2009 Literature Review Archives - Climate Projections
Boer, G.J. and Arora, V. 200. Temperature and concentration feedbacks in the carbon cycle. Geophysical Research Letters 36:L02704, doi:10.1029/2008GL036220, 2009.
Studies with the new Canadian Earth Systems Model indicate that including the carbon cycle into climate models enhances the projected rate of future warming. This supports similar findings from experiments with other models.
In recent years, a number of research studies have indicated that the climate system and the global carbon cycle are interconnected, and that any long term simulations of future climate should also consider carbon cycle feedbacks in a fully interactive manner. In a recent research article published in Geophysical Research Letters, Environment Canada researchers George Boer and Vivek Arora present their latest findings on these feedbacks. They used Canada's new Earth System Model (CanESM1, which is essentially the Canadian coupled climate model CGCM3 with interactive terrestrial and ocean carbon cycle processes added) to explore these feedbacks. They find that there are several components to these feedbacks that are important in a warming global climate. One is a positive 'carbon temperature' feedback that causes a rise in net carbon dioxide entering the atmosphere, raising atmospheric CO2 concentrations. This feedback is relatively linear with rising temperatures, and accelerates the rate of global warming. Another is the negative 'carbon-concentration' feedback (which includes the CO2 fertilization effect) that enhances vegetation and ocean carbon sinks. Since such carbon sinks eventually begin to saturate, this feedback weakens with time. The behaviour of this feedback was also shown to be non-linear, varying with the state of the climate, which differed when different emission scenarios were used in the model experiments. The important conclusion from this work is that since the carbon-temperature feedback is positive and the negative carbon-concentration feedback weakens as CO2 increases, together they act to enhance the rate of global warming.
Bonsal, B. and B. Kochtubjada. 2009. An assessment of present and future climate in the Mackenzie Delta and the near-shore Beaufort Sea region of Canada. Int. J. Climatol. 29:1780-1795
An Environment Canada study suggests that current changes in climate in the MacKenzie Basin and near shore Beaufort Sea region are quicker than models project.
This study by two Environment Canada researchers compares temperature and precipitation output from seven Global Climate Model (GCM) projections for the Mackenzie Delta and near-shore Beaufort Sea region for the 2010-2039 period with 1961-1990 station data and several gridded data sets. All climate projections showed increases in temperature and precipitation, although there is a considerable range over spatial and temporal scales. The ocean is projected to warm more than land, and highest seasonal temperature increases occurred during autumn (1.4-3.3 °C) and winter (1.2 -2.6 °C). Annual precipitation increases are projected to be between 4.8 and 10.7%, and are relatively consistent between seasons. Recent changes in temperature are occurring faster than the models have predicted and some stations are showing higher monthly values. The authors recommend that, in the interest of developing robust action plans for the North, both gridded data sets and model outputs need to be improved.
Eby, M., K. Zickfeld, A. Montenegro, D. Archer, K. J. Meissner and A. J. Weaver. 2009. Lifetime of anthropogenic climate change: millennial time scales of potential CO2 and surface temperature perturbation. Journal of Climate Vol 22, May 15, 2009, pp 2501-2511.
Study shows that anthropogenic CO2 and the resulting climate change are longer lived than previously thought. Even after 10,000 years, 15% to30% of the anthropogenic CO2 perturbation persists in the atmosphere. The resulting climate changes last even longer.-
A number of Canadian scientists at the University of Victoria, along with an American colleague, recently published the results of a study investigating the multi-millennial temperature response to a range of anthropogenic CO2 emissions, including very large emissions representing combustion of all known fossil fuel reserves. This study extends other research that has demonstrated that for relatively small amounts of emissions, anthropogenic CO2 may persist for a thousand years or longer, but that most of the CO2 is removed after a number of centuries. The study used the University of Victoria Earth System Climate Model to conduct a series of 10,000 year climate change simulations using either pulses of CO2 emissions, or transient CO2 emissions for a range of CO2 emissions from 160 - 5120 PgC (GtC). A number of important conclusions were drawn from the results of these experiments. First, it was shown that the long-term atmospheric CO2 response is nearly independent of the rate of CO2 emissions, confirming other recent work that has shown that it is the cumulative emissions amount that matters most. Secondly, the long lifetime of the atmospheric CO2 perturbation was clearly shown with 15-30% of emissions remaining in the atmosphere at the end of 10,000 years. However, the time to absorb a given percentage of emissions was strongly dependent on the total amount of emissions, and increased significantly for emissions greater than about 1000 GtC. For all but the lowest emission scenarios, average surface air temperature reached its maximum at least 550 years after the peak in atmospheric CO2. Furthermore, the temperature anomaly was even longer lived than the CO2 anomaly. In all experiments, at least 50% of the maximum temperature anomaly persisted after 10,000 years.
