Given the extreme precipitation and flooding that’s been experienced in some parts of the world recently, I thought I would comment briefly on the issue of attribution. Clearly the El Niño has played an important role in these extreme weather events, but an increase in the intensity and frequency of extreme precipitation events is what we expect in a warmer world. A typical response to this, however, is that we have yet to definitively attribute anthropogenic influences to these events.This may be true, but arguing about attribution can, in some sense, miss the basic point. Consider the figure on the right. It shows the basic energy flows. At the top of the atmosphere we have energy coming in from the Sun, some of which is reflected and some of which is absorbed, and we have energy radiated outwards from the planet. In equilibrium, these energy fluxes would be in balance (I realise this figure is for a situation where the planet is out of balance, but lets ignore that for now).
At the planet’s surface, there is energy absorbed from the Sun, and energy coming back to the surface because of the greenhouse gases in the atmosphere. From the surface, there is energy radiated away (which depends on the temperature), energy carried away by thermals, and energy transported from the surface to the atmosphere via evaporation. In equilibrium all these energy fluxes would balance and the evaporation rate would set the rate at which water precipitated out of the atmosphere.
What happens, however, if we add greenhouse gases? If we add greenhouse gases, the amount of energy reaching the surface goes up. To balance this the surface temperature will rise so as to increase the flux of energy radiated from the surface, but we’d also expect the energy transported via thermals and via evporation to also increase. Since the evaporation rate will set the precipitation rate, we’d expect precipitation to increase. There’s even some evidence for this. Between 1973 and 2008 there is an observed increase in downwelling longwavelength flux that exceeds the increase in outgoing flux via radiation; something else must be transporting energy from the surface.
So, even if we haven’t been able to definitively attribute a human influence to extreme precipitation events, basics physics tells us that we’d expect precipitation to increase in a warmer world and, in particular, that we’d expect an increase in the intensity and frequency of the extreme events. Is it possible that this might not happen? Well, as Victor points out, if the evaporation rate does not increase, then that implies that a larger fraction of the surface energy imbalance is closed by an increase in outgoing radiation (i.e., the change in surface temperature will be larger). Also, this would imply that the relative humidity has increased more than we expect (because the relative humidity also influences the evaporation rate).
Therefore, if one argues that we aren’t going to see an increase in precipitation in a warmer world, you’re essentially suggesting that the world will be warmer and more humid than a world in which precipitation does increase. You can’t really have it both ways. So, attribution might be an important part of convincing us that our understanding is correct, but it isn’t really an important part of trying to determine what will likely happen. That’s just physics.