The UK and California have agreed to work together to take "urgent action" to cut greenhouse gases and promote low carbon technologies.
Tony Blair and state governor Arnold Schwarzenegger signed the deal following a meeting of the two leaders and key business figures in Long Beach.
It will see the UK pass on information about its experiences in the EU emissions trading scheme to California, which is the seventh largest economy in the world and the 12th largest emitter of carbon dioxide.
The US has no such scheme, where firms are given carbon credits to use or sell on as they wish, but Mr Blair's spokesman said today's agreement could in the future pave the way for "formal linkages" between California and the EU on this issue.
In addition, the agreement promises to share research results on the economic effects of climate change, collaborate on new technologies, including clean energy and energy efficiency, and improve links between the UK and Californian scientific communities.
"California and the UK recognise the linkages between climate change, energy security, human health and robust economic growth," the resulting mission statement says.
"Working together, California and the UK commit to build upon current efforts, share experiences, find new solutions and work to educate the public on the need for aggressive action to address climate change and promote energy diversity."
The agreement is a snub to US president George Bush, whose administration has failed to take any strong action on tackling climate change, and refused to sign the Kyoto protocol.
When asked why the California deal was not being signed with Mr Bush, Mr Blair's spokesman replied: "Our differences with the US administration are well known on this."
He added: "The important thing is that you work with people who are like-minded and interested in pursuing the same sort of things where you can, and we will, whether the subject is stem cell or whether the subject is climate change."
Mr Blair also used his trip to California to woo researchers in stem cell science to come to the UK, after Mr Bush used his first presidential veto to block the further expansion of federal investment in this area of research.
Governor Schwarzenegger was also clear what he thought about Washington on the issue of climate change, telling reporters: "California will not wait for our federal government to take strong action on global warming.
"International partnerships are needed in the fight against global warming and California has a responsibility and a profound role to play to protect not only our environment, but to be a world leader on this issue as well."