Climate Variability

Climate variability means the alternation between the "normal climate" and a different, but recurrent, set of climatic conditions over a given region of the world (IPCC, 1998). Natural climate variability can produce floods, droughts, cyclones, heat waves, frosts, and other extremes in the humid and sub-humid tropics. GHG-induced climate change could further alter the frequency and magnitude of the climate extremes and associated disasters.

Over the past 100 years, the amount of changes of mean surface temperatures and precipitations has been less in the tropics than the global average, but either in the past climate or in the projected scenarios, climate variability, particularly multidecadal ones, are obvious (IPCC, 1998). Across the tropical Asia, there have not been observed identifiable changes in the number, frequency, or intensity of tropical cyclones in the two core regions of cyclogenesis over the past 100 years; but there is some evidence of substantial multidecadal variability of increases in the intensity or frequency of some extreme events on regional scales throughout the 20th century. Increased precipitation intensity, particularly during the summer monsoon, could increase flood-prone areas in temperate and tropical Asia. There is potential for drier conditions in sub-humid Asia during summer, which could lead to more severe drought. Many countries in temperate and topical Asia have experienced severe droughts and floods frequently in the 20th century (IPCC, 2001). In the Latin American region, there is ample evidence of climate variability and high confidence that ENSO is responsible for a large part of the climate variability at interannual scales (TAR). For example, El Niño is associated with dry conditions in northeast Brazil, northern Amazonia, the Peruvian-Bolivian Altiplano, and Pacific coast of Central America, The most severe droughts in Mexico in recent decades have occurred during El Niño years, whereas southern Brazil and northwestern Peru have exhibited anomalously wet conditions (Horel and Cornejo-Garrido, 1986). La Nina is associated with heavy precipitation and flooding in Colombia and drought in southern Brazil (Rao et al., 1986).

TABLE I

Climate change scenarios of humid and sub-humid tropics

TABLE I

Climate change scenarios of humid and sub-humid tropics

Regions

Temperature

Rainfall

Africa

Increase

-

Asia

South Asia

Increase

Increase

Southeast Asia

Increase

Increase

Latin America

Mexico

Increase

Decrease

Costa Rica

Pacific sector

Increase

Southeast Caribbean sector

Increase

Small increase

Nicaragua

Pacific sector

Increase

Decrease

Caribbean sector

Increase

Decrease

Brazil

Central and south central sector

Increase

Increase for autumn, decrease for summer

Source: Construction based on WGII TAR Sections 10.1.4, 11.1.3, 14.1.2.

Source: Construction based on WGII TAR Sections 10.1.4, 11.1.3, 14.1.2.

For the projected climate changes based on TAR, temperature will increase and precipitation increase or decrease in humid and sub-humid tropics (see Table I). High-resolution modeling studies suggest that over some areas the peak wind intensity of tropical cyclones is likely to increase by 5-10% and precipitation rates may increase by 20-30%, but none of the studies suggest that the locations of the tropical cyclones will change. There is little consistent modeling evidence for changes in the frequency of tropical cyclones (WGI TAR Box10.2). Current projects show little changes or a small increase in amplitude for El Niño events over the next 100 years. But many models show a more El Nino-like mean response in the tropical Pacific (particularly in Latin America). Even with little or no change in El Niño strength, global warming is likely lead to greater extremes of drying and heavy rainfall and increase the risk of droughts and floods that occur with El Niño events in many different regions (WGI TAR Section 9.3.5-6, WGII TAR Section 14.1.2). In short, there is a potential for an increased occurrence of droughts, floods and heavy rainfall and some other extreme weather events in most of humid and sub-humid tropics (see Table II).

It should be noted that there is great uncertainty in projections of likely changes in tropical cyclones, El Nino and monsoons so that vulnerabilities related to these hazards are qualitative and scientists do not discount possibilities of "climate surprises" in the future (IPCC, 2001).

TABLE II

The examples of climate variability and extreme climate (related to tropics) and examples of their impacts (related to agriculture and forestry)

Projected changes during the 21st century in extreme climate phenomena and their likelihood

Higher maximum temperature, more hot days and heat waves over nearly all land areas (very likely)

Higher minimum temperature, fewer cold days, frost days and cold waves over nearly all land areas (very likely)

More intense precipitation events (very likely, over some areas)

Increased in tropical cyclone peak wind intensities, mean and peak precipitation intensities (likely, over some areas) Intensified drought and floods associated with El Nino events in many different regions (likely) Increased Asian summer monsoon precipitation variability (likely)

Representative examples of projected impacts (all high confidence of occurrence in some areas)

Increased heat stress in livestock and wildlife

Increased risk of damage to a number of crops

Decrease risk of damage to a number of crops, and increased risk to others

Extend range and activity of some pest and disease vectors

Increased flood, landslide, and mudslide damage

Increase soil erosion

Increased damage to coastal ecosystems such as mangroves

Decreased agricultural and rangeland productivity in drought- and flood-prone regions

Increase in flood and drought magnitude and damages in tropical Asia

Source: Adapted from WGII TAR Table SPM-1.

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