Allison Crimmins

The research community devotes a lot of time and resources toward monitoring historic climate changes, tracking current impacts, and modeling future risks in the United States. But even as climate science has made significant advances over the last several decades, insights on how climate change has and will affect the nation as a whole are incomplete. That’s because much of the available information needed to understand climate risks and inform our response to the climate crisis typically… Show more

The research community devotes a lot of time and resources toward monitoring historic climate changes, tracking current impacts, and modeling future risks in the United States. But even as climate science has made significant advances over the last several decades, insights on how climate change has and will affect the nation as a whole are incomplete. That’s because much of the available information needed to understand climate risks and inform our response to the climate crisis typically omits key areas outside of the contiguous United States (OCONUS), i.e., Alaska, Hawai’i, US-Affiliated Pacific Islands, and the US Caribbean. The science community, including research funders, project managers, and local experts, needs to work together to close this gap, particularly if researchers want their work to remain relevant to climate mitigation and adaptation planning and policymaking. Show less

The Fifth National Climate Assessment is the US Government’s preeminent report on climate change impacts, risks, and responses. It is a congressionally mandated interagency effort that provides the scientific foundation to support informed decision-making across the United States.

Editorial on framing the first US National Nature Assessment

The use of calibrated language in climate assessments has evolved over time and is an important tool for better characterizing climate risk. Despite improvements in communication, likelihood and confidence terms continue to be misinterpreted by nontechnical audiences. Public understanding of scientific certainty in climate impacts, particularly at the extreme tails of likelihood distribution, is inadequate. This study evaluates the use of calibrated language in the four most recent U.S. climate… Show more

The use of calibrated language in climate assessments has evolved over time and is an important tool for better characterizing climate risk. Despite improvements in communication, likelihood and confidence terms continue to be misinterpreted by nontechnical audiences. Public understanding of scientific certainty in climate impacts, particularly at the extreme tails of likelihood distribution, is inadequate. This study evaluates the use of calibrated language in the four most recent U.S. climate assessments. Across the assessments, there is inconsistent use, definition, location, and formatting designating a term as calibrated. Authors include additional, undefined certainty categories bridging qualitative confidence categories (e.g. Medium to High) and likelihood terms with no associated statistical value (e.g. Highly Likely). Confidence and likelihood levels are more frequently reported for terms at the high end of certainty distributions, with at least 70% of all instances of confidence terms reporting High or Very High confidence in three of the assessments. Almost all (>98%) likelihood language reported is in the Likely (>66%) category or above and no likelihood terms below 50% are reported in any of the assessment's Key Messages. There is significant room for improvement in the representation of likelihood, as the word “likely” is frequently used with ambiguous intent; at least half of all instances of likelihood terms are used with non‐calibrated, potentially colloquial intent in three of the assessments. The study concludes with a set of recommendations for improving the use of calibrated language and communication of risk in the development of future assessment products. Show less

Update November 11, 2020: https://www.vox.com/2020/7/21/21332435/joe-biden-climate-change-department-epa

Increasing atmospheric concentrations of carbon dioxide (CO2) affect global nutrition via effects on agricultural productivity and nutrient content of food crops. We combined these effects with economic projections to estimate net changes in nutrient availability between 2010 and 2050.
Findings
Although technological change, market responses, and the effects of CO2 fertilisation on yield are projected to increase global availability of dietary protein, iron, and zinc, these increases are… Show more

Increasing atmospheric concentrations of carbon dioxide (CO2) affect global nutrition via effects on agricultural productivity and nutrient content of food crops. We combined these effects with economic projections to estimate net changes in nutrient availability between 2010 and 2050.
Findings
Although technological change, market responses, and the effects of CO2 fertilisation on yield are projected to increase global availability of dietary protein, iron, and zinc, these increases are moderated by negative effects of climate change affecting productivity and carbon penalties on nutrient content. The carbon nutrient penalty results in decreases in the global availability of dietary protein of 4·1%, iron of 2·8%, and zinc of 2·5% as calculated using one dataset, and decreases in global availability of dietary protein of 2·9%, iron of 3·9%, and zinc of 3·4% using the other dataset. The combined effects of projected increases in atmospheric CO2 (ie, carbon nutrient penalty, CO2 fertilisation, and climate effects on productivity) will decrease growth in the global availability of nutrients by 19·5% for protein, 14·4% for iron, and 14·6% for zinc relative to expected technology and market gains by 2050. The many countries that currently have high levels of nutrient deficiency would continue to be disproportionately affected.
Interpretation
This approach is an improvement in estimating future global food security by simultaneously projecting climate change effects on crop productivity and changes in nutrient content under increased concentrations of CO2, which accounts for a much larger effect on nutrient availability than CO2 fertilisation. Regardless of the scenario used to project future consumption patterns, the net effect of increasing concentrations of atmospheric CO2 will slow progress in decreasing global nutrient deficiencies. Show less

