SUGGESTED ALTERNATIVE
CAUSES
Scientific consensus on global warming now accepts
that most of the observed warming over the last 50 years has most
likely been attributable to human activities. In the journal Science
in December 2004, Dr Naomi Oreskes published a study of the abstracts
of 928 refereed scientific articles in the ISI citation database
identified with the keywords "global climate change".
This study concluded that 75% of the 928 articles either explicitly
or implicitly accepted the consensus view — the remainder of the
articles covered methods or paleoclimate and did not take any stance
on recent climate change. The study did not report how many of the
928 abstracts explicitly endorsed the hypothesis of human-induced
warming, but none of the 928 articles surveyed explicitly endorsed
an alternative hypothesis.
However, an objective approach should include
opposing views even if they conflict with the consensus of the scientific
community. Contrasting with the consensus view, alternative hypotheses
have been proposed to explain all or part of the observed increase
in global temperatures. Some of these hypotheses (listed here without
comment on their validity or lack thereof) include:
• The warming is within the range of natural variation
• The warming is a consequence of coming out of a prior cool period,
namely the Little Ice Age
• The warming is primarily a result of variances in solar irradiance
ie. modulation of cloud cover
• Most readings are done in heavily populated
areas which are expanding with growing population.
urban heat island
The observed warming actually reflects the Urban
Heat Island, as most readings are done in heavily populated areas
which are expanding with growing population. The vast majority of
scientists agree that this has been taken into account when taking
temperature readings across the globe.
Solar irradiance
The warming is primarily a result of variances
in solar irradiance, possibly via modulation of cloud cover. It
is similar in concept to the operating principles of the Wilson
cloud chamber, but on a global scale where earth's atmosphere acts
as the cloud chamber and the cosmic rays catalyze the production
of cloud condensation nuclei.
The solar variation theory
Modeling studies reported in the IPCC Third Assessment
Report (TAR) did not find that changes in solar forcing were needed
in order to explain the climate record for the last four or five
decades. These studies found that volcanic and solar forcings may
account for half of the temperature variations prior to 1950, but
the net effect of such natural forcings has been roughly neutral
since then. In particular, the change in climate forcing from greenhouse
gases since 1750 was estimated to be eight times larger than the
change in forcing due to increasing solar activity over the same
period.
Since the TAR, some studies (Lean et al., 2002,
Wang et al., 2005) have suggested that changes in irradiance since
pre-industrial times are less by a factor of 3 to 4 than in the
reconstructions used in the TAR (e.g. Hoyt and Schatten, 1993, Lean,
2000.). Other researchers (e.g. Stott et al. 2003) believe that
the effect of solar forcing is being underestimated and propose
that solar forcing accounts for 16% or 36% of recent greenhouse
warming. Others (e.g. Marsh and Svensmark 2000) have proposed that
feedback from clouds or other processes enhance the direct effect
of solar variation, which if true would also suggest that the effect
of solar variability was being underestimated. In general the level
of scientific understanding of the contribution of variations in
solar irradiance to historical climate changes is "very low".
The present level of solar activity is historically
high. Solanki et al. (2004) suggest that solar activity for the
last 60 to 70 years may be at its highest level in 8,000 years;
Muscheler et al. disagree, suggesting that other comparably high
levels of activity have occurred several times in the last few thousand
years. Solanki concluded based on their analysis that there is a
92% probability that solar activity will decrease over the next
50 years. In addition, researchers at Duke University (2005) have
found that 10–30% of the warming over the last two decades may be
due to increased solar output. In a review of existing literature,
Foukal et al. (2006) determined both that the variations in solar
output were too small to have contributed appreciably to global
warming since the mid-1970s and that there was no evidence of a
net increase in brightness during this period.
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