Environmental Engineering Reference
In-Depth Information
The ability of these techniques to replicate what our climate was like before and dur-
ing the industrial period means we have very good reasons to be confident in attributing the
abnormal and rapid global warming of the last 50 years to anthropogenic GHG emissions.
That message increases in strength the more we look at it. The same basic results tumble
out regardless of which model is used. Furthermore, the models are able to reproduce the
differences in the observed amplitude of temperature change seen between the continents
of North America, South America, Europe, Africa, Asia and Australia. They even replicate
the instrumental record to show that the warming trend is most intense in the mid- to high
latitudes of the Northern Hemisphere.
Here are the results from the use of some of the other attribution tools. We have
already noted that the most important GHG is carbon dioxide. Completely independent
methods show there is no doubt where this extra atmospheric carbon dioxide is coming
from. Carbon has two stable natural isotopes - 12 C and 13 C - that occur naturally at a ratio
of roughly 99:1. Carbon-13 is less common in vegetation and in fossil fuels because plants
prefer to take up the lighter isotope ( 12 C) and it is more abundant in the oceans and in vol-
canic or geothermal sources. Therefore, we would predict that if the “extra” carbon dioxide
in the atmosphere is derived from the burning of fossil fuels and vegetation, the relative
amount of atmospheric 12 C to 13 C should be increasing. This is indeed what is found and
this trend in carbon isotope ratios is also found in the coral core record over the last two
centuries. Another independent verification is that of the ratio of oxygen to nitrogen in the
atmosphere. This ratio is going down as carbon dioxide has increased - just as would be
expected, as oxygen is used up when fossil fuels are burned.
Another family oftechniques usedtoinvestigate attribution (which wewill notinvest-
igate here) is to compare the geophysical fingerprint of the expected response to a certain
GHG forcing with the fingerprint that would be expected from an alternative explanation
(such as changes in solar luminosity).
What natural processes could “force” the climate into a warmer or colder mode? Stud-
ies of climate in the geological past (paleoclimate) have identified a number of phenomena
that are associated with changes in the global climate.
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