Climate Feedback of bio-geochemical processes in the Arctic

Abstract

The Arctic warming affects the decreasing marine phytoplankton mass in future climate. The Arctic warming is also affected by changing the marine phytoplankton via absorbing more shortwave radiation and in turn radiative redistribution in the upper ocean layer, so-called bio-geophysical feedback. Thus, understanding the two-way interaction between marine biology and climate system is important to predict the Arctic climate change. This characteristic of absorbing shortwave heating of marine phytoplankton leads to conceive new natures and features in point of view moving on bio–climate interaction between marine biology, ocean, and atmosphere. However, previous works were majorly focusing on mean chlorophyll changes and its linear impact of bio-geophysical feedback and assuming the closed system of marine biogeochemical cycle. So, these studies couldn’t have clearly comprehended seasonal evolution of chlorophyll, nonlinear impact of bio-geophysical feedback, and the impact of nitrogen emission by human activity its Arctic climate feedback. In this dissertation, the evolutions, and responses of Arctic phytoplankton activity in sub-seasonal to seasonal timescales, and its bio-geophysical feedback processes in the present-day and future climates were investigated by model simulations using a Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 earth system model (ESM). This model shows one of the best model to represent the surface chlorophyll distributions. Results of historical run and Representative Concentration Pathway 4.5 (RCP4.5) scenario in the fifth’s Coupled Model Intercomparison Project (CMIP5) models are discussed in this dissertation to support the results of single model experiments. The thesis consists of three parts. The presence of interannual chlorophyll variability reduced the absorption rate of shortwave heating in the Arctic climate. This interannual chlorophyll variability was decreased in future climate, which amplified the Arctic warming. The human-induced emission of nitrogen fertilized the Arctic phytoplankton in the future climate, which amplified the previously suggested Arctic warming. In the first part, in present-day climate, we identified that in addition to the previously suggested effect of the mean chlorophyll change, an interannual chlorophyll variability substantially influences the Arctic mean climate state, even though the mean chlorophyll remains the same. We found that two nonlinear rectifications of chlorophyll variability induced Arctic cooling. One was due to the effect of a nonlinear shortwave heating term, which was induced by the positive ice–phytoplankton covariability in the boreal summer. The other was due to a cooling effect by rectification of a nonlinear function of the shortwave absorption rate, which reduced the shortwave absorption rate by interannually-varying chlorophyll. In the CMIP5 earth system models, included bio-geophysical feedback simulated a colder Arctic condition than models without a bio-geophysical feedback. This result suggests a possible mechanism in understanding how chlorophyll variability interacts with the Arctic climate system and its impact on the Arctic mean climate state. In the second part, in the future climate, the decreased interannual chlorophyll variability may amplify Arctic surface warming (+10% in both region) and sea ice melting (-13% and -10%) in Kara-Barents Seas and East Siberian-Chukchi Seas in boreal winter, respectively, while the decreased mean chlorophyll response exists. Future projections of CMIP5 ESMs show a future decrease in chlorophyll both mean concentration and interannual variability via sea ice melting and intensified surface-water stratification in summer. We found that these two nonlinear processes will be reduced by about 31% and 20% in the future, respectively, because the sea ice and chlorophyll variabilities, which control the amplitudes of nonlinear rectifications, are projected to decrease in the future climate. The Arctic warming is consequently enhanced by the weakening of the cooling effects of nonlinear rectifications. Thus, this additional biological warming will contribute to future Arctic warming. This