Nutrient cycling affects carbon uptake by the terrestrial biosphere and imposes controls on carbon cycle response to variation in temperature and precipitation, but nutrient cycling is ignored in ...most global coupled models of the carbon cycle and climate system. We demonstrate here that the inclusion of nutrient cycle dynamics, specifically the close coupling between carbon and nitrogen cycles, in a terrestrial biogeochemistry component of a global coupled climate system model leads to fundamentally altered behavior for several of the most critical feedback mechanisms operating between the land biosphere and the global climate system. Carbon‐nitrogen cycle coupling reduces the simulated global terrestrial carbon uptake response to increasing atmospheric CO2 concentration by 74%, relative to a carbon‐only counterpart model. Global integrated responses of net land carbon exchange to variation in temperature and precipitation are significantly damped by carbon‐nitrogen cycle coupling. The carbon cycle responses to temperature and precipitation variation are reduced in magnitude as atmospheric CO2 concentration rises for the coupled carbon‐nitrogen model, but increase in magnitude for the carbon‐only counterpart. Our results suggest that previous carbon‐only treatments of climate‐carbon cycle coupling likely overestimate the terrestrial biosphere's capacity to ameliorate atmospheric CO2 increases through direct fertilization. The next generation of coupled climate‐biogeochemistry model projections for future atmospheric CO2 concentration and climate change should include explicit, prognostic treatment of terrestrial carbon‐nitrogen cycle coupling.
Future changes in tropical cyclone properties are an important component of climate change impacts and risk for many tropical and midlatitude countries. In this study we assess the performance of a ...multimodel ensemble of climate models, at resolutions ranging from 250 to 25 km. We use a common experimental design including both atmosphere‐only and coupled simulations run over the period 1950–2050, with two tracking algorithms applied uniformly across the models. There are overall improvements in tropical cyclone frequency, spatial distribution, and intensity in models at 25 km resolution, with several of them able to represent very intense storms. Projected tropical cyclone activity by 2050 generally declines in the South Indian Ocean, while changes in other ocean basins are more uncertain and sensitive to both tracking algorithm and imposed forcings. Coupled models with smaller biases suggest a slight increase in average TC 10 m wind speeds by 2050.
Plain Language Summary
Tropical cyclones pose great risks to individuals and societies, particularly in terms of their local impacts, and how such risks may change in the future is a key question. In this work we use a common experimental framework with seven different state‐of‐the‐art global climate models, together with two different methods of identifying tropical cyclones. We find that the simulation of tropical cyclone frequency, spatial distribution, and intensity in some models approaches observed values with the model grid spacings of 20–50 km. Future projections to 2050 suggest that activity will generally decline in the South Indian Ocean while a more mixed picture is revealed in other regions.
Key Points
Biases in tropical cyclone distribution, frequency, and intensity are generally reduced in models at 25 km resolution
Northern Hemisphere basins show mixed responses to future forcing, while Southern Indian Ocean activity projected to decline
Future changes in 10 m wind speed in coupled models are mixed, and models with lower bias suggest small increases
This study examines how characteristics of tropical cyclones (TCs) that are explicitly resolved in a global atmospheric model with horizontal resolution of approximately 28 km are projected to change ...in a warmer climate using bias-corrected sea-surface temperatures (SSTs). The impact of mitigating from RCP8.5 to RCP4.5 is explicitly considered and is compared with uncertainties arising from SST projections. We find a
reduction
in overall global TC activity as climate warms. This reduction is somewhat less pronounced under RCP4.5 than under RCP8.5. By contrast, the frequency of very intense TCs is projected to increase dramatically in a warmer climate, with most of the increase concentrated in the NW Pacific basin. Extremes of storm related precipitation are also projected to become more common. Reduction in the frequency of extreme precipitation events is possible through mitigation from RCP8.5 to RCP4.5. In general more detailed basin-scale projections of future TC activity are subject to large uncertainties due to uncertainties in future SSTs. In most cases these uncertainties are larger than the effects of mitigating from RCP8.5 to RCP4.5.
Sensitivity to Glacial Forcing in the CCSM4 Brady, Esther C.; Otto-Bliesner, Bette L.; Kay, Jennifer E. ...
