Geoscience Reference
In-Depth Information
Weight of evidence
Science is not a belief system. For example you cannot decide that you believe in antibiot-
ics (as they may save your life) or that heavy metal tubes with wings can fly (because you
want to go on holiday), and yet at the same time deny that smoking causes cancer, or that
HIV causes AIDS, or that GHGs cause climate change. Science is based on a rational
methodology that moves forward by using detailed observation and experimentation to
constantly test ideas and theories. It is the very foundation of our global society. If we are
to understand climate change, we must understand how science works and the principle of
the 'weight of evidence', which prompts the continual need to compile new data and under-
take new experiments to test our ideas and theories regarding climate. Over the last 30
years the theory of climate change must have been one of the most comprehensively tested
ideas in science. There are six main areas of evidence that should be considered. First, as
described above we have tracked the rise in GHGs in the atmosphere and understand their
role in past climate variations. Second, we know from laboratory and atmospheric measure-
ments that these gases do indeed absorb heat when they are present in the atmosphere.
Table 1
summarizes the latest understanding of the main GHGs. Third, we have tracked
significant changes in global temperatures and sea level rise over the last century. Fourth,
we have analysed the effects of natural changes on climate including sun spots and volcan-
ic eruptions, and though these are essential to understanding the pattern of temperature
changes over the last 150 years, they can not explain the overall warming trend (
Figure 5
).
Fifth, we have observed significant changes in the Earth climate system including the re-
treat of sea ice in the Arctic, retreating mountain glaciers on all continents, and shrinking
permafrost and increased depth of its active layer. Sixth, we continually track global weath-
er and have seen significant shifts in the number and intensity of extreme events. In this
chapter evidence for changes in global temperature, precipitation, sea level, and extreme
weather events are examined.