There is a growing capability to project the impacts and economic effects of climate change across multiple sectors. This information is needed to inform decisions regarding the diversity and magnitude of future climate impacts and explore how mitigation and adaptation actions might affect these risks. Here, we summarize results from sectoral impact models applied within a consistent modelling framework to project how climate change will affect 22 impact sectors of the United States, including… Show more

There is a growing capability to project the impacts and economic effects of climate change across multiple sectors. This information is needed to inform decisions regarding the diversity and magnitude of future climate impacts and explore how mitigation and adaptation actions might affect these risks. Here, we summarize results from sectoral impact models applied within a consistent modelling framework to project how climate change will affect 22 impact sectors of the United States, including effects on human health, infrastructure and agriculture. The results show complex patterns of projected changes across the country, with damages in some sectors (for example, labour, extreme temperature mortality and coastal property) estimated to range in the hundreds of billions of US dollars annually by the end of the century under high emissions. Inclusion of a large number of sectors shows that there are no regions that escape some mix of adverse impacts. Lower emissions, and adaptation in relevant sectors, would result in substantial economic benefits. Show less

This retrospective data analysis augments previous studies on climate-induced changes in aeroallergen concentrations by providing, at a much larger geographical and temporal scale, quantitative values of temperature extremes (seasonal maxima and minima) that can be used to estimate recent shifts in
two important metrics: ongoing changes in aeroallergenic pollen season duration; and variation in pollen load, or Annual Pollen Integral (API). Thus, the current work integrates previous… Show more

This retrospective data analysis augments previous studies on climate-induced changes in aeroallergen concentrations by providing, at a much larger geographical and temporal scale, quantitative values of temperature extremes (seasonal maxima and minima) that can be used to estimate recent shifts in
two important metrics: ongoing changes in aeroallergenic pollen season duration; and variation in pollen load, or Annual Pollen Integral (API). Thus, the current work integrates previous, smaller-scale estimates and provides the first global (northern hemisphere) indication that recent temperature
increases are, in fact, contributing significantly to indices of pollen season duration and pollen load.
Implications of all the available evidence
The available data indicate that recent changes in maximum or minimum temperatures, or both, in association with anthropogenic climate change, are significantly correlated with both increasing airborne pollen loads and longer pollen seasonality across the northern hemisphere. Although smaller
regional assessments have shown that climate change will alter pollen seasonality, our data are, to our knowledge, the first to show two specific consequences—longer pollen seasons and more pollen—on a global, hemispheric scale. As such, these data synthesise and expand the current link between rising temperatures and aeroallergen concentrations and highlight the importance of future temperature increases on health impacts related to pollinosis, such as rhinitis and allergic asthma Show less

Pollen is an important environmental cause of allergic asthma episodes. Prior work has established a proof of concept for assessing projected climate change impacts on future oak pollen exposure and associated health impacts. This paper uses additional monitor data and epidemiologic functions to extend prior analyses, reporting new estimates of the current and projected future health burden of oak, birch, and grass pollen across the contiguous US. Our results suggest that tree pollen in the… Show more

Pollen is an important environmental cause of allergic asthma episodes. Prior work has established a proof of concept for assessing projected climate change impacts on future oak pollen exposure and associated health impacts. This paper uses additional monitor data and epidemiologic functions to extend prior analyses, reporting new estimates of the current and projected future health burden of oak, birch, and grass pollen across the contiguous US. Our results suggest that tree pollen in the spring currently accounts for between 25,000 and 50,000 pollen‐related asthma emergency department (ED) visits annually (95% C.I.: 14,000 to 100,000), roughly 2/3 of which occur among people under age 18. Grass pollen in the summer season currently accounts for less than 10,000 cases annually (95% C.I.: 4,000 to 16,000). Compared to a baseline with 21st century population growth but constant pollen, future temperature and precipitation show an increase in ED visits of 14% in 2090 for a higher greenhouse gas emissions scenario, but only 8% for a moderate emissions scenario, reflecting projected increases in pollen season length. Grass pollen, which is more sensitive to changes in climatic conditions, is a primary contributor to future ED visits, with the largest effects in the Northeast, Midwest, and Southern Great Plains regions. More complete assessment of the current and future health burden of pollen is limited by of the availability of data on pollen types (e.g, ragweed), other health effects (e.g. other respiratory disease), and economic consequences (e.g., medication costs). Show less

