Natural Carbon Sinks

IPCC models continue to predict an enormous future benefit from the fertilization of the terrestrial biosphere by CO2 – over 5 billion tons of carbon per year taken from the atmosphere by 2050 and approximately 2 billion tons per year today. This year, we published the last of a three-paper set, begun in Year One of the CMI project, showing that the terrestrial sink in the U.S. is large (currently ~0.5 billion tons of carbon per year), caused by land use change instead of by CO2 fertilization, and will disappear during this century. We have begun to extend this work to the rest of the northern hemisphere, and our preliminary carbon budget leaves little room for a global sink caused by CO2 fertilization, unless there is a corresponding missing source. There are also other new experimental and theoretical results that cast doubt on predictions of a large CO2 fertilization sink in the future. Emission cuts necessary to stabilize atmospheric CO2 are substantially more stringent without a future benefit from CO2 fertilization and must begin sooner.

We have also completed an analysis of the long-term fate of the oceanic sink under business as usual emissions. We have explored the multi-century consequences of an enduring fossil fuel era, where 5600 Gt(C) of fossil fuels are extracted over seven centuries, extending the IS92A emissions scenario with a logistic function. The maximum CO2 concentration in the atmosphere reaches 1840 ppm (6-7 times preindustrial levels), then falls to 1160 ppm at steady state. The steady-state level is so high because the ocean’s carbonate buffer becomes saturated, and this reduces the oceanic sink. This work provides interesting constraints on leakage from geologic sequestration reservoirs. The consensus view at the October 2002 International Conference on Greenhouse Gas Control Technologies, in Kyoto, was that geologic reservoirs need to hold sequestered CO2 for only 100-1000 years because most of the leakage (approximately 85% at equilibrium) would be absorbed by the oceans. Our new analysis indicates that this conclusion is true if leaks total less than about 1000 billion tons. Constraints on leakage are much tighter (up to a 10,000 year mean residence time), if leaks total more than 1000 billion tons, because they start to saturate substantially the oceanic carbon buffer.

Our measurement program is yielding new estimates of the size and inter -annual variability of the terrestrial and oceanic sinks. Our most recent estimate of the terrestrial sink is 1.3 +/- 0.6 billion tons of carbon. And for the oceanic sink it is 2.1 +/ – 0.5 billion tons of carbon. Three O2/N2 samplers are running well and improving the accuracy of our estimates of the current sinks. Our measurements of the seasonal variation of the atmospheric Ar/N2 ratio at six sites are providing an important constraint on models.

 


Earth System Model (ESM)

This past year, we devoted considerable effort to the building of new terrestrial and ocean biogeochemistry models for the new GFDL/Princeton ESM. The model provides our core capability to predict climate change and associated impacts. This coming year it will provide new estimates of the emissions cuts necessary to achieve stabilization. The new atmospheric and oceanic models developed by GFDL perform well, but work on the coupled system is still proceeding. Our land model includes a reactive biosphere and human land use. Thus, it can be used to investigate impacts affecting agricultural and natural ecosystems and solutions involving biofuels and ecological sequestration. Similarly, our ocean model includes iron biogeochemistry, nutrient limitation, and mechanistic phytoplankton dynamics and thus can be used to investigate some impacts affecting oceanic ecosystems as well as oceanic sequestration schemes.

We have also continued to develop our regional land-atmosphere model. This year we used this model to study the impact of historical land-cover change in the United States. Summer temperatures and rainfall in the US were seen to be affected by forest and grassland conversion in the 19th century and by farm abandonment and fire suppression in the 20th century. We plan to complete a similar global analysis with the new ESM.

 


Glacial Cycles and Atmospheric CO2

Confidence that the important mechanisms driving the Earth’s climate have been identified will increase dramatically once the ice ages are understood. A review of deep – sea sediment data suggests that a link between cold climates and polar ocean stratification is pervasive over the last thirty million years, leading to a critical hypothesis for the glacial-interglacial CO2 cycle focusing on polar ocean stratification. We are evaluating this hypothesis using new methods to retrieve biogeochemical information from deep-sea sediments in the Antarctic and Subarctic Pacific Oceans. With these new methods, we are able to measure the nitrogen isotope composition of organic matter in minute quantities of buried algal microfossils. These isotope measurements provide one of the key indications that the Antarctic Ocean was stratified during glacial times. Because of the critical nature of this result, we are also studying the generation of the isotope signal in modern polar surface waters, to determine once and for all whether our nitrogen isotope-based interpretation, that of polar stratification during cold climates, must be correct.

 


Frontier Scouts for Mitigation Technologies

We continue to scout the mitigation frontier, where many technological approaches are being investigated. Our modeling capability is allowing us to raise and answer important questions about feasibility in each instance. Three examples:

  • Deep ocean injection – We continued to study the sensitivity of the efficiency of direct deep ocean injection to uncertainties about vertical mixing in the oceans. Our most recent analysis suggests that over 90% of the carbon injected below 3000 m remains in the ocean after 500 years.
  • Wind energy – We have initiated a project on the possibility that large-scale development of wind energy would change climates. Although results are still very preliminary, both our new global models and our regional model indicate substantial effects.
  • Biofuels and terrestrial sequestration – We have initiated a project on the effects of large increases in plantation forestry on climate, surface hydrology, and air quality. This work builds on work completed this past year, showing that in the U.S. during the 1980’s and 1990’s forest management practices led to an increase in air emissions of precursors to tropospheric ozone (isoprene and monoterpenes). The magnitude of this increase exceeded the magnitude of the decrease in emissions brought about by EPA regulations on emissions from industry and transportation. In contrast, natural forest succession generally acts to improve air quality.