I'm not claiming anything against standard consensus on global warming. But if it does not wipe out every last single human being it is not an "existential risk".

I assure you that existentialist risk from climate change is both possible - even likely - without willful, forceful human intervention. I invite you to include this as the top-most threat of all human caused risks. Commentary I read on the climate models seem to give this about a 30% chance of full catastrophic extinction. Although some think this is possible by the year 2100 - forced warming continues for hundreds of years - and is certainly possible after that date.

My own chart is a very crude and simple view of data source connections localsteps.org/howbad.html - but I invite you to peruse some original research reports linked there. The referenced book "Six Degrees" is vetted by scientists at RealClimate realclimate.org/index.php/archives/2007/11/six-degrees/

The Intergovernmental Panel on Climate Change (IPCC) report is a UN sponsored, universally accepted document. It is a synthesis of work from thousands of scientists and political leaders... Designed to both reveal and suppress, it is dense and hard to comprehend - but totally public. The IPCC report lists a Permian type extinction as possible with the business as usual climate models. Specifically the scenario of A1FI saying that 6 C degrees of warming is possible. And this alone should suffice to place this on your chart.

There are some very wise computer scientists that you may resonate with... Steve Easterbrook at easterbrook.ca/steve/ and John Mashey - who does not blog directly:
warming101.blogspot.com/2009/08/john-masheys-greatest-hits-climate.html
amazon.com/review/R64ZZL9AHXWMK

One excellent IPCC report is
ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr_spm.pdf
You might search the within the document and on the Internet for the climate model of "SRES A1FI" which is one of the six climate models that is widely studied. [A1FI: Fossil-fuel Intensive, coal, oil, and gas continue to dominate the energy supply for the foreseeable future.] It is the scenario of "business-as-usual" and is the most severe - but completely includes the possibility of full catastrophic extinction - that can only be compared to the Permian extinction (see Paleoclimatology).
ipcc.ch/ipccreports/sres/emission/index.htm

Government publications
usgcrp.gov/usgcrp/default.php

Check the British Met Hadley Center for a report
timesonline.co.uk/tol/news/environment/article5371682.ece
with original government materials at metoffice.gov.uk/climatechange/guide/effects/high-end.html

MIT doubles the forecasts of the IPCC
reuters.com/article/idUSTRE54I6PF20090519?feedType=RSS&feedName=environmentNews
climateprogress.org/2009/05/20/mit-doubles-global-warming-projections-2/

Climate model discussion
realclimate.org/index.php/archives/2008/11/faq-on-climate-models/
RealClimate is the most respected science blog on global warming.
realclimate.org/index.php/archives/2005/08/on-record-high-temperatures/

climateprogress.org/2008/09/23/has-runaway-climate-change-begun/
ClimateProgress.org is perhaps the strongest political blog drawing on established science.
The founder, Joe Romm is flogging his book
climateprogress.org/2009/03/22/an-introduction-to-global-warming-impacts-hell-and-high-water/ but it is an excellent account of the dire possibilities.

Special note on Wikipedia articles: In global warming issues, Wikipedia is a battleground of acrimonious discussion that often leaves little information. I suggest checking revision history and original documents when possible.

Sorry to overwhelm you with links. I hope this is sufficient to include as a true risk for complete human extinction. Let me know if there is anything I can do.

Speaking on my own behalf, such examples may present a compelling case that climate change would pose a giant risk if there were little awareness of the issue. But in one perhaps ironic sense, the existence of widespread awareness (even if that does not always convert to action) reduces the risk from that threat.

On the other hand, there are other large risks natural and technological that seem to receive far less (and often, less serious) appraisal, and relatively negligible action.

Even given that the response to climate change is tragically less than ideal, the response is still overwhelming in proportion to the thought paid to some other global risks.

There are other aspects I may speak of, but on the basis of this, please understand the focus on less-celebrated but no less thought-worthy potential future factors.