Is there a petition of climatologists or something like that which shows a consensus on climate change?
The best you can probably do is to cite the reports of the Intergovernmental Panel on Climate Change, which represent the synthesis of the work of thousands of climate scientists.
You may want to look at the various reports on the site, particularly the Summary for Policymakers on the 2007 report.
Anyone who doesn’t think the IPCC report represents a consensus isn’t going to believe any other evidence you give to them.
Also reports from the National Academy of Science:
As well, the Royal Society, and various other national science academies around the world:
A list of all of the agencies and science organizations that endorse the global warming theory can be found in this thread. Googling those names in conjunction with the words “global warming” or “climate change” will probably come up with a press release, or worst case you have to search about their website.
“EPA climate change”:
“NASA climate change”:
“royal society climate change”:
Here’s an article from Science, the most prestigious US magazine on scientific research: