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Chapter 14 Past and Present Climate
Climate determines how we dress, what we do for recreation, and how we make a living. Climate is important for a variety of applications, weather forecasting being only one. The climate of a region is an important feature in city planning. You would not want to locate a waste processing plant upwind of a city center. Housing designs would benefit if architects can take advantage of solar illumination to maximize energy gains during the winter and minimize them in summer. The climate of a region also determines agriculture of a region. The spread of certain diseases is influenced by climate conditions.
Describing today's climates is rather straightforward because of the large number of observations we have available for analysis. Determining past climates is more of a challenge. But there is one thing for sure, there is abundant evidence that climate of a region is not constant; it changes in space and in time. Sometimes the change is gradual, others sudden. The challenge is to discover these past changes.
Determining past climates is like solving a mystery. We must search for evidence that identifies the meteorological character of a given time period. The data must offer a consistent story while obeying physical laws. Fossils, tree rings, ocean sediments, and air bubbles trapped in glaciers all provide clues to the mystery. This combined evidence indicates that today's climate is fundamentally different than that of a million years ago. But, knowing that climate has varied does not explain the cause of these differences. Explaining the reasons why the climate of a region has changed is like discovering the motive behind a crime. This chapter explores some of the methods that are used to make these discoveries. So in the words of Sherlock Holmes
Come, Watson, come! The game is afoot.[1]
Definition of Climate
Climate is the collective state of the atmosphere for a given place, over a specified interval of time.
Climate is defined as the collective state of the atmosphere for a given place over a specified interval of time. There are three parts to this definition. The first deals with the state of the atmosphere. The collective state is classified based on some set of statistics. The most common statistic is the mean, or average. Climate descriptions are made from observations of the atmosphere and are described in terms of averages (or norms) and extremes of a variety of weather parameters, including temperature, precipitation, pressure and winds.
The second part of the climate definition deals with a location. It could be a climate the size of a cave, the Great Lakes region, or the world. In weather and climate studies we are most interested in micro-scale, regional, and global climates. The climate of a given place should be defined in terms of your purpose. For example, in Chapter 11 we used climate data to study thunderstorms. We compared thunderstorm frequency over North America with the frequency of occurrence of large hail. This comparison helped us to understand the difference between ordinary and severe thunderstorms in that location.
Time is the final aspect of the definition of climate. A time span is crucial to the description of a climate. Weather and climate both vary with time. Weather changes from day to day. Climate changes over much longer periods of time. Variations in climate are related to shifts in the energy budget and resulting changes in atmospheric circulation patterns.
Many factors determine the climate of a region. The five basic climate controls on the global and region scale are latitude, elevation, topography, proximity to large bodies of water, and prevailing atmospheric circulation. Latitude determines solar energy input. Elevation influences air temperature and whether precipitation falls as snow or rain. Mountain barriers up wind can affect precipitation of a region as well as temperature. Topography also affects the distribution of cloud patterns and thus solar energy reaching the surface. The thermal stability of water moderates the temperature of regions downwind of the region. Atmospheric circulation is somewhat less regular than the previous controls. Large-scale circulation patterns exert a systematic impact on the climate of a region, such as the position of the ITCZ or the descending branch of the Hadley cell. These controls produce variations in temperature and precipitation that bring about our changing patterns of climate and weather.
Observations
A fundamental challenge of modern science is to predict climate. Recent concerns about global warming and the effect of greenhouse gases added to the atmosphere by humans have heightened the need to understand the natural processes that cause climate variations. To gain this knowledge climatologists have turned to the past. Let's raise some questions about current and past climates that this chapter should investigate (Figure 14.1). Fossils indicate that the global climate during the dinosaur era, some 65 million years ago, was warmer than it is today. Is there additional evidence that supports the idea that the dinosaurs lived in a climate warmer than today's? Fossil evidence also indicates that the dinosaurs quickly died off on a global scale, suggesting an environmental catastrophe of a global nature. Is there other evidence to support this environmental cataclysm that led to the demise of the thunder lizards? Pre-historic paintings of hippopotami have been found in caves located in what are now the deserts of North Africa. Today, the climates of the North African deserts could not sustain such animals. Can we classify climates by the animals that live in the region? What is a simple way to classify climate regimes?
Temporal Scale of Climate
The most information about the variations of a parameter over a given time period is provided by a plot of the frequency of occurrence, or the number of observations in a given interval. Figure 14.2 is a plot of the frequency of occurrence, or histogram, of the annual average temperature of Madison WI, in intervals of 5F. The average, standard deviation, and the maximum and minimum values are also given. This type of plot is very useful, but it is difficult to plot over a large region, which makes the mean of a variable very useful. Plotting the mean over time allows us to quickly identify fluctuations in time.
