Harvard Kennedy School

Wind power can impact the climate by altering the atmospheric boundary layer, with at least 40 papers and 10 observational studies now linking wind power to climatic impacts. We make the first comparison between the climatic impacts of large-scale wind power and site-scale observations, finding agreement that warming from wind turbines is largest at night. Wind power's climatic impacts will continue to expand as more are installed.

Do these impacts matter? How do these impacts compare to the climate benefits of reducing emissions? We offer policy-relevant comparisons: wind's climatic impacts are about 10 times larger than solar photovoltaic systems per unit energy generated. We explore the temporal trade-off between wind's climatic impacts and the climate benefits it brings by reducing emissions as it displaces fossil fuels. Quantitative comparisons between low-carbon energy sources should inform energy choices in the transition to a carbon-free energy system.

Solar radiation management (SRM), a form of geoengineering, might be used to offset some fraction of the anthropogenic radiative forcing of climate as a means to reduce climate change, but the risks and effectiveness of SRM are uncertain. We examine the possibility of testing SRM through sub-scale deployment as a means to test models of climate response to SRM and explore risks prior to full-scale implementation. Contrary to some claims, this could provide meaningful tests of the climate’s response to SRM within a decade. We use idealized simulations with the HadCM3L general circulation model (GCM) to estimate the response to SRM and signal-to-noise ratio for global-scale SRM forcing tests, and quantify the trade-offs between duration and intensity of the test and it’s ability to make quantitative measurements of the climate’s response to SRM forcing. The response at long time-scales would need to be extrapolated from results measured by a short-term test; this can help reduce the uncertainty associated with relatively rapid climate feedbacks, but uncertainties that only manifest at long time-scales can never be resolved by such a test. With this important caveat, the transient climate response may be bounded with 90% confidence to be no more than (1.5^{\circ} C) higher than it’s estimated value, in a single decade test that used roughly 1/10th the radiative forcing perturbation of a (CO_2)-doubling. However, tests could require several decades or longer to obtain accurate response estimates, particularly to understand the response of regional hydrological fields which are critical uncertainties. Some fields, like precipitation over land, have as large a response to short period forcing as to slowly- varying changes. This implies that the ratio of the hydrological to the temperature response that results from a sustained SRM deployment will differ from that of either a short-duration test or that which has been observed to result from large volcanic eruptions.

If implementation of proposals to engineer the climate through solar-radiation management (SRM) ever occurs, it is likely to be contingent on climate sensitivity. However, modelling studies examining the effectiveness of SRM as a strategy to offset anthropogenic climate change have used only the standard parameterizations of atmosphere–ocean general circulation models that yield climate sensitivities close to the Coupled Model Intercomparison Project mean. Here, we use a perturbed-physics ensemble modelling experiment to examine how the response of the climate to SRM implemented in the stratosphere (SRM-S) varies under different greenhouse-gas climate sensitivities. When SRM-S is used to compensate for rising atmospheric concentrations of greenhouse gases, its effectiveness in stabilizing regional climates diminishes with increasing climate sensitivity. However, the potential of SRM-S to slow down unmitigated climate change, even regionally, increases with climate sensitivity. On average, in variants of the model with higher sensitivity, SRM-S reduces regional rates of temperature change by more than 90% and rates of precipitation change by more than 50%.

We perform engineering cost analyses of systems capable of delivering 1–5 million metric tonnes (Mt) of albedo modification material to altitudes of 18–30 km. The goal is to compare a range of delivery systems evaluated on a consistent cost basis. Cost estimates are developed with statistical cost estimating relationships based on historical costs of aerospace development programs and operations concepts using labor rates appropriate to the operations. We evaluate existing aircraft cost of acquisition and operations, perform in-depth new aircraft and airship design studies and cost analyses, and survey rockets, guns, and suspended gas and slurry pipes, comparing their costs to those of aircraft and airships. Annual costs for delivery systems based on new aircraft designs are estimated to be $1–3B to deliver 1 Mt to 20–30 km or $2–8B to deliver 5 Mt to the same altitude range. Costs for hybrid airships may be competitive, but their large surface area complicates operations in high altitude wind shear, and development costs are more uncertain than those for airplanes. Pipes suspended by floating platforms provide low recurring costs to pump a liquid or gas to altitudes as high as ~ 20 km, but the research, development, testing and evaluation costs of these systems are high and carry a large uncertainty; the pipe system's high operating pressures and tensile strength requirements bring the feasibility of this system into question. The costs for rockets and guns are significantly higher than those for other systems. We conclude that (a) the basic technological capability to deliver material to the stratosphere at million tonne per year rates exists today, (b) based on prior literature, a few million tonnes per year would be sufficient to alter radiative forcing by an amount roughly equivalent to the growth of anticipated greenhouse gas forcing over the next half century, and that (c) several different methods could possibly deliver this quantity for less than $8B per year. We do not address here the science of aerosols in the stratosphere, nor issues of risk, effectiveness or governance that will add to the costs of solar geoengineering.

Estimates of the global wind power resource over land range from 56 to 400 TW. Most estimates have implicitly assumed that extraction of wind energy does not alter large-scale winds enough to significantly limit wind power production. Estimates that ignore the effect of wind turbine drag on local winds have assumed that wind power production of 2–4 W m2 can be sustained over large areas. New results from a mesoscale model suggest that wind power production is limited to about 1 W m2 at wind farm scales larger than about 100 km2. We find that the mesoscale model results are quantitatively consistent with results from global models that simulated the climate response to much larger wind power capacities. Wind resource estimates that ignore the effect of wind turbines in slowing large-scale winds may therefore substantially overestimate the wind power resource.