M.I. Hegglin and T. Shepherd. 2009. Large climate-induced changes in ultraviolet index and stratosphere-to-troposphere ozone flux. Nature Geoscience online doi10.1038/NGE0604.
Climate warming is changing the upper air circulation, shifting the global ozone distribution towards the poles. In doing so, it is changing the amount of ultraviolet radiation received at the Earth's surface as well as the rate of exchange of ozone between the stratosphere and the troposphere.
Global computer models indicate that if, all countries follow the control measures of the Montreal Protocol, stratospheric ozone depletion due to the anthropogenic release of ozone depleting substances (ODS) will disappear by the middle to the end of this century. Attention is now turning to what additional effects global warming will have on the ozone layer by the end of this century. Past model studies have indicated that global warming will increase the strengths of the Brewer- Dobson circulation, which brings ozone from the tropics to the poles in the stratosphere. Any shift in the ozone distribution in the stratosphere will correspondingly affect the distribution of ultraviolet radiation (UV) received at the Earth's surface with related consequences for ecosystem and human health. Hegglin and Shepherd used the Canadian Middle Atmosphere model to isolate the effects of climate change on ozone depletion from those caused by ODS. They also examine how the ozone recovery expected in future decades will influence the distribution of ozone within the stratosphere (and the clear-sky UV Index) as well as the stratosphere-to-troposphere ozone flux. Under the IPCC moderate emission scenario A1B, the equator to pole redistribution of ozone within the stratosphere caused by the effects of climate change produces a corresponding decrease in the UV Index in the northern high latitudes by 9% between 1960-1970 and 2090-2100. The UV Index, however, increased by 4% in the tropics and by up to 20% in the southern high latitudes in late spring and early summer. The global stratospheric-to-troposphere ozoneflux is also projected to increase by 23% between 1965 and 2095, with significant consequences for tropospheric ozone budget and the tropospheric radiative forcing affecting air quality, and human and ecosystem health.
Howell, S.E.L., Duguay, C.R. and Markus, T. 2009. Sea ice conditions and melt season duration variability within the Canadian Arctic Archipelago: 1979-2008. GRL 36, L10502, doi:1029/2009GL037681, 2009; Sou, T. and Flato, G. 2009. Sea ice in the Canadian Arctic Archipelago: Modeling the past (1950-2004) and the future (2041-60). J. Climate 22:2181-2197.
Recent Canadian studies indicate that the amount of ice in the Canadian Arctic Archipelago is decreasing, but suggest that the region will continue to be an important refuge for in situ and Arctic Ocean multi-year ice throughout at least the next half century.
Two recent studies have provided some new insights into trends and projections for sea ice conditions within the various channels of the Canadian Arctic Archipelago. The first, published in Geophysical Research Letters by a team of researchers headed by Stephen Howell of the University of Waterloo, notes that melt season duration in the region has increased by an average 7 days per decade since 1979 and that average multi-year ice concentrations in late summer has been declining by 6.4%/decade. However, invasion of old ice from the Arctic Ocean into the region replaces much of the disappearing multi-year ice, and the authors suggest that the key shipping channels will therefore continue to be susceptible to its presence until the Arctic Ocean becomes entirely ice free in late summers. The second study, undertaken by Canadian modelers Tessa Sou and Greg Flato and published in the Journal of Climate, uses a high resolution ice-ocean regional model to examine how ice conditions in the archipelago region respond to climate projections generated by the Canadian GCM under the IPCC A2 greenhouse gas emission scenario. Results suggest that, by mid-century, the region's waterways will continue to be ice covered in winter, but that summer concentrations will decrease by 45%. Mean ice thickness will decrease by 17% in winter and 36% in summer. Thus, while reduced summer ice cover would facilitate commercial shipping through the channels, a completely ice free archipelago by 2050 is unlikely.
Lean, J.L. and D.H. Rind. 2009, How will Earth's surface temperature change in future decades? Geophysical Research Letters, Vol 36, L15708, doi: 10.1029/2009GL038932.
