Furthermore, there is solid scientific evidence that we should act now on climate change, and this is reflected in the statements by these definitive scientific authorities.

Joint statement from 11 national academies of science
Issued 7 June 2005 by the national science academies of the United States, United Kingdom, France, Russia, Germany, Japan, Italy, Canada, Brazil, China and India, the statement begins with:

Climate change is real
There will always be uncertainty in understanding a system as complex as the world’s climate. However, there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades can be attributed to human activities (IPCC 2001). This warming has already led to changes in the Earth's climate.

The statement goes on to conclude:

We urge all nations, in line with the principles of the United Nations Framework on the Convention on Climate Change (UNFCCC),
to take prompt action to reduce the causes of
climate change, adapt to its impacts and ensure that the
issue is included in all relevant national and international
strategies.

Full text of statement on NAS website (pdf)

Royal Society - climate change


Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change (IPCC) was set up by the United Nations in 1988 to assess the scientific, technical and socio-economic information relevant for the understanding of the risk of human induced climate change.  About 1,000 experts from around the world are involved in drafting, revising and finalizing IPCC reports.  About 2,500 experts take part in the report review process. Thus, the IPCC represents a global consensus of the world's climate change experts. From the IPCC's most recent scientific assessment:

Most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations".

• "There is new and stronger evidence that most of the observed warming over the last 50 years is attributable to human activities".
• "About three-quarters of the anthropogenic [human caused] emissions of CO2 to the atmosphere during the past 20 years are due to fossil fuel burning”.

The assessment goes on to warn that there is a risk of feedback loops, which could cause runaway climate change, and that the global warming to date is already having an effect on the biosphere. 

More about the IPCC

Greenpeace briefing paper on the IPCC Third Assessment Report (pdf)
Intergovernmental Panel on Climate Change (IPCC)