Natural Ecosystems

This project aims to understand pathways in which ocean ecosystems are subject to change and to quantify states of ecological vulnerability. The researchers propose to use existing satellite measurements, particularly ocean color, in situ datasets, along with numerical model output and theory, to address spatial, temporal and depth-dependent changes to marine ecosystems with a focus on how best to detect these changes and characterize vulnerability using satellite measurements.

Assessing Ecosystem Vulnerability to Climate Change Through Optics, Imagery & Models

Recent studies have suggested that the combined effect on climate of increases in the concentrations of several trace gases (principally CH4 and N2O) could rival or even exceed that of the increasing concentration of carbon dioxide (CO2), due to their much stronger abilities in absorbing infrared radiation. Despite their high potential for climate feedback, the specific sources, magnitudes and changes of these trace-gas emissions are still not well understood.

Use of Soil-Moisture Retrievals to Refine Global Land Trace Gases Emissions and Their Climate Feedbacks

The terrestrial biosphere as a net source of greenhouse gases to the atmosphere

The terrestrial biosphere can release or absorb the greenhouse gases, carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), and therefore has an important role in regulating atmospheric composition and climate. Anthropogenic activities such as land-use change, agriculture and waste management have altered terrestrial biogenic greenhouse gas fluxes, and the resulting increases in methane and nitrous oxide emissions in particular can contribute to climate change. The terrestrial biogenic fluxes of individual greenhouse gases have been studied extensively, but the net biogenic greenhouse gas balance resulting from anthropogenic activities and its effect on the climate system remains uncertain. Here we use bottom-up (inventory, statistical extrapolation of local flux measurements, and process-based modelling) and top-down (atmospheric inversions) approaches to quantify the global net biogenic greenhouse gas balance between 1981 and 2010 resulting from anthropogenic activities and its effect on the climate system. We find that the cumulative warming capacity of concurrent biogenic methane and nitrous oxide emissions is a factor of about two larger than the cooling effect resulting from the global land carbon dioxide uptake from 2001 to 2010. This results in a net positive cumulative impact of the three greenhouse gases on the planetary energy budget, with a best estimate (in petagrams of CO₂ equivalent per year) of 3.9 ± 3.8 (top down) and 5.4 ± 4.8 (bottom up) based on the GWP100 metric (global warming potential on a 100-year time horizon). Our findings suggest that a reduction in agricultural methane and nitrous oxide emissions, particularly in Southern Asia, may help mitigate climate change.

The dynamical landscape of marine phytoplankton diversity

Observations suggest that the landscape of marine phytoplankton assemblage might be strongly heterogeneous at the dynamical mesoscale and submesoscale (10–100 km, days to months), with potential consequences in terms of global diversity and carbon export. But these variations are not well documented as synoptic taxonomic data are difficult to acquire. Here, we examine how phytoplankton assemblage and diversity vary between mesoscale eddies and submesoscale fronts. We use a multi-phytoplankton numerical model embedded in a mesoscale flow representative of the North Atlantic. Our model results suggest that the mesoscale flow dynamically distorts the niches predefined by environmental contrasts at the basin scale and that the phytoplankton diversity landscape varies over temporal and spatial scales that are one order of magnitude smaller than those of the basin-scale environmental conditions. We find that any assemblage and any level of diversity can occur in eddies and fronts. However, on a statistical level, the results suggest a tendency for larger diversity and more fast-growing types at fronts, where nutrient supplies are larger and where populations of adjacent water masses are constantly brought into contact; and lower diversity in the core of eddies, where water masses are kept isolated long enough to enable competitive exclusion.

How well do global ocean biogeochemistry models simulate dissolved iron distributions?

