Observed Climate Variability and Change

One of the most obvious characteristics of climate is its variability, especially in areas with strong gradients of climate zones, e.g. in the semi-arid tropics and in higher mid-latitudes. The mean temperature of one of the coldest days in July and one of the warmest in December in Hamburg do not differ. The rain in parts of the Northern Sahel from one year to the next may differ by more than a factor 3. Therefore climate - as the synthesis of weather - is not only characterized by averages of parameter values but also by their frequency distributions (see figure 2. 1 and box 2.2). Although strong deviations from the average value are rare, they get most of the attention because they represent weather extremes to which our infrastructures are often not well adapted. As Figure 2.1 also clarifies, new extremes on the side to which the distribution is shifted must accompany climate change. The only exception would be the case with a strongly narrowing frequency distribution (what has not been observed). Do we already observe manifestations of climate change? Yes, there are numerous ones, besides the obvious mean global near surface air temperature increase over the recent 150 years (see figure 2. 2). These are (only examples):

• Accelerated mean retreat of mountain glaciers worldwide,

• Strong decrease of multi-year sea ice in the Arctic Ocean (-7 percent per decade since 1979 when satellite observations began),

• Reduced snow cover over North America, less pronounced over Eurasia,

• More rain per event in nearly all areas with slightly decreasing, constant or increasing total precipitation,

• Reduced daily temperature amplitude which can be caused by higher water vapour content, increased cloudiness and higher atmospheric turbidity,

• Mean global sea level rise, about 1.5 to 2.0 mm/a in the 20th century, recently increased to ~3 mm/a, as observed by satellite altimeter measurements since 1991,

• Increased yearly precipitation in most high latitude areas, decreased yearly precipitation in the semi-arid subtropics,

• Decreased temperatures in the stratosphere and mesosphere,

• Increased vegetation period length (about 2 weeks) in the Northern Hemisphere higher latitudes,

• Changed optical properties of clouds caused by air pollution.

In the present rapid climate change era single evaluations of very long time series are therefore partly misleading as the recent decades might have shown a changed frequency distribution of a climate parameter, e.g. rain amounts per event. Therefore subsections of long time series have to be evaluated separately.

Strong climate variability on time-scales up to millennia, as oceans and ice sheets are involved in creating it, makes it difficult to separate climate variabil ity from real climate change. Observations are always the result of variability and change. For more details see section 4.4.

Figure 2.1: Schematic frequency distribution of climate parameters both for present climate and changed climate. Also a broadened distribution for a changed climate is shown (Grassl, 2002).

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Figure 2.2: Global mean near surface air temperature since 1856 (Meteorological Office of the United Kingdom)

Box 2.2: Climate, its Variability and Change

The World Meteorological Organization (WMO) in Geneva, a Specialized Agency of the United Nations, defines climate as the synthesis of weather extracted from frequent atmospheric and surface parameter observations over at least 30 years. This synthesis must contain the probability of deviations from the mean, i.e., the number of events deviating for example 3 standard deviations a from the mean must be known. If mathematical functions are fitted to the observations the frequency distri-

butions turn into probability density functions. If the frequency distribution is close to a Gaussian distribution, like for temperature, a 3 a-event is close to what most would call a hundred year event, observed only once per century. However, for such long periods the frequency distribution might have changed when climate change has occurred.

Climate change occurs if external parameters change, like solar radiation flux density. It has become customary to speak of climate change also if volcanic eruptions reach the stratosphere or mankind changes atmospheric composition inadvertently, although we and volcanoes are part of the Earth system.

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