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Recent Trends in Carbon Dioxide Concentrations
One of the first indications that humans have altered the composition of
the global atmosphere comes from the measurements of atmospheric carbon
dioxide. Figure 2
shows that from 1973 to 1989, carbon dioxide
concentrations, expressed in parts per million by volume, in the earth's
atmosphere increased from 320 to 350. More recent measurements now
show 2005 values in excess of 370 parts per million. Figure 2 shows that the upward
trend is independent of location and occurs in both the Northern and Southern
Hemispheres. The main differences in the curves are the different amplitudes
of the annual cycle. Careful examination shows that the concentration reaches
a peak in April or May in the Northern Hemisphere and a minimum in July,
corresponding to the absorption by plants during the growing season and the net
release back to the atmosphere due to decaying vegetation outside the growing
season. Global vegetation maps discussed later in this unit will allow
you to explain the different amplitudes at different locations on the earth.
However, regardless of location, there is compelling evidence that the trend in
concentrations is upward.
If we examine proxy data (data that indirectly reveals past CO2 levels), we can get very good estimates of the carbon dioxide levels over longer periods of time. At the time of the industrial revolution in the late 1700s, the amount of carbon dioxide in the earth's atmosphere was about 270 parts per million. The record showed that concentrations grew slowly until the 20th century but have grown very rapidly since then, particularly in the last 50 years (Figure 3).
We now have the capability of looking at an even longer record by examining ice cores taken from a 2-km deep hole drilled in the Antarctic ice sheet. A team of scientists from France and the former Soviet Union analyzed tiny bubbles of air trapped in this core at various levels below the present ice surface. Deeper layers correspond to times in the more distant past. These bubbles can be analyzed for the relative abundance of carbon dioxide to estimate atmospheric carbon dioxide levels at times extending back 160,000 years. Concentrations of isotopes of oxygen trapped in these bubbles also are measured, and their ratio, being temperature dependent, gives an estimate of the surface temperature on the Antarctic continent at the time the ice was formed. Figure 4 shows that carbon dioxide concentrations have risen from about 180 parts per million up to about 300 parts per million over the last 160,000 years. And you can see from the same diagram that present concentrations exceed 350 parts per million, which is higher than it has been in the last 160,000 years. Figure 5 extends the CO2 and temperature record back 400,000 years. Note the correspondence between CO2 and temperature over this period.
