This was the first study to use data related to greenhouse gas emissions from the construction and operation of different types of power plants to examine the greenhouse gas and climate consequences of specific energy system transitions. The central cases examined was a 40-year transitions from a coal-based electricity generation system to lower-greenhouse-gas-emission alternatives. A primary conclusion was that to achieve substantial climate benefits this century, we must transition to the very lowest emitting electricity generation technologies.
Ban-Weiss, G A., G. Bala, L. Cao, J. Pongratz, and K. Caldeira (2011) Climate forcing and response to idealized changes in surface latent and sensible heat. Environ. Res. Lett. 6, 034032.
It has long been known that evaporation of water can cause local cooling, but that water must condense somewhere else in Earth’s climate system, and wherever it condenses there will be local heating. Previously, it was not known whether evaporation of water, on average, causes the Earth to cool globally or whether it simply represents a transfer of energy within Earth’s climate system. This study showed that, because increased evaporation tends to create more low clouds that reflect sunlight to space, for each unit of energy that goes into evaporating the water, an amount of sunlight equivalent to about 3/4 of that energy is reflected to space. Thus, this study showed for the first time that, on average, evaporation of water causes global and not merely local cooling.
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Davis, S.J., G.P. Peters and K. Caldeira (2011) The Supply Chain of CO2 Emissions. PNAS www.pnas.org/cgi/doi/10.1073/pnas.1107409108
This was the first study to follow the international supply chain of CO2 emissions from the time the carbon-based fuel is extracted from the ground, to the time it is burned in a power-plant to provide power to produce products (perhaps in another country), to the time when the product is consumed (perhaps in a third country). This study illustrated the way actors all along the supply change have incentives to keep the current system operating. The extraction businesses want to profit by selling fuels into the marketplace; businesses producing consumer products want to benefit from low fossil fuel prices; and the consumer wants to benefit from lower prices of products. Nevertheless, everyone along this supply chain has an interest in reducing the climate risk that the current energy system entails. This work was built on Davis and Caldeira (Consumption-based accounting of CO2 emissions, PNAS, 2010), which was one of the first studies to examine carbon dioxide emissions embodied in international trade.
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Ban-Weiss, G. A., & K. Caldeira (2010) Geoengineering as an optimization problem. ERL, doi:10.1088/1748-9326/5/3/034009
This was the first study to treat intentional manipulation of Earth’s climate system as a mathematical optimization problem. Previous studies of intentional climate engineering asked the question “What would happen if we did X?”. This study was the first to ask, “How would we manipulate Earth’s climate system if we want Y?” This study showed that it is impossible to achieve all climate goals simultaneously, and that, for example, that deflecting sunlight in a pattern that would more closely restore Earth’s pre-industrial hydrological cycle will degrade ability to restore Earth’s pre-industrial temperature patterns. The decision of whether and how to intervene in Earth’s climate system will depend on value judgments about what is most important to us.
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Davis, S. J., K. Caldeira, and H. D. Matthews (2010) Future CO2 emissions and climate change from existing energy infrastructure, Science, doi:10.1073/pnas.0906974107
There has been a lot of discussion of trying to limit global warming at 2 C above the temperatures that prevailed a few centuries ago. This study was the first to ask: If we allow existing CO2 emitting devices (e.g., power plants, automobiles) to live out their normal useful lives, have we already built enough stuff to push us over this 2 degree benchmark? The study found that existing CO2-emitting devices are unlikely to be sufficient to reach that 2 degree benchmark, and that atmospheric CO2 concentrations would like remain well below 450 ppm. Thus, the largest threats from climate change come from infrastructure that is yet to be built.
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Cao, L; Caldeira, K (2008) Atmospheric CO2 stabilization and ocean acidification, Geophysical Research Letters 35 (19), DOI: 10.1029/2008GL035072.
Cao and Caldeira performed a study of effects of ocean acidification for CO2 stabilization at different levels responding to a specific request from Congressional staffers. They looked at how corals might be affected at different CO2 stabilization levels. Corals make their skeletons out of aragonite and their ability to construct skeletons is related to the degree to which sea water is saturated with respect to aragonite. Cao and Caldeira find that several centuries ago 99.8% of corals were surrounded by open ocean waters that were > 3.25 times saturated with respect to aragonite. By the time atmospheric CO2 content reaches 650 ppm, which could occur later this century, no such water will exist in the oceans, suggesting that coral reefs may not be sustainable under business as usual emission scenarios.
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Matthews, HD; Caldeira, K (2008) Stabilizing climate requires near-zero emissions, Geophysical Research Letters 35 (4), DOI: 10.1029/2007GL032388
This study showed that, to a close approximation, each emission of carbon dioxide produces another increment of warming, and thus to prevent further increments of warming we would need to prevent further CO2 emissions. This is a fundamental result that was later taken up by others who proposed setting limits on cumulative emissions to stay within specific temperature boundaries.
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Bala G, Caldeira, K., Wickett M, et al. (2007) Combined climate and carbon-cycle effects of large-scale deforestation, Proceedings of the National Academy of Sciences of the United States of America 104 (16): 6550-6555.
