Bednar, J., Obersteiner, M., Baklanov, A., Thomson, M., Wagner, F., Geden, O., Allen, M., Hall, J. W. 2021. Operationalizing the net-negative carbon economy. Nature.
Abstract. The remaining carbon budget for limiting global warming to 1.5°C will likely be exhausted within this decade. Carbon debt generated thereafter will need to be compensated by net negative emissions. However, economic policy instruments to guarantee potentially very costly net carbon-dioxide removal (CDR) have not yet been devised. Here, we propose intertemporal instruments to provide the basis for widely applied carbon taxes and emission trading systems to finance a net negative carbon economy. We investigate an idealized market approach to incentivize repayment of previously accrued carbon debt by establishing emitters’ responsibility for net carbon removal through ‘Carbon Removal Obligations’ (CROs). Inherent risks, such as the default risk of carbon debtors, are addressed by pricing atmospheric CO2 storage through interest on carbon debt. In contrast to the prevailing literature on emission pathways, we find that interest payments for CROs induce substantially more ambitious near-term decarbonization complemented by earlier and less aggressive deployment of CDR. We conclude that CROs will need to become an integral part of the global climate policy mix if we are to ensure the viability of ambitious climate targets and an equitable distribution of mitigation efforts across generations.
Thomson, M.J. and MacDonald, G.M., 2020. Climate and growing season variability impacted the intensity and distribution of Fremont maize farmers during and after the Medieval Climate Anomaly based on a statistically downscaled climate model. Environmental Research Letters.

Abstract. The rise and decline of many complex, pre-European maize-farming cultures in the American Southwest coincides with the warm, climatically quiescent Medieval Climate Anomaly (MCA, ca. 850-1350 CE) and transition to the cool, hydrologically variable Little Ice Age (LIA, ca. 1350-1850 CE). The effects of drought on early subsistence agriculture in the Southwest is well studied, but the impact of temperature stress and variability on the growth of maize crops and which areas were most resilient to such stress remain open questions. We statistically downscaled outputs from a paleo-climate model experiment (CESM1 LME) to map changes to cumulative growing degree days for maize (GDD, 30/10°C) and precipitation over Utah between 850 and 1449 CE, and downscaled GDD changes to local Fremont Culture archaeological site occupations from radiocarbon-dated contexts mapped as spatially discrete kernel density estimates of summed probability distributions (SPDs). We then analyzed correspondences between Fremont SPDs and GDD/precipitation between 850 and 1449 CE. In general, we found (1) high Fremont occupation intensity coincident with GDD that is less volatile than the long term average, and low occupation intensity coincident with, or following, periods of volatile GDD; (2) intensified occupation of high-elevation sites during the MCA, followed by a retreat to lower elevation sites coincident with a sudden rise in annual temperature volatility and increasing drought conditions; and (3) these occupation changes occurred in spite of the greater temperatures and variability in GDD at low-elevation sites. We found evidence that increased inter-annual variability of growing seasons prior to the onset of the LIA, was likely a determinant of Fremont subsistence strategy decision making, and high-elevation site occupation. The most resilient Fremont occupations in the face of these challenges were sited where growing season lengths were least variable.
Thomson, M. J. (In press). Integrated Assessment Models. The International Encyclopedia of Geography: People, the Earth, Environment, and Technology. Wiley.

Abstract: Integrated assessment models (IAMs) are analytical frameworks that combine computer models of physical and socio-economic phenomena to provide insights on their interactions. There are two major classes of IAMs, CB-IAMs and DP-IAMs. CB-IAMs are highly reduced representations of socioeconomic and climate sectors used for cost-benefit analyses of economic growth balanced against future economic damage due to climate change. DP-IAMs are higher complexity models used to determine pathways to future greenhouse gas mitigation targets, or analyze impacts of environmental change on sectors they represent. IAMs play a central role in the economic, social, and political science of climate change research and policy-making. Ensuring geographical variation and connectivity is properly incorporated into IAMs is an important challenge.

FOLU Coalition. (2019). Growing Better: Ten Critical Transitions to Transform Food and Land Use. The Global Consultation Report of the Food and Land Use Coalition September 2019.
Mosnier, A., et al. (The FABLE Consortium) (2019). 2019 Report of the FABLE Consortium: Pathways to Sustainable Land-Use and Food Systems. Laxenburg and Paris: International Institute for Applied Systems Analysis (IIASA) and Sustainable Development Solutions Network (SDSN).

Summary. This first report by the FABLE Consortium presents preliminary pathways towards sustainable land-use and food systems prepared by the 18 country teams from developed and developing countries, including the European Union. The aim of these pathways is to determine and demonstrate the technical feasibility of making land-use and food systems sustainable in each country. They can also inform mid-century low-emission development strategies under the Paris Agreement on Climate Change. FABLE country teams have aimed for consistency with the SDGs and the Paris Agreement objectives. At this early stage, not all target dimensions have been considered. The report does not discuss policy options for transforming these systems, their implementation, or associated costs and economic benefits. These critical issues will be addressed in the global report by the Food and Land-Use Coalition, which will be published in September 2019 ahead of the Climate Summit convened by UN Secretary-General António Guterres. This executive summary outlines the need for long-term pathways towards sustainable land-use and food systems and why a global FABLE network is needed. It then presents the FABLE approach, summarizes key findings, and describes the way forward.
Thomson, M. J., Pérez-Guzmán, K., Sperling, F., Frank, S. (2019). European Union. Chapter in the 2019 Report of the FABLE Consortium. p. 180-191.
Colven, E. and Thomson, M.J., 2019. Bridging the divide between human and physical geography: Potential avenues for collaborative research on climate modeling. Geography Compass, 13(2), p.e12418.

