March 22, 2004

Ups and Downs

Media hypes a jump in carbon dioxide, ignoring recent years’ CO2 growth rate fluctuations. In fact, there has been no significant trend for 27 years.

The Associated Press newswire lit up over the weekend with reports that atmospheric carbon dioxide (CO2) concentration grew last year at a record pace. But AP neglected to mention the year-to-year fluctuations that characterize the CO2 growth rate and that in fact no significant trend has existed for 27 years.

Measurements made atop Hawaii’s Mauna Loa volcano showed that this winter’s CO2 level was greater than last year’s readings by about 3 parts per million (ppm). Missing from AP’s report was that growth the year before was only about 2 ppm, and the year before that was 1.5 ppm, and the year before that was 1.2 ppm, and two years prior to that, the growth rate was 2.9 ppm. In other words, there is a fair degree of fluctuation in the year-to-year to year values of the growth rate of atmospheric CO2 concentration. So a year with a large growth rate is no more or less newsworthy than a year (such as 2000) that had a low growth rate.

The time series in Figure 1 depicts the annual growth rate in atmospheric CO2 since the start of Mauna Loa measurements in the late 1950s. Over the past 27 years, there is no significant trend in the growth rate, which during that time has averaged 1.62 ppm per year.

Mauna Loa CO2 concentrations

Figure 1. Annual growth rate of atmospheric CO2 from 1950 to 2003, from measurements taken at Hawaii’s Mauna Loa Observatory.

The trend in the overall record results from a change that occurred in the mid-1970s. NASA’s James Hansen comments on this change in his 2001 paper published in the Proceedings of the National Academy of Sciences. In that paper, Hansen and a co-author extended the CO2 growth rate back to the mid-1800s (Figure 2). They state:

The annual growth rate of CO2 increased rapidly between World War II and the oil crisis of the mid-1970s, an interval during with fossil fuel CO2 emissions increased exponentially at more than 4% per year. Since then the annual CO2 growth has been relatively flat at about 1.5ppm per year. As fossil fuel CO2 emissions have increased at about 1% per year during the past 25 years, the flat growth rate implies some increase in the net terrestrial and/or oceanic uptakes of CO2.

CO2 Forcing

Figure 2. Annual growth rate of atmospheric CO2 extended back to 1850 (from Hansen and Sato, 2001).

That the earth’s terrestrial CO2 uptake has been growing is evidenced in the remarkable enhancement of global vegetation during the past 20 years, as reported by in a recent paper by Ramakrishna Nemani and colleagues. Nemani et al. report that this growth enhancement results from a combination of two major influences—the increased fertilization effect from rising concentrations of atmospheric carbon dioxide and the patterns of change in the earth’s climate during the study period.

An enhanced global uptake of CO2 means that atmospheric CO2 concentrations are not increasing as fast as most forecasts of their rise suggest. The Intergovernmental Panel on Climate Change (IPCC), in their 2001 Third Assessment Report, gave a range of possible CO2 concentrations by the year 2100 from their six marker scenarios as being between 549 ppm and 970 ppm. An extension of the average 1977–2003 growth rate of 1.62 ppm per year produces a value of about 531 ppm—below the lowest IPCC possibility.

That the CO2 buildup is progressing more slowly than the IPCC projects has led researchers such as Hansen to downplay the feasibility of many of the IPCC predictions: “The IPCC includes CO2 growth rates that we contend are unrealistically large,” wrote Hansen and Sato in 2001.

When observations, rather than IPCC scenarios, are used as a basis for climate forecasting, the results are decidedly less remarkable. In employing observations, Hansen finds that “future global warming can be predicted much more accurately then is generally realized…we predict additional warming in the next 50 years of 3/4ºC ± 1/4ºC, a warming rate of 0.15ºC ± 0.05ºC per decade.” Independently, a similar value was reported by Patrick Michaels and colleagues a year later.

Recall that this warming rate is just barely above the lowest rate of warming for the possible temperature rise during the 21st century projected by IPCC—a range given as 1.4ºC to 5.8ºC by the year 2100.

Less CO2 buildup, less warming, less impact. That is what the Associated Press left out of the story. The remarkable thing is not that atmospheric CO2 concentrations are continuing to grow, but that they are continuing to grow so slowly—and one year of data does nothing to change the indications established by more than a quarter-century of behavior.

References:

Hansen, J.E., and M. Sato, 2001. Trends of measured climate forcing agents. Proceedings of the National Academy of Sciences, 98, 14778-14783.

Intergovernmental Panel for Climate Change (IPCC), 2001. Climate Change 2001: The Scientific Basis, Cambridge Univ. Press, New York.

Michaels, P.J., et al., 2002. Revised 21st-century temperature projections. Climate Research, 23, 1-9.

Nemani, R.R., et al., 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science, 300, 1560-1563.




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