A statistical model projects global temperature increases of 0.17 ± 0.03°C/decade over the next two decades which is consistent with the projected warming of about 0.2°C/decade in the IPCC Fourth Assessment Report.
As an alternative to numerical model simulations, Lean and Rind (2009) use a simple empirical technique to project temperature changes over the next two decades (2009-2030). Multiple linear regression analysis is used to develop equations that describe global and regional mean temperatures as a product of variations in time series representative of ENSO, solar irradiance, anthropogenic influences and volcanic aerosols. Future temperatures are then projected on the basis of best estimates of how each of these four factors will change in the near future. The authors note that this approach may be 'perturbed' by non-linearities in the climate system, assumes that modeled relationships hold in the future and is dependent on the quality of the projections of the forcing variables. The model forecasts a global temperature increase of 0.17 ± 0.03°C/decade over the next two decades (consistent with the projected 0.2°C/decade warming in IPCC 2007), based on changes in solar and anthropogenic factors alone. Within this interval, the warming is anticipated to be greatest from 2009-2014 (0.15 ± 0.03°C) and decrease to 0.03 ± 0.01°C from 2014-2019 because of declining solar irradiance (as part of the projected solar cycle). The spatial patterns of warming projected are fairly similar to the long-range forecasts in the IPCC AR4 with maximum warming over land and at most high northern latitudes. Future scenarios involving a major volcanic eruption and super ENSO are also investigated in order to estimate the maximum possible future impact of volcanic and ENSO variability on decadal time scales.
Long, Z., W. Perrie, J. Gyakum, R. Laprise and D. Caya, 2009. Scenario changes in the climatology of winter midlatitude cyclone activity over eastern North America and the Northwest Atlantic, J. Geophys. Res., 114, D12111, doi:10.1029/ 2008JD010869.
A new exploratory study using the Canadian Regional Climate Model (CRCM) finds that midlatitude winter cyclone frequency along the Canadian east coast could decrease in the future (2040-2059).
Midlatitude cyclones are important for North American coastal areas, because the most intense ones are often associated with high winds, heavy precipitation and high ocean surface waves. In a recent study, a team of Canadian researchers performed simulations with the Canadian Regional Climate Model (CRCM) to look at midlatitude winter cyclones under current (1975-1994) and future (2040-2059) climates. To validate the CRCM simulations, comparisons are made with data from the North American Regional Reanalysis (NARR) and the Second Generation Coupled Global Canadian Climate Centre model (CGCM2) simulations. Results from the two models are also compared to evaluate if higher resolution simulations are better. For the current climate, compared to NARR, both models reproduce the overall pattern of the usual Atlantic storm track, but underestimate the number of intense cyclones; however, better results are given by the CRCM. In addition, the CRCM offers an improvement in simulations of the most intense cyclones. Under a future high CO2 scenario (IPCC-IS92a), a northwest shift of the dominant Atlantic storm track is found, but is not statistically significant. Both models show a decrease in the total cyclone frequency along the Canadian east coast, with the CRCM showing a decrease in almost all pressure bands, particularly the weak cyclones. For the CRCM, the total estimated reduction is by up to 11% for the area of study.
Lowe, J.A., C. Huntingford, S.C.B. Raper, C.D. Jones, S.K. Liddicoat and L.K. Gohar. How difficult is it to recover from dangerous levels of global warming? Environmental Research Letters 4 (2009) 014012, 9pp.
Experiments with a complex global climate model add robustness to the conclusion from earlier work that it will take a very long time to reduce global temperatures from elevated levels.
