Philip B. Duffy

Fine-resolution climate projections enhance regional climate change impact studies

A new data set enhances the abilities of researchers and decision-makers to assess possible future climates, explore societal impacts, and approach policy responses from a risk-based perspective. The data set, which consists of a library of 112 fine-resolution climate projections, based on 16 climate models and three greenhouse gas emissions scenarios, is now publicly available. Monthly climate projections from 1950 to 2099 were downscaled to a spatial resolution of ⅛° (about 140 square kilometers per grid cell) covering the conterminous United States and portions of Canada and Mexico. For the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, climate modeling groups produced hundreds of simulations of past and future climates. The colocation of these simulations in a single archive (at the Program for Climate Model Diagnosis and Intercomparison at Lawrence Livermore National Laboratory (LLNL), established to facilitate assessment of general circulation models, or GCMs) and the conversion of all results to a common data format have made probabilistic, multi-model projections and impact assessments practical. A remaining issue is that the spatial scale of climate model output is typically too coarse for regional impact studies. Multiple downscaling approaches exist for deriving regional climate from coarse-resolution model output; these approaches are typically applied on an ad hoc basis to a particular region.

Impact of geoengineering schemes on the global hydrological cycle.

The rapidly rising CO2 level in the atmosphere has led to proposals of climate stabilization by “geoengineering” schemes that would mitigate climate change by intentionally reducing solar radiation incident on Earths surface. In this article we address the impact of these climate stabilization schemes on the global hydrological cycle. By using equilibrium climate simulations, we show that insolation reductions sufficient to offset global-scale temperature increases lead to a decrease in global mean precipitation. This occurs because solar forcing is more effective in driving changes in global mean evaporation than is CO2 forcing of a similar magnitude. In the model used here, the hydrological sensitivity, defined as the percentage change in global mean precipitation per degree warming, is 2.4% K−1 for solar forcing, but only 1.5% K−1 for CO2 forcing. Although other models and the climate system itself may differ quantitatively from this result, the conclusion can be understood based on simple considerations of the surface energy budget and thus is likely to be robust. For the same surface temperature change, insolation changes result in relatively larger changes in net radiative fluxes at the surface; these are compensated by larger changes in the sum of latent and sensible heat fluxes. Hence, the hydrological cycle is more sensitive to temperature adjustment by changes in insolation than by changes in greenhouse gases. This implies that an alteration in solar forcing might offset temperature changes or hydrological changes from greenhouse warming, but could not cancel both at once.

Land use changes and northern hemisphere cooling

Recent reconstructions of Northern Hemisphere mean temperatures over the past millennium show a long-term cooling of about 0.25K between 1000 and 1900 AD, prior to the 20th centurys warming. In this paper, we present the results of equilibrium climate model simulations that indicate that the land-use change occurring over this period may largely explain this observed cooling, although other factors also could have played a significant role. The simulated annual mean cooling due to land-use change is 0.25K globally and 0.37 K for the Northern Hemisphere, suggesting that the cooling of the prior centuries could have been largely the result of anthropogenic interference in the climate system.

Geoengineering Earth's radiation balance to mitigate climate change from a quadrupling of CO2

It has been suggested that climate change induced by anthropogenic CO2 could be counteracted with geoengineering schemes designed to diminish the solar radiation incident on Earth’s surface. Though the spatial and temporal pattern of radiative forcing from greenhouse gases differs from that of sunlight, it was shown in a recent study that these schemes would largely mitigate regional or seasonal climate change for a doubling of the atmospheric CO2 content. Here, we examine the ability of reduced solar luminosity to cancel the effects of quadrupling of CO2 content. In agreement with our previous study, geoengineering schemes could markedly diminish regional and seasonal climate change. However, there are some residual climate changes: in the geoengineered 4CO2 climate, a significant decrease in surface temperature and net water flux occurs in the tropics; warming in the high latitudes is not completely compensated; the cooling effect of greenhouse gases in the stratosphere persists and sea ice is not fully restored. However, these residual climate changes are much smaller than the change from quadrupling of CO2 without reducing solar input. Caution should be exercised in interpretation because these results are from a single model with a number of simplifying assumptions. There are also many technical, environmental and political reasons not to implement geoengineering schemes. D 2003 Elsevier Science B.V. All rights reserved.

Detection, attribution, and sensitivity of trends toward earlier streamflow in the Sierra Nevada

[1] Observed changes in the timing of snowmelt dominated streamflow in the western United States are often linked to anthropogenic or other external causes. We assess whether observed streamflow timing changes can be statistically attributed to external forcing, or whether they still lie within the bounds of natural (internal) variability for four large Sierra Nevada (CA) basins, at inflow points to major reservoirs. Streamflow timing is measured by ‘‘center timing’’ (CT), the day when half the annual flow has passed a given point. We use a physically based hydrology model driven by meteorological input from a global climate model to quantify the natural variability in CT trends. Estimated 50-year trends in CT due to natural climate variability often exceed estimated actual CT trends from 1950 to 1999. Thus, although observed trends in CT to date may be statistically significant, they cannot yet be statistically attributed to external influences on climate. We estimate that projected CT changes at the four major reservoir inflows will, with 90% confidence, exceed those from natural variability within 1–4 decades or 4–8 decades, depending on rates of future greenhouse gas emissions. To identify areas most likely to exhibit CT changes in response to rising temperatures, we calculate changes in CT under temperature increases from 1 to 5. We find that areas with average winter temperatures between 2C and 4C are most likely to respond with significant CT shifts. Correspondingly, elevations from 2000 to 2800 m are most sensitive to temperature increases, with CT changes exceeding 45 days (earlier) relative to 1961–1990.

