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2011 Literature Review Archives - Climate Change Projections

Amstrup, S.C., E.T. DeWeaver, D.C. Douglass, B.G. Marcot, G.M. Durner, C.M. Bitz and D.A. Bailey. 2010. Greenhouse gas mitigation can reduce sea-ice loss and increase polar bear persistence. Nature, Vol 468, p. 955-958, doi: 10.1038/nature09653. See also: Derocher, A.E. 2010. Climate Change: The prospects for polar bears. Nature, Vol 468, p. 905-906.

A new study makes the point that previously predicted declines in polar bear populations based on projected increases in global temperatures are not unavoidable. Greenhouse gas mitigation efforts that limit the increase in global mean surface air temperature to less than around 2OC above pre-industrial were found to greatly improve the likelihood that sustainable polar bear populations will exist throughout this century.

Observed declines in polar bear health, survival and population size have been linked with declines in summer sea ice (a key habitat requirement for access to prey, for mating and traveling) which have in turn been associated with increasing global surface temperatures.  Projections of Arctic sea ice loss published in 2007 by the United States Geological Survey suggested that under a moderate ‘business as usual’ greenhouse gas emission scenario (IPCC SRES A1B) two-thirds of the world’s polar bears could be lost by 2050.  Recent emissions trends suggest that without mitigation efforts there will be little divergence from the 2007 projections.  Amstrup et al use the Community Climate System Model version 3 (CCSM3) to investigate the potential for greenhouse gas (GHG) mitigation efforts to prevent the loss of sea ice habitat essential for polar bear survival.  The authors also explore the relationship between sea ice extent and temperature increases with the purpose of identifying possible tipping points (i.e. a threshold to irreversible ice loss).  If such a tipping point were surpassed future mitigation of GHG emissions would confer no benefits to polar bear populations.  Projections of important polar bear habitat features were developed based on global mean surface air temperature projections derived from five emissions scenarios.  The authors identified a linear relationship between global mean surface air temperature and sea ice habitat and no evidence for a threshold beyond which Arctic sea-ice collapses irreversibly.  The authors suggest that rapid ice losses (both in observations and simulations) are related to increased volatility of a thinning sea ice cover and can be reversed but  do caution that their results are based on a single model and that tipping points may still exist in the real world. The authors conclude that mitigation efforts (in conjunction with wildlife management efforts) that keep the global mean surface air temperature increase to <1.25oC above the 1980-1999 mean are likely to maintain sufficient sea-ice habitat for polar bears to persist at sustainable, albeit lower than present, levels throughout the century. 

Arora, V.K., J.F. Scinoca, G.J. Boer, J.R. Christian, K.L. Denman, G.M. Flato, V.V. Kharin, W.G. Lee and W.J. Merryfield. 2011. Carbon emission limits required to satisfy future representative concentration pathways of greenhouse gases. Geophysical Research Letters, Vol 38, L05805, doi: 10.1029/2010GL046270.

 Scientists from Environment Canada have published the first climate simulations based on the next generation of emission scenarios developed for climate change research (Representative Concentration Pathways or RCPs).  The results indicate that to limit the global temperature increase by 2100 to 2oC, rapid reductions of greenhouse gas emissions must begin immediately and negative emissions (i.e. carbon sequestration greater than emissions) must occur from ~2060 onwards.