A comprehensive assessment of the impacts of climate change on human health in the United States concluded that climate change exacerbates existing climate-sensitive health threats and creates new challenges, exposing more people in more places to hazardous weather and climate conditions.1 This chapter builds on that assessment and considers the extent to which modifying current, or implementing new, health system responses could prepare for and manage these risks. Please see Chapter 13: Air… Show more

A comprehensive assessment of the impacts of climate change on human health in the United States concluded that climate change exacerbates existing climate-sensitive health threats and creates new challenges, exposing more people in more places to hazardous weather and climate conditions.1 This chapter builds on that assessment and considers the extent to which modifying current, or implementing new, health system responses could prepare for and manage these risks. Please see Chapter 13: Air Quality for a discussion of the health impacts associated with air quality, including ozone, wildfires, and aeroallergens. Show less

The goal of this study is to reframe the analysis and discussion of extreme heat projections to improve communication of future extreme heat risks in the United States. We combine existing data from 31 of the Coupled Model Intercomparison Project Phase 5 models to examine future exposure to extreme heat for global average temperatures of 1.5, 2, 3, and 4 °C above a preindustrial baseline. We find that throughout the United States, historically rare extreme heat events become increasingly common… Show more

The goal of this study is to reframe the analysis and discussion of extreme heat projections to improve communication of future extreme heat risks in the United States. We combine existing data from 31 of the Coupled Model Intercomparison Project Phase 5 models to examine future exposure to extreme heat for global average temperatures of 1.5, 2, 3, and 4 °C above a preindustrial baseline. We find that throughout the United States, historically rare extreme heat events become increasingly common in the future as global temperatures rise and that the depiction of exposure depends in large part on whether extreme heat is defined by absolute or relative metrics. For example, for a 4 °C global temperature rise, parts of the country may never see summertime temperatures in excess of 100 °F, but virtually all of the country is projected to experience more than 4 weeks per summer with temperatures exceeding their historical summertime maximum. All of the extreme temperature metrics we explored become more severe with increasing global average temperatures. However, a moderate climate scenario delays the impacts projected for a 3 °C world by almost a generation relative to the higher scenario and prevents the most extreme impacts projected for a 4 °C world. Show less

BACKGROUND:
The public health community readily recognizes flooding and wildfires as climate-related health hazards, but few studies quantify changes in risk of exposure, particularly for vulnerable children and older adults.
OBJECTIVES:
This study quantifies future populations potentially exposed to inland flooding and wildfire smoke under two climate scenarios, highlighting the populations in particularly vulnerable age groups (≤4 y old and ≥65 y old).
METHODS:
Spatially… Show more

BACKGROUND:
The public health community readily recognizes flooding and wildfires as climate-related health hazards, but few studies quantify changes in risk of exposure, particularly for vulnerable children and older adults.
OBJECTIVES:
This study quantifies future populations potentially exposed to inland flooding and wildfire smoke under two climate scenarios, highlighting the populations in particularly vulnerable age groups (≤4 y old and ≥65 y old).
METHODS:
Spatially explicit projections of inland flooding and wildfire under two representative concentration pathways (RCP8.5 and RCP4.5) are integrated with static (2010) and dynamic (2050 and 2090) age-stratified projections of future contiguous U.S. populations at the county level.
RESULTS:
In both 2050 and 2090, an additional one-third of the population will live in areas affected by larger and more frequent inland flooding under RCP8.5 than under RCP4.5. Approximately 15 million children and 25 million older adults could avoid this increased risk of flood exposure each year by 2090 under a moderate mitigation scenario (RCP4.5 compared with RCP8.5). We also find reduced exposure to wildfire smoke under the moderate mitigation scenario. Nearly 1 million young children and 1.7 million older adults would avoid exposure to wildfire smoke each year under RCP4.5 than under RCP8.5 by the end of the century.
CONCLUSIONS:
By integrating climate-driven hazard and population projections, newly created county-level exposure maps identify locations of potential significant future public health risk. These potential exposure results can help inform actions to prevent and prepare for associated future adverse health outcomes, particularly for vulnerable children and older adults. https://doi.org/10.1289/EHP2594 Show less

