Art Mirin

Attribution of Declining Western U.S. Snowpack to Human Effects

Observations show snowpack has declined across much of the western United States over the period 1950–99. This reduction has important social and economic implications, as water retained in the snowpack from winter storms forms an important part of the hydrological cycle and water supply in the region. A formal model-based detection and attribution (D–A) study of these reductions is performed. The detection variable is the ratio of 1 April snow water equivalent (SWE) to water-year-to-date precipitation (P), chosen to reduce the effect of P variability on the results. Estimates of natural internal climate variability are obtained from 1600 years of two control simulations performed with fully coupled ocean–atmosphere climate models. Estimates of the SWE/P response to anthropogenic greenhouse gases, ozone, and some aerosols are taken from multiple-member ensembles of perturbation experiments run with two models. The D–A shows the observations and anthropogenically forced models have greater SWE/P reductions than can be explained by natural internal climate variability alone. Model-estimated effects of changes in solar and volcanic forcing likewise do not explain the SWE/P reductions. The mean model estimate is that about half of the SWE/P reductions observed in the west from 1950 to 1999 are the result of climate changes forced by anthropogenic greenhouse gases, ozone, and aerosols.

Regional Differences in the Influence of Irrigation on Climate

Abstract A global climate model experiment is performed to evaluate the effect of irrigation on temperatures in several major irrigated regions of the world. The Community Atmosphere Model, version 3.3, was modified to represent irrigation for the fraction of each grid cell equipped for irrigation according to datasets from the Food and Agriculture Organization. Results indicate substantial regional differences in the magnitude of irrigation-induced cooling, which are attributed to three primary factors: differences in extent of the irrigated area, differences in the simulated soil moisture for the control simulation (without irrigation), and the nature of cloud response to irrigation. The last factor appeared especially important for the dry season in India, although further analysis with other models and observations are needed to verify this feedback. Comparison with observed temperatures revealed substantially lower biases in several regions for the simulation with irrigation than for the control, sug...

Detection and Attribution of Temperature Changes in the Mountainous Western United States

Abstract Large changes in the hydrology of the western United States have been observed since the mid-twentieth century. These include a reduction in the amount of precipitation arriving as snow, a decline in snowpack at low and midelevations, and a shift toward earlier arrival of both snowmelt and the centroid (center of mass) of streamflows. To project future water supply reliability, it is crucial to obtain a better understanding of the underlying cause or causes for these changes. A regional warming is often posited as the cause of these changes without formal testing of different competitive explanations for the warming. In this study, a rigorous detection and attribution analysis is performed to determine the causes of the late winter/early spring changes in hydrologically relevant temperature variables over mountain ranges of the western United States. Natural internal climate variability, as estimated from two long control climate model simulations, is insufficient to explain the rapid increase in...

Quantifying the effects of CO2‐fertilized vegetation on future global climate and carbon dynamics

Climate and the global carbon cycle are a tightly coupled system where changes in climate affect exchange of atmospheric CO{sup 2} with the land biosphere and the ocean, and vice-versa. In particular, the response of the land biosphere to the ongoing increase in atmospheric CO{sup 2} is not well understood. To evaluate the approximate upper and lower limits of land carbon uptake, we perform simulations using a comprehensive climate-carbon model. In one case the land biosphere is vigorously fertilized by added CO{sup 2} and sequesters carbon throughout the 21st century. In a second case, CO{sup 2} fertilization saturates in year 2000; here the land becomes an additional source of CO{sup 2} by 2050. The predicted atmospheric CO{sup 2} concentration at year 2100 differs by 40% between the two cases. We show that current uncertainties preclude determination of whether the land biosphere will amplify or damp atmospheric CO{sup 2} increases by the end of the century.

Increase of carbon cycle feedback with climate sensitivity: results from a coupled climate and carbon cycle model

Coupled climate and carbon cycle modelling studies have shown that the feedback between global warming and the carbon cycle, in particular the terrestrial carbon cycle, could accelerate climate change and result in greater warming. In this paper we investigate the sensitivity of this feedback for year 2100 global warming in the range of 0 to 8 K. Differing climate sensitivities to increased CO2content are imposed on the carbon cycle models for the same emissions. Emissions from the SRES A2 scenario are used. We use a fully coupled climate and carbon cycle model, the INtegrated Climate and CArbon model (INCCA), the NCAR/DOE Parallel Climate Model coupled to the IBIS terrestrial biosphere model and a modified OCMIP ocean biogeochemistry model. In our integrated model, for scenarios with year 2100 global warming increasing from 0 to 8 K, land uptake decreases from 47% to 29% of total CO2emissions. Due to competing effects, ocean uptake (16%) shows almost no change at all. Atmospheric CO2 concentration increases are 48% higher in the run with 8 K global climate warming than in the case with no warming. Our results indicate that carbon cycle amplification of climate warming will be greater if there is higher climate sensitivity to increased atmospheric CO2 content; the carbon cycle feedback factor increases from 1.13 to 1.48 when global warming increases from 3.2 to 8 K.

Biogeophysical effects of CO2 fertilization on global climate

CO2 fertilization affects plant growth, which modifies surface physical properties, altering the surface albedo, and fluxes of sensible and latent heat.We investigate how such CO2-fertilization effects on vegetation and surface properties would affect the climate system. Using a global three-dimensional climate-carbon model that simulates vegetation dynamics, we compare two multicentury simulations: a ‘Control’ simulation with no emissions and a ‘Physiol-noGHG’ simulation where physiological changes occur as a result of prescribedCO2 emissions, but whereCO2-induced greenhousewarming is not included. In our simulations, CO2 fertilization produces warming; we obtain an annual- and global-mean warming of about 0.65 K (and land-only warming of 1.4 K) after 430 yr. This century-scale warming is mostly due to a decreased surface albedo associated with the expansion of the Northern Hemisphere boreal forests. On decadal timescales, the CO2 uptake by afforestation should produce a cooling effect that exceeds this albedo-based warming; but if the forests remain in place, the CO2-enhanced-greenhouse effect would diminish as the ocean equilibrates with the atmosphere, whereas the albedo effect would persist. Thus, on century timescales, there is the prospect for net warming from CO2 fertilization of the land biosphere. Further study is needed to confirm and better quantify our results.

Simulated climate near steep topography: Sensitivity to numerical methods for atmospheric transport

[1] We present the sensitivity of the simulated climate near steep topographical regions when the numerical method for atmospheric transport in the Community Climate System Model (CCSM3) is changed from spectral to a finite volume (FV) transport. Our analysis of the circulation and precipitation shows significant local improvement in three aspects: 1) The Gibbs oscillations present in the cloudiness and shortwave radiative forcing fields in the spectral simulation are absent in the FV simulation. 2) The alongshorecomponentofwindstressinthewesterncoastalregions of North and South America increases in the FV simulation. This tends to reduce the persistent biases in sea surface temperature through enhanced oceanic upwelling. 3) The FV simulation shows improvement in the wet-dry contrast of orographically forced precipitation. These local improvements have impact on continental and larger scales and are critical to the confident use of information from climate predictions in adaptation to climate change. Citation: Bala, G., R. B. Rood, D. Bader, A. Mirin, D. Ivanova, and C. Drui (2008), Simulated climate near steep topography: Sensitivity to numerical methods for atmospheric transport, Geophys. Res. Lett., 35, L14807, doi:10.1029/2008GL033204.