Climate model simulations in the Fifth Assessment Report of the IPCC (due in 2014) will be based on Representative Concentration Pathways (RCPs) which specify changes in atmospheric concentrations of greenhouse gases and aerosol emissions over the course of the century.  Arora and colleagues from the Canadian Centre for Climate Modelling and Analysis present the first published results for the new RCPs based on the second-generation Canadian earth system model (CanESM2) which represents the physical climate system as well as biogeochemical cycles (carbon and sulphur cycles).  The model effectively simulates climate changes based on historical forcing and was therefore used to generate five simulations for each of three RCPs (RCP2.6, RCP 4.5 and RCP 8.5 where the RCP numbers indicate radiative forcing in W/m2 at the end of the century).  Projected global temperature increases over the period 2006-2100 for RCPs 2.6, 4.5 and 8.5 were 1.4, 2.3 and 4.9oC respectively which equate to increases of 2.3, 3.2 and 5.8oC over the period 1850-2100.  Global precipitation increases of 3.6, 4.5 and 7.8% are projected for RCPs 2.6, 4.5 and 8.5 for the period 2006-2100.  The authors also estimated the cumulative fossil-fuel based CO2 emissions consistent with the future concentration pathways as 182, 643, and 1617 Pg C (=GtC) for RCPs 2.6, 4.5 and 8.5 respectively over the period 2006-2100.  These “allowable” cumulative fossil fuel emissions limits are lower than those presented in previous studies because the effects of non-CO2 greenhouse gases (GHGs) and aerosols are accounted for in the CanESM2 simulations.  Over the period 2001-2100, the impact of non-CO2 GHGs dominates that of aerosols (which are prescribed to decline over this period) contributing to climate warming. The authors emphasise that even the most aggressive of the pathways (RCP 2.6) is unlikely to limit global temperature increases to 2oC above pre-industrial (the target set to avoid dangerous human interference with the climate system in the Copenhagen Accord).  Furthermore to meet the “allowable” emission limits needed to follow RCP 2.6, unprecedented emission reductions must begin immediately followed by negative emissions globally (i.e. removal of CO2 from the atmosphere) over the second half of the century.  If global fossil fuel emissions continue at their current rate, the “allowable” 182 Pg C (2006-2100 limit for RCP 2.6) would be used up in roughly two decades.

Beaumont, L.J., A. Pitman, S. Perkins, N.E. Zimmerman, N.G. Yoccoz and W. Thuiller. 2011. Impacts of climate change on the world’s most exceptional ecoregions. PNAS, Vol 108, pp 2306-2311, doi: 10.1073/pnas.1007217108.

Climate model simulations indicate that by 2070 up to ~80% of the world’s most exceptional terrestrial and aquatic ecoregions will routinely experience monthly temperatures considered extreme compared to the 1961-1990 period. Twentieth century warming has been linked to changes in biological systems worldwide and these projected changes may therefore place increasing stress on these iconic ecoregions.

The ‘Global 200’ is a set of terrestrial and freshwater ecoregions identified as irreplaceable or distinct because of their biological diversity and global uniqueness (amongst other attributes).  Protection of these ecoregions would be of value to conservation efforts worldwide but the majority are threatened by habitat loss, fragmentation and degradation.  Beaumont and colleagues investigate the impacts projected climate changes may also have on a subset of these regions by exploring changes in their exposure to monthly precipitation and temperature extremes (defined as >±2 standard deviations from the 1961-1990 mean) simulated for 2070.  The authors developed ensembles of climate model simulations from 23 climate models for three emission scenarios (IPCC SRES B1, A1B and A2, with ~200 simulations each).  The results indicate that by 2030, the entire range of 12-22% of the world’s most important ecoregions may be subjected to extreme monthly temperatures and an additional 38-39% will likely be exposed over parts of their range.  By 2070, these numbers increase dramatically with 60-86% of terrestrial and 51-83% of aquatic ecoregions projected to be exposed to extreme monthly temperatures over their entire range over some part of the year.  It is generally thought that high-latitude ecosystems may be more vulnerable to climate change because projected temperature increases are greatest for these regions.  However, the absolute range of temperature variability is lower in the tropics than high-latitude regions therefore a smaller absolute temperature increase in the tropics can be more extreme in terms of deviation from the mean.  As a consequence, the authors find that tropical and subtropical ecoregions and mangroves are projected to experience extreme monthly temperatures earliest with lower temperature increases (in some cases with <1oC local warming).  Unlike temperature, projected monthly precipitation does not exceed the threshold defined as extreme (i.e. >±2 standard deviations from the 1961-1990 mean) over the 21st century for any of the ecoregions under the three emission scenarios (the authors note high variability amongst the simulations). However, changes in other properties of precipitation such as changes in daily extremes not captured in this study may impact these ecoregions.  Ultimately ecosystem response to changes in climate is a result of many factors but these results indicate that changes in temperature extremes may place increasing stress on these key ecoregions.

