Climates on a Rotating Earth
Required Reading: Thomas et al. (2004) Nature 421: 145-148 Hotlink.
We can divide the study of climate into a number of sub-areas. The first of these is the global pattern of penetration and absorption of solar energy. That energy is the driving force for most of what follows. It is, therefore, where we begin.
The solar energy flux which reaches the outer limit of the atmosphere (and on a surface held perpendicular to the sun's rays) is 2 calories/cm2/minute. That's called the solar constant. It isn't really constant, however, since the orbit of the earth around the sun is elliptical, so that the distance from the earth to the sun varies seasonally. The so-called solar constant, therefore, varies by about 15% from season to season. However, only about 1/2 the energy striking the outer surface of the earth's atmosphere actually reaches ground level; the remainder is reflected, re-radiated, or absorbed within the atmosphere. In addition the relative energies in the colour spectrum which reaches the surface are much different than those which strike the upper atmosphere. To give you an idea of where the energy goes:
21% is reflected by clouds back into space
5% is reflected by dusts and aerosols
6% is reflected by the earth's surface
3% is absorbed by clouds
15% is absorbed by dust, water vapour and CO2
The next question we ask is: Why are the tropics, i.e. the low latitudes, warmer than the poles? To start with, the total number of daylight hours in a year is constant for all points on earth; over the year every site averages 12 hours of daylight and 12 hours of night per day over the entire year. The distribution of daylight hours during the year differs dramatically, of course. But the answer is still simple and obvious: the input of solar energy, measured in calories, is not evenly distributed over latitude; the rate of input, and total input, are both higher in the tropics. The real question is why? Before considering the earth's surface and what happens there, you should also note that the thickness of the atmosphere through which light must penetrate to reach the surface differs with 'solar' latitude. At high latitudes, where the surface is 'tilted away' from the sun, the effective thickness of the atmosphere is greater, and therefore, all those absorptions and reflections listed above are occurring to a greater extent, and less of the input solar energy reaches the surface.
It isn't day length that's important, but the intensity of sunlight, measured as calories per unit area. The solution to the problem is simple trigonometry. Let's use as an example a comparison of solar heating effects at the equator and at 45o N latitude. Let's be precise, and say that we're making this comparison on the date of one of the equinoxes, when the sun shines directly down on the equator. Let's compare square sunbeams 1 cm on a side. Since the sun is much larger than the earth, the solar constant applies to both beams as they arrive at the outer atmosphere. Glossing over both differences in the thickness of the atmosphere which must be penetrated to strike the earth's surface and the complexities of spherical trigonometry, we'll temporarily assume the sunbeams strike flat surfaces. At the equator the 1 square cm sunbeam is absorbed by an earth surface which also measures 1 cm2. At 45o N latitude the surface of the earth is at a tilt with respect to the beam. In fact, at that latitude the angle of incidence of the beam (90 - L) and the latitude are equal. To be completely accurate representation would require spherical geometry. Without spherical trig, on our artificially ‘flat’ earth, energy input is spread over an area 1 cm wide, but 1.414 (1 x sqrt(2)) cm long. Where 1 calorie per cm2 was available at the equator, even disregarding atmospheric thickness and spherical trig only 0.707 calories/cm2 are available at 45o latitude. Since we live at about 42o latitude, that is a fair approximation. The effect on temperature of movement away from the solar equator averages out to about 1o C/degree latitude. These approximations become more suspect as we move closer to the poles, because the disregarded complexities become more significant. Further, we have made our comparison at an equinox. How does this comparison shift as seasons change?
Figure 1 The areas of sunbeams of equal size as they spread over the surface of the earth at differing latitudes. To be completely accurate representation would require spherical geometry.
