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Amplified Arctic Warming by Phytoplankton Under Greenhouse Warming

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Amplified Arctic Warming by Phytoplankton Under Greenhouse Warming Amplified Arctic warming by phytoplankton under greenhouse warming Jong-Yeon Parka, Jong-Seong Kugb,1, Jürgen Badera,c, Rebecca Rolpha,d, and Minho Kwone aMax Planck Institute for Meteorology, D-20146 Hamburg, Germany; bSchool of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, South Korea; cUni Climate, Uni Research & Bjerknes Centre for Climate Research, NO-5007 Bergen, Norway; dSchool of Integrated Climate system sciences, University of Hamburg, D-20146 Hamburg, Germany; and eKorea Institute of Ocean Science and Technology, Ansan 426-744, South Korea Edited by Christopher J. R. Garrett, University of Victoria, Victoria, BC, Canada, and approved March 27, 2015 (received for review September 1, 2014) Phytoplankton have attracted increasing attention in climate suggested substantial future changes in global phytoplankton, with science due to their impacts on climate systems. A new generation the opposite sign of their trends in different regions (15–17). Such of climate models can now provide estimates of future climate climate change-induced phytoplankton response would impact cli- change, considering the biological feedbacks through the development mate systems, given the aforementioned biological feedbacks. of the coupled physical–ecosystem model. Here we present the geo- Previous studies discovered an increase in the annual area- physical impact of phytoplankton, which is often overlooked in future integrated primary production in the Arctic, which is caused by the climate projections. A suite of future warming experiments using a fully thinning and melting of sea ice and the corresponding increase in coupled ocean−atmosphere model that interacts with a marine ecosys- the phytoplankton growing area (18, 19). Considering that the tem model reveals that the future phytoplankton change influenced by biogeophysical feedback of phytoplankton leads to a warmer greenhouse warming can amplify Arctic surface warming considerably. ocean surface layer, the increased area of Arctic phytoplankton The warming-induced sea ice melting and the corresponding increase bloom can induce first-order warming in the Arctic Ocean. The in shortwave radiation penetrating into the ocean both result in a Arctic is a particularly vulnerable region where various positive longer phytoplankton growing season in the Arctic. In turn, the in- feedback processes are involved, such as ice albedo feedback, crease in Arctic phytoplankton warms the ocean surface layer through temperature lapse rate feedback, and water vapor/cloud feedback direct biological heating, triggering additional positive feedbacks in the – (20 22). Thus, Arctic warming is generally greater than the glob- EARTH, ATMOSPHERIC, Arctic, and consequently intensifying the Arctic warming further. Our ally averaged warming, a phenomenon known as Arctic amplifi- AND PLANETARY SCIENCES results establish the presence of marine phytoplankton as an important cation (23). Due to such strong positive feedback processes, an potential driver of the future Arctic climate changes. initial warming induced by the Arctic phytoplankton increase can biogeophysical feedback | phytoplankton−climate interaction | be amplified and can potentially contribute to the Arctic ampli- Arctic climate changes fication. In this sense, recent trends in annual mean surface temperature, sea ice, and chlorophyll may suggest the potential role of biogeophysical feedback on the current Arctic warming hytoplankton, aquatic photosynthetic microalgae, play a key trend (Fig. 1). The strong trends of surface warming and the re- role in marine ecology, forming the foundation of the marine P lated sea ice reduction appear over Barents, Kara, Laptev, and food chain. Besides this ecological importance, the climatic im- Chukchi Seas. The pattern of increased annual mean chlorophyll, portance of phytoplankton is also evident, given their role in which can be caused by stronger Arctic phytoplankton bloom or by carbon fixation, which potentially reduces human-induced carbon extended growing season, is closely linked with that of surface dioxide (CO ) in the atmosphere (1–3). The great strides made 2 warming and ice melting trends. Although, in this observational in the field of biogeochemical modeling have improved climate result, the causality of their relationship and the quantitative models, enabling the investigation of carbon−climate feedback, such as diagnosing the strength of biogeochemical feedback and quantifying its importance in total carbon cycle responses. In fact, Significance several modeling groups provide future climate projections in- cluding the biogeochemical process, as seen