study suggests that chlorophyll activities of both mean and variability should be considered to the sensitivity of Arctic warming via bio-geophysical feedback processes in future projections using earth system models. Lastly, the impacts of the anthropogenic reactive nitrogen (Nr) on the Arctic bio-geochemical processes and climate feedback in the present-day and future climates were investigated by comparing idealized experiments of different sets based on external forcings of preindustrial and contemporary amounts of the atmospheric Nr and runoff Nr fluxes. The anthropogenic Nr fluxes generate the positive chlorophyll anomaly in the future Arctic Ocean where the limiting condition of nitrogen dominates for the phytoplankton activity. The shortwave absorption rate on the Arctic Ocean surface is reinforced by the less depletion of the Arctic nitrate inventory and more concentration of chlorophyll response. Thus, the additional shortwave heating is generated by anthropogenic Nr fluxes, that enhances the previously suggested Arctic warming amplified by the bio-geophysical feedback. This study firstly suggested the possible mechanism that the accelerating nitrogen cycle by human activities can influence on the Arctic climate change via the marine phytoplankton response and its bio-geophysical feedback process. This thesis represents the significant roles of bio-climate interaction on Arctic climate. This result suggests that bio–climate interaction would be crucial role in controlling the temperature and sea ice responses in future Arctic in addition to well-known positive sea ice-albedo feedbacks. The chlorophyll is attracting climate scientist because of the role of coupling coefficient in biology and climate system, while the chlorophyll had been comprehended by marine biologists as a basis of biological productivity in marine food web. Investigating the two-way interaction of bio-climate interaction should not be dealt with only by-product in earth system simulation runs but also be considered as a new “climate-modulator” in the Arctic.The Arctic warming affects the decreasing marine phytoplankton mass in future climate. The Arctic warming is also affected by changing the marine phytoplankton via absorbing more shortwave radiation and in turn radiative redistribution in the upper ocean layer, so-called bio-geophysical feedback. Thus, understanding the two-way interaction between marine biology and climate system is important to predict the Arctic climate change. This characteristic of absorbing shortwave heating of marine phytoplankton leads to conceive new natures and features in point of view moving on bio–climate interaction between marine biology, ocean, and atmosphere. However, previous works were majorly focusing on mean chlorophyll changes and its linear impact of bio-geophysical feedback and assuming the closed system of marine biogeochemical cycle. So, these studies couldn’t have clearly comprehended seasonal evolution of chlorophyll, nonlinear impact of bio-geophysical feedback, and the impact of nitrogen emission by human activity its Arctic climate feedback. In this dissertation, the evolutions, and responses of Arctic phytoplankton activity in sub-seasonal to seasonal timescales, and its bio-geophysical feedback processes in the present-day and future climates were investigated by model simulations using a Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 earth system model (ESM). This model shows one of the best model to represent the surface chlorophyll distributions. Results of historical run and Representative Concentration Pathway 4.5 (RCP4.5) scenario in the fifth’s Coupled Model Intercomparison Project (CMIP5) models are discussed in this dissertation to support the results of single model experiments. The thesis consists of three parts. The presence of interannual chlorophyll variability reduced the absorption rate of shortwave heating in the Arctic climate. This interannual chlorophyll variability was decreased in future climate, which amplified the Arctic warming. The human-induced emission of nitrogen fertilized the Arctic phytoplankton in the future climate, which amplified the previously suggested Arctic warming. In the first part, in present-day climate, we