Numerical models of ocean biogeochemistry are relied upon to make projections about the impact of climate change on marine resources and test hypotheses regarding the drivers of past changes in climate and ecosystems. In large areas of the ocean, iron availability regulates the functioning of marine ecosystems and hence the ocean carbon cycle. Accordingly, our ability to quantify the drivers and impacts of fluctuations in ocean ecosystems and carbon cycling in space and time relies on first achieving an appropriate representation of the modern marine iron cycle in models. When the iron distributions from 13 global ocean biogeochemistry models are compared against the latest oceanic sections from the GEOTRACES program, we find that all models struggle to reproduce many aspects of the observed spatial patterns. Models that reflect the emerging evidence for multiple iron sources or subtleties of its internal cycling perform much better in capturing observed features than their simpler contemporaries, particularly in the ocean interior. We show that the substantial uncertainty in the input fluxes of iron results in a very wide range of residence times across models, which has implications for the response of ecosystems and global carbon cycling to perturbations. Given this large uncertainty, iron fertilization experiments based on any single current generation model should be interpreted with caution. Improvements to how such models represent iron scavenging and also biological cycling are needed to raise confidence in their projections of global biogeochemical change in the ocean.

Climate and Land: Tradeoffs and Opportunities

Land management for carbon sequestration offers an opportunity to avoid about 0.5°C of warming if landowners have full economic incentives to participate in a global greenhouse gas mitigation policy. In an energy-only policy aimed at about 550 ppm CO₂- eq stabilization the additional 0.5°C of avoided warming brings the world close to staying below the 2°C above preindustrial target. While greater incentives for mitigation from energy and land would be needed to actually meet the 2°C target even holding temperature to 2.2°C or so would be a great improvement over the path we are on now. Even with success at the 21^st meeting of the Conference of the Parties (COP21) in Paris in 2015, we would still be heading toward a global mean surface temperature increase is in the range of 1.9 to 2.6°C (central estimate 2.2°C) by mid-century relative to the pre-industrial level (1860–1880 mean), and 3.1 to 5.2°C (central estimate 3.7°C) by 2100. This preliminary assessment awaits a full interpretation of the implications of agreements at COP21 with greater clarity on just what countries pledges mean and how they will be implemented. But this first look estimates that the COP21 pledges would shave about 0.2°C more from warming compared with previous international commitments. From that perspective, a half-degree of avoided warming from land carbon sequestration and avoided deforestation, if we could achieve it, is significant. The relatively small contribution from the COP21 agreement says less about the significance of the commitments—many countries stepped up and offered important goals that will mean changes in their energy systems—but rather more about the challenge of weaning the global economy from its dependence on greenhouse gas emitting activities. It also highlights that no source (or potential sink) can be ignored.

Biogeochemical drivers of the fate of riverine mercury discharged to the global and Arctic oceans

Rivers discharge 28 ± 13 Mmol yr⁻¹ of mercury (Hg) to ocean margins, an amount comparable to atmospheric deposition to the global oceans. Most of the Hg discharged by rivers is sequestered by burial of benthic sediment in estuaries or the coastal zone, but some is evaded to the atmosphere and some is exported to the open ocean. We investigate the fate of riverine Hg by developing a new global 3-D simulation for Hg in the Massachusetts Institute of Technology ocean general circulation model. The model includes plankton dynamics and carbon respiration (DARWIN project model) coupled to inorganic Hg chemistry. Results are consistent with observed spatial patterns and magnitudes of surface ocean Hg concentrations. We use observational constraints on seawater Hg concentrations and evasion to infer that most Hg from rivers is sorbed to refractory organic carbon and preferentially buried. Only 6% of Hg discharged by rivers (1.8 Mmol yr⁻¹) is transported to the open ocean on a global basis. This fraction varies from a low of 2.6% in East Asia due to the barrier imposed by the Korean Peninsula and Japanese archipelago, up to 25% in eastern North America facilitated by the Gulf Stream. In the Arctic Ocean, low tributary particle loads and efficient degradation of particulate organic carbon by deltaic microbial communities favor a more labile riverine Hg pool. Evasion of Hg to the Arctic atmosphere is indirectly enhanced by heat transport during spring freshet that accelerates sea ice melt and ice rafting. Discharges of 0.23 Mmol Hg yr⁻¹ from Arctic rivers can explain the observed summer maximum in the Arctic atmosphere, and this magnitude of releases is consistent with recent observations. Our work indicates that rivers are major contributors to Hg loads in the Arctic Ocean.

MIT Global Change

Natural Ecosystems

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