Cheng, C.S., G. Li, and H. Auld. 2011. Possible impacts of climate change on freezing rain using downscaled future climate scenarios: Updated for eastern Canada. Atmosphere-Ocean, Vol 49(1), pp. 8-21.

The number of freezing rain events during the coldest winter months in eastern Canada is projected to increase in the future.  During warmer months the frequency of freezing rain events is likely to decrease.   

Freezing rain events are one of the most costly hydrometeorological hazards in Canada with ice accumulation causing damage to infrastructure and agriculture and creating dangerous driving conditions. How their frequency may change in response to projected increases in global temperatures is important in terms of informing adaptive planning (e.g. infrastructure design). Cheng et al project future changes in the frequency and severity of daily freezing rain events from the historical average in eastern Canada (Winnipeg to the Atlantic Ocean and from the U.S. border to as far north as Churchill, Manitoba).  Severity of freezing rain events in this study is defined as the number of hours during a day when freezing rain occurs (not accumulation).  Daily outputs of surface and upper air meteorological variables were obtained from eight General Circulation Models (GCMs) simulations (for IPCC SRES scenarios A2, B1 and B2 obtained from 4, 3 and 1 different climate models) and were analysed for three time periods: 2016-2035, 2046-2065 and 2081-2100.  These data were downscaled (spatially and temporally) to produce future hourly station-scale climate data (Nov-Apr and Oct-May for southern and northern stations respectively) for 42 stations in the study region.  A synoptic weather typing technique was then used to identify current and future weather ‘types’ associated with freezing rain and examine projected changes in their frequency in six subregions.  Major findings are that freezing rain events are expected to increase in the coldest months and decrease in the warmest months under study.  The magnitude of decreases in the warmer months is less pronounced than the increases in the coldest months.  The greatest changes were found in the more northerly regions and generally increased from southwest to northeast.  As one would anticipate, changes became more pronounced through time and with higher emission scenarios (as warming becomes more pronounced in both cases).  The observed changes were anticipated because warming temperatures will result in a northward movement of freezing rain-related weather systems in the coldest months.  In the warmer months, precipitation that fell as freezing rain in the historical climate may occur as liquid rain in a warmer climate.

Diffenbaugh, N.S. and M. Scherer. 2011. Observational and model evidence of global emergence of permanent, unprecedented heat in the 20th and 21st centuries. Climatic Change 107:615-624.

 A new study shows that, should greenhouse gas concentrations continue to increase similar to those under a mid-range business-as-usual emission scenario, many areas of the globe are likely to move into a new heat regime over the next four decades where the coolest warm-season of the 21st century is hotter than the hottest warm-season of the late 20th century.  Tropical areas are shown to experience the most immediate and robust emergence of unprecedented heat.

A robust prediction associated with continued increases in human emissions of greenhouse gases is that the frequency and intensity of extreme hot events will increase. A study by two scientists with Stanford University explored one aspect of this projected change: the timing of emergence of a novel heat regime in which the new minimum is hotter than the baseline maximum. To do this, they analyzed surface air temperature from the CMIP3 global climate model archive, using a total of 52 realizations from 24 models that contributed both 20th century simulations and projections based on the IPCC SRES A1B emission scenario. They estimate 3 metrics of severe heat emergence, calculated separately for June-July-August (JJA) and Dec-Jan-Feb (DJF) for a number of time periods: 1) the percentage of seasons warmer than the late 20th century (1980-1999 period) maximum, 2) the ‘time of emergence’ when the ensemble mean warming above the late 20th century maximum emerges above the ensemble spread, and 3) the timing of the last occurrence in each model realization of a season cooler than the late 20th century maximum. Overall, they find that tropical areas exhibit the most immediate and robust emergence of unprecedented heat. Up to 70% of seasons in the early 21st century period (2010-2039) exceed the late 20th century maximum for both JJA and DJF, a percentage that increases to more than 90% for much of the tropics by late in the 21st century. In other regions, exceedance is generally greater in JJA than DJF, and extends to greater than 90% of seasons over much of extra-tropical Africa, southern Eurasia and western North America in the late 21st century. The median date (from the 52 realizations) marking the last occurrence of a season that is cooler than the late 20th century maximum occurs by the end of the 2050s over most of the tropics and large areas of northern Africa and southern Eurasia. The authors note that since actual GHG emissions over the early 21st century exceed those in the A1B emission scenario, these results may be conservative estimates of the emergence of unprecedented heat.