As scientific evidence mounts that even small increases in global average temperature will likely lead to significant impacts, there has been growing interest in the idea of 'overshoot scenarios' which envision temperature, or atmospheric GHG concentrations peaking at some level above a desired target before being brought down to 'safer' levels. One of the key questions about overshoot scenarios is "how long would it take to lower global temperature to the target level"? A number of recent papers have provided evidence that it is much more difficult to reduce atmospheric CO2 concentrations and global temperature than previously thought. These results have been obtained with relatively simple climate models. Lowe and colleagues investigate the issue of recovery time in overshoot scenarios using, for the first time, a complex global climate model with a fully coupled carbon cycle model (HADCM3LC model). They use idealized experiments in which emissions followed SRES A2 values until being set abruptly to zero for the next 100 years at years 2012 (CO2 concentration of ~400 ppm), 2050 (CO2 concentration of ~550 ppm) or 2100 (CO2 concentration of ~1000ppm). The experiments continued until the year 2200. They find that only very low rates of decline in atmospheric CO2 concentration occur even when emissions are abruptly stopped. Rates of change for the 100 year period following the halt in emissions were -0.2 ppm/yr, -0.4 ppm/yr and -0.75 ppm/yr for the three scenarios. Temperature continued to rise out to 2200. The second part of the paper reports on experiments with a simple climate model (MAGICC) where a large number of simulations were run to explore the impact of different settings for climate sensitivity, ocean mixing rate and a carbon cycle feedback factor on global temperature reduction. These experiments were run until the year 2500 and an additional, more realistic, multi-gas emission scenario was included, with emissions peaking in 2015 and then reducing at a rate of 3% per year. The reduction in CO2 equivalent emissions in this scenario are around 47% of the 1990 value by 2050. The outcome of this large set of simulations were probability estimates of the amount of time for which global average surface temperature might exceed warming thresholds of 1.7°C, 2°C and 3°C under different model parameterizations. They find that for the multi-gas emission scenario that peaks in 2015, there is a 55% probability of exceeding a 2°C threshold above pre-industrial level, and a 30% probability that temperature would remain above that level for at least 100 years. Overall the suite of simulations provide strong evidence of the long timescales required for global temperatures to decline and remind us of how difficult it will be to return back to 'safer' levels following peak levels of global warming.
Matulla, C., E. Watson, S. Wagner and W. Schöner. Downscaled GCM projections of winter and summer mass balance for Peyto Glacier, Alberta, Canada (2000-2100) from ensemble simulations with ECHAM5-MPIOM. International Journal of Climatology, Vol 29, doi:10.1002/joc.1796).
Projections of seasonal mass balance for Peyto Glacier, Alberta indicate that moderate increases in winter accumulation will not compensate for increased summer melt and that little of the glacier will likely remain by AD 2100.
Matulla et al. (2009) provide the first projections of summer and winter mass balance for an individual glacier in the Canadian Rockies. The projections (AD 2000-2100) are based on transient ensemble simulations from a coupled atmosphere-ocean GCM (ECHAM5-MPIOM) forced with IPCC emission scenarios A1B and B1. Winter and summer mass balance at Peyto Glacier are related to large scale atmospheric circulation features. An empirical technique is used to downscale the coarse-scale GCM projections to changes in winter and summer mass balance based on relationships between glacier mass balance and atmospheric circulation in the observational record. The results suggest that modest increases in winter mass balance will be accompanied by a substantial reduction of summer mass balance: the net effect will be glacier wastage. In the absence of a detailed model of glacier behaviour, the statistical relationship (established over the observed period) between net mass balance and the equilibrium line altitude (ELA; net accumulation occurs above and net ablation below this line) is used to approximate the effect the projected changes in mass balance may have on glacier area. The results indicate that the ELA may shift at least 100 m upwards suggesting that little of the glacier will remain by 2100. The authors note that since past variations in the extent of Peyto Glacier are similar to those for other glaciers in the southern Canadian Rockies (including those that provide flow to the Bow, Athabasca and Saskatchewan Rivers), the projections may provide useful insights into future glacier behaviour across the region.
Scinocca, J.F., M.C. Reader, D.A. Plummer, M. Sigmond, P.J. Kushner, T.G. Shepherd. and A.R. Ravishankara.2009. Impact of sudden Arctic Sea-ice loss on stratospheric polar ozone recovery. GRL, Vol 36, L24701, doi:10.1029/2009GL041239.
The rapid loss of Arctic summer sea ice in the first part of this century could influence the stratosphere such that Arctic ozone recovery could be delayed by about a decade.
Computer model studies indicate that recovery of the Arctic ozone layer is likely after the middle of this century if countries follow the control measures of the Montreal Protocol on substances that deplete the ozone layer. Climate change is expected to have influence on the recovery by introducing changes to chemical and temperature processes that could either advance the time of the recovery or induce a delaying factor. This study by Canadian and American scientists investigates the sensitivity of the NH Arctic ozone recovery to a rapid loss of Arctic summer sea ice from 2025 to 2050. The study used the Canadian Middle Atmosphere Model (CMAM) coupled with a modified global ocean circulation model to perform simulations of ozone recovery with, and without, conditions that lead to a loss of Arctic summer sea ice. The model simulations indicate that the gradual loss of Arctic sea-ice will lead to a stratospheric cooling as a result of changes in the stratospheric Brewer-Dobson circulation. This circulation, where rising air from the tropics moves northward to sink at the poles, slows down in response to changes in ocean circulation brought on through the disappearance of summer Arctic sea-ice. In the simulations, the atmospheric response was determined to be caused through feedbacks involving the North Atlantic meridional over turning circulation. The overall changes result in a 10 Dobson Units reduction in the Arctic ozone column for about two decades after summertime sea-ice is lost in 2025 and could potentially delay estimates of Arctic ozone recovery to pre-1980 values by about one decade.
