Drafts are reviewed by experts
inside and outside the world's governments,
and independent editors assure
that the authors provide comprehensive
responses to all reviews. Final
approval is by consensus among representatives
of the world's governments
(about 120 for the 2007 reports),
including word-by-word approval for
the Summary for Policymakers. The
IPCC reports represent the gold standard
of scientific credibility (and as
such, are rather conservative).
IPCC reports in 2007 make it clear
that climate has warmed in recent
decades and that human actions are a
primary driver of most of the warming.
In the words of the IPCC:
"Warming of the climate system is
unequivocal, as is now evident from
observations of increases in global
average air and ocean temperatures,
widespread melting of snow and ice,
and rising global average sea level."
"Most of the observed increase in
global average temperatures since the
mid-20th century is very likely due to
the observed increase in anthropogenic
During the 20th century, the global
mean temperature increased by 0.74°C
(1.3°F).Most of the globe warmed, and
only limited areas have experienced
cooling,mostly over oceans (Figure II).
The rate of warming has been very
rapid in the last few decades. Globally,
11 of the last 12 years are in the
warmest 12 years of the last 150 years,
since the beginning of the record based
on accurate thermometers.
Impacts of the climate changes that
have already occurred are also clear.
Mountain glaciers are shrinking
worldwide. Summer disappearance of
Arctic sea ice is reaching record levels.
An increasing fraction of tropical
storms is reaching a very damaging
level of intensity.
In North America, sea levels have
risen in most coastal regions,
to increased coastal flooding.
Over the last 50 years, the frost-free
season in the western U.S. has lengthened
by several days per decade,
resulting in increased risk of
Warmer temperatures in the western
U.S. have led to decreased spring
snowpack and distribution shifts of
many plants and animals toward the
cooler parts of their ranges.
HOW DO WE KNOW THAT PEOPLE
ARE REALLY CHANGING THE CLIMATE,
AND NOT NATURAL CYCLES?
In a study examining 928 peerreviewed
scientific papers from 1993-
2003, N. Oreskes found that not a single
paper disagreed with the consensus
position that human activity is the
cause of the recent and currently
occurring climate change.
Observations and climate models
both point to the role of human activities
in causing recent climate changes.
Some observational evidence is indirect.
For example, it is clear from measurements
that changes in the
output of energy from the sun are not large
enough to explain the observed warming.
Other evidence involves direct
observation of "fingerprints," specific
elements of the changes that are consistent
with human contributions to
climate change, but not with other
mechanisms. The best fingerprint
example is the observed high-atmosphere
cooling, an observation
expected with warming from greenhouse
gases but not from other mechanisms.
In the vineyard or winery, if you
want to know how a new irrigation
regime or a new strain of yeast will
affect your grapes or wine, it's possible
to conduct an experiment. You can try
the new regime on a few rows of vines
or a small lot of wine and compare the
results to the business-as-usual control,
then decide which results you
The current trends of human greenhouse
gas emissions were first recognized
as "the great geophysical experiment"
50 years ago,
but it is an
Figure V. Probability distribution of average temperatures under baseline climate (broken
line) and a warmer climate (solid line). Even a small shift towards warmer average
temperatures leads to greater incidence of hot weather and record hot weather.