Research Programs

Global Biogeochemical Cycles
FY2006 activities

TIIMES Theme:
BGS

Compiled by Elisabeth Holland - ACD - TIIMES

Research Team: Elisabeth Holland-ACD & TIIMES, Joanie Kleypas-SERE & TIIMES, Julia Lee-Taylor-ACD & TIIMES, Samuel Levis-CGD & TIIMES, Keith Lindsay-CGD & TIIMES, Natalie Mahowald-CGD & TIIMES, Nan Rosenbloom-CGD & TIIMES, Brit Stephens-EOL & TIIMES, Peter Thornton-CGD & TIIMES

Global Biogeochemical Cycles

Midday vertical CO2 profiles measured at 12 global locations averaged over different seasonal intervals. Summer profiles (A) are averages over the months July – September, annual-mean profiles (B) are averages over all months, and winter/spring profiles (C) are averages over the months January – May. Thick gray lines in each panel represent Northern Hemisphere average profiles for the same times.

As is clearly articulated in the Fourth Assessment report of the Intergovernmental Panel on Climate Change (IPCC), to be released in 2007, there is increasingly strong motivation to examine terrestrial and oceanic carbon fluxes on regional to continental scales, to understand the coupling of the carbon cycle to the climate system and to other biogeochemical cycles, to understand the processes responsible for present uptake of anthropogenic carbon, to predict future trends in these fluxes under various climate change scenarios, and to assess potential strategies for increasing carbon uptake and storage into the future. The challenges of scaling up from local measurements and scaling down from global constraints are being addressed in TIIMES through the development and application of advanced observational and modeling tools.

There are systematic biases in atmospheric models that predict large northern terrestrial CO₂ uptake and large tropical CO₂ releases based on analysis of light-aircraft vertical CO₂ profiles measured at 12 global sites by 6 international laboratories [Figure 1].

Northern Hemisphere sites include:

• Poker Flat, Alaska, USA (PFA)
• Harvard Forest, Massachusetts, USA (HFM)
• Briggsdale, Colorado, USA (CAR)
• Park Falls, Wisconsin, USA (LEF)
• Estevan Point, British Columbia, Canada (ESP)
• Molokai Island, Hawaii, USA (HAA)
• Orleans, France (ORL)
• Zotino, Russia (ZOT)
• Sendai/Fukuoka, Japan (SEN)
• Surgut, Russia (SUR)

  Southern Hemisphere sites include:

• Rarotonga, Cook Islands (RTA)
• Bass Straight/Cape Grim, Australia (AIA)

These new findings suggest that previous estimates of the Northern Hemisphere “missing terrestrial carbon sink” were overestimated.

Carbon only and carbon+nitrogen sensitivity to changing air temperature and precipitation patterns using CCSM CLM C and CLM C-N models. The figure is from Thornton, P. E., et al. Effects of terrestrial carbon-nitrogen cycle coupling on climate-carbon cycle feedbacks in a global model, submitted to Global Biogeochemical Cycles.

Carbon-nitrogen (C-N) coupling reduces by about a factor of four the global terrestrial carbon uptake response to increasing atmospheric CO₂ concentration, compared to the carbon-only response. Global integrated responses of Net Ecosystem Exchange (NEE) to variation in temperature and precipitation are damped by C-N coupling [Figure 2]. The carbon-only model predicts that the NEE responses to variation in temperature and precipitation increase in magnitude under increasing atmospheric CO₂ concentration, while the C-N model predicts that these responses decrease in magnitude. Fertilization responses in the C-N model to increasing CO₂ and increasing mineral nitrogen deposition are in qualitative agreement with results from field experiments.

The changes in the North and South Atlantic sea surface temperatures (SSTs) resulting from North African atmospheric dust transport may account for up to 50% of the Sahel  precipitation reduction, but this effect could be a duet of factors other than desert dust aerosols (e.g., biomass burning aerosols and other low-frequency processes). Vegetation loss in the Sahel region may explain about 10% of the observed drying, but this effect is statistically insignificant due to the small number of years in the simulation. Greenhouse gas warming seems to have an impact to increase Sahel precipitation that is opposite to the observed change. Although the estimated values of impacts are likely to be model dependent, our analyses suggest the importance of direct radiative forcing of dust and feedbacks in modulating Sahel precipitation (Yoshioka, Mahowald et al. submitted to Journal of Climate).

FY07 plans are to continue moving towards more fully coupled representations of biogeochemical cycles in the Community Climate System Model (CCSM). Key foci for the next year include: a) examination of the role of biomass burning, b) working towards coupling the chemistry and biogeochemistry through emission and deposition models, c) a continuing commitment to the carbon model inter-comparison, d) continued development of the coupled carbon nitrogen model, e) the CCSM Community Land Model (CLM) C-N model, and f) combining observations and models to understand both trends and inter-annual variability in the biogeochemical exchanges.