The Modification Of Ocean Oxygen Cycle Under Climate Change

Oxygen is an important factor influencing the habitability of the Earth.This material basis for the survival and development of all aerobic organisms is inextricably linked to the formation and evolution of complex life.In the ocean oxygen exists mainly in the form of dissolved oxygen,which is an essential component for maintaining the prosperity of marine ecosystems.The current oxygen-rich environment in seawater is the result of billions of years of evolution of the Earth system.However,with increasing levels of human activity since the industrial revolution,the Earth system oxygen cycle has been gradually pushed off its established trajectory-the human footprint has become increasingly visible in the modern Earth oxygen cycle,significantly altering the processes within the cycle and upsetting its balance.For the oceans,one of the most obvious manifestations is the substantial decrease in dissolved oxygen content,that is,“the ocean is losing its breath”.As a key subset of the Earth’s oxygen cycle,disturbances in the ocean oxygen cycle and changes in seawater dissolved oxygen concentrations could have profound effects on the marine biota and ecosystes.It is therefore of great importance to investigate the response of the ocean oxygen cycle to climate change.This paper presents a systematic study of the ocean oxygen cycle based on observational data as well as climate models.First,this paper quantifies the global ocean oxygen balance using the results derived from the CMIP5.In this part,the paper provides an overall assessment of the distributions and long-term trends of ocean dissolved oxygen content under climate change,clarifies the contribution of various processes to the ocean oxygen budget,and predicts the oxygen concentration changes under different emission pathways.Second,this paper explores the disagreement between climate model simulations and observations,and discusses the reasons for these deficiencies.The CESM is used here to explore how to reduce the model error.The results show that,by applying a bubble-mediated parameterization scheme for the air-sea oxygen flux,the significant improvement has been found in simulating ocean oxygen cycle.Third,we focus on the low-oxygen extreme events under climate change,and establish the linkages between low-oxygen extreme and marine heatwaves.The frequency,duration and other characteristics of these extreme events are investigated in this paper.Finally,based on the close relationship between the carbon and oxygen cycle,this paper quantitatively estimates the carbon uptake in the ocean and on land,and predicts the changes of carbon sinks in different emission scenarios in the future.The major conclusions are summarized as follows:（1）Under the human-induced climate change,there are significant decreases of dissolved oxygen content in the oceans.The analysis of the global ocean oxygen budget show that dissolved oxygen is continuously escaping from the ocean to the atmosphere,which is the direct cause of the decrease in ocean oxygen content.The paper finds that the average air-sea oxygen flux over the historical period was about1.6 Gt yr^-1,i.e.,the ocean expelled about 1.6 Gt of oxygen to the atmosphere each year.However,the net rate of replenishment of oceanic oxygen stocks by processes such as internal ocean biology and sedimentation is only 0.6 Gt yr^-1;These processes together eventually result in a decline in oceanic dissolved oxygen of approximately1.0 Gt yr^-1 during this period.A series of changes in physical and dynamical processes were found to be the dominant factors in the oxygen decline:first,ocean warming leads to a decrease in oxygen solubility;second,the warming modifies ocean stratification and circulation.These changes impede the transport and circulation of dissolved oxygen within the ocean,which ultimately results in an accelerated"escape"of oxygen to the atmosphere,leading to a decrease in ocean oxygen levels.Climate model projections show that the air-sea oxygen flux will increase to 4.3 Gt/yr by the end of the century under RCP8.5.（2）Although climate models have simulated the decline in ocean dissolved oxygen,there are still considerable differences and discrepancies between models and observations.By analyzing the processes affecting the ocean oxygen budget,the article finds that the model’s inadequate ability to simulate the air-sea oxygen flux might be one of the most important reasons for this problem.To this end,a bubble-mediated parameterization scheme for air-sea oxygen fluxes is applied in this paper with the use of CESM.The results show that,the performance of the air-sea oxygen fluxes simulaitons is improved by about 15%overall and up to more than 40%locally in the Southern Ocean after the implementation of the bubble-mediated scheme.The bubble-mediated parameterization can better simulate the oxygen exchange process between the ocean and the atmosphere under high wind and wave conditions,and thus improve the model simulation of the ocean oxygen cycle.（3）Low-oxygen extreme events in the ocean have significantly increased under climate change,which could general extreme and exceptional conditions that would not occur for decades in the average climate change,with severe impacts on marine organisms and ecosystems.The article finds that the frequency,duration,and intensity of low-oxygen extreme events have increased significantly.More importantly,there is intensified synergy of heatwave and low-oxygen extreme over the past few decades,with the probability of the co-occurrence of the two extremes increasing from about40%in the 1980s to more than 50%after 2000.Under the combined effects of anthropogenic forcing and natural variability,this intensfifation is more pronounced in regions with abundant marine biomass,which indicates that most marine organisms will suffer more severe compounding effects of extreme environmental events such as low oxygen and high temperature in the future.（4）Studying the response of the oxygen cycle to climate change can also provide unique clues for insight into the carbon cycle.The article quantifies the oceanic and terrestrial carbon sinks on the basis of the oxygen budget.Our estimations show that for the 1990--2000 period,the averaged net ocean and land sinks are 2.10±0.43 and1.14±0.52 Gt C yr^-1 respectively,overall consistent with estimates derived by the Global Carbon Project（GCP）.An enhanced carbon uptake is found in both land and ocean after year 2000,which reflects the modification of carbon cycle under human activities.The article also points out the changes in the global carbon uptake systems under the combined influence of natural internal variability and anthropogenic forcing.The results corroborate the existing estimates of carbon sinks and predict their future changes,which provide important implications for the pathways of carbon neutral.In summary,this thesis focuses on the modification of ocean oxygen cycle to climate change.These results provide an important scientific reference for further research on the biochemical cycles of the Earth system,especially the oceans,and have a positive effect on the development of sustainable development strategies for the oceans.