Frölicher, T.L. and F. Joos. 2010. Reversible and irreversible impacts of greenhouse gas emissions in multi-century projections with the NCAR global coupled carbon-cycle climate model. Climate Dynamics 35:1439-1459.

Study demonstrates that climate change impacts associated with emissions of non-CO2 climate forcers are largely reversible while those from 21st century CO2 emissions are, on human timescales, irreversible. This leads the authors to suggest that emission trading schemes based on CO2-equivalencies of various GHGs should be re-examined, given the fundamentally different nature of the climate response to continuing emissions of anthropogenic CO2 vs those of non-CO2 GHGs.

A number of studies have been published to date examining the ‘legacy’ of anthropogenic CO2 emissions in experiments in which emissions are abruptly ceased and the climate system response evaluated. Most such studies have used CO2-only scenarios and have consistently found that global mean temperature remains roughly constant at near peak levels in the millennium following cessation of CO2 emissions (see summary of Gillett et al. above, for example). Frölicher and Joos also explore the impact of zero emission commitment scenarios but they include all relevant anthropogenic GHG emissions (not just CO2) as well as sulphate aerosols.  The study investigated the multi-century changes (out to year 2500) in temperature, precipitation, sea level rise and terrestrial and marine carbon storage in a comprehensive carbon-climate model (NCAR CSM1.4) following cessation of anthropogenic emissions in (1) the year 2000 (Historical emissions), or (2) the year 2100 following either High (SRES A2) or Low (SRES B1) 21st century emission scenarios. In response to all three multi-gas scenarios, global mean surface temperature peaks immediately after emissions cease followed by a relatively rapid decrease in temperature lasting some decades after which global surface temperature slowly stabilizes at levels that are still significantly elevated. The authors note that sulphur forcing is small in their model thus the short-term increase in global mean surface temperature due to the removal of the cooling effects of sulphate aerosols after cessation of emissions is small. The projected rapid initial decrease in global temperature is a response to the relatively rapid decay in atmospheric concentrations of shorter-lived non-CO2 GHGs when emissions of these substances are eliminated and demonstrates the benefits of mitigating emissions of these substances. The impact of human emissions of non-CO2 substances is largely reversible within a few weeks (for very short lived substances such as aerosols) to a century. The residual persistent long-term warming occurs in response to CO2 emissions. 21st century CO2 emissions are shown to perturb temperature, precipitation, sea level and ocean chemistry (ocean acidification) irreversibly on human timescales, consistent with other studies. By demonstrating that the long term consequences of 21st century emissions of CO2 are fundamentally different from those of other GHGs (with the possible exception, they say, of SF6), the authors contend that the Kyoto Protocol approach of pooling emissions of the major GHGs into a single ‘basket’, and allowing trade-offs in emissions among these, is no longer supportable. Rather, parallel mitigation strategies are required for CO2 and for non-CO2 GHGs.

Gillet, N. P., V.K. Arora, K. Zickfeld, S.J. Marshall and W. J. Merryfield. 2011. Ongoing climate change following a complete cessation of carbon dioxide emissions. Nature Geoscience. Published online January 9. 5pp.

Environment Canada scientists demonstrate that irreversible and ongoing climate changes occur at regional scales even after human CO2 emissions are eliminated, as a result of the long atmospheric lifetime of anthropogenic CO2. Following a cessation of emissions in 2100, ongoing warming of the Southern Ocean means that West Antarctic ice shelves may be vulnerable to long term melting, and much of the West Antarctic Ice sheet may be vulnerable to eventual collapse.

