Goal:

Simulate the growth in atmospheric mixing ratios of CO2 as a result of fossil fuel emissions. Compare the simulated growth rate to observed growth rates and estimate what fraction of the emitted CO2 stays in the atmosphere.

Introduction

The atmospheric concentration of CO2 has been increasing from about 330 ppm in 1974 to about 380 ppm in 2006 (see figure 1).There is little doubt that the CO2 emissions that are related to fossil fuel use are responsible for the increase in atmospheric CO2 concentrations. But what would be the increase in atmospheric concentrations if all the CO2 would remain in the atmosphere? And what is the gradient between the Northern Hemisphere (most emissions) and the Southern hemisphere (less emissions). To test this, CO2 will be added as an non-reactive tracer with only fossil fuel emissions. Other processes, such as uptake by the biosphere, oceans, and emission as a result of biomass burning, are ignored.

Figure 1: CO2 concentration (ppm or micro-mol/mol) measured at Barrow (Alaska) between 1974 and 2006.

In order to compare to model to a real atmosphere, measurements taken by NOAA/CMD will be used. The link is given below. Also, the Carbon Dioxide Information Analysis Center provides estimates of world-wide CO2 emissions. The web-addresses are used throughout the exercises.

Table 1: relevant web-addresses for the exercises

In the following exercises, it will be attempted to:

1. Quantify the relation between fossil fuel emissions and CO2 growth rates
2. Investigate if the North-South gradient is well simulated by the model
3. Estimate what fraction of the fossil fuel emissions stays in the atmosphere
4. Investigate the interannual variations of CO2
5. Study the impact of uptake by oceans and biosphere

Exercise 9: CO2 rate of change

First, the MOGUNTIA model has to initialised. A prelimanary input file fossil.in is provided as a starting point. Note that the MOLMASS is defined as 12, since all data are normally given in Millions of Tonnes (1 pG = 1000 Million Tonnes) of carbon. Note also that the CO2 emission distribution is read from a file. Logically, highest emissions are found over industrialised and densily populated areas.

Initially, the model will be run for four years (2000, 2001, 2002, 2003) with the CDIAC fossil fuel emissions.

• Run MOGUNTIA with the input file fossil.in.
• Where do you observe the highest concentrations at the surface?
• Discuss the growth rate observed at the stations.
• How does the growth rate compare to the observed growth-rate at stations at comparable latitudes?
• Estimate the North-South gradient
• How does this gradient compare to the observed gradient?

In order to get a first estimate of the CO2 that is taken up by the oceans and biosphere, try to reduce the emissions and rerun the model to get a better correspondance to observations.

Exercise 10: Uptake of CO2

In order to estimate the long-term average uptake of CO2 in a more sophisticated way, the following diagram is used:

Figure 2: Diagram showing the estimated budget for CO2. Blue numbers indicate fluxes in GtC/yr and black numbers indate reservoirs in GtC. 1 GtC corresponds to 10 to the power 12th gram, or 1 Pg.

From the net uptake of CO2, e.g. to the oceans, a lifetime can be calculated by dividing the atmospheric burden by the net-uptake. The task is now to test these numbers in MOGUNTIA. For that reason, the input file is modified in the following way:

• Extend the simulation period, e.g. to 1995-2003 by looking up the relevant emission data
• Define some stations that can be directly compared to measurements (NOAA/CMD)
• Define a correct initial concentration, based on measured atmospheric concentrations
• Examine fossil2.in to see how to use SINK to calculate the uptake by oceans and/or the biosphere
• Calculate the CO2 growth with and without the biosphere uptake

Address the following issues

• How does the biosphere affect the growthrate of CO2?
• How is the seasonal cycle affected on the northern and southern hemispheres?
• How do the new simulations compare to measurements?

Repeat the calculations for:

• The sink to the oceans (SINK FILE co2_sink_oc.dat)
• The sink to both the oceans and the biosphere (SINK FILE co2_sink_ocbio.dat)
• Discuss the results and compare to measurement time series.