The papers published in this issue investigate agriculture and forestry impacts across multiple climate forcing scenarios at national, multi-national, and global scales. In addition, a number of the papers in this focus issue (Monier et al 2016, Sue Wing et al 2015, Beach et al 2015, Kim et al 2017, and Tian et al 2016) represent modeling contributions to the Climate Change Impacts and Risk Analysis (CIRA) project (Martinich et al 2015), which utilizes a consistent analytic framework to… Show more

The papers published in this issue investigate agriculture and forestry impacts across multiple climate forcing scenarios at national, multi-national, and global scales. In addition, a number of the papers in this focus issue (Monier et al 2016, Sue Wing et al 2015, Beach et al 2015, Kim et al 2017, and Tian et al 2016) represent modeling contributions to the Climate Change Impacts and Risk Analysis (CIRA) project (Martinich et al 2015), which utilizes a consistent analytic framework to estimate climate change impacts across sectors and regions. Show less

Future climate change is expected to lengthen and intensify pollen seasons in the U.S., potentially increasing incidence of allergic asthma. We developed a proof-of-concept approach for estimating asthma emergency department (ED) visits in the U.S. associated with present-day and climate-induced changes in oak pollen. We estimated oak pollen season length for moderate (RCP4.5) and severe climate change scenarios (RCP8.5) through 2090 using five climate models and published relationships between… Show more

Future climate change is expected to lengthen and intensify pollen seasons in the U.S., potentially increasing incidence of allergic asthma. We developed a proof-of-concept approach for estimating asthma emergency department (ED) visits in the U.S. associated with present-day and climate-induced changes in oak pollen. We estimated oak pollen season length for moderate (RCP4.5) and severe climate change scenarios (RCP8.5) through 2090 using five climate models and published relationships between temperature, precipitation, and oak pollen season length. We calculated asthma ED visit counts associated with 1994-2010 average oak pollen concentrations and simulated future oak pollen season length changes using the Environmental Benefits Mapping and Analysis Program (BenMAP-CE), driven by epidemiologically-derived concentration-response relationships. Oak pollen was associated with 21,200 (95% confidence interval, 10,000-35,200) asthma ED visits in the Northeast, Southeast, and Midwest U.S. in 2010, with damages valued at $10.4 million. Nearly 70% of these occurred among children age <18 years. Severe climate change could increase oak pollen season length and associated asthma ED visits by 5% and 10% on average in 2050 and 2090, with a marginal net present value through 2090 of $10.4 million (additional to the baseline value of $346.2 million). Moderate versus severe climate change could avoid >50% of the additional oak pollen-related asthma ED visits in 2090. Despite several key uncertainties and limitations, these results suggest that aeroallergens pose a substantial U.S. public health burden, that climate change could increase U.S. allergic disease incidence, and that mitigating climate change may have benefits from avoided pollen-related health impacts. Show less

Multiple studies have identified links between climate and West Nile virus disease since the virus arrived in North America. Here we sought to extend these results by developing a Health Impact Function (HIF) to generate county-level estimates of the expected annual number of West Nile neuroinvasive disease (WNND) cases based on the county’s historical WNND incidence, annual average temperature, and population size. To better understand the potential impact of projected temperature change on… Show more

