## Lab: Modern Atmospheric CO2 Record

### I. Introduction

The measurements of atmospheric carbon dioxide (CO2) concentrations at the Mauna Loa Observatory are derived from the Scripps Institution of Oceanography's continuous monitoring program. This record constitutes the longest continuous record of atmospheric CO2 concentrations available in the world. Monthly average mole fractions of CO2 in water-vapor-free air are given from March 1958 through 1992, except for a few interruptions. For comparison, CO2 records for two other stations, Barrow (Alaska), and South Pole (Antarctica) are given. The comparison of the records yield insights into natural and anthropogenic sources of CO2 and its atmospheric transport.

#### A. Atmospheric CO2 Data

Open the atmospheric CO2 data. Familiarize yourself with the location of the stations where the data were collected by checking out the related web sites: Mauna Loa, South Pole, Barrow. Note that there is one parameter, CO2, measured monthly at three locations: Mauna Loa (Hawaii), South Pole (Antarctica), and Barrow (Alaska).

Task 1: What are the general characteristics of this data set? Consider the units of CO2, and the maximum and minimum CO2 concentrations for each station. Overall, are the values increasing or decreasing with time?

Task 2: What is the cause of the short-period oscillation of CO2? To answer this question, make two charts of the data: one showing all the data and the other showing the last 3 years of the CO2 records. Include data from all three stations. Recall that this is an "xy scatter plot" and not a "line plot" when using Excel's chart function.

• How long is each oscillation?
• When do maxima and minima occur at the individual stations? Why?
• What are the differences in amplitude and timing in the oscillations in the three CO2 records? Can you explain the differences?

Now review this animation showing monthly variations in net primary productivity. Discuss these seasonal changes in the context of the time series.

Task 3: Calculate the annual averages for each of the last three years for the three sites and compare them. What causes the differences in annual average CO2 concentrations at the sites?

Task 4: What is responsible for the long-term change in CO2 concentrations measured at Mauna Loa? Go back to the graph showing the entire time series. Determine the long-term rate of change of the Mauna Loa CO2 concentrations by adding a trendline and displaying the equation of that line on the graph. What is the rate of change (including units)? Using the regression equation, extrapolate CO2 concentrations to the year 2100. How long will it take for CO2 to rise by 50% of the last measurement at this rate?

So far, you have assumed that the rate of increase in CO2 is constant. Is there evidence that the rate is actually systematically increasing or decreasing over the years? Do you have an explanation of this phenomenon? Based on the shape of the curves, do you think that you over- or underestimated the time it will take to exceed the latest measurement by 50%?

#### B. Greenhouse Gases

Task 5: Examine this table showing the concentrations, global warming potentials, and lifetimes for various greenhouse gases (GHGs). Answer the following questions:

• Which GHGs are most abundant? Note which are measured in ppm and which in ppb? In what units are the others measured?
• The Global Warming Potential (GWP) is typically used to contrast different greenhouse gases relative to CO2. GWP is an index for estimating relative global warming. It is a measure of warming contribution of a kg of a particular GHG as compared to the warming contibution of a kg of carbon dioxide. The GWP provides a simple measure of the relative radiative effects of the emissions of various greenhouse gases (see footnote 2 in the table for more information). What are the Global Warming Potentials of CO2 and CH4? Which 3 GHGs have the highest GWP? Which 3 have the lowest?
• Atmospheric lifetime is the time-scale characterizing the decay of an instantaneous pulse input into the reservoir (see footnote 3). What are the lifetimes of CO2 and CH4? Which 3 GHGs have the longest lifetimes? Which 3 have the shortest?

Task 6: Examine the projected emissions in CO2 and CH4 for the next century (From: "Climate Change 2001: The Scientific Basis").

• How do you account for the wide range in emission scenarios?
• Which one follows the current trend?
• Which scenario(s) ultimately result in reductions in GHG emissions below current values? For these scenarios, about what time do emissions stop increasing and start to decrease?

Task 7: Examine these two plots showing potential changes in CO2 and CH4 concentrations (From: "Climate Change 2001: The Scientific Basis").

• Why do CO2 concentrations increase for all scenarios, even though emissions have decreased for some of them? What is the minimum CO2 concentration projected for 2100? How does this compare with your projection for 2100 based on extrapolation of the Mauna Loa trend(s)?
• Why do CH4 concentrations show a decrease for some scenarios? Do the decrease in CH4 concentration and CH4 emission occur at the same time? Why or why not?

Task 8: HCFC-22 and HFC-23 were designated acceptable replacements for CFCs-11/12/113 in the initial Montreal Protocol because they have less potential to deplete stratospheric ozone than the CFCs. However, HCFC-22 and HFC-23 are greenhouse gases.

• Look at the metadata for HFC-23. What is the growth rate of HFC-23 in the atmosphere?
• Go back to the Current Greenhouse Gas chart. How do the present tropospheric concentrations, GWP, and atmospheric lifetimes of HCFC-22 and HFC-23 compare to those of the CFCs (11/12/113)?
• What are the major chemical differences between CFCs, HCFCs and HFCs? (see Kump Chapter 17 especially pages 357-359) Why are HCFCs and HFCs used as substitutes for CFCs? What are the environmental consequences of using HCFC-22 and/or HFC-23?

### VI. Lab Report Instructions

Write a lab report (as per the Lab Report Format) summarizing the major findings of your investigation including the answers to the questions given in the lab instructions. Include the following questions in your discussion.

1. What processes control atmospheric CO2 concentrations on an annual basis at different latitudes?
2. What processes might explain the long-term trend in atmospheric CO2 concentration?