This study was the first study to look at effects of deforestation in a three dimensional coupled-atmosphere-ocean carbon-climate model to look at tradeoffs and synergies involving carbon storage and biophysical climate forcing (i.e., changes in reflectivity, evapotranspiration, etc). This study found that tropical forests tend to have strong cooling influences because the store a lot of carbon and the evapotranspiration helps maintain clouds that reflect sunlight to space. In contrast, at high latitudes, snowy fields reflect much more sunlight than snowy forests, and this reflectivity effect is more important than the carbon storage effect yielding a warming influence for high latitude forests. These results have been supported by later studies performed by other teams of researchers.
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Matthews, H.D., and K. Caldeira, Transient climate-carbon simulations of planetary geoengineering, Proceedings of the National Academy of Sciences of the United States of America 104 (24): 9949-9954, 2007.
This was the first study to look at effects of solar geoengineering (i.e., intentionally reflecting sunlight away from the Earth to counteract some effects of high atmospheric greenhouse gas concentrations) in a time-dependent model. This was the first study to show how fast climate could cool if such a system were suddenly implements and how rapidly the Earth could warm if such a system were suddenly to fail.
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Bala, G., K. Caldeira, A. Mirin, M. Wickett and C. Delire, Multicentury changes to the global climate and carbon cycle: Results from a coupled climate and carbon cycle model. Journal of Climate 18 (21) 4531-4544, 2005.
This study was the first study to use a coupled three-dimensional atmosphere-ocean carbon-climate model to address the question of what would occur to the global climate system over the next centuries if greenhouse gas emissions were not abated. Similar simulations are now being done by many other modeling groups.
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Caldeira, K., and M.E. Wickett, Anthropogenic carbon and ocean pH, Nature 425, 365-365, 2003.
This study predicted future changes in ocean pH and compared these changes with changes inferred for Earth’s geologic past. This study was the first study to point out that anthropogenic CO2 released to the atmosphere, and later absorbed by the ocean, is causing ocean chemistry changes that are extremely unusual even when viewed from the perspective of broad expanses of geologic time. This study has been credited with playing a large role in developing current concern about ocean acidification, “the other CO2 problem”.
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Caldeira, K., A.K. Jain, and M.I. Hoffert, Climate sensitivity uncertainty and the need for energy without CO2 emission, Science 299, 2052-2054, 2003.
This was the first study to predict carbon-emission-free energy needs needed to stabilize climate at 2 degrees C. This study looked at the amount of carbon-emission-free energy would be needed to stabilize climate at a 2 C warming and how that estimated amount would vary as a function of climate sensitivity to added atmospheric CO2.
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Caldeira, K., and G.H. Rau, Accelerating carbonate dissolution to sequester carbon dioxide in the ocean: Geochemical implications, Geophysical Research Letters, 27, 225–228, 2000.
This was the first study to use a model of the atmosphere and ocean to evaluate the potential to store carbon in the ocean by adding alkalinity to the ocean from dissolved limestone. This approach could potentially also be used to offset effects of ocean acidification.
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Govindasamy, B., and K. Caldeira, Geoengineering Earth’s radiation balance to mitigate CO2-induced climate change, Geophysical Research Letters 27, 2141-2144, 2000.
This was the first three-dimensional climate modeling study to simulate proposed sunshade geoengineering. The basic conclusion was that such schemes could potentially offset most climate change in most places most of the time.
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Hoffert M.I., K. Caldeira, A.K. Jain, E.F. Haites, L.D.D. Harvey, S. D. Potter, M.E. Schlesinger, S. H. Schneider, R.G. Watts, T. M. L Wigley, and D. J. Wuebbles. Energy implications of future stabilization of atmospheric CO2 content. Nature 395, 881–884, 1998.
This was the first study ever published to estimate the amount of carbon-emission-free power that would be needed to stabilize climate at various CO2 levels under a standard set of economic assumptions. It also showed how increasing efficiency of energy use would diminish the amount of carbon-emission-free power that would be needed. The basic conclusion is that to stabilize atmospheric CO2 content, we will need to construct carbon-emission free energy systems over the next decades that is larger in scale than today’s CO2-emitting energy systems. Caldeira believes this to be the most important study of his career.
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Caldeira, K. and J.F. Kasting, Insensitivity of global warming potentials to carbon dioxide emission scenarios, Nature 366, 251–253, 1993.
At higher CO2 concentration, CO2 emissions tend to remain in the atmosphere longer because of non-linearities in ocean carbonate chemistry. However, that CO2 in the atmosphere has less of a radiative effect because of the log-like relationship between atmospheric concentrations and radiative forcing of CO2. Thus, this chemical mechanism partitioning more CO2 into the atmosphere is opposite in sign to the effect of diminishing radiative effects of increased CO2 at higher concentrations. To a first approximation, these two effects cancel making the amount of warming per unit CO2 addition relatively insensitive to the background CO2 scenario. Decades later this concept has been developed into the concept of limits on cumulative CO2 emissions.
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