Precipitation is an important way that Earth “keeps its cool”. Precipitation processes account for about a quarter (estimated 80 W/m3) of the 340W/m3 our planet must remove to avoid warming.
For perspective, contrast that 80W/m3 with the 4W/m3 impact of doubling CO2 (ignoring CO2’s possible knock-on effects). Precipitation, besides this direct heat transfer role, places water vapor, a greenhouse gas, into the mid- and upper-atmosphere, affecting IR radiation from lower levels. And, condensed water vapor (cloud) plays a role in Earth’s reflection of a portion of incoming sunlight as well as a portion of upwelling IR. A change in global precipitation processes could, in concept, affect Earth’s heat removal effectiveness in several ways. That change, given the various roles of precipitation, could be significant.
How has Earth’s precipitation trended? Below is a plot of global precipitation rate since 1979 (per ERA reanalysis data). As it indicates, Earth saw a mostly downward trend until around 2005, at which time an uptrend occurred. In the last several years that uptrend seems to be reversing. In absolute amount, the ERA data (1979-2010) averaged 2.92 mm/day (1979-2010). It’s important to note that precipitation estimates, whether by ERA, other reanalyses or TRMM (a sophisticated measurement program), are all estimates – there is no single unquestionable source for historical global rain and snow amounts.
L et’s take a look at the trends in the tropics and extratropics. Below is a plot of trends in the tropics (20N-20S) as well as the poleward areas north and south. The three areas are of about the same size. What stands out is the large role of tropical rain. My analogy is that the tropics are like the troposphere’s “furnace”, providing considerable heat for the more poleward regions, with significant heat exhausting from the tropics to the midlatitudes via the middle and upper troposphere.
What about trends? Below is a plot of the precipitation anomalies in the three regions (annual cycles removed), based on ERA reanalysis data. The most significant feature (based on visual appearance) is the ramp-up that began about 2006. and apparently peaked four or five years later. That increase, roughly 3 or 4% in global precipitation, equates to an extra 2 or 3 W/m2 of latent heat moving from the surface into the troposphere. (I realize that my last statement involves many assumptions.) That increased energy movement is of a magnitude similar to the likely to-date effect of the CO2 added to the atmosphere to-date. No causation is implied – rather, mine is just an observation.
Finally, let’s take a look at data from TRMM, a high-quality Japan/US program which studies tropical rainfall. Below is TRMM’s map of global rainfall changes over the last 35 years. It shows an increase in the tropics (West Pacific and Indian Oceans) and some decrease in the central tropical Pacific (which I think of as the El Nino region).
Here’s a TRMM plot of tropical ocean precipitation anomalies (red) since 1996. It seems to show an increase 2007-2011 followed by a leveling. At a crude level that is similar to the ERA pattern but certainly differs in detail.
Below, simply for reference, is a global map of the precipitation observed by TRMM, with the three latitude bands marked:
In summary, there is evidence that tropical rainfall has increased over the last decade, especially in the very warm Indian and West Pacific Oceans, the mid- and upper-troposphere’s “furnace”. Characterization of the mid-2000 shift to increasing precipitation is worth pursuing.