in a recent version of One of the important impacts of marine phytoplankton on the Coupled Model Intercomparison Project (CMIP), i.e., CMIP5. climate systems is the geophysical feedback by which chloro- In addition to their biogeochemical feedback, phytoplankton phyll and the related pigments in phytoplankton absorb solar also modify physical properties of the ocean. Chlorophyll and re- radiation and then change sea surface temperature. Yet such lated pigments in phytoplankton affect the radiant heating in the biogeophysical impact is still not considered in many climate ocean by decreasing both the ocean surface albedo and shortwave projections by state-of-the-art climate models, nor is its impact on penetration (4–6). Thus, higher phytoplankton biomass generally the future climate quantified. This study shows that, by conduct- ing global warming simulations with and without an active ma- results in warmer ocean surface layer. This biogeophysical feed- rine ecosystem model, the biogeophysical effect of future back is known to significantly impact the global climate (7–10) and phytoplankton changes amplifies Arctic warming by 20%. large-scale climate variability, such as the El Niño–Southern Oscil- – Given the close linkage between the Arctic and global climate, lation (11 13) and Indian Ocean dipole (14). Unlike biogeochemical the biologically enhanced Arctic warming can significantly feedback, however, biogeophysical feedback has been overlooked in modify future estimates of global climate change, and there- many future climate projections simulated by state-of-the-art climate fore it needs to be considered as a possible future scenario. models, even in projections by so-called Earth System Models that include interactive marine ecosystem components. Author contributions: J.-S.K. and M.K. designed research; J.-Y.P. and R.R. performed re- Greenhouse warming generally involves changes in physical search; J.-Y.P. analyzed data; and J.-Y.P., J.-S.K., J.B., and R.R. wrote the paper. fields that inevitably affect growth factors of phytoplankton such as The authors declare no conflict of interest. temperature, light, and nutrients. Hence, they lead to changes in This article is a PNAS Direct Submission. phytoplankton responding to future climate warming. Recent ana- 1To whom correspondence should be addressed. Email: [email protected]. lyses based on the historical observation of phytoplankton and the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. future phytoplankton population estimated by modeling works have 1073/pnas.1416884112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1416884112 PNAS Early Edition | 1of6 Downloaded by guest on September 30, 2021 Fig. 1. Observational Arctic climate trends in recent warming. Annual mean trends of observational (A) surface temperature, (B) sea ice concentration, (C) ice-free days, and (D) chlorophyll over the period 1998–2013 when the satellite-retrieved chlorophyll data are available. The ice-free days are defined as the number of days when sea ice concentration is lower than 5%. impact of phytoplankton cannot be established, the similar pat- marine ecosystem model. In the other experiment, named terns between the different variables suggest the possibility of in- ECO.off, the marine ecosystem model is turned off, and instead, teractive phytoplankton−climate feedback in the Arctic. the chlorophyll concentration is prescribed with the long-term The same feedback loop may hold for the future. If global climatology of the present climate simulation. Therefore, ECO.off warming continues with increased ice-free regions as projected by cannot consider future changes in marine phytoplankton, and most climate models, the phytoplankton growing area would in- thus the difference between the two experiments implies the crease in the Arctic Ocean, and this, in turn, may intensify the impact of biogeophysical feedback on future climate projections. Arctic amplification. However, most future climate projections have The five-member ensemble mean of two experiments shows been produced without considering the impact of biogeophysical a stronger Arctic surface warming in ECO.on compared with feedback, as mentioned earlier. Therefore, the effects of phyto- ECO.off (Fig. 2A). Although ECO.on results in a globally warmer plankton feedback need to be investigated in the context of the climate than ECO.off, the relative warming in ECO.on is most phytoplankton−Arctic warming hypothesis. evident in Arctic regions (SI Appendix,Fig.S2). Interestingly, the Arctic surface warming pattern with the most remarkable warming Results near the Kara and Chukchi Seas is consistent with the observa- To examine the biological impact on the climate responses to tional results (compare Figs. 2A and 1A). Moreover, the patterns greenhouse warming, a coupled
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