There is a growing body of evidence from modeling studies demonstrating that even if human CO2 emissions were to cease abruptly, global temperature would remain elevated for millennia. Results such as these support the notion that anthropogenic global warming from CO2 emissions is irreversible, on time scales that are relevant to human lifespans. A newly published study by scientists at Environment Canada’s Canadian Centre for Climate Modeling and Analysis and the University of Calgary adds to our understanding of the spatial scales of irreversible climate change by demonstrating that some regional changes in climate are not only effectively irreversible, but these regional changes continue even as global temperature stabilizes. Environment Canada’s Canadian Earth System Model was used to simulate the response of the climate system over the coming millennium to idealized scenarios of either an immediate cessation of CO2 emissions (in the year 2010), or cessation in the year 2100 after a century of ‘business-as-usual’ increasing emissions of CO2. The results indicate that global temperatures stabilize almost immediately following a cessation of emissions, and remain for the next thousand years at roughly the level reached when emissions cease. Regional responses vary, however. Following a cessation of emissions in 2100, the Northern Hemisphere land masses and the Arctic Ocean gradually cool, and Arctic sea ice recovers. In contrast, the Southern Ocean and Antarctica continue to warm, with warming up to 9°C by the year 3000 relative to preindustrial climate in some coastal areas. The warming of the Southern Hemisphere and cooling of the Northern Hemisphere drive other shifts in climate, some of which, such as ongoing drying in parts of North Africa, exacerbate negative climate change impacts. The penetration of warming to mid-depths of the Southern Ocean, and ongoing regional warming, highlight the long-term vulnerability of the Antarctic ice shelves to melting.

Kodra, E., K. Steinhaeuser, and A.R. Ganguly. 2011. Persisting cold extremes under 21st-century warming scenarios. Geophysical Research Letters, Vol 38, L08705, doi: 10.1029/2011GL047103.

Cold extremes are projected to be less frequent by the end of the century, globally, although events of equal or greater intensity and duration than at present are still expected to occur on occasion.  Despite the projected mean global warming, planning for future warm extremes should not compromise our ability to deal with cold extremes.

Kodra and colleagues explore the likelihood of short-lived cold events (i.e. those lasting one day to several weeks) under future anthropogenic global warming.  Simulations of future climate from nine climate models (all used in the IPCC AR4) for the moderate SRES A1B emission scenario are used to examine cold extremes for the decade 2091-2100 versus 1991-2000.  Three attributes of future cold temperature extremes are examined: intensity (coldest 3-day average of daily maximum temperature in a given year), duration (annual maximum of consecutive frost days) and frequency (the total number of frost days in a given year).  The reliability of the cold extreme projections is assessed through comparisons of model hindcasts (for the recent past) with observations and evaluations of the extent of multi-model agreement of future conditions.  The majority of the models show that, over a substantial portion of the global land-mass, 1-2 events of greater intensity and 1 of greater duration than the 1991-2000 average may be experienced by the last decade of the 21st century.  Despite variations in the intensity and duration of cold extremes by region and by model, agreement over continental scales and for some regions is good.  In accordance with previous studies, cold extremes are projected to occur less frequently on average by the end of the 21st century than the end of the 20th century.  The authors conclude that planning for increased frequency of warm extremes expected in a warmer climate should not come at the expense of cold-weather preparedness.

Lang, C. and D.W. Waugh. 2011. Impact of climate change on the frequency of Northern Hemisphere cyclones. Journal of Geophysical Research, Vol 116, D04103, doi: 10.1029/2010JD014300.

Climate model simulations reveal weak and inconsistent trends in the projected future frequency of extratropical Northern Hemisphere cyclones during summer. This result is unlike that for extratropical Northern-Hemispheric winter cyclones which are consistently projected to decrease in number.