identified that in addition to the previously suggested effect of the mean chlorophyll change, an interannual chlorophyll variability substantially influences the Arctic mean climate state, even though the mean chlorophyll remains the same. We found that two nonlinear rectifications of chlorophyll variability induced Arctic cooling. One was due to the effect of a nonlinear shortwave heating term, which was induced by the positive ice–phytoplankton covariability in the boreal summer. The other was due to a cooling effect by rectification of a nonlinear function of the shortwave absorption rate, which reduced the shortwave absorption rate by interannually-varying chlorophyll. In the CMIP5 earth system models, included bio-geophysical feedback simulated a colder Arctic condition than models without a bio-geophysical feedback. This result suggests a possible mechanism in understanding how chlorophyll variability interacts with the Arctic climate system and its impact on the Arctic mean climate state. In the second part, in the future climate, the decreased interannual chlorophyll variability may amplify Arctic surface warming (+10% in both region) and sea ice melting (-13% and -10%) in Kara-Barents Seas and East Siberian-Chukchi Seas in boreal winter, respectively, while the decreased mean chlorophyll response exists. Future projections of CMIP5 ESMs show a future decrease in chlorophyll both mean concentration and interannual variability via sea ice melting and intensified surface-water stratification in summer. We found that these two nonlinear processes will be reduced by about 31% and 20% in the future, respectively, because the sea ice and chlorophyll variabilities, which control the amplitudes of nonlinear rectifications, are projected to decrease in the future climate. The Arctic warming is consequently enhanced by the weakening of the cooling effects of nonlinear rectifications. Thus, this additional biological warming will contribute to future Arctic warming. This study suggests that chlorophyll activities of both mean and variability should be considered to the sensitivity of Arctic warming via bio-geophysical feedback processes in future projections using earth system models. Lastly, the impacts of the anthropogenic reactive nitrogen (Nr) on the Arctic bio-geochemical processes and climate feedback in the present-day and future climates were investigated by comparing idealized experiments of different sets based on external forcings of preindustrial and contemporary amounts of the atmospheric Nr and runoff Nr fluxes. The anthropogenic Nr fluxes generate the positive chlorophyll anomaly in the future Arctic Ocean where the limiting condition of nitrogen dominates for the phytoplankton activity. The shortwave absorption rate on the Arctic Ocean surface is reinforced by the less depletion of the Arctic nitrate inventory and more concentration of chlorophyll response. Thus, the additional shortwave heating is generated by anthropogenic Nr fluxes, that enhances the previously suggested Arctic warming amplified by the bio-geophysical feedback. This study firstly suggested the possible mechanism that the accelerating nitrogen cycle by human activities can influence on the Arctic climate change via the marine phytoplankton response and its bio-geophysical feedback process. This thesis represents the significant roles of bio-climate interaction on Arctic climate. This result suggests that bio–climate interaction would be crucial role in controlling the temperature and sea ice responses in future Arctic in addition to well-known positive sea ice-albedo feedbacks. The chlorophyll is attracting climate scientist because of the role of coupling coefficient in biology and climate system, while the chlorophyll had been comprehended by marine biologists as a basis of biological productivity in marine food web. Investigating the two-way interaction of bio-climate interaction should not be dealt with only by-product in earth system simulation runs but also be considered as a new “climate-modulator” in the Arctic.

