Precipitation changing with climate change
Figure 1: Observed change in annual precipitation over land
Precipitation is the general term for rainfall, snowfall and other forms of frozen or liquid water falling from the clouds. The type of precipitation that occurs is greatly dependent on the temperature and weather at the time (IPCC, 2007).
Precipitation varies from year to year, constantly changing the amount, intensity, frequency and types. Precipitation has increased in many regions of the world and decreased in others (figure 1), with little or no net change in the total amount of precipitation (Trenberth , 2011).
Observed changes
'Precipitation has increased in the Eastern parts of North and South America, Northern Europe, and Northern and Central Asia. Precipitation decreases have been observed in the sub tropics and the tropics outside of the monsoon trough, namely the Sahel, the Mediterranean, Southern Africa and Southern Asia' (Trenberth, 2007).
Some areas have experienced widening swings between the two precipitation extremes. An example of this is the East African region. This region typically has a bi-modal annual rainfall cycle. The past 30 years has seen a gradual drying of the long rain season, and increased rainfall in the short rain season (figure 2). To read more on this Visit my Blog Post this week on Climate Change in East Africa.
Figure 2: Percent of normal precipitation in East Africa, 2011
Source: NOAA Climate Prediction Center via Weather Underground
Can this not be natural variability?
Local and regional changes in precipitation largely depend on atmospheric circulation patterns determined by El Nino, the North Atlantic Oscillation and other patterns of variability. Trenberth (2010) found that the global patterns of variations are dominated by large-scale patterns of change in precipitation associated with ENSO. A study based on the rainfall trends in the Philippines between 1951- 2010 found that the statistically significant drier conditions are related to El Nino, and the statistically significant wetter conditions are related to La Nina. The results suggested a trend of the dry season (January- March) becoming drier and the west season (July- September) becoming wetter.
Due to the natural variability in rainfall, it can be difficult to detect changes in rainfall patterns. Not only does El Nino make wet areas water, it also contracts storm tracks towards the equator, and that is the opposite effect of climate change. So, to separate the signal of climate change from natural variability, Marvel (2013) looked for both increased rainfall in wet areas and a shift in storm tracks away from the equator, which cannot happen naturally. This is exactly what they found.
Therefore, as also found in Scott (2010), natural variability arguably cannot explain the observed intensity and distributional changes in precipitation. Rather these are consistent with a combination of natural factors and human influence.
How is climate change affecting Rainfall?
There is more and more evidence that human- induced climate change, is changing the hydrological cycle, especially extreme weather events.
A study on the Increased record- breaking precipitation events under global warming has shown that in 'the last 3 decades the number of record breaking events has significantly increased in the global mean' (Figure 3). They found that there was 12% more of these events from 1981- 2010, compared to a scenario without global warming. In 2010 there was a 26% chance that the increase rainfall events were caused by climate change.
Southeast Asia had an increase of up to 56%, Europe 31% and Central United States 24%.
A study on the Increased record- breaking precipitation events under global warming has shown that in 'the last 3 decades the number of record breaking events has significantly increased in the global mean' (Figure 3). They found that there was 12% more of these events from 1981- 2010, compared to a scenario without global warming. In 2010 there was a 26% chance that the increase rainfall events were caused by climate change.
Southeast Asia had an increase of up to 56%, Europe 31% and Central United States 24%.
Figure 3: Annual observed record-breaking anomaly between 1981 and 2010
What is the relationship between rainfall and temperature?
The Clausius- Clapeyron equation is a good predictor for changes in extreme rainfall intensities- The higher the temperature of the air, the higher its potential to hold more moisture. This greater amount of moisture in the air translates into stronger downpours. The relationship predicts a 7% increase in water vapour per ℃ of warming. Not only does increased moisture mean heavier rainfall, but also less frequent and shorter rains, as it takes longer to recharge the atmosphere with moisture.
The future?
Climate models suggest that increasing GHG's in the atmosphere will change precipitation patterns in 2 main ways:
- There will be a shift in a strengthening of existing precipitation patterns- The wet areas will get wetter and the dry areas will get drier.
- Changes in atmospheric circulation- Storm tracks will move away from the equator and towards the poles
IPCC Models
IPCC models indicate that with a warmer climate, precipitation will increase in the areas of regional tropical precipitation maxima. Specifically over the tropical pacific, with general decreases in the sub-tropics, and increases at high latitudes (IPCC, 2007).
Globally the average mean water vapour, evaporation and precipitation are projected to increase.
Figure 4:Changes in extremes based on multi-model simulations from nine global coupled climate models
As shown in Figure 4, intensity of precipitation events is projected to increase, particularly in tropical and high latitude areas. where there are general increases in mean precipitation. Even in areas where mean precipitation decreases, precipitation intensity is projected to increase, with variability. The mid- continental regions are at great risk of drought due to the tendency for drying during summer.
Concluding thoughts
Rainfall patterns are constantly changing; however increasing temperatures has meant that these precipitation events are becoming more and more extreme. Extreme precipitation events are typically defined as floods and droughts. Floods are associated with extremes in rainfall, while droughts are associated with a lack of precipitation and often very high temperatures (Trenberth, 2011). Todays post aimed to provide an insight into how rainfall patterns have changed and how they are projected to change in the future.
Next week's blog will focus on how these changes have affected North East Brazil in terms of Drought.
Just to clarify,Are you saying that ENSO is completely ruled out as the cause or that it can not be the single cause of changes in precipitation?
ReplyDeleteHi Emily, thank you for you comment. The point i was trying to make was that ENSO cannot be the sole cause of these changes in precipitation. Some of these changes are driven by changes in climate for example, storm tracks drifting away from the equator. However, ENSO is still an important factor and a warmer climate is also intensifying the affects of ENSO for example in the Philippines.
ReplyDeleteSo yes overall i was saying that ENSO cannot be the sole cause of changes in precipitation around the globe.