Siddall, M., T.F. Stocker and P. U. Clark. 2009, Constraints on future sea-level rise from past sea-level change. Nature Geoscience, doi: 10.1038/NGEO587
A new paleo-calibrated model for sea-level change supports the projected sea-level rises presented in the Fourth Assessment Report of the IPCC (IPCC AR4).
Siddall and colleagues outline the development of a new model useful for projecting future sea-level rise in response to warming temperatures. Their model is derived from the history of sea-level variations in response to changing climate since the last glacial maximum (LGM, ~21 ka) reconstructed from fossil data. The model integrates the contributions to sea-level variations from ice sheets, glacier melt and thermal expansion, incorporating non-linear responses to temperature changes. When informed with paleo-temperature variability (from ice core records) over the past ~22,000 years, the model simulates known centennial-scale sea-level fluctuations. When forced with changes in mean annual temperature over the 20th century, the modeled sea-level rise of 4-24 cm is in general agreement with observations. Modeled estimates of sea-level change with projected temperature increases by 2100 are in line with those reported for the minimum (1.1°C) and maximum (6.4°C) warming scenarios presented in the IPCC AR4. In particular, Siddall et al. estimate sea-level rises for the minimum and maximum warming scenarios of 7 and 82 cm, compared with the 18-76 cm estimated by the IPCC AR4 (when accelerated ice sheet dynamics are included). The authors conclude that these similarities increase confidence in the IPCC projections as they were achieved using different approaches.
Solomon, S., G-K. Plattner, R. Knutti and P. Freidlingstein. 2009. Irreversible climate change due to carbon dioxide emissions. PNAS 106:6:1704-1709.
Climate change is irreversible for at least a millennium even if emissions were to cease. Among the irreversible impacts that will accompany even moderate global warming, are dry-season rainfall reductions in subtropical land areas and substantial sea level rise.
The fact that carbon dioxide has a very long atmospheric lifetime is not a new discovery. However, a number of recent scientific papers have reinforced the message about our commitment to irreversible climate change due to the persistence of CO2 in the atmosphere. This paper by Solomon et al. does just this, by investigating how long it would take for carbon dioxide concentrations to fall were anthropogenic emissions to cease immediately following attainment of a range of peak concentration levels. Two types of models are used in the experiments: atmosphere-ocean general circulation models (GCMs) and Earth System Models of Intermediate Complexity (EMICs). The authors then explore the "irreversible" warming, precipitation changes and sea-level rise associated with the slow decline in carbon dioxide concentrations out to the year 3000, defining "irreversible change" as any change on a time scale exceeding the end of the millennium. They find that about 40% of the peak CO2 concentration above pre-industrial levels remains in the atmosphere by the end of the millennium. Global average temperatures increase while CO2 is increasing and then remain approximately constant (within about 0.5°C) out to the year 3000 despite zero emissions after the CO2 peak. The authors provide an estimate of the lower limits to committed sea level rise, based on thermal expansion alone (i.e. not accounting for additional contributions from melting land ice). This analysis shows a commitment to eventual sea level rise of as much as 1.0m if 21st century CO2 concentrations exceed 600 ppm, and as much as 2.0 m if concentrations exceed 1000 ppm. Solomon et al. also present a map of expected dry-season precipitation trends per degree of global warming and conclude that despite large uncertainties for some areas, the projection of increased drying throughout much of the subtropics is robust. Furthermore, the magnitude of projected rainfall reductions is similar to that associated with major droughts in the past. Overall, the paper provides strong support for limiting emissions of CO2 since once emitted, the longevity of CO2 in the atmosphere means that the climatic consequences will persist for at least a thousand years.
Vermeer, M. and Rahmstorf, S. 2009. Global sea level linked to global temperature. PNAS. org/cgi/doi/10.1073/pnas.0907765106
European researchers use a simple model of temperature-sea level relationships to project sea level rise of 75 to 190 cm by 2100. This is a much larger rise than projected in the most recent IPCC science assessment report.