Effects of Subgrid-Scale Mixing Parameterizations on Simulated Distributions of Natural 14C, Temperature, and Salinity in a Three-Dimensional Ocean General Circulation Model

Abstract The effects of parameterizations of subgrid-scale mixing on simulated distributions of natural 14C, temperature, and salinity in a three-dimensional ocean general circulation model are examined. The parameterizations studied are 1) the Gent–McWilliams parameterization of lateral transport of tracers by isopycnal eddies; 2) horizontal mixing; 3) a parameterization of vertical mixing in which the amount of mixing depends on the local vertical density gradient; and 4) prescribed vertical mixing. The authors perform and analyze four ocean GCM simulations that use different combinations of these parameterizations. It is confirmed that the Gent–McWilliams parameterization largely eliminates the tendency of GFDL-based models to overestimate temperatures in the thermocline. However, in the authors’ simulations with the Gent–McWilliams parameterization the deep ocean is too cold, in places by more than 3 degrees. Our results are the first known to assess the effects of the Gent–McWilliams parameterization...

Increasing prevalence of extreme summer temperatures in the U.S.

Human-caused climate change can affect weather and climate extremes, as well as mean climate properties. Analysis of observations and climate model results shows that previously rare (5th percentile) summertime average temperatures are presently occurring with greatly increased frequency in some regions of the 48 contiguous United States. Broad agreement between observations and a mean of results based upon 16 global climate models suggests that this result is more consistent with the consequences of increasing greenhouse gas concentrations than with the effects of natural climate variability. This conclusion is further supported by a statistical analysis based on resampling of observations and model output. The same climate models project that the prevalence of previously extreme summer temperatures will continue to increase, occurring in well over 50% of summers by mid-century.

Radiocarbon as a diagnostic tracer in ocean and carbon cycle modeling

Models which are used to study the evolution of oceanic anthropogenic CO2 uptake in a warmer climate need to be adequately tested before their results are used to influence science and national policy, let alone predict future regional climate. In order to study circulation and the redistribution of carbon in the ocean, we have simulated the uptake and redistribution of bomb radiocarbon in a state of the art ocean general circulation model. The model does reasonably well in simulating the gross surface prebomb distribution of radiocarbon as well as some of the finer details resolved during Geochemical Ocean Section Study and World Ocean Circulation Experiment observations. In areas of downwelling the simulated surface-ocean bomb 14C maxima occur too early with too much amplitude. This indicates inadequate vertical mixing and coupling between the wind driven and deeper regions of the ocean with a piling up of radiocarbon and CO2 in the upper ocean. This inadequacy will hamper efforts to predict ocean CO2 uptake on decadal to centennial timescales, the equivalent ventilation time of the respective water masses. These results are not specific to our model and indicate a systemic problem in many ocean models.

Sensitivity of simulated salinity in a three-dimensional ocean model to upper ocean transport of salt from sea-ice formation

We show that explicit representation of sinking of salt rejected during sea-ice formation dramatically improves simulated salinity in an ocean general circulation model (OGCM). In our “control” simulation, rejected salt goes into the top model layer, and simulated salinities are typical of OGCMs: the deep ocean is too fresh, and the intermediate-depth salinity minimum associated with Antarctic Intermediate Water is absent. These problems are eliminated in our “test” simulation, in which we distribute rejected salt uniformly over the upper 160 m. Also, the strength of the Antarctic Circumpolar Current is more realistic in this simulation. These results show the need for, but do not provide, a better representation of sinking of rejected salt. The sensitivity of our model to sinking of rejected salt suggests that a similar sensitivity may exist in the real ocean, and that loss of Antarctic sea ice might have major effects.

Effects of sinking of salt rejected during formation of sea ice on results of an ocean‐atmosphere‐sea ice climate model

We show that results of an ocean-atmosphere-sea-ice model are sensitive to the treatment of salt rejected during formation of sea ice. In our Control simulation, we place all rejected salt in the top ocean-model level. In the Plume simulation, we instantaneously mix rejected salt into the subsurface ocean, to a maximum depth which depends on local density gradients. This mimics the effects of subgrid-scale convection of rejected salt. The results of the Plume simulation are more realistic than those of the Control simulation: the spatial pattern of simulated salinities (especially in the Southern Ocean), deep-ocean temperatures, simulated sea-ice extents and surface air temperatures all agree better with observations. A similar pair of simulations using horizontal tracer diffusion instead of the Gent-McWilliams eddy parameterization show similar changes due to instantaneous mixing of rejected salt.