The Arctic warming affects the decreasing marine phytoplankton mass in future climate. The Arctic warming is also affected by changing the marine phytoplankton via absorbing more shortwave radiation and in turn radiative redistribution in the upper ocean layer, so-called bio-geophysical feedback. Thus, understanding the two-way interaction between marine biology and climate system is important to predict the Arctic climate change. This characteristic of absorbing shortwave heating of marine phytoplankton leads to conceive new natures and features in point of view moving on bio–climate interaction between marine biology, ocean, and atmosphere. However, previous works were majorly focusing on mean chlorophyll changes and its linear impact of bio-geophysical feedback and assuming the closed system of marine biogeochemical cycle. So, these studies couldn’t have clearly comprehended seasonal evolution of chlorophyll, nonlinear impact of bio-geophysical feedback, and the impact of nitrogen emission by human activity its Arctic climate feedback. In this dissertation, the evolutions, and responses of Arctic phytoplankton activity in sub-seasonal to seasonal timescales, and its bio-geophysical feedback processes in the present-day and future climates were investigated by model simulations using a Geophysical Fluid Dynamics Laboratory (GFDL) CM2.1 earth system model (ESM). This model shows one of the best model to represent the surface chlorophyll distributions. Results of historical run and Representative Concentration Pathway 4.5 (RCP4.5) scenario in the fifth’s Coupled Model Intercomparison Project (CMIP5) models are discussed in this dissertation to support the results of single model experiments. The thesis consists of three parts. The presence of interannual chlorophyll variability reduced the absorption rate of shortwave heating in the Arctic climate. This interannual chlorophyll variability was decreased in future climate, which amplified the Arctic warming. The human-induced emission of nitrogen fertilized the Arctic phytoplankton in the future climate, which amplified the previously suggested Arctic warming. In the first part, in present-day climate, we identified that in addition to the previously suggested effect of the mean chlorophyll change, an interannual chlorophyll variability substantially influences the Arctic mean climate state, even though the mean chlorophyll remains the same. We found that two nonlinear rectifications of chlorophyll variability induced Arctic cooling. One was due to the effect of a nonlinear shortwave heating term, which was induced by the positive ice–phytoplankton covariability in the boreal summer. The other was due to a cooling effect by rectification of a nonlinear function of the shortwave absorption rate, which reduced the shortwave absorption rate by interannually-varying chlorophyll. In the CMIP5 earth system models, included bio-geophysical feedback simulated a colder Arctic condition than models without a bio-geophysical feedback. This result suggests a possible mechanism in understanding how chlorophyll variability interacts with the Arctic climate system and its impact on the Arctic mean climate state. In the second part, in the future climate, the decreased interannual chlorophyll variability may amplify Arctic surface warming (+10% in both region) and sea ice melting (-13% and -10%) in Kara-Barents Seas and East Siberian-Chukchi Seas in boreal winter, respectively, while the decreased mean chlorophyll response exists. Future projections of CMIP5 ESMs show a future decrease in chlorophyll both mean concentration and interannual variability via sea ice melting and intensified surface-water stratification in summer. We found that these two nonlinear processes will be reduced by about 31% and 20% in the future, respectively, because the sea ice and chlorophyll variabilities, which control the amplitudes of nonlinear rectifications, are projected to decrease in the future climate. The Arctic warming is consequently enhanced by the weakening of the cooling effects of nonlinear rectifications. Thus, this additional biological warming will contribute to future Arctic warming. This study suggests that chlorophyll activities of both mean and variability should be considered to the sensitivity of Arctic warming via bio-geophysical feedback processes in future projections using earth system models. Lastly, the impacts of the anthropogenic