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 [human-caused] greenhouse gas concentrations."

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, contributing 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

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 prefer.

The current trends of human greenhouse gas emissions were first recognized as "the great geophysical experiment" 50 years ago, but it is an uncontrolled experiment.