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Effects of Doubling Carbon Dioxide
from 300 ppm to 600 ppm
by Dr. Douglas V. Hoyt

(El artículo original está en: http://users.erols.com/dhoyt1/)


The effects of a doubling of carbon dioxide can be calculated theoretically.

1. Richard Lindzen (1995) obtains a theoretical value of 0.30 C. More recently he appears to have revised this number to 0.55 C (i.e., 1 F).

2. Calculations by Stephen Schneider in 1971 gave a value of about 0.67 to 0.8 C.

3. Newell and Dopplick (1979) gave a number of 0.25 C.

4. These results are generally equivalent to moving towards the equator by about 25 to 50 miles.

Empirical Determinations of the Greenhouse Effect
There are about a dozen ways of calculating the effect of doubling carbon dioxide by empirical studies, which is the same as using the Earth-atmosphere system as your climate model and performing diagnostic studies to back out the expected warming. All the empirical results indicate a few tenths of degree warming can be expected and despite the large differences in the ways these calculations are carried out, the results are very similar. Below we give summary results of the derived values since complete details on any one method would cover 10 to 20 pages of text.

1. 0.64 C (empirical, based upon sensitivity of climate to solar forcing of 0.16 C/W/m2 derived by Crowley and Kim [1996] and 4 W/m2 forcing with a greenhouse gas warming; this empirical determination assumes the climate is equally sensitive to solar and greenhouse gas forcing, which is customarily assumed, but is probably not strictly true - there are good reasons to believe climate is more sensitive to solar forcing than to greenhouse gas forcing.

2. 0.44 C +/- 0.12 C (empirical, based upon sensitivity of climate to solar forcing of 0.11 +/- 0.03 derived by White et al. [1997] and 4 W/m2 forcing with a greenhouse gas warming)

3. Richard Lindzen derived a value of 0.3 to 0.5 C empirically based upon responses of the Earth's temperature to volcanic forcing (see Lindzen, PNAS, 1994; JGR, 1998).

4. 0.30 to 0.60 C (empirical, based upon observed change in global annual temperature cycle in the 1900's by Mann et al.)

5. 0.36 C (empirical, based upon comparisons of observed and calculated changes in sea level).

6. 0.45 C (empirical, calculated from ice core carbon dioxide and temperature variations, after de-trending both time series to remove orbital effects).

7. 0.80 C, based upon surface temperature observations since 1940 (see Greenhouse Warming Scorecard).

The following two results are included for completeness without endorsement or objection:

8. 0.25 C (empirical, based upon three techniques as given by Sherwood Idso).

9. About 0.21 C (empirical, based upon six techniques by John Daly.)

The mean increase for a doubling of carbon dioxide from these nine empirical techniques is 0.46 C with an one standard deviation uncertainty of about 0.19 C.

This implies a warming of about 0.04 degrees should have been seen from 1979 to 1996. MSU observed about a 0.05 C cooling, but the climate noise means the uncertainty in this trend is 0.08 C. Therefore, we expect it will take another 20 to 30 years of observations by MSU before a greenhouse effect signal is observed.

The above empirical results would also suggest that a greenhouse gas warming of 0.27 C should have already have occurred over the last century and a further warming of about 0.18 C can be expected over the next 70 years or so as greenhouse gases double.

Another way to look at this problem: Assume that the 0.5 C warming from 1880 to the present is due entirely to a 50% increase in greenhouse gases. Then a doubling of greenhouse gases will give a 0.85 C total warming. This doubling from the pre-industrial levels will occur around 2070. That means there will be an additional 0.35 C warming over the next 70 years or 0.050 C/decade. The value 0.050 C/decade is consistent with surface observations provided half of the increase in temperatures from 1979 to 1996 is caused by changes in spatial sampling, as discussed on the home page.

Bottom line: A doubling of greenhouse gases will probably cause a global warming between 0.5 and 0.9 C. Since 60% of the warming should have already have occurred, the remaining 40% of the warming will be between 0.2 and 0.4 C, which will occur over the next 70 years. This warming is equivalent to about 0.03 to 0.06 C/decade. The rate of change is less than previous climate changes; its total magnitude is small; and ecological systems can adapt as they have in the past.

Douglas V. Hoyt, dhoyt1@erols.com



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