INTRODUCTION: SOCIOECONOMIC ASPECTS OF BIOMASS CONVERSION - Biomass as a Sustainable Energy Source for the Future - page 9

(c)

330

300

0.1

300

270

240

1800 1900

Year

2000

270

0

10,000 5,000

Time (before 2005)

0

FIGURE 1.3 ( Continued )

and household activities have a measurable impact. Scientists largely agree on the
point that in the last few centuries, the activities of humans have directly or indirectly
caused the concentrations of the major GHG to increase. This is exemplified by
Figure 1.3. The atmospheric CO 2 concentration varies to some extent from place
to place and from season to season. It has been shown that concentrations are some-
what higher in the northern hemisphere than in the southern hemisphere as most of the
anthropogenic sources of CO 2 are located north of the equator. The difference in land
surface covered with forests, being more concentrated north of the equator, causes
larger seasonal fluctuations due to comparatively shorter growth periods than in the
generally milder southern hemisphere locations that are under the influence of larger
oceanic surfaces.
Oscillations of atmospheric CO 2 concentrations between about 180 and 280 ppm v
have occurred in the past approximately 480,000 years in cycles of 100,000 years,
but it appears now we have abandoned this cycling behavior in a remarkably short
time frame.
Studies at the NASA Goddard Institute for Space Studies in New York (United
States) have shown that over the past few decades, the combined warming effect
of non-CO 2 GHG should have been comparable to that of CO 2 alone. However, while
each of the GHGmentioned earlier acts to warm the surface of the Earth, the long-term
climatic effects of the other GHG differ from those of CO 2 . Methane, e.g., has an
atmospheric lifetime of only about 12 years. By comparison, newly added CO 2 will
remain for a time span of tens to thousands of years. As a result, about 65% of the
carbon dioxide that human activities have generated since the start of the Industrial
Revolution is in the air we breathe today. A historical record of the amount of
CO 2 in the atmosphere can be found in bubbles of air in arctic ice layers, dating back
as far as 600,000 years. The depth of such a layer is a measure of its time of formation.