Day length at the solar equator is 12 hours each day of the year. However, because the earth's axis is at a 23o tilt with respect to the plane of the earth's orbit around the sun, the solar equator moves seasonally, unlike the one we draw on maps. The solar equator moves from the Tropic of Capricorn (approximately 23o S latitude) on December 21 to the Tropic of Cancer (23o N) on June 21. The map equator is the latitude at which the variance in day length is smallest. Meanwhile, north of the Arctic Circle and south of the Antarctic Circle (each at approximately 67o latitude) there are 'days' and 'nights' 24 hours long. In fact, the circles mark the map latitude where the 'solar latitude' reaches 90o on at least 1 day of the year. A fair part of seasonal temperature variation is explained by considering seasonal patterns in solar latitudes. Let's consider our summer period. On June 21 the solar equator is at 23oN. That makes our effective solar latitude not what the map says (42o), but rather 42 - 23, or a little less than 20oN. Thus, not only are our summer days longer, but it is as if we were temporarily living at lower latitudes, at least with regard to the solar energy input per unit area per unit time. Of course, the opposite applies during winter; our effective solar latitude on December 21 is 42 + 23, or about 65oN, and our days are shorter. Both the energy input per unit area per unit time and the duration during which we receive that input are reduced.
As a last comment on energy inputs, atmospheric heating results from 2 energy inputs:
1. absorption of incoming radiation, which accounts for 18% of incoming energy, and is unlikely to change much over geological time scales on earth;
2. re-radiation of infra-red energy from the earth's surface, and its absorption by CO2 in the atmosphere.
That absorption is what's termed the 'greenhouse effect', and adds significantly to the heat load of the atmosphere. We can change the pattern of re-radiation in two ways. 1) By changing the albedo (reflectance) of the earth's surface (building asphalt parking lots increases the amount of absorption and re-radiation as infra-red; in winter a snow cover increases the albedo and decreases absorbance, cooling the climate even more than decreased day length and increased solar latitude might suggest alone). 2) By changing the chemical composition of the atmosphere. Atmospheric CO2 has approximately doubled since the beginning of the industrial age, i.e. from around 200 ppm in the 1700's to approximately 360 ppm today, but much larger increases may follow if we unbalance the solubility equilibrium of atmospheric and aquatic CO2 by continuing and expanded combustion of fossil fuels. Current models for the increase in greenhouse gases project a possible atmospheric CO2 concentration of 600 ppm by 2050 (this is an extreme estimate for this short time). A part of the increase is due to increasing rates of fossil fuel combustion as the developing world becomes more industrialized through the use of fossil fuels. A part comes from anticipated increases in deforestation of tropical areas, and a part comes from a shift in the equilibrium between the atmospheric and larger oceanic CO2 pools. For purposes of our study of current climate and geographical patterns in climate, the greenhouse effect is not important, but the atmospheric heating resulting from absorption of solar energy and re-radiation is. That atmospheric heating drives the global patterns of air circulation. Global warming is a separate topic to be considered at the end of this climate lecture.
To understand air circulation patterns let's begin at the equator, and momentarily forget that the surface of the earth is covered by irregular land masses as well as water, and that the earth rotates on its axis. And let's again begin by considering the pattern at an equinox, when the solar equator and the map equator coincide. For the moment, consider the flow as if it were two-dimensional, just rising and falling on a plane in the atmosphere. At the equator the intensity of solar energy input is at its maximum, and the atmosphere is warmed most. What happens to hot air? Think no further than a hot air balloon. Hot air rises. As the hot air rises it expands in the more rarefied atmosphere of higher altitude (that's the reason barometric pressure is compensated to sea level, and the difference used to drive simple altimeters). To expand, any parcel of air must push neighbouring parcels aside. To expand, the air does work, spends energy. That energy has to come from the parcel's own energy supply. Spending it means the parcel cools. That cooling occurs at a characteristic rate, called the adiabatic lapse rate. We'll discuss adiabatic cooling more later. For now, we only need to know that air heated at the earth's surface at the equator rises, and cools as it rises and expands. The rising creates a low pressure area at the equator; it's occurring continuously, thus forcing a flow in the upper atmosphere away from the equator. The rising air is deflected towards increasing latitudes. The combination of cooling caused by rising in the atmosphere and additional cooling caused by displacement from the equator causes a gradual increase in the density of the air mass we're following. If hot air rises, then cold air sinks; it's just the result of changes in the densities of air masses as their 'temperature' changes. By the time the air mass has reached about 30o latitude (N or S), its density is higher than that of the surface-warmed atmosphere beneath, and the air mass sinks back to the surface. That produces a band of consistently high pressure at what are termed the 'horse latitudes'. The reverse of what happened when the air rose happens when it falls. The parcel of air is compressed by parcels surrounding it, which are at higher pressure at lower elevation; work is done upon the falling air; that energy input warms the falling air at the adiabatic lapse rate. But the descending air has to go somewhere, it can't just pile up. A portion of the descending air is deflected toward the equator, and completes a circulation cell. That air produces what we call 'trade-winds' (about which more later).