Multiple studies have identified links between climate and West Nile virus disease since the virus arrived in North America. Here we sought to extend these results by developing a Health Impact Function (HIF) to generate county-level estimates of the expected annual number of West Nile neuroinvasive disease (WNND) cases based on the county’s historical WNND incidence, annual average temperature, and population size. To better understand the potential impact of projected temperature change on WNND risk, we used the HIF to project the change in expected annual number of WNND cases attributable to changing temperatures by 2050 and by 2090 using data from five global climate models under two representative concentration pathways (RCP4.5 and RCP8.5). To estimate the costs of anticipated changes, as well as to enable comparisons with other public health impacts, projected WNND cases were allocated to nonfatal and fatal outcomes, then monetized using a cost-of-illness estimate and the U.S. Environmental Protection Agency’s value of a statistical life, respectively. We found that projected future temperature and population changes could increase the expected annual number of WNND cases to ≈2000 - 2200 cases by 2050 and to ≈2700 - 4300 cases by 2090, from a baseline of 970 cases. Holding population constant at future levels while varying temperature from a 1995 baseline, we estimated projected temperature change alone is responsible for ≈590 and ≈960 incremental WNND cases in 2050 and 2090 (respectively) under the RCP4.5 scenario, and ≈820 and ≈2500 cases in 2050 and 2090 (respectively) for the RCP8.5 scenario, with substantial regional variation. The monetized impact of these temperature-attributable incremental cases is estimated at $0.5 billion in 2050 and $1.0 billion in 2090 under the RCP4.5 scenario, and $0.7 billion in 2050 and $2.6 billion in 2090 under the RCP8.5 scenario (undiscounted 2015 U.S. dollars). Show less

The United States (U.S.) Environmental Protection Agency (EPA) has established voluntary programs to reduce methane (CH4) emissions, and regulations that either directly reduce CH4 or provide co-benefits of reducing CH4 emissions while controlling for other air pollutants. These programs and regulations address four sectors that are among the largest domestic CH4 emissions sources: municipal solid waste landfills, oil and natural gas, coal mining, and agricultural manure management. Over the… Show more

The United States (U.S.) Environmental Protection Agency (EPA) has established voluntary programs to reduce methane (CH4) emissions, and regulations that either directly reduce CH4 or provide co-benefits of reducing CH4 emissions while controlling for other air pollutants. These programs and regulations address four sectors that are among the largest domestic CH4 emissions sources: municipal solid waste landfills, oil and natural gas, coal mining, and agricultural manure management. Over the 1993-2013 time period, 127.9 Tg of CH4 emissions reductions were attributed to these programs, equal to about 18% of the counterfactual (or potential) emissions over that time, with almost 70% of the abatement due to landfill sector regulations. Reductions attributed to the voluntary programs increased nearly continuously during the study period. We quantified how these reductions influenced atmospheric CH4 concentration and global temperature, finding a decrease in concentration of 28 ppb and an avoided temperature rise of 0.006 °C by 2013. Further, we monetized the climate and ozone-health impacts of the CH4 reductions, yielding an estimated benefit of $255 billion. These results indicate that EPA programs and policies have made a strong contribution to CH4 abatement, with climate and air quality benefits. Show less

USGCRP, 2016: The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment. Crimmins, A., J. Balbus, J.L. Gamble, C.B. Beard, J.E. Bell, D. Dodgen, R.J. Eisen, N. Fann, M.D. Hawkins, S.C. Herring, L. Jantarasami, D.M. Mills, S. Saha, M.C. Sarofim, J. Trtanj, and L. Ziska, Eds. U.S. Global Change Research Program, Washington, DC, 312 pp. http://dx.doi.org/10.7930/J0R49NQX

Increasing atmospheric carbon dioxide levels, higher temperatures, altered precipitation patterns, and other climate change impacts have already begun to affect US agriculture and forestry, with impacts expected to become more substantial in the future. There have been numerous studies of climate change impacts on agriculture or forestry, but relatively little research examining the long-term net impacts of a stabilization scenario relative to a case with unabated climate change. We provide an… Show more

Increasing atmospheric carbon dioxide levels, higher temperatures, altered precipitation patterns, and other climate change impacts have already begun to affect US agriculture and forestry, with impacts expected to become more substantial in the future. There have been numerous studies of climate change impacts on agriculture or forestry, but relatively little research examining the long-term net impacts of a stabilization scenario relative to a case with unabated climate change. We provide an analysis of the potential benefits of global climate change mitigation for US agriculture and forestry through 2100, accounting for landowner decisions regarding land use, crop mix, and management practices. The analytic approach involves a combination of climate models, a crop process model (EPIC), a dynamic vegetation model used for forests (MC1), and an economic model of the US forestry and agricultural sector (FASOM-GHG). We find substantial impacts on productivity, commodity markets, and consumer and producer welfare for the stabilization scenario relative to unabated climate change, though the magnitude and direction of impacts vary across regions and commodities. Although there is variability in welfare impacts across climate simulations, we find positive net benefits from stabilization in all cases, with cumulative impacts ranging from $32.7 billion to $54.5 billion over the period 2015–2100. Our estimates contribute to the literature on potential benefits of GHG mitigation and can help inform policy decisions weighing alternative mitigation and adaptation actions. Show less