Numerous studies have examined the impacts of projected climate change on the frequency, strength and position of mid-latitude cyclones in the Northern Hemisphere.  These studies have focused on winter months and have consistently projected a decrease in the number of extratropical storms averaged over the Northern Hemisphere.  Lang and Waugh evaluate climate model output for possible changes in the expected frequency of extratropical (30-90oN) summer cyclones in the Northern Hemisphere.  Extratropical surface cyclone frequency is calculated for climate model simulations (16 models from the Coupled Model Intercomparison Project phase 3 multi-model dataset) of mean daily sea level pressure for part of the 20th century (1960-2000) and for three future scenarios of greenhouse gas emissions (B1, A1B and A2 from IPCC SRES).  Over the 1960-2000 period, the multi-model mean frequencies and spatial distributions of extratropical cyclones agree reasonably well with observations (reanalysis data) and previous studies (indicating the simulations should provide a reasonable assessment of future conditions).  Unlike for winter months, there is no consistency in the sign of the trend in summer cyclones across the models and therefore no significant trend in the multi-model mean.  The multi-model trend in the intensity of extratropical summer cyclones is also weak and non-significant.  When examined at a smaller scale, there are some regions where trends in summer cyclone frequency exhibit the same sign across the majority of models (and the multi-model mean trend is significant) but there is still a large spread in the actual magnitude of the trends.  The authors conclude that the lack of a consistent and robust signal across models indicates that care should be taken when interpreting projected changes in summer weather systems particularly if they are based on a single model.

Mladjic, B., L. Sushama, M.N. Khaliq, R. Laprise, D. Caya and R. Roy. 2011. Canadian RCM projected changes to extreme precipitation characteristics over Canada. Journal of Climate 15 May, 2011, pp2565-2584.

Experiments with the Canadian regional climate model show that, over most regions of Canada, there will be increases in single and multi-day extreme precipitation amounts for a given return period; that is, a 1 in 20 yr, 50 yr or 100 yr event in the future will be associated with more rainfall than it is now. This increase in short and long duration extreme precipitation has implications for many water management activities.

Extreme precipitation events and related impacts such as floods are of particular concern for societies in planning for future climate change given the potential threats to infrastructure, the environment and human life. A new study has recently been published reporting on experiments with the fourth generation of the Canadian Regional Climate Model (CRCM) to simulate characteristics of precipitation extremes and their projected changes over Canada. Single and multi-day (i.e. 1-,2-,3-,5-,7- and 10-day) maximum precipitation amounts over the period April-September (to minimize the chance of mixing rain and snow events) are studied using an ensemble of five 30-yr integrations each for current climate (1961-1990) and future climate (2040-2071) using two methods of extreme event analysis (regional frequency analysis, RFA, and gridbox analysis, GBA). Projected changes to return levels (i.e. changes in amounts of rainfall) associated with 20-, 50- and 100-yr return periods for the period 2040-71 relative to 1961-90 are derived both in terms of percentage change and absolute changes. The results of the regional scale analysis, based on 12 climatic regions, suggest an increase in return level in a future climate for all regions, with the largest percentage changes but lowest absolute changes projected for northern regions. In general, similar patterns but larger changes were projected for the longer return periods although these results were also shown to be less statistically significant. Analyses at the gridcell level with both RFA and GBA confirm the projected increases in extreme single and multi-day precipitation amounts for most areas of Canada. As expected, analysis at this finer spatial scale revealed a more complex pattern of response, with small areas of large projected percent changes in extreme precipitation appearing in southern as well as northern regions. The increase in return levels of short- and long-duration extreme precipitation events has implications for management of water resources over the coming century.

Perrie, W., Y. Yonghong, and W. Zhang. 2010. On the impacts of climate change and the upper ocean on midlatitude northwest Atlantic landfalling cyclones. Journal of Geophysical Research, Vol 115, D23110, doi: 10.1029/2009JD013535.

The impacts of climate change on aspects of midlatitude North Atlantic cyclones that make landfall in North America during the autumn were investigated.  In future simulations these landfalling cyclones are found to be slightly less intense and there is a tendency for their tracks to shift slightly north of present.

