What if there is no rebirth? Recently, I found a rather interesting quote of HH Dalai Lama XIV, which he said in Bodhgaya interviews, 1981:


  “Buddhists believe in rebirth. But suppose through various investigative means, science comes one day to a definite conclusion that there is no rebirth. If this is definitely proven, then we must accept it and we will accept it.” — see, for example, here


First of all, it's worth mentioning that this phrase seems to be one of the most progressive thought I have ever seen.

However, I would like to know, how exactly the Buddhist community will behave in case if, indeed, scientific method proves some day that one of key concepts of Buddhism (not necessarily, rebirth) is not correct. Obviously, it's impossible to rewrite sacral books, rethink important consequences of a mentioned concept, restructure rituals, etc.

Also, many people are happy with current paradigms, and such discovery may have disastrous effect on many of them.
There is a freely available talk by Vessantara, an order member in the sangha I practice with (Triratna) that addresses this kind of issue. If one entirely rejected rebirth (or if it were proven to be untrue) then what would this do to the Dharma? My previous opinion was rebirth was a cultural accretion not a core teaching so if rebirth wasn't true the Dharma would remain essentially the same. There might be some reframing and reemphasising but the essentials would be the same. However Vessantara disagrees. The following parts of the Dharma would be shaken, possibly irreparably damaged (list not exhaustive)


The Bodhisattva Ideal. It is difficult to see how one could hope to aim for the enlightenment of all beings in one lifetime. It's not going to happen
The Buddha enlightenment experience. A central part of this was his visions of previous rebirth. Are we saying that this is not true. What other parts of his enlightenment experience are also not true
Death proximate Karma. There is no rebirth so the powerful karmic effects at the point of death are unreal.
The niddanas. There are one lifetime interpretations of these admittedly but other interpretations (3 lifetimes) would be thrown out
Enlightenment as the deathless. This is common imagery in texts such as the Dhammapada. Without rebirth what could deathless mean? It's unclear (to me)
Nirvana as 'blowing out'. Enlightenment as the concept of stepping off the wheel of samsara. Without rebirth there is no endless round so what are we stepping off from.


I could go on (i won't). I was just impressed with the talk and I think it's an interesting exercise to see where a rejection or disprove of rebirth might end up. I'm certainly not banging the drum for rebirth, in fact I remain at best agnostic. I know that rebirth-less interpretation of each of the six points are available but the question is - are we twisting the Dharma completely out of shape? This talk argues that we would be.

As a footnote an alternative view is available from another order member, Nagapriya in his book Exploring Karma and Rebirth. This book takes the opposite to Vessantara and argues that all rebirth-less interpretations are valid and do not lose their meaning. The talk and the book taken together give excellent (and opposing) views - companion pieces almost.