Studying climate requires caution in comparing observations from different regions of the world. Observations must be over the same time periods. Recent climate norms are determined by averaging weather elements over a 30-year time period. The average temperature in January is the average temperature of 30 consecutive years of the temperature during January at a location. The most recent averaging period is the period 1961 through 1990. The previous period was 1931-1960. An exceptionally warm winter may be termed abnormal because it falls outside the typically observed temperatures during this period.
Climatology is concerned with averages and variations about the average value. The annual total precipitation of Portland, Oregon (USA) and Montreal, Canada is similar; Portland has 39.8 inches and Montreal has 40.8 inches. The average precipitation for a month is nearly the same for the two cities. Portland's average monthly precipitation is 3.32 inches, while that of Montreal is 3.4 inches. Figure 14.3 demonstrates that the distribution of this precipitation throughout the year is very different for these two cities. Portland has distinct rainy (winter) and dry (summer) seasons while precipitation is evenly distributed throughout the year over Montreal. Variation about the mean is often expressed in terms of the standard deviation. Small standard deviations indicate little variations while large values suggest broad variations. The following statement provides more information about the annual precipitation of these two cities. The monthly mean precipitation for Portland is 3.3 with a standard deviation of 2.2; Montreal has a monthly mean precipitation of 3.4 inches with a standard deviation 0.3.
Extreme values (e.g., record maximum and minimum temperatures) are observations that occur only rarely. They are of particular interest in engineering. For example, in designing a building it is important to know the highest winds a building will have to withstand. When burying water pipes the maximum penetration of frost must be known to avoid any possibility of bursting pipes. Record temperatures for a particular day that are reported in weather reports are additional examples of extreme values of temperature.
Spatial Scale
As with the different scales of weather, a discussion of climate must also specify the size of the area under discussion. The different climate scales, global, regional, and microscale, are indicated in Figure 14.4
Atmospheric circulation patterns are of critical importance in determining the climate of a location. On a global scale, atmospheric motions transport heat from the topics towards the poles. Evaporation over the oceans supplies the water molecules that support precipitation over land. These circulation patterns are in large part driven by energy differences between regions of the globe. As discussed in Chapter 7, dry climates are associated with the descending branch of the Hadley Cell while moist climates coincide with the ascending branch. On a regional scale, precipitation on the lee side of a mountain is typically less than on the windward side. On a still smaller scale, the amount of snow downwind of a snow fence is on average larger than the amount upwind (Figure 14.4).
Global
Global climate is the largest spatial scale. We are concerned with the global scale when we refer to the climate of the globe, its hemispheres, and differences between land and oceans. Energy input from the sun is largely responsible for our global climate (Box 14.1). The solar gain is defined by the orbit of Earth around the sun and determines things like the length of seasons. The distribution of land and ocean is another import influence on the climatic characteristics of the Earth. Contrasting the climate of the Northern Hemisphere, which is approximately 39% land, with the Southern Hemisphere, which only has 19% land, demonstrates this. The yearly average temperature of the Northern Hemisphere is approximately 15.2C, while that of the Southern Hemisphere is 13.3C. The presence of the water reduces the annual average temperature. The land reduces the winter average temperature while increasing the average temperature during summer. As a result, the annual amplitude of the seasonal temperature is nearly twice as great for the Northern Hemisphere. The Northern Hemisphere has a large variation in the monthly mean temperature.
The land absorbs and loses heat faster than the water. Over land, the heat is distributed over a thin layer, while conduction, convection and currents mix the energy over a fairly thick layer of water. Soil, and the air near it, therefore follow radiation gains more closely than water. For this reason, continental climates have a wider temperature variation. We observed this in Chapter 3 by comparing the seasonal cycles of temperatures for different regions of the globe.
Table 14.1 The average temperatures of the Northern Hemisphere and Southern Hemisphere for winter, summer and the year. The Annual Range is give as well as the differences between the Hemispheres. Differences between the Hemispheres are caused by the differences in the distribution of land and water.
