There are several responses the theist could make. Firstly, he could try to reduce all natural evil to moral evil, by suggesting that natural disasters were caused by the free choices of supernatural beings. Cf. Maverick philosopher:
If someone can see his way clear to accepting the existence of a purely spiritual being, then the belief in angels, fallen or otherwise, will present no special problem. Given the existence of fallen angels, the Free Will Defense may be invoked to account for natural evils such as tsunamis: natural evils turn out to be a species of moral evils.
I suppose believing in fallen angels (or gremlins, or *insert supernatural villain of choice*) is no worse for our ontology than believing in the theistic God. But it's more than a bit lame to appeal to such entities when explaining natural phenomena. I mean, it'd be understandable if you were the shaman of an isolated tribe that'd just discovered the wonders of the wheel. But, really, hasn't our civilization learnt a thing or two about how the world works since then? *shakes head in disgust*
An even worse response would be to deny that anything bad had happened. Norm Weatherby writes:
The Tsunami is "Evil"? How so? It's a natural phenomena. It's neutral in the good vs evil game. Lots of people who were eventually going to die, died. That's bad? Maybe inconvenient or sad for some. Hardly "evil".
Yes, that's bad. How morally obtuse are you? We're supposed to believe that the incredible suffering of untold thousands (millions?) is "maybe inconvenient" (a gracious concession, to be sure), but not really anything bad? Perhaps one would prefer to restrict use of the word 'evil' to agents - I sure hope that's the sole reason for the scare quotes - but there can be no question that natural catastrophes are indeed harmful.
More generally, for those who deny that human suffering is unjust (perhaps by appealing to 'original sin'), this raises the question of why it matters when we harm each other. Aren't rape and murder wrong? Even in the case of natural disasters, why should we help the purported 'victims' if they weren't wronged? If God doesn't think they're worth helping, why should we? But surely we should help them. So this response can't work.
Another misled (though at least not misanthropic) response was suggested by 'Jeremy' in the comments at Jason's blog:
I don't hold myself to that standard. I have some degree of relative power (being a citizen of a relatively wealthy country), yet I don't think the choice NOT to contribute to the tsunami relief effort is evil. Presumably god is more moral than any human, but I'm still not convinced that requires that he be an intercessory deity.
1) We don't claim omnibenevolence - and just as well! It's permissible for us not to give to charity, but it sure isn't morally perfect of us.
2) A key disanalogy is that it costs us to help. But God (we're told) is omnipotent, so there are no opportunity costs to his actions. If you could relieve the suffering (or, better yet, prevent the disaster in the first place) just by clicking your fingers, surely we agree that it would be morally despicable to fail to do so.
More interesting is the 'Moorean' response seemingly favoured by the Archbishop of Canterbury, and perhaps Brandon of Siris. In short: God exists, therefore the problem of evil must have a solution. Taken alone this response is patently question-begging. I guess it wouldn't be so bad if supplemented by an independent proof of God's existence. But I've never come across a plausible one of those before (especially not one that proves benevolence), so I currently don't think much of this strategy.
The usual response is simply to plead ignorance. We can't see why God would allow such an awful event, but perhaps he had some reason that we are unaware of. There are several problems with this. Justifying harm in the name of "God's plan" or the "greater good" would seem to assume utilitarianism, which I'd expect most theists to oppose. Further, if God is omnipotent then he shouldn't need to employ bad means to good ends. He presumably could bring about those ends directly. Plans are for the powerless - what need would God have of them?
Moreover, Chris Kane at FBC has previously argued that God can't make use of incomprehensible justifications, for in doing so he would fail to be a perfect moral example. In the comments there, Clayton (see also here) makes the point that the ignorance response would seem to deprive us of moral responsibility, for we can no longer reliably tell good from bad, right from wrong. What seems atrocious to us might be part of God's plan. To respin an earlier question: if God doesn't think someone is worth helping, why should we?
Indeed, I think this response is just a more sophisticated version of the 'no harm was done' response - the key change being that we're instead supposed to believe 'the harm was worth it'. I guess it's possible, but I sure don't find it plausible.
Another interesting approach is the slippery slope argument against interference, which is similar to the claim that there is no 'best possible world' so God cannot be faulted for not creating it. Opiniatrety exposes the invalid logic employed here:
The argument is a reductio of the antecedent of (2)--that it's legitimate to criticize God for creating a decent enough world if He could have created a better world instead. But that reductio only establishes the following:
(6a) There is some decent enough world such that God would be exempt from criticism for creating it, even though He could have created a better world instead.
It does not establish that this world is such a world. There may be a principled way of picking out the worlds such that God can be [criticized] for not creating a better one (even though they are decent enough). And our world can be such a one.
Blogosophy offers an even more forceful refutation:
If No Best Possible World were sound, then since the argument doesn't depend on any actual features of the world--and particularly doesn't depend on the nature or amount of evil and suffering in the world--it would be sound even if the world were literally Hell on Earth... But if that's the case there's something horribly wrong with any definition of benevolent that's compatible with the No Best Possible World defense; we wouldn't ordinarily call a being benevolent if it created only Hell and condemned *everyone* to it. So either NBPW requires that you adopt an unacceptable definition of benevolent, or it is unsound.
The modal realism solution seems to work, except that modal realism itself is so implausible.
Lastly, there's an argument I heard once (but not recently), suggesting that God shouldn't ever break the laws of nature. I don't recall the exact argument, but the gist of it was that living in an unpredictable/chaotic world would be really bad, so we need exceptionless laws of nature to prevent this. But that doesn't sound very plausible either. Surely God could have prevented the tsunami without anyone realising the laws of nature had been broken, so nobody would be upset. Perhaps this is meant to be a 'slippery slope' argument, but then it falls victim to the above criticisms.
To sum up then, I don't think there is any good answer to the problem of natural evil. The best 'solution' is probably just to plead ignorance, but we saw that this is problematic in several ways. Even if the problem of evil isn't a decisive refutation, I think it's about as close as philosophical arguments ever get. If anyone wasn't already entirely convinced of the existence of the theistic God, it should be more than enough to lead them to skepticism. And even for the convinced theist, I think it should at least raise some significant doubts.