Figure 2 The positions of the Hadley upper air circulation cells with respect to latitude. The other part of the descending air is forced towards more extreme latitudes.
At a point in the general neighbourhood of 45-50o latitude (exact position here is much hazier) the warmer air from that equatorial circulation cell meets air masses from a cold, polar circulation cell. The polar cell results from the descent of very cold, dense air masses near the poles, and their spread to lower latitudes. The poles are thus another zone of fairly steady high pressure. Where these two masses meet (equatorial air moving to higher latitude and polar air to lower latitude), air 'piles up' at the surface, causing an upward displacement. Because transient shifts in flow cause this zone to move slightly in latitude, there is a zone here of unstable pressure. We live about there, and, as you'll see, the unstable pressure leads this region to be characterized by storms. We now have the basic N-S global circulation cells mapped. We have disregarded, to this point, the effects of warming and cooling on atmospheric moisture content and the effects of the earth's spin on the direction of air movement (wind) at the surface of the earth.
The next step is to understand the effects of atmospheric heating and cooling on relative humidity, and thus on rainfall. As hot air rises at the equator, it cools according at the adiabatic lapse rate. In saying that, we are assuming no further input of energy (i.e. absorption of solar energy) as the air mass rises. Thus, what follows is an approximation which disregards the slight energy input during rise. The atmosphere decreases in pressure at a constant rate with altitude, and thus the rate of cooling is constant with altitude. That rate of cooling, called the adiabatic lapse rate, is defined as: "the rate at which air cools as it rises freely through the atmosphere." For dry air, that rate is 10oC/km. We must specify dry air because water vapour has a higher thermal capacity than the gases which compose dry air. In addition, warm air has a higher capacity for water vapour than cool air; if a rising air mass cools sufficiently it may become saturated. If it cools further, water vapour will condense to a liquid state on particulate matter in the atmosphere. Should condensation occur, the heat of vaporization of the condensing water vapour (approximately 585 cal/gm) is released into the air mass, slowing the rate of cooling to 6oC/km.
Now consider the atmosphere. As it cools, the transition in temperature is opposite that of relative humidity. As air cools, it takes less and less water vapour to saturate it, i.e. it can hold less in solution. Saturation is what the weatherman calls 100% relative humidity. Relative humidity is a measure of how much water vapour is in solution in comparison with a saturated solution. Thus the same amount of water vapour that represents 50% relative humidity at 20oC inside your house condenses on windows because that more than saturates the colder air against the window. As the air mass rises at the equator, it reaches 100% relative humidity, and further rise and cooling causes water to condense out on particulates in the air (dust), forming clouds. As the water droplets get bigger, they fall as rain.
Assuming you now understand adiabatic cooling and warming as air masses rise and fall, we can now explain the large scale global latitudinal 'bands' of high rainfall and deserts. In the tropics, at very low latitudes (really with respect to the solar equator), say 5o on either side, there is a low pressure zone where solar heating causes a rising flow of air. As this air rises and cools adiabatically, water vapour r condenses. The result is almost daily rainfall, usually in the evening when cooler surface temperatures and a lack of direct solar heating aids in lowering the temperature of the rising air to the condensation point. For the same reasons, a more than proportional share of our local summer rainfall occurs as evening showers. Note that tropical rain forests lie along the equator include the Amazon Basin in South America, the Congo Basin in Zaire in Africa, and the rain forests of New Guinea and parts of southeast Asia. Each land mass lying at equatorial latitude has a region of tropical rain forest.