This paper provides the first quantitative synthesis of the rapidly growing literature on future tropical and extratropical cyclone damages under climate change. We estimate a probability distribution for the predicted impact of changes in global surface air temperatures on future storm damages, using an ensemble of 478 estimates of the temperature-damage relationship from nineteen studies. Our analysis produces three main empirical results. First, we find strong but not conclusive support for… Show more

This paper provides the first quantitative synthesis of the rapidly growing literature on future tropical and extratropical cyclone damages under climate change. We estimate a probability distribution for the predicted impact of changes in global surface air temperatures on future storm damages, using an ensemble of 478 estimates of the temperature-damage relationship from nineteen studies. Our analysis produces three main empirical results. First, we find strong but not conclusive support for the hypothesis that climate change will cause damages from tropical cyclones and wind storms to increase, with most models predicting higher future storm damages due to climate change. Second, there is substantial variation in projected changes in losses across regions. Potential changes in damages are greatest in the North Atlantic basin, where the multi-model average predicts that a 2.5 °C increase in global surface air temperature would cause hurricane damages to increase by 63 %. The ensemble predictions for Western North Pacific tropical cyclones and European wind storms (extratropical cyclones) are +28 % and +23 %, respectively. Finally, our analysis shows that existing models of storm damages under climate change generate a wide range of predictions, ranging from moderate decreases to very large increases in losses. Show less

ABSTRACT This article is part of a Special Issue on “A Multi-Model Framework to Achieve Consistent Evaluation of Climate Change Impacts in the United States” edited by Jeremy Martinich, John Reilly, Stephanie Waldhoff, Marcus Sarofim, and James McFarland.

This paper develops and applies methods to quantify and monetize projected impacts on terrestrial ecosystem carbon storage and areas burned by wildfires in the contiguous United States under scenarios with and without global greenhouse gas mitigation. The MC1 dynamic global vegetation model is used to develop physical impact projections using three climate models that project a range of future conditions. We also investigate the sensitivity of future climates to different initial conditions of… Show more

This paper develops and applies methods to quantify and monetize projected impacts on terrestrial ecosystem carbon storage and areas burned by wildfires in the contiguous United States under scenarios with and without global greenhouse gas mitigation. The MC1 dynamic global vegetation model is used to develop physical impact projections using three climate models that project a range of future conditions. We also investigate the sensitivity of future climates to different initial conditions of the climate model. Our analysis reveals that mitigation, where global radiative forcing is stabilized at 3.7 W/m2 in 2100, would consistently reduce areas burned from 2001 to 2100 by tens of millions of hectares. Monetized, these impacts are equivalent to potentially avoiding billions of dollars (discounted) in wildfire response costs. Impacts to terrestrial ecosystem carbon storage are less uniform, but changes are on the order of billions of tons over this time period. The equivalent social value of these changes in carbon storage ranges from hundreds of billions to trillions of dollars (discounted). The magnitude of these results highlights their importance when evaluating climate policy options. However, our results also show national outcomes are driven by a few regions and results are not uniform across regions, time periods, or models. Differences in the results based on the modeling approach and across initializing conditions also raise important questions about how variability in projected climates is accounted for, especially when considering impacts where extreme or threshold conditions are important. Show less

Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO2, pco2)1. Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (~12–5 Myr ago), an interval with pco2 values of… Show more

Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO2, pco2)1. Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (~12–5 Myr ago), an interval with pco2 values of only 200–350 parts per million by volume but nearly ice-free conditions in the Northern Hemisphere2, 3 and warmer-than-modern temperatures on the continents4. Here we present quantitative geochemical sea surface temperature estimates from the Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth persisted throughout the interval of low pco2 ~12–5 Myr ago. We also present new stable isotope measurements from the western equatorial Pacific that, in conjunction with previously published data5, 6, 7, 8, 9, 10, reveal a long-term trend of thermocline shoaling in the equatorial Pacific since ~13 Myr ago. We propose that a relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapour feedbacks may help to explain the warmth of the late Miocene. Additional shoaling of the thermocline after 5 Myr ago probably explains the stronger coupling between pco2, sea surface temperatures and climate that is characteristic of the more recent Pliocene and Pleistocene epochs11, 12. Show less

DOI: 10.13140/2.1.1614.5280