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2011 Literature Review Archives - Climate System Studies

Flanner, M.G., K.M. Shell, M. Barlage, D.K. Perovich ad M.A. Tschudi. 2011. radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008. Nature Geoscience published online January 16, 2011.

 A new analysis of changes in the reflectivity (albedo) of the Northern Hemisphere due to declines in snow and ice cover show that the impact of this reduced reflectivity on climate warming is more than double what state-of-the-art climate models estimate. This suggests that the role of the cryospheric albedo feedback in amplifying Arctic warming could be greater than thought.

 The importance of changes in snow and ice to the Earth’s reflectivity – and hence its cooling capacity – has long been recognized. However, measuring changes in this reflectivity over large scales is difficult. Flanner and colleagues synthesize a variety of remote sensing and field measurements of snow and sea-ice cover to provide estimates of changes in cryospheric radiative forcing for the Northern Hemisphere (NH) over the period 1979-2008. Their aim was to understand the radiative impact of observed changes in snow and sea-ice on surface albedo and top of the atmosphere (TOA) energy balance. Their study only examined impacts of changes in the cryosphere on short-wave (solar) radiative forcing although they acknowledge that long-wave feedbacks associated with changes in the cryosphere can be important under some circumstances. Cryospheric radiative forcing (CrRF) was estimated using a number of different methods to obtain a range of estimates. From these, they found that on average, ‘cryospheric cooling’ (change in NH CrRF) has declined by 0.45 W/m2 from 1979 to 2008 with nearly equal contributions from changes in land snow cover and sea ice. Combining their estimates of changes in CrRf with estimates of NH warming over the same period yielded estimates of the strength of the NH cryosphere albedo feedback (DCrRF/Dsurface temperature) of on average 0.62 W/m2 (range 0.33 to 1.07). This estimate is more than double the average cryosphere albedo feedback in the CMIP3 multi-model dataset. This discrepancy indicates that either other surface processes are offsetting the strong cryospheric feedback in models, or that the cryosphere is responding more sensitively to, and driving stronger climate warming than current models indicate. The authors note that recent observations that NH sea ice is declining faster than projected would seem to support the latter theory. 

Hawkins, E. R. S. Smith, L. C. Allison, J. M. Gregory, T. J. Woollings, H. Pohlmann, and B. de Cuevas3. 2011. Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport. Geophysical Research Letters, Vol 38, L10605, doi: 10.1029/2011GL047208.

 Simulations using a coarse resolution atmosphere-ocean coupled global climate model are the most sophisticated to date to demonstrate that a sudden collapse of the Atlantic meridional overturning circulation (part of the so-called global ocean conveyer belt) is a possible response to freshening of the North Atlantic.

 In brief, the term Atlantic meridional overturning circulation (AMOC) describes the density-driven circulation in the Atlantic Ocean whereby warm surface waters are transported northwards where they eventually cool, become more dense and sink, returning southward at depth.  AMOC is an important component of the climate system and studies based on simplified climate models (Earth-system models of intermediate complexity) suggest that its behaviour is bistable (i.e. it has two stable regimes, ‘on’ or ‘off’).  More complex state-of-the-art atmosphere-ocean coupled global climate models (AOGCMs) have not found evidence of this behaviour.  Experiments with AOGCMS suggest that a rapid collapse (i.e. transition to ‘off’) in response to projected freshening of the North Atlantic, anticipated with warming-related changes in the hydrological cycle, is unlikely in the future although a gradual weakening of AMOC is projected.  To further test for bistability in AMOC behaviour, Hawkins et al. explore the AMOC response to simulated freshwater perturbations in the North Atlantic using the FAMOUS AOGCM (a lower resolution version of HadCM3).  The long term (thousands of years) experiments under pre-industrial GHG levels were not designed to evaluate the response of the AMOC to anthropogenic warming per se, but to test for bistable behaviour in response to influxes of freshwater to the North Atlantic (20o-50oN) over various timescales. They show that (at 26°N) the strength of AMOC drops rapidly to zero (i.e shuts down) when the fresh water flux added to the North Atlantic equals 0.4 Sv.  For comparison, projected freshwater input to the North Atlantic from changes in precipitation and river runoff by the end of the 21st century is 0.1 Sv. Their modeling results suggest that changes in the hydrological cycle related to ongoing climate warming may induce a sudden collapse in AMOC.  The authors conclude that their simulations using FAMOUS are the most physically comprehensive dynamical modeling simulations to date to show bistable AMOC behaviour.

Melillo, J.M., S. Butler, J. Johnson et al. 2011. Soil warming, carbon-nitrogen interactions, and forest carbon budgets. PNAS, June 7, 2011. Vol. 108 #23. 5pp.

 An experimental soil-warming study in a deciduous forest shows that warming leads to enhanced carbon emissions to the atmosphere (i.e. a positive feedback). However, this feedback is dampened by warming-triggered release of soil nitrogen which contributes to enhanced growth of - and storage of carbon in - trees.

How the world’s forests will respond to rising atmospheric concentrations of CO2 and related climate changes is one of the central questions behind how sensitive the climate system is to human perturbations. Feedbacks between the carbon cycle and climate change are expected to be positive overall (i.e. increase the fraction of anthropogenic emissions that remain in the atmosphere) but the strength of this response, and the various processes that affect it are all active areas of research. Melillo and colleagues present empirical evidence that soil warming-induced increases in the availability of nitrogen can affect the carbon budget of forest ecosystems.  The study was based on a long-term (7 year) soil-warming experiment conducted in a large (30m x 30m) experimental plot (with a similarly sized control site) in a mixed hardwood temperate forest in the U.S. Soil temperature was artificially raised to maintain a 5°C temperature difference between the heated and control site year-round. Over the 7 years, the cumulative warming-induced carbon flux was from the forest to the atmosphere (primarily from enhanced soil respiration), but the magnitude of the flux diminished over time as a result of an increase in tree growth in the heated area. Investigating the processes that contributed to this change, they found a progressive decrease in fine-root biomass in the heated area over the 7 years, consistent with increases in nitrogen availability (since this allows trees to reallocate more carbon to above-ground growth rather than to roots).  They show also that net nitrogen mineralization in the heated area increased by 45% relative to the control area (to ~27 kg N/ha/yr) and estimate that ~12% of this is used to store carbon in woody tissue while the remainder is recycled rapidly between leaves and soils. They conclude that their findings support increased carbon storage in trees due to a warming-induced acceleration of the nitrogen cycle but note that an observed lengthening of the growing season (as indicated by the timing of bud-break) in the heated area is a factor which may also have contributed to enhanced tree growth in their experiment.

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