over the north Pacific in the mid 1970s, which increased the amount of heat coming off the Pacific and increased the amount of heat transported into the MRB region. This, according to the author, is why the region has experienced a greater warming trend than other regions of Canada. This amplification, and change in Pacific circulation does not explain all the warming, nor can climate change be excluded as a reason for the change in the conditions over the north Pacific. This study is a reminder that regional temperature trends will be strongly affected both by changes in circulation and climate forcings operating on regional and local scales, as well as global scales.

Torn, M.S. and J. Harte. 2006. Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming. GRL VOl 33, L10703, doi:10.1029/2005GL025540.
This paper, which examines the strength of the carbon cycle feedback on climate, is the second such paper recently published in GRL. (The other paper, by Scheffer et al., was reviewed in ASAI's review of the science last week.) Torn and Harte estimate the magnitude of the feedback from CO2 and CH4 using empirical ice-core information from the Vostok ice core and climate sensitivity estimated from GCMs. When this 'ecosystem' feedback is added to that of other known feedbacks in the climate system (e.g. the water vapour feedback, among other things), they find that the warming of 1.5-4.5 C associated with a doubling of CO2 is amplified to 1.6 - 6.0 C. The feedback is clearly shown to be asymmetric with the expected temperatures skewed toward higher temperatures. The authors also note that while there are uncertainties in the feedbacks, the consequences of the uncertainty are tilted toward more warming rather than less.

Walter, K.M., Zimov, S.A., Chanton, J.P. et al. 2006. Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming. Nature 443:71-75.
While researchers have long known that shallow lakes can be a significant natural source of methane emissions, accurate measurements, particularly from lakes in northern environments, has been hampered by the presence of ice cover during much of the year and the fact that much of the emissions occur are abrupt releases of large bubbles. In this new study, Walter et al. report on the development of a new technique for measuring emissions from several Siberian thaw lakes that address these challenges. The results show that 95% of the methane emissions from these lakes occur through bubbling processes, and that emission levels from Arctic thaw lakes may be some five times that from previous studies. Furthermore, much of the released methane comes from thawing permafrost at the margins of these lakes. Carbon dating of the methane confirms that it originates from the decay of 40,000 year old carbon stored in the soils buried in the decaying permafrost along lake margins, not from much younger lake sediments. While their new estimates for current release of methane from all Siberian thaw lakes remain small relative to global annual release (about 1%), the authors suggest that future methane releases from decaying Arctic permafrost may create a new significant positive climate feedback that has not been considered by climate modellers.

Wolf, J., and D.K. Woolf. 2006. Waves and Climate Change in the north-east Atlantic. Geophys. Res. Lett., 33, L06604, doi:10.1029/2005/GL025133.
Over the past quarter century, wave heights in the North Atlantic have been observed to be increasing. The increase could be related to a change in the North Atlantic Oscillation (NAO) as well as changes in other potential mechanisms, all of which could be influenced by climate change. The authors of this study used the North East Atlantic wave model to investigate the effects of idealized storm conditions on wave height. In addition, they looked at the impact that changes in the intensity and frequency of storms and the strength of the background westerly wind field would have on wave heights. They found that the strength of the westerly winds was most effective in increasing the mean and maximum monthly wave heights. Thus, they suggest that the recent observed increase in wave heights was likely caused by an intensification of the background westerly atmospheric circulation under a positive NAO influence rather than changes in storminess.

Wyser, K, C.G. Jones, P. Du et al. 2008. An evaluation of Arctic cloud and radiation processes during the SHEBA year: simulation results from eight Arctic regional climate models. Clim. Dyn., 30: 203-223.
A new international study confirms that regional climate models (RCMs), including the Canadian model (CRCM), need improvements in the representation of clouds and surface albedo before they can realistically simulate sea-ice evolution in the Arctic.
Between September 1997 and October 1998, a major observation campaign on the Surface Heat Budget of the Arctic Ocean (SHEBA) took place in the western Arctic. The large amounts of observations taken during SHEBA offered the opportunity, within the Arctic Regional Climate Model Intercomparison Project (ARCMIP) to evaluate regional climate model (RCM) simulations over the Arctic and to use the observed data to identify and improve deficiencies in the RCM parameterizations. In this recently published study, the authors present the results of ARCMIP simulations for cloud and radiation variables. Eight atmospheric RCMs were run during the study, including the Canadian model (CRCM) developed at l'Université du Québec à Montréal. All RCMs used the same initial and boundary conditions and every simulated variable from each model is evaluated against the SHEBA observation data. The results show that despite relatively good agreement between monthly and daily observed and simulated surface radiation in most models, there is much less agreement when it comes to the atmospheric factors that control radiative transfer - clouds and surface albedo. The most striking difference is found for the cloud cover where even for monthly averages, many models do not reproduce the annual cycle correctly. The inter-model spread is very large and no model appears superior to the others. These results are important as cloud and radiation processes are the main controls on sea-ice evolution in the Arctic. Thus, improvements in the representation of cloud and surface albedo in RCMs are needed to improve projections of Arctic sea ice.

Zhang, X., A. Sorteberg, J. Zhang, and R. Gerdes. 2008. Recent radical shifts of atmospheric circulations and rapid changes in Arctic climate system. GRL, Vol. 35, L22701, doi:10,1029/2008GL035607.
Understanding changes in the Arctic atmospheric circulation may help explain the rapidity of recent Arctic warming, and could be useful in future predictions of Arctic climate change.
Changes in the Arctic climate system have accelerated tremendously over the past few decades with the prime example being the extreme sea-ice loss observed in the late summers of 2007 and 2008. Anthropogenic climate change has been shown to have been a contributing factor in the recent reduction of Arctic sea ice. However, the overall acceleration in Arctic climate changes are not well correlated with the slow rise in greenhouse gases nor with the Arctic / North Atlantic Oscillation, which has showed a general positive trend since 2000 but has weakened in recent years. The fundamental physical process for the accelerated change in the Arctic remains unknown but could be due to a change in circulation, which is the hypothesis investigated in this study. Zhang et al. attempt to understand this process by comparing the recent climate behaviour with alterations in a broad spectrum of atmospheric circulation patterns over the Arctic. To do so, they use an analysis applied to mean sea level pressure data that provides simple representations of spatial states of atmospheric circulations that evolve over a 30-month running wintertime window. Using data from 1958 to 2006, the researchers found a shift in the centres of maximum climate variability from the North Atlantic northeast into the Barents Sea. They also found systematic spatial changes in the atmospheric circulation reflecting a change in the AO/NAO pattern. These changes in circulation pattern could be a result of the polar shift of storm tracks and the intensification of Arctic storm activity. The shifts in circulation could represent an impetus for the recent acceleration in the arctic climate system's response to climate change, and appear to support arguments for a tipping point of the Arctic climate system. The radical spatial shifts may also be used as a precursor to extreme change and therefore useful in future atmosphere prediction.

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