Intense extratropical cyclones are often associated with high winds, heavy precipitation and flooding. Those that make landfall in North America often cause extensive economic and social damage.  Perrie and colleagues investigate the influence of climate change on midlatitude North Atlantic storms that landfall in North America during autumn, a period of high frequency for extra-tropical cyclones in the northwest Atlantic area. High-resolution simulations of autumn storms are developed using a coupled mesoscale atmosphere-ocean model system.  Boundary conditions for these simulations are derived from climate scenario outputs from the Canadian second generation Coupled Global Climate Model (CGCM2) following the IPCC IS92a scenario for 2040-2059 (warming over the North Atlantic by this period is projected to be 2oC in this business-as-usual scenario).   Composites (derived from ensemble populations produced by the models) of landfalling autumn storms for the present day (1975-1994) and the future scenario are compared in terms of intensity, trajectory and vertical structure.  The authors find that in the future climate scenario the landfalling storms are weaker than present storms in terms of minimum sea level pressure and wind speeds.  The authors explain that although climate warming tends to intensify storms this is balanced by modest dynamic cooling around the storm centre that is related to enhanced upward motion.  The future storm tracks of the landfalling autumn storms (south of 50oN) also show a tendency to move slightly poleward (100-200 km) towards the North American coastline (consistent with IPCC results).

Radic, V. and R. Hock. 2011. Regionally differentiated contribution of mountain glaciers and ice caps to future sea-level rise. Nature Geoscience Vol 4:2:91-94.

New projections of global glacier and ice cap losses over the 21st century reveal that Antarctica, Arctic Canada, Alaska and Svalbard are the main contributors to global sea-level rise as a consequence of large ice volumes in these regions. The European Alps and New Zealand are identified as the regions most vulnerable to losing their glaciers. However, owing to small ice volumes in these regions, their contributions to sea-level rise are negligible although the implications for regional water resources are serious.

Over recent decades, the retreat of glaciers and ice caps has contributed about one quarter of the observed global sea-level rise (SLR) and these sources are expected to continue to make substantial contributions to SLR over the coming century. Given that current information about the status of global glaciers is incomplete, and that existing projections of mass loss by mountain glaciers and ice caps show discrepancies, there remains considerable uncertainty about the magnitude of the contribution of these sources to SLR this century. Radic and Hock use a new modeling approach that incorporates inventory data on each individual glacier and ice cap in the World Glacier Inventory (WGI-XF) and uses projections of future climate change (driven by the IPCC SRES A1B scenario) from ten global climate models to provide updated estimates of the contribution of glaciers and ice caps, by region, to global SLR over the coming century. They find that all projections for the 21st century show substantial mountain glacier and ice cap volume losses, although the magnitude of those losses is sensitive to the choice of climate model. The multi-model mean global volume loss produces an estimated 0.124 ± 0.037 m SLR by 2100 (in line with IPCC projections based on multiple emission scenarios). Percent volume changes varied considerably among 19 differentiated regions, with multi-model means indicating the smallest losses from Greenland (8 ± 4%) and High Mountain Asia (10 ± 16%) and the largest losses from the European Alps (75 ± 15%) and New Zealand (72 ± 7%). Despite the large relative losses from these latter two regions, their contributions to SLR over the coming century are negligible given their small total ice volumes. The main contributors to global SLR by 2100 are Arctic Canada (0.027 ± 0.012m sea level equivalent (SLE), Alaska (0.026 ± 0.007m SLE), Antarctica (0.021 ± 0.012m SLE) and Svalbard (0.014 ± 0.004m SLE). The authors note that their projected SLR from glacier wastage is probably low given that the surface mass balance model they used neglects mass loss through iceberg calving which is known to be a significant cause of ice mass loss from marine-terminating glaciers.

Schaefer, K., T. Zhang, L. Bruhwiler, and A.P. Barret. 2011. Amount and timing of permafrost carbon release in response to climate warming. Tellus, Vol, 63B p. 165-180 doi:10.1111/j.1600-0889.2011.00527.x.

Under a business as usual projection, the extent of northern hemisphere permafrost is expected to decrease by 29-59% by 2200, releasing 190 ± 64 Gt of carbon, currently frozen in permafrost, to the atmosphere.  This release corresponds to an increase in atmospheric CO2 concentration of roughly 87 ± 29 ppm.

