May 2002

Assimilation competition: Experts on the trail of CO2

One of the iconic images in atmospheric science is the steady climb in global carbon dioxide levels, as shown by the famous 45-year trace from Mauna Loa (see http://www.cmdl.noaa.gov/ccg/figures/co2mm_mlo.jpg).Partitioning that global trend into regional sources and sinks hasn’t been easy, but it’s critical in creating climate change policy. In a workshop at the Mesa Lab this month, two teams of scientists will match wits, using virtual instruments and atmospheres, to see how our understanding of the CO2 budget might be improved.

The unusual contest was conceived by Britt Stephens (ATD). It’s the centerpiece of a summer institute cosponsored by NCAR and Colorado State University (CSU) and organized by Dave Schimel (CGD) and Britt. Some 30 to 40 participants will be on hand from 20 to 31 May.

	Dave Schimel and Britt Stephens. (Photo by Carlye Calvin.)

Long-term global averages for CO2 are quite solid, but the finer-scale details about the most prevalent human-produced greenhouse gas remain murky. Due to differences in ground cover, CO2 concentrations over continents can vary by as much as several percent across only a few miles. Levels of CO2 can also rise and fall with the passage of fronts.

The bulk of air motion worldwide is along east-west rather than north-south belts, so it’s been possible to narrow down the latitudinal zones that serve as overall sources or sinks. "We think there’s a large amount of CO2 being removed from the atmosphere by land ecosystems between 30 and 60 degrees north," says Britt. But any results more specific than that, he adds, are "highly model dependent." For instance, recent studies—including one by Dave—have disagreed sharply on the percentage of global CO2 absorbed by U.S. forests. The result is major uncertainty for policymakers trying to assess regional contributions to global warming.

Part of the motivation behind the institute is to tackle the chicken-or-egg aspect of the CO2 problem, says Britt. "It’s clear we need better modeling and much more data. What’s not clear is the best approach to the modeling scheme and how best to expand the observing network."

After several days of introductory talks, the attendees will be split into two teams. Each team will be granted an imaginary budget for use in deploying imaginary instruments—aircraft, towers, ships, and buoys—around the globe. Ahead of time, organizers have assigned realistic costs for each imaginary instrument and woven a few biases and uncertainties into their data.

With the help of SCD’s computers, the teams will bring data from their instrument fleet into a global model. The idea is to construct a region-by-region picture of CO2 over a year’s worth of model time, with the picture taking a different form for each team based on the location and type of its chosen instrument fleet. Each team’s performance will be checked against a pseudo-atmosphere created by Dave Baker (ASP) and Roger Dargaville (CGD). Building blocks include an ocean model from CGD’s Scott Doney and models from the Vegetation/Ecosystem Modeling and Analysis Project, furnished by CSU and

other sites. Face-offs like these have taken place in mesoscale weather research, says Dave, but in the world of biogeochemistry, "as far as I know this has never been done before. What we hope is that we can come up with a fundamentally new approach for integrating land-surface data, atmospheric observations, and models."

Plans for the institute crystallized after a North American carbon workshop organized last September by Bob Harriss (ESIG). Support is coming from the Integrated Research Challenge of NSF’s biological sciences directorate and from the NCAR director’s office as part of NCAR’s strategic initiative in the biogeosciences.

With funding limited at first, Dave and Britt worked with CSU colleagues Dennis Ojima and Tomi Vukicevic to pull together a grassroots team of enthusiastic volunteers. Organizers from across NCAR, along with those noted above, include Steve Aulenbach (CGD), Beth Holland (ACD), Don Middleton and Dave Brown (SCD), and Jen Oxelson (ESIG/ATD). According to Dave Schimel, "Everybody perceived this would be a great scientific problem, a technological opportunity, or just plain fun."

Institute speakers run the gamut from satellite experts to ice-core specialists. Dave is especially pleased that Inez Fung (University of California, Berkeley) will give a keynote talk. A pioneer in carbon-cycle science with roots in meteorology, Inez will lay out the evolution of data assimilation in numerical weather prediction and how it can evolve into a broader tool for Earth system science.

There won’t be any prize offered in this contest, other than the inherent reward of making headway on a tough problem. It may take some heat to shed light on the issues, Dave adds, so a few friendly sparks could fly. "This is a very collegial yet extremely competitive bunch of people." • Bob Henson

Edited by David Hosansky, hosansky@ucar.edu
Prepared for the Web by Carlye Calvin
Last revised: Tues May 28 17:08:40 MST 2001