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Quantifying Uncertainty in Climate Projections including Biogeochemical Feedbacks

Final Report Summary - CLIMB (Quantifying Uncertainty in Climate Projections including Biogeochemical Feedbacks)

Project’s objectives
State-of-the-art earth system models used for long-term climate projections, which originally dealt with only large-scale physical processes, are becoming ever more complex in terms of not only spatial resolution but also the number of processes included in the models. Biogeochemical processes are beginning to be incorporated into these complex models. However, the implementation of biogeochemical processes into earth system models is incomplete due to a difficulty in resolving heterogeneous biogeochemical processes. Moreover, upon implementation, subjective assumptions (albeit based on expert judgments) are often made on the estimates of uncertain parameters introduced in the added processes, without being combined with a more objective mathematical method. The incomplete representation of biogeochemical processes and subjective assumptions on the estimates of uncertain parameters influences the projections of both the past and future climate.
The objectives of the project are two-fold:
- To understand how biogeochemical cycles interact and influence the climate system over the past millennium
Previous millennium studies such as the Community Simulations of the last Millennium project (http://www.mpimet.mpg.de/en/wissenschaft/working-groups/millennium.html) include only the carbon cycle. A large uncertainty remains in the historical carbon budget, which is hypothesized due to missing carbon cycle processes (e.g. soil erosion and coastal zone processes) or the absence of coupling with other elements cycles through biological production of the marine and terrestrial ecosystem.
- To include biogeochemical feedbacks in the projections of future climate including associated uncertainties
State-of-the-art climate models do not fully account for biogeochemical cycles, which may provide significant feedbacks to climate system. Uncertain parameters in climate and biogeochemical models are often tuned solely based on the expertise of model developers, without employing an objective inverse estimation technique.

Description of the work performed
The researcher tested the response of the coupled carbon-nitrogen-phosphorus biogeochemical cycle model Terrestrial-Ocean-aTmosphere Ecosystem Model version 2 (TOTEM2) to various types of perturbation. These results were compared with those obtained from other models. He performed sensitivity studies by decoupling the carbon cycle from the nitrogen and phosphorus cycles. Furthermore, as an example of studies investigating specific climate-biogeochemical feedbacks, the researcher looked into how permafrost-carbon feedbacks influence emission and temperature pathways to achieve different levels of climate stabilization by using the Aggregated Carbon Cycle, Atmospheric Chemistry, and Climate (ACC2) model coupled with a newly developed simple parameterization of such feedbacks. Another example is his study looking into the change in the subtropical nitrogen cycle under future climate change by using the Kaneohe Bay ECOsystem Model (KECOM). Related to uncertainties in climate projections, the researcher explored how the uncertainty in climate sensitivity changes over a historical period as more observations are acquired.

Description of the main results achieved during the specific period
The results for the TOTEM2 model response to various perturbations contributed to the CO2 Impulse Response Function Intercomparison Project (http://www.climate.unibe.ch/~joos/IRF_Intercomparison/) and have been published in the following peer-reviewed paper:
- Joos F, Roth R, Fuglestvedt JS, Peters GP, Enting IG, von Bloh W, Brovkin V, Burke EJ, Eby M, Edwards NR, Friedrich T, Frölicher TL, Halloran PR, Holden PB, Jones C, Kleinen T, Mackenzie FT, Matsumoto K, Meinshausen M, Plattner G-K, Reisinger A, Segschneider J, Shaffer G, Steinacher M, Strassmann K, Tanaka K, Timmermann A, Weaver AJ (2013) Carbon dioxide and climate impulse response functions for the computation of greenhouse gas metrics: a multi-model analysis. Atmospheric Chemistry and Physics 13:2793-2825
The carbon cycle parameterizations, which the results from this Marie Curie project contributed to, will be widely used in climate research and policy analyses, in particular computations of emission metrics (i.e. indices to compare emissions of different greenhouse gases in climate policies).
The paper addressing the nitrogen cycle in the subtropics is in revision.
- Tanaka K, Guidry MW, Gruber N (2013) Ecosystem responses of the subtropical Kaneohe Bay, Hawaii to climate change: a nitrogen cycle modeling approach. Aquatic Geochemistry, in revision

Expected final results and their potential impact and use
The researcher plans to develop the model further by improving the representation of riverine transport of elements to rivers and oceans. Then how C-N-P biogeochemical cycles provide feedbacks to the climate will be explored by using the revised model TOTEM3. On this theme, the project aims to produce two more papers as follows:
- Tanaka K, Mackenzie FT, Knutti R, Bouchez J, Lerman A, Ver M (in preparation) Model description of the Terrestrial-Ocean-aTmosphere Ecosystem Model version 3 (TOTEM3).
- Tanaka K, Knutti R, Mackenzie FT (in preparation) Carbon-nitrogen-phosphorus biogeochemical feedbacks to the climate.
This study may have an implication for future climate projections and serve as a pilot study to inform the climate research community as to whether coupled C-N-P cycle processes are important to be included in other climate models.