In a recent issue of the Proceedings of the National Academies of Science, two European researchers proposed a simple two-component sea level rise model that uses global mean temperature as the primary driver for change in sea levels. The first component describes the slow response of sea levels to net change in mean temperatures over time, relative to a base temperature where sea level is in equilibrium with climate. The second component considers the more rapid sea level response to variations in the rate of temperature change. When tested against historical data, the model captures changes in about 98% of the observed variations in sea levels since 1880. The model was then applied to projections for future changes in temperature as reported in the IPCC Fourth Assessment Report. Model results suggest a potential sea level rise by 2100 of between 75 and 190 cm, relative to 1990. The upper bound of this range is more than double the upper bound reported in the recent IPCC assessment (AR4) of ~80 cm which was based on contributions from accelerated ice sheet flow growing linearly with global average temperature change.
Yeh, S.-W., J.-S. Kug, B. Dewitte, M.-H. Kwon, B. P. Kirtman and Jin, F.-F. El Niño in a changing climate. Nature, Vol 46, doi:10.1038/nature08316.
Related reference: Ashok, K. and Yamagata, T. The El Niño with a difference. Nature, Vol 461, p. 481-484.
A new study suggests that the spatial structure of El Niño events and associated circulation anomalies has changed over the late 20th century and that this 'new' type of El Niño event will be more common under future global warming.
Recent studies have identified a different 'kind' of El Niño event characterized by maximum SST anomalies concentrated in the central equatorial Pacific (CP) near the International Dateline (as opposed to the eastern Pacific (EP) during typical El Niños) and flanked by cooler waters to the east and west along the equator. In this paper, Yeh et al explore changes in the historical frequency of occurrence of EP versus CP El Niños in El Niño indices and use GCM output from 11 coupled climate models to explore possible future changes in this ratio. In the historical record, the authors detected an increased occurrence of CP El Niños after 1990 which they hypothesize is related to anthropogenic climate change. Simulations from 8 of the 11 models show an increased ratio of CP to EP El Niños in the future warming scenario versus the corresponding control run. The change in the ratio for the 11 model ensemble mean is statistically significant. The authors attribute the increased occurrence of CP El Niños under the anthropogenic warming scenario to changes in the thermocline structure of the equatorial Pacific and conclude that the associated changes are sufficient to modify tropical-extratropical teleconnection patterns and hence impact local climates in a manner different to the conventional El Niño. Prediction of the global impacts of such future changes in El Niño behaviour is important from an adaptation perspective.
Zeng, N. and Yoon, J. Expansion of the world's deserts due to vegetation-albedo feedback under global warming. Geophysical Research Letters, Vol 36, L17401, doi:10.1029/2009GL039699, 2009.
A new modeling study suggests that vegetation-albedo feedbacks may accelerate the expansion of the world's warm deserts under future global warming by about 30% by the end of the century.
Zeng and Yoon (2009) explore the role of vegetation-albedo feedbacks in the size of the world's warm deserts under future warming. They use a single coupled atmosphere-ocean-land model with a dynamic vegetation component that predicts changes in surface albedo. The model was forced with observed variations in CO2, solar irradiance, anthropogenic and volcanic forcings over the 20th century and by IPCC scenario A1B after the year 2000. The model simulates a warm desert area of 25 million km2 (Mkm2) by year 2000 which compares well with the observed area of 23 Mkm2. When the model is run into the future without vegetation-albedo feedbacks, the desert area expands by 2.5 Mkm2 (10%) by the end of the 21st century. When the vegetation-albedo feedback is included in the model, the projected expansion of warm desert area is considerably larger (8.5 Mkm2 or 34%). Most of the projected expansion is in the existing sub-tropical deserts: the Sahara, Kalahari, Gobi and the Great Sandy Desert. To corroborate their results, the authors used precipitation and temperature projections from 15 models presented in IPCC AR4 (multi-model ensembles with SRES A1B) to drive the vegetation model. They found warm desert expansion for all 15 models (average ~12% expansion) but considerable differences in magnitude (from negligible change to 9 Mkm2 expansion). The authors relate the expansion to a combination of: (1) drying in the subtropics; (2) warming-induced soil moisture decreases and related vegetation loss; and (3) the positive feedback to atmospheric circulation as increased surface albedo modifies the atmospheric energy balance.
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