Projections for Extreme Weather

6.1.1.d

Extreme weather and climate events are likely to change in several ways. They can change in frequency and in intensity, and would result in mostly adverse impacts on the biophysical systems.

Look at why this is the case (see Figure 1). It shows how climate can change in three different situations

• if the average (mean) temperature increases
• if the variance (variability) increases
• if both the mean and the variance increase.

Situation 1: Panel (a) shows the normal curve statistical distribution for temperature as a schematic diagram. The dashed line curve represents the distribution for our current climate. Notice the points on the x-axis at the points identified as “hot” and as “cold”. The area to the left of the “cold” limit (blue) represents the relative number of cold days or events. The area to the right of the “hot” (yellow) is the frequency of hot days or events. Note the location of the average or mean temperature. If this mean temperature increases, the entire curve shifts to the left, as shown by the solid line curve labeled “new climate”. Notice how the area to the right of the “hot” limit (yellow) increases greatly. We also have days in the new area (red), which are temperatures hardly ever seen in the previous climate. That is, we have more record-breaking hot days.

Because of the shift to a warmer average temperature, the blue area almost disappears. There is less cold weather.

Situation 2: Panel (b) shows the temperature range for the new climate extending farther into the hot and cold regions. Note that the mean stays the same. This produces an increase in the probability of both hot and cold extremes as well as the absolute value of the extremes. That is the areas under the curve shaded in yellow and in blue. Both the increases and the values are farther from the average.

Situation 3: Panel (c) shows a new climate with both an increased average temperature and an increase in the variability, or extent, of the temperature from the average. In this new climate, we have more frequent hot events with more extreme high temperatures, (yellow, red) and fewer cold events (blue).

Figure 1. Schematic diagrams showing the effects on extreme temperatures when (a) the mean increases, leading to more record hot weather, (b) the variance increases, and (c) when both the mean and variance increase, leading to much more record hot weather.

Source: IPCC Climate Change 2001 Synthesis Report p. 81

Temperature-related Changes:

More hot days and heat waves, and fewer cold and frost days are very likely over nearly all land areas. Increases in mean temperature will lead to increases in hot weather and record hot weather, with fewer frost days and cold waves. Table 1 shows a summary of recently observed changes in weather and climate events with their projected future changes. Scientists have made a judgment as to how likely these changes are a result of climate change rather than of natural variation.

Increases in daily minimum temperature are projected to occur over nearly all land areas and are generally larger where snow and ice retreat. Frost days and cold waves are very likely to become fewer. The changes in surface air temperature and surface absolute humidity are projected to result in increases in the heat index (which is a measure of the combined effects of temperature and moisture).
The changes in temperature extremes are likely to result in increased crop and livestock losses, higher energy use for cooling and lower for heating, and increased human illness and heat-stress-related deaths. Table 2 shows examples of the kind of impact that this climate variability and these extreme events can produce.

Fewer frost days will result in decreased cold-related human health effects, and decreased risk of damage to some crops, but the risk to other crops may increase. Benefits to agriculture from a small temperature increase could include small increases in the GDP of temperate zone countries.

Precipitation-related Changes:

The strength and frequency of extreme precipitation events is projected to increase almost everywhere. This would lead to more frequent floods and landslides. Along with this comes loss of life, health impacts such as epidemics and infectious diseases, property damage, loss to infrastructure and settlements, soil erosion, pollution loads, with insurance and agriculture losses. (See Table 2.)

A general drying of the mid-continental areas during summer is likely to lead to increases in summer droughts and could increase the risk of wildfires. This general drying is due to a combination of increased temperature and potential evaporation that is not balanced by increases in precipitation.

It is likely that global warming will lead to an increase in the variability of Asian summer monsoon precipitation. Tropical cyclones are likely to have increased peak and average precipitation intensities. That is, the total volume of water and maximum precipitation of these storms will be higher.

Table 2 Examples of climate variability and extreme climate events and examples of their impacts. Likely means 66-90% chance and very likely means 90-99% chance.

CHART NEEDED

Source: IPCC Climate Change 2001 Synthesis Report p. 82