Journal of climate,
03/2013, Letnik:
26, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Results are presented from the Community Climate System Model, version 4 (CCSM4), simulation of the Last Glacial Maximum (LGM) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) at the ...standard 1° resolution, the same resolution as the majority of the CCSM4 CMIP5 long-term simulations for the historical and future projection scenarios. The forcings and boundary conditions for this simulation follow the protocols of the Paleoclimate Modeling Intercomparison Project, version 3 (PMIP3). Two additional CCSM4 CO₂ sensitivity simulations, in which the concentrations are abruptly changed at the start of the simulation to the low 185 ppm LGM concentrations (LGMCO₂) and to a quadrupling of the preindustrial concentration (4×CO₂), are also analyzed. For the full LGM simulation, the estimated equilibrium cooling of the global mean annual surface temperature is 5.5°C with an estimated radiative forcing of −6.2 W m−2. The radiative forcing includes the effects of the reduced LGM greenhouse gases, ice sheets, continental distribution with sea level lowered by approximately 120 m from the present, and orbital parameters, but not changes to atmospheric aerosols or vegetation biogeography. The LGM simulation has an equilibrium climate sensitivity (ECS) of 3.1(±0.3)°C, comparable to the CCSM4 4×CO₂ result. The LGMCO₂ simulation shows a greater ECS of 4.2°C. Other responses found at the LGM in CCSM4 include a global precipitation rate decrease at a rate of ∼2% °C−1, similar to climate change simulations in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4); a strengthening of the Atlantic meridional overturning circulation (AMOC) with a shoaling of North Atlantic Deep Water and a filling of the deep basin up to sill depth with Antarctic Bottom Water; and an enhanced seasonal cycle accompanied by reduced ENSO variability in the eastern Pacific Ocean’s SSTs.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
An overview of a simulation referred to as the ‘‘Last Millennium’’ (LM) simulation of the Community Climate System Model, version 4 (CCSM4), is presented. The CCSM4 LM simulation reproduces many ...large-scale climate patterns suggested by historical and proxy-data records, with Northern Hemisphere (NH) and Southern Hemisphere (SH) surface temperatures cooling to the early 1800s Common Era by ∼0.5°C (NH) and ∼0.3°C (SH), followed by warming to the present. High latitudes of both hemispheres show polar amplification of the cooling from the Medieval Climate Anomaly (MCA) to the Little Ice Age (LIA) associated with sea ice increases. The LM simulation does not reproduce La Niña–like cooling in the eastern Pacific Ocean during the MCA relative to the LIA, as has been suggested by proxy reconstructions. Still, dry medieval conditions over the southwestern and central United States are simulated in agreement with proxy indicators for these regions. Strong global cooling is associated with large volcanic eruptions, with indications of multidecadal colder climate in response to larger eruptions. The CCSM4’s response to large volcanic eruptions captures some reconstructed patterns of temperature changes over Europe and North America, but not those of precipitation in the Asian monsoon region. The Atlantic multidecadal oscillation (AMO) has higher variance at centennial periods in the LM simulation compared to the 1850 nontransient run, suggesting a long-term Atlantic Ocean response to natural forcings. The North Atlantic Oscillation (NAO), Pacific decadal oscillation (PDO), and El Niño–Southern Oscillation (ENSO) variability modes show little or no change. CCSM4 does not simulate a persistent positive NAO or a prolonged period of negative PDO during the MCA, as suggested by some proxy reconstructions.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We tested a variety of methods for determining steady state solutions for Biome-BGC, a coupled model of terrestrial water, carbon, and nitrogen dynamics. Our objective was to identify methods that ...could reduce the computational cost of model spin-up relative to simulations running under the model's native dynamics, while retaining or improving upon the simulation quality, where quality is judged by comparison to assumed values for the true steady state solution. Two classes of methods were tested: ad hoc methods that approximate steady state by taking advantage of specific characteristics of the modeled dynamics to produce individual time trajectories through the model state space, and general multivariate minimization methods that iteratively explore multiple time trajectories through state space in search of a reasonable steady state solution. We examined the behavior of these methods for both woody and herbaceous vegetation simulations. We found that both the ad hoc and the generalized methods, parameterized appropriately, could provide reductions in computational cost of 50–75% compared to the model's native dynamics. With the exception of the generalized methods for the woody vegetation case, we also found that the quality of the simulated steady state solution was as good as or better than the native dynamics approach. The one method which performed consistently well across sites and vegetation types involved an acceleration of decomposition rates for the spin-up phase, resulting in 73 and 66% reductions in computational cost for woody and herbaceous vegetation types, respectively.