Figure 3 Latitudinal variation in evaporation and precipitation for the earth as a whole.
Look now at the next latitudinal zone whose weather pattern is clearly determined by the global latitudinal pattern of air circulation, the zones surrounding 30oN and S latitude. Here cold air masses are descending toward the surface and warming adiabatically as they descend. As air warms, its relative humidity decreases. Therefore, the air mass will only very rarely reach 100% relative humidity, only rarely will condensation produce clouds, and rain is unlikely. Instead, the warm air at the land surface will 'absorb' evaporation from the warm surface into the unsaturated atmosphere (at least when it’s warm; these areas tend to have the largest diurnal fluctuations in temperature on the globe). The result is the world's great deserts, and a water deficit, i.e. there is less precipitation than there is evaporation. In the southern hemisphere these are the Atacama desert in Chile (where more than 20 years once passed between measurable rainfalls), the Kalihari desert of southern Africa, and the Central Desert of Australia. In the northern hemisphere they are the Gobi desert of Manchuria (at high elevation this is a cold desert), the Sonoran desert of the southwestern U.S. and Mexico, and the Sahara of Africa (in which there are also areas with no recorded rainfall over periods of 20 years or more).
At both the northern and southern latitudinal boundaries of the desert zones are fairly narrow zones in which precipitation shows consistent patterns of seasonal variation. Along the equatorial region is a zone which receives most of its precipitation in the summer (i.e. becomes effectively equatorial), and little precipitation during its winter period (i.e. becomes effectively desert in terms of solar latitude). At the high latitude margins of desert regions, the pattern is exactly the opposite. In these zones rain (or precipitation, whatever its form) falls principally during the winter season when these regions are at a solar latitude corresponding to the temperate, stormy latitudes; during the summer their solar latitudes produce a moderate, desert-like climate.
At somewhat more extreme latitudes, where warm equatorial and cold polar air masses meet, exists a zone of unstable low pressure. The meeting of the air masses causes a general rising flow, and adiabatic cooling during the rise leads to rainfall, but this is a diffuse belt, and rainfall is not predictable at any specific location or time. The weather pattern is inconsistent and stormy. Southern England lays in this latitudinal zone and so does the latitude dividing the Canadian plains and the upper Midwest of the U.S. A look at winter weather patterns on the news indicates the frequency with which precipitation patterns move along or close to the U.S.-Canadian border. Remember that this zone does shift somewhat seasonally. During the northern winter this zone reaches down to about 40o N, while during the summer it is shifted northwards to about 60oN (map latitudes). Also, note from the diagram that during the winter our latitudes are more under the influence of polar air moving south (at least at the surface), while during summer the pattern of influence is reversed and surface flows up to about 60o N are more influenced by warm equatorial air moving northward. Even though the predominant influence may shift, the instability leads this zone to receive precipitation in all seasons.
Finally, at extreme latitudes we find Arctic and Antarctic polar deserts. These areas receive extremely low amounts of precipitation annually; they are zones of stable high pressure where cold air descends back toward the surface, and where rainfall (or snowfall) is therefore unlikely. While we think of them as icy wastelands, the ice has built up over extremely long periods, and little is added in any year. They are truly deserts according to global circulation patterns and their influence over precipitation. These areas cover latitudinal zones from around the polar circles (65o or so) to the poles.
There are other air movements of great importance in our weather. Surface winds, produced directly or indirectly by the daily rotation of the earth on its axis, will be considered later. For now we will assume that surface winds exist. As surface winds pass over terrestrial topography, the air masses comprising them necessarily must rise and fall. Those upward and downward movements subject air masses to the same adiabatic changes in temperature, and therefore are also of great importance in determining precipitation patterns. As a result, on the leeward side of every mountain chain there is a 'rain shadow', a region of low rainfall; and on the windward side, particularly along mountain slopes, there is typically a fairly 'wet' climate and biological community.