reactive nitrogen (Nr) on the Arctic bio-geochemical processes and climate feedback in the present-day and future climates were investigated by comparing idealized experiments of different sets based on external forcings of preindustrial and contemporary amounts of the atmospheric Nr and runoff Nr fluxes. The anthropogenic Nr fluxes generate the positive chlorophyll anomaly in the future Arctic Ocean where the limiting condition of nitrogen dominates for the phytoplankton activity. The shortwave absorption rate on the Arctic Ocean surface is reinforced by the less depletion of the Arctic nitrate inventory and more concentration of chlorophyll response. Thus, the additional shortwave heating is generated by anthropogenic Nr fluxes, that enhances the previously suggested Arctic warming amplified by the bio-geophysical feedback. This study firstly suggested the possible mechanism that the accelerating nitrogen cycle by human activities can influence on the Arctic climate change via the marine phytoplankton response and its bio-geophysical feedback process. This thesis represents the significant roles of bio-climate interaction on Arctic climate. This result suggests that bio–climate interaction would be crucial role in controlling the temperature and sea ice responses in future Arctic in addition to well-known positive sea ice-albedo feedbacks. The chlorophyll is attracting climate scientist because of the role of coupling coefficient in biology and climate system, while the chlorophyll had been comprehended by marine biologists as a basis of biological productivity in marine food web. Investigating the two-way interaction of bio-climate interaction should not be dealt with only by-product in earth system simulation runs but also be considered as a new “climate-modulator” in the Arctic.

미래 북극해 식물플랑크톤 농도는 기후변화로 발생한 북극 온난화로 인해 감소할 것으로 예상된다. 하지만 기후시스템 또한 북극온난화에 따른 해양 식물플랑크톤 반응으로 인해 영향을 받는데, 소위 생지역학 피드백 효과에 의해 태양복사에너지 흡수량이 증가하여 해양을 추가적으로 데우기 때문이다. 그러므로, 기후시스템 및 해양 식물플랑크톤의 양방향 상호작용은 북극해 기후변화를 이해함에 있어서 매우 중요한 요소이다. 해양 식물플랑크톤의 태양복사에너지 흡수량 증가는 지구시스템에서 생물-해양-대기 상호작용의 연구에 대한 필요성을 이끌었다. 그러나, 선행연구들에서는 주로 클로로필의 연평균 변화와 이에따른 선형적인 생지역학 피드백 피드백 연구에 초점을 두고있으며, 클로로필의 계절변화 및 그에 따른 북극 기후피드백에 대한 이해는 부족하다. 본 학위논문에서는, GFDL CM2.1 지구시스템 모형을 이용하여 북극 식물플랑크톤의 계절변화와 그에따른 생지역학 피드백 과정을 연구하였다. 이 모형은 CMIP5 지구시스템 모형들 중에 해양 표층 클로로필 농도를 가장 잘 모의하는 모형중 하나이다. 또한 다른 CMIP5 모형 분석을 통해 본 연구의 결과를 보충하였다. 본 학위논문은 총 세가지 파트로 구성되었다. 클로로필의 경년변동성의 존재가 북극의 태양복사에너지 흡수 효율을 떨어뜨릴 수 있다. 이러한 클로로필 경년변동성 효과는 미래기후로 가면 감소할 것이며, 이 때문에 북극 온난화가 증폭된다. 인간활동에 의한 질소배출은 미래기후의 북극 식물플랑크톤 생장을 촉진하며 앞선 연구에서 제시한 북극 온난화 증폭을 강화시킬 수 있다. 첫번째 파트에서는, 현재기후에서 기존에 제시되었던 클로로필 농도 평균장의 효과뿐만 아니라, 평균장은 같음에도 불구하고 클로로필 경년변동성 또한 북극 기후 평균장에 영향을 줄 수 있음을 확인하였다. 차가운 북극 기후 평균장을 유지하는데에는 클로로필 경년변동성의 두가지 비선형적인 보정효과가 존재함을 발견하였다. 첫번째는 태양복사에너지 흡수량의 비선형 항 때문인데, 이것은 여름철 양의 해빙-식물플랑크톤 상호작용 때문이다. 두번째는 태양복사에너지 흡수율의 비선형성 때문인데, 태양복사에너지 흡수율은 클로로필이 경년변동성을 가짐에 따라 감소되기 때문이다. CMIP5 지구시스템 모형에서, 생지역학 피드백을 모의하는 모형의 경우 북극 기후가 그렇지않은 모형에 비해 더 차갑다는 것을 확인하였다. 이 결과는 클로로필 변동성이 어떻게 북극 기후시스템과 상호작용하며 북극 기후 평균장에 영향을 미치는지에 대한 매커니즘을 제시하였다. 두번째 파트에서는, 미래기후에서는 클로로필 평균장은 감소하는 반면에, 클로로필 경년변동성이 감소하여 그에따른 북극 카라-바렌츠해 및 동시베리아-척치해의 온난화를 각각 10%정도 상승시키고 해빙을 각각 13% 및 10% 정도 감소시키는 것으로 나타났다. 다른 CMIP5 지구시스템 모형들의 미래전망 역시 여름철 평균장의 감소 및 경년변동성의 감소가 나타났다. 본 연구에서는 첫번째 파트에서 소개된 비선형적인 보정효과가 미래에 각각 31% 및 20% 감소함을 확인하였고, 이는 북극의 해빙 및 클로로필 경년변동성이 감소하여 비선형적인 보정효과의 진폭을 낮추기 때문이다. 따라서, 이러한 추가적인 생물작용의 온난화 현상은 북극 온난화에 기여할 수 있다. 본 연구에서는 클로로필의 평균장 및 경년변동성에 의한 생지역학 피드백 작용이 지구시스템 모형을 이용한 미래 북극온난화 전망에 있어 중요한 요소임을 제시하였다. 마지막 파트에서는, 산업화 이전 수준의 질소플럭스와 인간활동에 의해 증가된 질소플럭스를 지구시스템 모형 외부강제력으로 처방한 이상화실험을 분석하여, 인간활동에 의한 질소효과로 인해 북극 온난화가 가속화 될 수 있음을 밝혔다. 인간에 의해 배출된 질소플럭스는 질소가 고갈된 미래기후의 양의 클로로필 편차를 만든다. 태양복사에너지흡수율은 이렇게 질소가 고갈된 환경에서 증가된 클로로필 반응에 의해 강화된다. 따라서, 인간활동에 의한 질소플럭스가 추가적인 태양복사흡수량을 증가시키고, 선행연구에서 제시된 생지역학 피드백에 의해 북극 온난화가 가속화 된다. 이 연구는 인위적인 질소순환의 가속화가 해양 식물플랑크톤 반응 및 이에따른 생지역학 피드백 작용에 의해 북극 기후변화에 영향을 미칠 수 있음을 제안하는 첫번째 연구이다. 본 연구는 대기-해양-생물권의 생물-기후 상호작용이 북극 기후에 유의미한 영향을 줌을 제시하였다. 잘 알려진 해빙-알베도 양의 피드백뿐만 아니라 해양생물의 이에따른 북극 기후의 생지역학 효과의 이해 또한 북극 온도 및 해빙농도 미래전망에 중요한 역할을 할 것이다. 클로로필 계절성은 과거 해양생물학자들에 의해 해양의 기초생산자로의 역할로 종합적인 이해가 선행되었지만, 또한 생물-기후시스템의 상호작용 강도를 결정하는 중요한 요소이기 때문에 기후학자들에게 매력적인 연구대상이다. 기후시스템 및 해양 식물플랑크톤의 양방향 상호작용 연구는 지구시스템 시뮬레이션의 부산물이 아닌 새로운“기후조절자”로 깊이 연구해야할 대상이다.