How warm was the last interglacial? New model-data comparisons Otto-Bliesner, Bette L.; Rosenbloom, Nan; Stone, Emma J. ...
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
10/2013, Letnik:
371, Številka:
2001
Journal Article
Recenzirano
Odprti dostop
A Community Climate System Model, Version 3 (CCSM3) simulation for 125 ka during the Last Interglacial (LIG) is compared to two recent proxy reconstructions to evaluate surface temperature changes ...from modern times. The dominant forcing change from modern, the orbital forcing, modified the incoming solar insolation at the top of the atmosphere, resulting in large positive anomalies in boreal summer. Greenhouse gas concentrations are similar to those of the pre-industrial (PI) Holocene. CCSM3 simulates an enhanced seasonal cycle over the Northern Hemisphere continents with warming most developed during boreal summer. In addition, year-round warming over the North Atlantic is associated with a seasonal memory of sea ice retreat in CCSM3, which extends the effects of positive summer insolation anomalies on the high-latitude oceans to winter months. The simulated Arctic terrestrial annual warming, though, is much less than the observational evidence, suggesting either missing feedbacks in the simulation and/or interpretation of the proxies. Over Antarctica, CCSM3 cannot reproduce the large LIG warming recorded by the Antarctic ice cores, even with simulations designed to consider observed evidence of early LIG warmth in Southern Ocean and Antarctica records and the possible disintegration of the West Antarctic Ice Sheet. Comparisons with a HadCM3 simulation indicate that sea ice is important for understanding model polar responses. Overall, the models simulate little global annual surface temperature change, while the proxy reconstructions suggest a global annual warming at LIG (as compared to the PI Holocene) of approximately 1°C, though with possible spatial sampling biases. The CCSM3 SRES B1 (low scenario) future projections suggest high-latitude warmth similar to that reconstructed for the LIG may be exceeded before the end of this century.
The relationship between African dust and the climatology of tropical cyclones (TCs) in the North Atlantic is explored using the Community Atmosphere Model at a global horizontal resolution of 28 km. ...A simulation in which the aerosol model is modified to significantly reduce the amount of airborne dust is compared to a standard simulation. The simulation with reduced dust increases TC frequency globally, with the largest increase occurring in the North Atlantic. The increase in TC activity in the North Atlantic is consistent with an environment that is more conducive for the genesis and intensification of storms. TCs are more frequent (27%) and on average significantly longer lived (13%) in the low dust configuration but only slightly stronger (3%). This results in a 57% increase in accumulated cyclone energy per hurricane season on average. This work has implications for projections of future climate and resulting changes in TC activity.
Plain Language Summary
Dust from the Sahara desert in Africa can impact hurricane formation and development in the North Atlantic. Carried by the wind over the North Atlantic and around the globe, the dust contributes to dry and stable air that can make it harder for thunderstorms over the ocean to develop into stronger tropical cyclones or hurricanes. In this study, we look at how this effect shows up in a climate model with sufficient resolution to represent hurricanes. After the control experiment (1980–2012), we reduced the amount of airborne dust in the model over the same historical period as a low dust experiment. In the low dust simulation, tropical cyclone frequency increases globally. In the North Atlantic where storms can be impacted directly by African dust, storms become more frequent, longer lived, and slightly stronger, increasing their destructive potential. Understanding how dust interacts with tropical cyclones in climate models will help us understand and prepare for the potential changes in hurricanes of the future.
Key Points
Global and North Atlantic TC counts increase in a CAM5 global simulation with a large reduction in dust
The increase in North Atlantic TC genesis activity in the low dust simulation is consistent with the change in the genesis potential index
Simulated TCs are more intense and longer in duration in the low dust configuration
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