Winter / Summer / Year / Annual RangeNH / 8.1C (46.6F) / 22.4C (72.3F) / 15.2C (59.4F) / 14.3C (25.7F)
SH / 9.7C (49.5F) / 17.0C (62.6F) / 13.3C (55.9F) / 7.3C (13.1F)
Difference / -1.6C (-2.9F) / 5.4C (9.7F) / 1.9C (3.5F) / 7.0C (12.6F)
Regional climates
The major factors that determine global climate also influence climate on a regional scale. Regional climates are influenced by water bodies and mountain ranges. Lakes exert a moderating influence on local climate, in a manner similar to how oceans affect larger climate. The Great Lakes are a good example for demonstrating the impact of lakes on climate. We saw in Chapter 7 how the Great Lakes effect snow fall. The Great Lakes also influence the temperature of the region. Figure 14.5 shows the average land temperature versus the average surface lake temperature in the Southern Lake Michigan region. The temperature of the water is lower than the land from mid-March to August. Largest differences occur from mid-May to early June. The water temperature is greater than that of the land from late August to mid March, with the largest differences in late November and early-December in late autumn and winter. Exchanges of heat and moisture above the lakes is the key to weather modification by the Great Lakes. The influence of large water bodies on the weather of surrounding regions is most marked when the temperature differences are greatest. .
Large mountains influence regional climates. They provide barriers for the air. Large mountain ranges that are oriented east-west can block cold air outbreaks from reaching regions that are more southern. You can observe this by comparing the annual mean temperature of a city south of large mountain barrier with a city at a similar latitude but with no mountain barrier. Lahore, India, (31.5N) located south of the Himalayan Mountain Range has an average temperature of 12.8C, while the temperature of Austin, TX USA (30.25N) has an average temperature of only 10.4C.
Vegetation also affects regional climate (Figure 14.6), an observation made obvious when comparing the wind speed within a forest with the wind speed at the same height over an open field. Friction reduces the wind speed in the forest, so open areas have greater winds. The relative humidity is usually greater in a forest than in the surrounding open country. Forests depress the summer temperatures by 1 to 2 C (2-4F) below the annual mean in their vicinity. This temperature difference is driven by heat budget differences; less solar energy reaches the forest floor than the open field.
Microclimate
Variations in climate can be observed over a short distance. Small-scale climates are referred to as microclimates. Examples of microscale climates include the climate of a cornfield, a house, a patio, or a sand dune. Microclimates can be very different across a particular region. Topography, presence or absence of water, exposure to the sun, and soil conditions are important factors that determine microclimates.
The presence or absence of snow can be a good indication of differences in microclimates. Variations in temperature due to differences in exposure to the sun affect accumulation and melting. South facing slopes generally retain smaller snow amounts than north facing slopes (Chapter 4). Snowdrifts are generated by rapid changes in wind speed due to the interaction of winds with obstacles.
Determining Past Climates
Paleoclimatology is the study of climate of the past and the causes for observed variations.
A fuller understanding of past climates enables scientists to better predict future climate, including the impact of humans. Uncovering the global and regional climates of the past is like a solving a mystery. We look for evidence and compile this evidence into a consistent story. Paleoclimatology is the study of climate and climate change throughout geologic time. This section discusses some of the methods paleoclimatologists use to collect evidence.
Bubbles in ice
Glacier a mass of perennial ice that originates on land through the accumulation of snow.
Bubbles trapped in ice provide windows to the past for atmospheric chemists. Air bubbles get trapped in glaciers and ice sheets (Box 14.2) as snow gets compressed. These trapped bubbles provide a record of the concentration of trace gases such as carbon dioxide (CO2) and methane (CH4) over the past 200,000 years. CO2 and CH4 are trace gases and thus only occupy a small fraction of the molecules in the atmosphere.
Methane concentrations during the last ice age were approximately 350 ppbv (parts per billion by volume). Figure 14.7 shows the concentration of atmospheric CO2 and CH4 obtained for a 2,083 meter long ice core cut from Vostk, Antarctica. Also shown on this figure are estimates of temperature changes during this period. The warmer temperatures are clearly related to higher concentrations of CO2 and CH4. Approximately 150,000 years ago the concentration of CO2 was less than 200 ppmv (parts per million by volume) and CH4 amounts were approximately 300 ppbv. Both these gases are greenhouse gases. Increased concentration of a greenhouse gas can lead to a warming of the atmosphere. The amount of methane has approximately doubled from about 10,000 years ago. This was a warm period in the history of our planet and is associated with increased concentrations of the greenhouse gases CO2 and CH4.
Dust in ice
Ice sheets also provide valuable information on the frequency of volcanic eruptions. Strong eruptions can inject dust into the atmosphere where it is transported across the globe and then settles onto glaciers. Snowfall then covers the dust providing a long-term record of an eruption.
Dry conditions can lead to soil erosion and the transport of the soil by the winds in the form of dust storms. Dust storms (Figure 14.8) from the Sahara can transport dust as far as Greenland. So, dust deposits on ice may result from a change in precipitation, or a change in wind direction that is favorable for dust transport and deposition. Either way they indicate something happened! Dust on ice sheets is, like many observations discussed in this section, a piece of the climate puzzle, not the complete answer. Sediments on the ocean floor provide another clue to past climates.