Figure 4 The effect of adiabatic lapse on the pattern of rainfall and temperature across a mountain range.
The classic diagram to examine this phenomenon is an examination of the passage of air over the Cascade and Rocky Mountains of western North America. The Rockies reach slightly more than 2 km in peak elevation, and the Cascades do not quite reach those elevations, but that's a fairly good estimate of the rise over surrounding terrain. Assume (correctly) that a westerly air flow occurs (winds are properly described using the direction from which they come, so that a westerly wind is one moving west -> east). For numerical simplicity we'll follow an air mass which begins at the western edge of the rise at a comfortable 20oC, and a moderately high relative humidity. As the air rises up the western slope, it cools adiabatically, initially as unsaturated air at the 'dry' adiabatic lapse rate of 10oC per km. Again for simplicity assume that the air reaches saturation (100% relative humidity, the condensation point) at 10oC. Then, when the air has risen halfway up the mountain(s), or at an elevation of 1 km, it is saturated, and, as it continues to rise and cool from 1 to 2 km elevation, clouds will form and rain will fall. You now know why mountain peaks are so frequently bathed in clouds. Of greater importance is the difference in rate of cooling in the 1-2 km zone. Here condensation releases the heat of vaporization, so that this rising air cools at the lower, saturated adiabatic rate of 6oC/km. Therefore, air which had cooled to 10o at 1 km cools to 4o at the peak of the mountain. Now that air begins to fall, warming adiabatically as it descends. As it warms, the relative humidity drops. Since the air is now unsaturated, the rate of warming is the 10o unsaturated rate. When this air has descended to 1 km, its temperature is 14oC, and at the eastern base of the mountains it is 24oC. The leeward side is warmer, and since the air is unsaturated, rainfall is an uncommon occurrence. The leeward areas of the Rockies are the dry plains of Colorado, the Great Basin of the U.S., the Great Plains, and the Canadian prairies; it is the rain shadow of the Rockies which explains their dry climates. Similarly, the steppes of central Russia are the rain shadow of the Ural Mountains.
The rain shadow phenomenon and the adiabatic temperature changes which promote or make rainfall unlikely, are important in many areas of the world (including oceanic islands). However, there are complications traceable to the more complicated surface circulation patterns of air and water resulting from the earth's spin and the location of major land masses. For now we can suggest that the effects of rain shadows and differences in thermal capacity of saturated and unsaturated air broadly explain the general climatic pattern described as 'continental'. Regions in the middle of continents typically undergo seasonal extremes in climate – hot summers and cold winters. There are 2 parts to a more complete explanation of the extreme seasonal fluctuations at mid-continent:
1. Water has a high thermal capacity. The presence of large bodies of water nearby (e.g. oceans, the Great Lakes) tends to moderate temperature fluctuations. The temperatures of these water masses change very slowly, and air passing over such bodies of water tends to reach temperature equilibrium with them, as well as becoming saturated (reaching 100% relative humidity at the equilibrated air temperature). Thus, temperature extremes are surprisingly moderate at Anchorage, Alaska.
2. Temperatures can fluctuate more rapidly and to wider extremes when air is 'dry' (i.e. unsaturated) than when air is at or very near saturation. That is evident in the adiabatic lapse rates. Continental topography virtually assures that areas in the centers of continents are on the leeward side of some significant topographic feature, thus air will likely be unsaturated. The effect can be extreme: Our mean monthly temperatures differ by about 25oC between warmest and coldest months. In central Eurasia (the world's largest landmass, with no central Great Lakes) the parallel difference in monthly means exceeds 55oC.