Estimates indicate that 1672 gigatonnes of carbon (GtC) are stored in Northern hemisphere permafrost regions (where soil is at or below freezing for at least two consecutive years).  Recent observations of widespread permafrost degradation in the northern hemisphere suggest that rising global temperatures may cause the thaw and subsequent release of part of this stored carbon to the atmosphere, exacerbating warming (a positive feedback).  Schaefer et al explore impacts of a changing climate on the strength and timing of potential permafrost carbon feedbacks.  Simulations from three climate models for the IPCC A1B emission scenario (a ‘middle of the road’ scenario) are used to run projections of the terrestrial carbon cycle (using a land surface model) for the period 1973-2200 for permafrost and discontinuous permafrost regions poleward of 45oN. The models project that by 2200, there will be a 29-59% decrease in permafrost extent and a 53-97 cm average increase in the depth of the active layer (i.e. the upper layer of permafrost that thaws seasonally will be deeper).  The cumulative permafrost carbon flux to the atmosphere by 2200 is projected to be 190 ± 64 GtC (equivalent to an increase of atmospheric CO2 concentration of roughly 87 ± 29 ppm). A lag of roughly 70-100 years between the start of warming and the expected increase in atmospheric CO2 is demonstrated, due to slow microbial decay.  The authors note that their estimates may actually be on the low side because their modeling approach does not account for amplified warming related to the permafrost carbon feedback and exclude some regions of discontinuous permafrost as well as methane emissions from wetlands.  The results indicate that the arctic would shift from a carbon sink to source by the mid-2020s canceling as much as 42-88% of the total global land carbon sink.  The authors conclude that since carbon release from permafrost thaw and decay is irreversible it should be considered when projecting fossil fuel emission targets consistent with particular atmospheric CO2 concentrations.

Tietsche, S., D. Notz, J.H. Jungclaus, and J. Marotzke. 2011. Recovery mechanisms of Arctic summer sea ice. Geophysical Research Letters, Vol 38, L20707, doi:10.1029/2010GL045698.

Tipping points in Arctic sea ice are explored through simulations of the response of Arctic sea ice extent to periodic ice-free summers during the 21st century.  Summer sea ice extent is projected to recover rapidly (typically within 2 years). This suggests that as sea ice continues to decline over this century, anomalous ice-free conditions in a single summer are not irreversible. However, in the long term, with ongoing Arctic warming, eventual disappearance of summer sea ice is projected.

Summer sea ice extent has declined rapidly over recent years.  Previous studies have suggested that because the positive ice-albedo feedback effect (which occurs because open water absorbs more solar radiation than ice) enhances warming and associated ice melt, there may be a tipping point beyond which summer sea ice extent would not recover.  Tietsche et al used a coupled atmosphere-ocean global circulation model (AOGCM; ECHAM5/MPIOM) and a sea-ice model to evaluate whether rapid losses of summer sea ice cover is in itself sufficient to trigger an irreversible change in Arctic sea ice extent.  The Arctic energy budget is evaluated to assess important sea ice recovery mechanisms that can counteract the ice-albedo effect after rapid ice losses.  Rapid, complete losses of early summer sea ice (the largest ice-albedo feedback possible) are artificially introduced every twenty years to AOGCM projections of 21st century climate based on the IPCC-A1B emission scenario.  The authors note that in ‘normal’ simulations with this scenario, ECHAM5/MPIOM shows a ~10oC increase in mean surface air temperature in the Arctic from the 1900s to 2090s and a continued trend of decreasing summer sea-ice extent with ice free conditions occurring after 2070.  The results show that in response to imposed ice-free conditions in early summer, summer ice extent recovers quickly (typically within two years) even within the prevailing warming climate.  It then continues a more linear decline in response to ongoing Arctic warming in the A1B scenario. This pattern is repeated after each artificially imposed summer with no sea ice. The analysis of the Arctic energy budget reveals that in the simulated ice-free perturbations, the surface of the open water quickly gains heat during the first summer.  However, the ice free conditions cause enhanced heat loss to the atmosphere during the subsequent fall and winter when the insulating sea-ice cover is anomalously thin. The excess heat in the atmosphere leaves the Arctic through increased longwave emission at the top of the atmosphere.  The ocean therefore cooled sufficiently to favour ice recovery. The authors conclude that tipping-point behaviour is unlikely to occur over the 21st century while summer sea-ice extent continues to decline in response to projected increases in global temperatures. The ocean therefore cooled sufficiently to favour ice formation.

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