Vol. 47: 123–138, 2011 CLIMATE RESEARCH Published March 31 doi: 10.3354/cr00953 Clim Res Contribution to CR Special 25 ‘Climate services for sustainable development’ OPENPEN ACCESSCCESS Changes in precipitation with climate change Kevin E. Trenberth* National Center for Atmospheric Research, Box 3000, Boulder, Colorado 80307, USA ABSTRACT: There is a direct influence of global warming on precipitation. Increased heating leads to greater evaporation and thus surface drying, thereby increasing the intensity and duration of drought. However, the water holding capacity of air increases by about 7% per 1°C warming, which leads to increased water vapor in the atmosphere. Hence, storms, whether individual thunderstorms, extratropical rain or snow storms, or tropical cyclones, supplied with increased moisture, produce more intense precipitation events. Such events are observed to be widely occurring, even where total precipitation is decreasing: ‘it never rains but it pours!’ This increases the risk of flooding. The atmo- spheric and surface energy budget plays a critical role in the hydrological cycle, and also in the slower rate of change that occurs in total precipitation than total column water vapor. With modest changes in winds, patterns of precipitation do not change much, but result in dry areas becoming drier (generally throughout the subtropics) and wet areas becoming wetter, especially in the mid- to high latitudes: the ‘rich get richer and the poor get poorer’. This pattern is simulated by climate mod- els and is projected to continue into the future. Because, with warming, more precipitation occurs as rain instead of snow and snow melts earlier, there is increased runoff and risk of flooding in early spring, but increased risk of drought in summer, especially over continental areas. However, with more precipitation per unit of upward motion in the atmosphere, i.e. ‘more bang for the buck’, atmo- spheric circulation weakens, causing monsoons to falter. In the tropics and subtropics, precipitation patterns are dominated by shifts as sea surface temperatures change, with El Niño a good example. The volcanic eruption of Mount Pinatubo in 1991 led to an unprecedented drop in land precipitation and runoff, and to widespread drought, as precipitation shifted from land to oceans and evaporation faltered, providing lessons for possible geoengineering. Most models simulate precipitation that occurs prematurely and too often, and with insufficient intensity, resulting in recycling that is too large and a lifetime of moisture in the atmosphere that is too short, which affects runoff and soil moisture. KEY WORDS: Climate change · Precipitation · Storms · Drought · Extremes · Floods · Geoengineering · Climate models Resale or republication not permitted without written consent of the publisher 1. INTRODUCTION short period of time may cause local flooding and runoff, leaving soils much drier at the end of the day (Fig. 1). Heated by the sun’s radiation, the ocean and land Snow may remain on the ground for some months before surface evaporate water, which then moves around it melts and runs off. Even with identical amounts, the with winds in the atmosphere, condenses to form climate can be very different if the frequency and inten- clouds, and falls back to the Earth’s surface as rain or sity of precipitation differ, as illustrated in Fig. 1, and in snow, with the flow to oceans via rivers completing the general the climate is changing from being more like global hydrological (water) cycle. that at Station (Stn) B in Fig. 1 to that at Stn A. These ex- Precipitation varies from year to year and over amples highlight the fact to the characteristics of decades, and changes in amount, intensity, frequency, precipitation are just as vital as the amount, in terms and type (e.g. snow vs. rain) affect the environment and of the effects on the soil moisture and stream flow. society. Steady moderate rains soak into the soil and Hydrological extreme events are typically defined as benefit plants, while the same amounts of rainfall in a floods and droughts. Floods are associated with ex- *Email: [email protected] © Inter-Research 2011 · www.int-res.com 124 Clim Res 47: 123–138, 2011 2. OBSERVED CHANGES IN PRECIPITATION 40 Drought Wild fires Flooding A: Wilting plants Monthly amount: 20 75 mm 2.1. Mean fields Intensity: 37.5 mm d–1 0 Frequency: 6.7% There is a very strong relationship between total col- 1 6 11 16 21 26 umn water vapor (TCWV, also known as precipitable 40 B: Soil moisture continually replenished; water) and sea-surface temperatures (SSTs) over the virtually no runoff Monthly amount: oceans (Trenberth et al. 2005) in both the overall Precipitation (mm) Precipitation 20 75 mm patterns and their variations over time (e.g. Fig. 2 for –1 Intensity: 3.75 mm d January and July). The Clausius-Clapeyron (C-C) 0 Frequency: 67% equation describes the water-holding capacity of the 1 6 11 16 21 26 atmosphere as a function of temperature, and typical Fig. 1. Hypothetical daily amounts of precipitation for a month at Stations A and B, including the monthly amounts, average values are about 7% change for 1°C change in temper- intensity per day conditional on rain occurring, and frequency. ature. For SST changes, the TCWV varies with slightly Consequences for runoff and plants are noted in each panel larger values owing to the increase in atmospheric temperature perturbations with height, especially in the tropics. Accordingly, the largest average TCWV tremes in rainfall (from tropical storms, thunderstorms, values occur over the tropical Pacific Warm Pool, orographic rainfall, widespread extratropical cyclones, where highest large-scale values of SSTs typically etc.), while droughts are associated with a lack of pre- reside. The annual mean and January and July values cipitation and often extremely high temperatures that are given in Table 1 for the global, ocean, and land contribute to drying. Floods are often fairly local and domains. Global surface air temperatures are higher in develop on short time scales, while droughts are exten- Northern Hemisphere summers, and this is reflected in sive and develop over months or years. Both can be much higher TCWV values over land. mitigated; floods by good drainage systems and drought The observed TCWV and precipitation mean annual by irrigation, for instance. Nonetheless, daily news- cycles also show very strong relationships in the trop- paper headlines of floods and droughts reflect the ics and subtropics, but not in the extratropics (Fig. 2). critical importance of the water cycle, in particular pre- Atmospheric convergence at low levels tends to occur cipitation, in human affairs. World flood damage esti- in close association with the highest SSTs, although the mates are in the billions of U.S. dollars annually, with factors extend beyond the SST values alone to include 1000s of lives lost; while drought costs are of similar the gradients in SSTs, which are reflected in surface magnitude and often lead to devastating wildfires and pressure gradients and thus in the winds. Accordingly, heat waves. The loss of life and property from extreme in the tropics, the precipitation patterns tend to mimic hydrological events has therefore caused society to the patterns of TCWV and thus SSTs, but with much focus on the causes and predictability of these events. more structure and sharper edges, as overturning cir- Tropical cyclones typically have the highest property culations, such as those associated with the Hadley damage loss of any extreme event, and are therefore of Cell and monsoons, develop and create strong areas of great interest to state and local disaster preparedness subsidence that dry out the troposphere above the organizations, as well as to the insurance industry boundary layer. (Murnane 2004). The mean TCWV falls off at higher latitudes in both Evidence is building that human-induced climate hemispheres, along with the SSTs, but precipitation change (global warming), is changing precipitation has a secondary maximum over the oceans associated and the hydrological cycle, and especially the ex- with mid-latitude storm tracks. In January these are tremes. This article first discusses the observed changes strongly evident over the North Pacific and Atlantic (Section 2) and then considers the processes involved Oceans and, to a lesser extent, throughout the south- and the conceptual basis for understanding changes in ern oceans. The southern ocean storm tracks are precipitation, floods, and drought, and future pros- strongest in the transition seasons and broadest in the pects (Sections 3 and 4). Climate models (Section 5) southern winter, while the northern ocean storm tracks have been used as a guide to future changes, but become weaker in the northern summer. The absence are challenged in their ability to correctly simulate of relationships between mean TCWV and precipita- patterns, seasonal variations, and characteristics of tion in the extratropics highlights the very transient precipitation, and hence their results must be used nature of precipitation events that are associated with with caution (e.g. Kharin et al. 2007, Liepert & Pre- extratropical cyclones, while in the tropics there is a vidi 2009). Outstanding issues are briefly discussed in much stronger mean-flow component associated with Section 6. monsoons and the Hadley circulation. Trenberth: Changes in precipitation with climate change 125 Fig. 2. Mean sea-surface temperature (SST), total column water vapour (TCWV), and precipitation for January (left