Climatologists use a complex measure called 'continentality' to indicate the combination of variation in temperature and humidity which demonstrates the difference between a 'continental' and a 'marine' climate. This measure also corrects for an expected latitudinal increase in temperature variation. It produces an interesting map for North America. Values are highest in mid-continental arctic areas, i.e. the Northwest Territories and prairie provinces, and nearly as high on the U.S. Great Plains and prairies. Values are lowest all along the Pacific coast, and considerably lower along the Pacific coast than at parallel latitudes along the Atlantic. The reason is the prevailing westerly winds in our latitudes, so that the effects of marine influence are felt most strongly along the west coast. Much of the weather along the east coast arises from the movement of air across the entire continent until reached its eastern margin or by northward movement along the coast. Finally, note the 'bump' in the isoclines for continentality, pushing significantly northward (i.e. less continental climates) in the region of the Great Lakes.
Figure 5 The pattern of a 'continentality' over North America. Lines on the map are isoclines of constant continentality. Note the dip to lower latitudes in the mid-region of the continent.
The last major factor needed to understand large scale, global patterns in climate is the daily rotation of the earth on its axis, and concomitant effects on air and water circulation. Rotation produces what is called the Coriolis effect (or Coriolis force). Basically, the Coriolis force represents conservation of momentum for objects moving over the surface of a rotating earth. Its effect causes air masses moving latitudinally to deflect from the simple N-S patterns indicated in considering global air circulation previously. To understand the Coriolis effect its easiest to think of yourself as standing at the equator. At the equator the earth is approximately 40,000 km in circumference. Standing quite still for 24 hours at the equator, the rotation of the earth over that 24 hours will have caused you to move 40,000 km. The air which surrounds you moves at the same 40,000 km per day, assuming you feel no wind.
Figure 6 The pattern of surface air circulation driven by the earth's rotation.
We already know that equatorial air is heated by solar energy, rises, expands, and is forced aside to higher latitudes by more rising air behind it, and that this air spreads to 30o latitude before it has cooled sufficiently to descend back to the surface. This air has both mass and momentum. What happens when it moves away from the equator? What is the circumference of the earth at 30o latitude? A little trigonometry produces the result; the circumference is about 34,600 km at 30o. Thus, if you'd been standing at 30o instead of the equator, you would have been moving 34,600 km per day due to the rotation of the earth. Equatorial air descending at 30o is moving faster than that, even including friction, which would decrease its actual velocity as the air spread northward and southward to less than the ~40000 km per day it was moving as it began to rise. Rotation of the earth is from west to east (that's why the sun rises in the east and sets in the west, in case you've lost track). Therefore, that's the direction of deflection of winds at 30o (i.e. from west to east, or westerly), or at the latitude of descent of air moving toward higher latitude from the equator. Frictional drag as air spreads from the solar equator is important. Otherwise the difference in velocity (167.7 km/hr) would produce continuous hurricane force winds. Even though the velocity is far less, the Coriolis force deflection is a major cause of surface wind patterns.
The westerly deflection (west east) continues in that portion of the descending air which moves toward more extreme latitudes. In the southern hemisphere, with few land masses to block or slow these winds, this deflection produces the 'roaring 40's'. Those strong winds made sailing westward (east west) around South America through the Straits of Magellan difficult for everyone from early explorers attempting to circumnavigate the globe to Darwin's voyage on the Beagle. In the northern hemisphere this deflection is somewhat less evident due to the blocking effect of the many large land masses. Is there any local evidence of Coriolis effect? Water currents circulate demonstrating the Coriolis force (see below). The same forces produce cyclonic storms.
Let's return now to global wind patterns determined by Coriolis forces. Remember we started by looking at poleward flows arising from the equatorial (Hadley) circulation cell. The air flow which descends at 30o then moves back toward the equator is affected differently. Frictional drag near the surface (in fact almost entirely in the 1 km nearest the surface) has slowed this air mass to within a few km per hour of the rotational velocity at 30o, i.e. the velocity differential or wind speed is only a few km/hr. As this air moves toward the equator, it now has a horizontal velocity which is lower than the rotational velocity of the earth's surface over which it is passing. Frictional drag tends to accelerate this air, but it is nevertheless deflected from east to west. These northeasterly winds (north -> south as a result of Hadley cell circulation, east -> west from Coriolis deflection) are called the trade winds, or NE trades, in the northern hemisphere, and accordingly, SE trades in the southern hemisphere. Near the equator frictional drag has caused surface winds to essentially catch up with surface velocity, and there is little N-S velocity; air movements are dominated by the Hadley cell vertical circulation. As a result surface winds are usually weak near the equator, and result in a zone known as the doldrums.