Abstract. Despite repeated calls for greater collaboration between physical and human geographers, the unique interdisciplinary potential of geography remains largely underutilized. Yet geographers are well positioned to take a leading role in the interdisciplinary turn in climate‐related research. This paper explores the possibilities for physical and human geographers to collaborate within and beyond the discipline, specifically on the topic of climate modeling. We first examine geographical research critically examining the production and circulation of climate knowledge. Drawing on insights from a recent literature called “Critical Physical Geography,” we then outline how geographers might engage in collaborative and interdisciplinary work in order to promote more democratic practices of producing climate knowledge, enrich understandings of climate change, and more effectively serve goals of social and environmental justice. We argue that both the discipline of geography and the field of climate research stand to gain enormously from geographers’ efforts to talk across the divide between social and natural science within and beyond the boundaries of geography.
Thomson, M.J., Balkovič, J., Krisztin, T. and MacDonald, G.M., 2019. Simulated impact of paleoclimate change on Fremont Native American maize farming in Utah, 850–1449 CE, using crop and climate models. Quaternary International, 507, pp.95-107.

Abstract. The Fremont were members of an expansive maize-based Ancestral Puebloan (AP) cultural complex who disappeared from Utah between the 12th and 13th centuries CE. This period brackets that of a climatic transition in the Southwest from the warm, dry Medieval Climate Anomaly (MCA, ca. 850–1350 CE) to the cool, hydro-climatically variable Little Ice Age (LIA, ca. 1350–1850 CE). We simulated maize (Zea mays) crop productivity for Fremont AP archaeological sites in Utah between 850 and 1449 CE using a process-based crop model driven by climatologies from a statistically downscaled a climate model. We compared the model-simulated crop yields to time-series of archaeological site occupations given by spatially discrete, chronologically summed probability distributions (SPDs) of radiocarbon-dated Fremont artifacts. We found that the anomalous abandonment of different sites throughout Utah may be explained by site-specific combinations of reduced mean yield due to volatile year-to-year yields caused by increasing temperature variability, increasing hydro-climatic variability, and loss of soil quality, which depended on crop management strategy. In other words, we model the elimination of the Fremont AP ecological niche by exogenous influences of temperature and precipitation variability at the MCA-LIA transition and endogenous degradation of soil from organic carbon and nitrogen loss. Our method has broad applicability to contexts of low-technology, dryland farming human-environmental interactions.
Thomson, M. J. 2018. Paleoclimatology, Reference Module in Earth Systems and Environmental Sciences. Elsevier.

Abstract. Paleoclimatology is the study of the climate history of the Earth. This article summarizes major methods used in the reconstruction of paleoclimates for practical use by ecologists. Special attention is given to terrestrial proxies, isotopic indicators of environmental change, and age-modeling for the quaternary period.
Loisel, J., MacDonald, G.M. and Thomson, M.J., 2017. Little Ice Age climatic erraticism as an analogue for future enhanced hydroclimatic variability across the American Southwest. PloS one, 12(10), p.e0186282.

Abstract. The American Southwest has experienced a series of severe droughts interspersed with strong wet episodes over the past decades, prompting questions about future climate patterns and potential intensification of weather disruptions under warming conditions. Here we show that interannual hydroclimatic variability in this region has displayed a significant level of non-stationarity over the past millennium. Our tree ring-based analysis of past drought indicates that the Little Ice Age (LIA) experienced high interannual hydroclimatic variability, similar to projections for the 21st century. This is contrary to the Medieval Climate Anomaly (MCA), which had reduced variability and therefore may be misleading as an analog for 21st century warming, notwithstanding its warm (and arid) conditions. Given past non-stationarity, and particularly erratic LIA, a ‘warm LIA’ climate scenario for the coming century that combines high precipitation variability (similar to LIA conditions) with warm and dry conditions (similar to MCA conditions) represents a plausible situation that is supported by recent climate simulations. Our comparison of tree ring-based drought analysis and records from the tropical Pacific Ocean suggests that changing variability in El Niño Southern Oscillation (ENSO) explains much of the contrasting variances between the MCA and LIA conditions across the American Southwest. Greater ENSO variability for the 21st century could be induced by a decrease in meridional sea surface temperature gradient caused by increased greenhouse gas concentration, as shown by several recent climate modeling experiments. Overall, these results coupled with the paleo-record suggests that using the erratic LIA conditions as benchmarks for past hydroclimatic variability can be useful for developing future water-resource management and drought and flood hazard mitigation strategies in the Southwest.
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