Climate change. It is going to affect most things we care about.
Many people think that if it's real, it's just a matter that things will be a little warmer, and what's the big deal about that?
But warming is only one facet. It's a serious facet, because important crops such as rice and wheat are already at their thermal limits in places such as India, which means that significant warming--and significant further warming is unavoidable at this point--will lower crop yields. And it's a serious facet, because 'wet bulb' temperatures have about 5 Celcius degrees leeway before human heat stress becomes a really pervasive problem.
But it's not *just* that. It's drought and flooding, both of which (paradoxically) will increase under a warming climate, and both of which hurt agricultural production. (The Texas/Mexico drought of 2011 cost over $10 billion, and hurt production enough to affect food prices all over North America.) So climate change puts food security at risk--especially since we are, most experts agree, headed for a global population peak of around 9 billion people.
It's also extreme weather, which is expected to increase. It's sea level rise, currently about 3 mm per year and accelerating. (Before the time the new levees protecting New Orleans reach the end of their expected lifetimes, we may be sorry we didn't build them higher.) And there may be 100 million people or more in coastal areas low enough to be at risk.
There's more, but putting together what I've already said with human nature, it's a fair prediction that political and military security is at risk, too--hungry people will fight before they starve, and refugees in massive numbers always mean trouble.
So the big challenge right now--while we still have time to avoid the worst--is, to transform our energy economy onto a sustainable footing.