Ocean currents are also directed by Coriolis forces. Generally the direction of ocean currents is determined by the effects of surface winds moving surface waters, and the effects of land masses and Coriolis forces deflecting that water movement. Note that, like the winds, ocean currents in the northern and southern hemispheres are basically mirror images of each other. In the northern hemisphere currents generally have a clockwise flow, and in the southern hemisphere flows are counterclockwise.
Figure 7 Water circulation in the world's oceans: driven by the positions of the continents, the rotation of the earth, and surface air circulation.
We should not neglect the flows which accompany and return these major currents, for they too have important effects. Where Coriolis forces draw surface waters away from continental margins, that water must be replaced. The replacement water is cold, nutrient-rich upwelling. These waters are the world's great fishing grounds. Off Newfoundland, where the Gulf Stream is deflected towards Europe by its northward movement, lays the Grand Banks. Off California and northern Mexico where the Japan Current in its return flow is deflected westward across the Pacific by southward movement, and off Peru, where a southern Pacific cell has a return flow deflection, are similar, important fishing grounds. These upwellings and coastal flows also have a major impact on the rainfall patterns over nearby continental areas.
Figure 8 - Patterns of rainfall along the west coast of North America in winter, indicating effects of the relative temperature of water and land.
In the Great Lakes region, some areas experience what is often called lake-effect precipitation. The Great Lakes represent a huge thermal mass which cools much more slowly than land during winter. In fact, the lower lakes generally don't freeze over during winter, meaning their surface temperatures are equal to or greater than 0oC; they are thus clearly warmer than the land surface during much of the winter. Winds passing over the lakes are equilibrated to the water temperature, and saturated at that temperature. When this saturated air is cooled by coming over land, precipitation in the form of snow is deposited. The air has thermal mass, and does not cool instantaneously. Therefore the snow belts are typically at least 20-30 km from shoreline. This explains the snowbelts between Chatham and London and beyond (at the appropriate distance from Lake Huron and with respect to the typical northwesterly winds of winter). For Lake Erie this snow belt is south and east of the center of Buffalo, for Lake Ontario it's south of Syracuse, N.Y., for Lake Michigan (oriented N-S) it's a long band along western Michigan and Indiana, and for Superior it's the skiing areas of the northern Peninsula of Michigan.
One of the grave concerns facing humanity is our effect on climates around the world, due largely to our combustion of fossil fuels. Scientists project increase in the global average temperature of ~2ºC, but important differences across latitude and the spread of individual continents. Why do they project increased temperature? Is there historical evidence that leads to these projections? How do projections differ among latitudinal zones? We will try to answer those questions, at least in broad terms. If the projections are accurate, the effects on species diversity, the patterns of species' distributions, agricultural production, and even sea level will by vast.
1. The Historical Evidence - we know that there has been variation in the atmospheric concentration of CO2 over the last few hundred thousand years. The quantitative evidence comes from air trapped in the ice of Greenland and the Antarctic. Cooperative studies of a very long ice core (2,000 m), representing the last 160,000 years, from a Soviet station at Vostok (78º S) (Lorius et al. 1985) have shown a relationship between global climate and carbon dioxide levels. Around 150,000 years ago the CO2 level increased fairly dramatically from about 200 ppm to around 280 ppm. It then dropped more or less gradually back to about 200 ppm by around 20,000 years ago, and rose rapidly back to 280 ppm thereafter. The rapid rises are associated with deglaciation. Thus, the rises are also associated with periods of rapid temperature increase.
see Figure 9.
The Vostok ice core does not provide evidence for the last 2000 years, since the core was too cracked and fragmented over the last few hundred meters. The estimates for the last 2000 years use a variety of source information. In part, some of the data from the top of the Vostok ice core was used; palynology provides some information, and data from other ice cores was also used. The assembly of information was compiled by Post (1990). In sum, the data suggest a relatively constant atmospheric CO2 concentration from 2000 years ago until the middle of the 18th century. Since then the concentration has been increasing exponentially.
see Figure 10.
