Constraints on future changes in climate and the hydrologic cycle Myles R. Allen* & William J. Ingram† http://www.climateprediction.net/science/pubs/nature_insight_120902.pdf
"Changes in precipitation extremes Long-term mean precipitation is a useful summary indicator of the intensity of the hydrologic cycle, but details of the distribution of precipitation over time, including the peak intensity of precipitation events and duration of prolonged droughts, are likely to be the most important issues in determining impacts of precipitation changes9. Although global-mean precipitation is primarily constrained by the energy budget, the heaviest rainfall events are likely to occur when effectively all the moisture in a volume of air is precipitated out, suggesting that the intensity of these events will increase with the availability of moisture7 (that is, significantly faster than the global mean). Thus we might expect the uppermost quantiles of the rainfall distribution to increase by about 6.5% per kelvin (ref. 44) if the ambient flows change (most likely at higher latitudes). In the tropics, where the flows leading to precipitation are themselves driven largely by the latent heat released by precipitation, larger increases still might occur7. In particular, the maximum thermodynamically possible rainfall and winds in hurricanes are predicted to increase rapidly with warming56. The red and purple curves in Fig. 4 show the magnitude of daily precipitation as a function of percentile of the precipitation distribution in one AOGCM under climate change and control conditions, respectively. The green curve (and right-hand axis) shows the ratio between them. At the highest end of the distribution (all tropical cases), it appears to be converging to about a 25% increase, which is indeed slightly more than we would expect from the Clausius–Clapeyron relationship. Because such increases are more than double the increase in global-mean precipitation (that is, the change summed over all percentiles, which is constrained by the energy budget, not by the Clausius–Clapeyron relation), there must be decreases lower down the distribution. Indeed, the increase at the heaviest rain events is large enough that the energy constraint on the total implies that only on one day in ten does precipitation increase (the control and perturbed distributions cross around the 90th percentile in Fig. 4). It would be interesting to know how other models compare — if this were be another emergent constraint generic over all models in regions of interest, then we could use forecast temperature changes to constrain extreme as well as mean precipitation. Given the acknowledged difficulties in relying on model simulations of extreme events and in observing changes in extremes directly57, this could be a powerful result. In particular, this convergence to a particular fractional increase, possibly related to the Clausius–Clapeyron limit, could even improve as we move to the highest percentiles, which are generally the least tractable under more direct approaches." There is probably better papers to quote than that. When I did quote that before, this exchange which followed seemed quite odd: >[You should be aware that the claims of more weather extremes are often >over-hyped; our weather will change somewhat, with the climate, and hence >become more unexpected; but there is no great evidence for it becoming more >variable -W] Regarding temperature extremes and wind extremes, I have seen nothing to disagree with what you are saying. However with rainfall, I thought there was agreement that warmer atmosphere holds more water and due to some constraint (energy budget?) it was fairly clear that the heavy downpours are going to get heavier. Have I got that wrong, is this highly contested? [I agree; and so does WJI, given the paper you've quoted above. But there is another way of looking at it: most stuff is approx normal, but rainfall isn't. If you look at extremes of its log distribution, which is approx normal, you may find its changes revert to what everything else does, ie the distribution just shifts -W] W Being William Connolley and the exchange at http://scienceblogs.com/stoat/2009/01/arctic_sea_ice_retreat_when_wi.php My thoughts shouldn't be counted as meaning anything. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/globalchange -~----------~----~----~----~------~----~------~--~---