By 1953 the concentration had reached 315 ppm. The level is now 350-360 ppm. The most recent measurements have been made on Mount Mauna Loa in Hawaii, and date back far enough to show an almost perfect correspondence with other sources of data.
We are convinced that human activity has caused the recent increase in carbon dioxide in the atmosphere. Why are we so convinced? There are two sources of evidence:
1) The exponential increase over the last 150 years has three "breaks". Those breaks match with the First and Second World Wars and the Great Depression. They are the three breaks in global economic activity. That is only indirect evidence. 2) Isotopic ratios between C12 and C14 indicate fossil fuel combustion is the source of increasing CO2 concentration. The half life of the radioactive isotope is 5,730 years. Fossil fuels were formed millions of years ago. C14 in them will have decayed. If increasing carbon dioxide in the atmosphere is due to fossil fuel combustion, then the isotopic ratio should have changed over time due to the release of carbon with an enormously reduced C14 content. The reduced ratio is called the Suess effect. It is clearly evident in Figure 11.
2. How do effects vary across latitudes? The general pattern is that the temperature increase will be greatest at high latitudes, in polar environments, and the change will be much smaller in the tropics. However, the details of atmospheric and water circulation affect the results of models attempting to predict change. Even the depth of the ocean, and the depth to which surface circulation of warmer waters descends, affects results. GCM (global circulation models) can only be calculated on supercomputers. One such model, which seems to predict somewhat larger temperature change globally than most, was developed by AES Canada. In arctic extremes this model predicts 2-4ºC warming. In isolated pockets of North America and Asia greater increases of 6-8ºC are predicted. These are generally dry areas, and subject to greater increase due to the lower thermal energy capacity of dry air. The key to seeing the concerns about polar environments comes from looking at the predicted change in Antarctic regions, where changes as large as 10-14ºC may occur during the winters. The same thermal capacity arguments explain why larger changes are predicted over land than over the oceans, independent of latitude.
Finally, what are the predicted biological impacts of the changes associated with global warming, and are some of these changes already apparent? Hughes (2000) argued that changes are evident, and presented some of the patterns evident and expected from global warming.
Why are these changes important? Where local populations decline as a result of average poleward range expansion, open space is left that is likely to be invaded (at least first) by weedy species. Changes in physiology and the timing of life cycle events may lead to decoupling of important interactions. For example, many plants, whenever they initiate seasonal growth, are cued to flowering by day length. Associated insect pollinators are generally cued by accumulated thermal energy degree-days). Global warming does not affect day length, but does change the rate of energy accumulation. As a result of global warming key pollinators may emerge before their required food resources (pollen, nectar) are available, and die of starvation without pollinating the plants. Decoupling of mutualistic, competitive, and predatory interactions can clearly lead to extinctions, either at the local or global level.
1) Mammals on Mountaintops (and associated communities)
Predicted climate change should have specific implications for particular species and communities. We will consider biodiversity loss in a more general context later, but one of the more neatly worked out predictions is for loss in diversity of boreal small mammals from montane forests of the isolated mountain ranges in the Great Basin of the western U.S. (Brown 1995, 1998). Using a prediction of approximate carbon dioxide doubling from pre-industrial levels by near the end of this century, and a 3ºC warming as a result, the distribution of montane forest was predicted.
Figure 12 : the current and predicted forest community. In the diagram desert shrubland is unshaded, piñion-juniper forest is gray, and mixed coniferous boreal woodland is dark.
It is not that boreal woodland will necessarily disappear on all mountain ranges, but that it will move up the mountain by 500m, and that significantly decreases the habitat area available to the small mammals. Using species-area relationships for these species that Brown had determined earlier, he was able to predict species losses on each range.
Figure 13 shows predicted changes. The open circle is the current condition, arrows point down in parallel, at the slope of the species-area curve for these small mammals, to a solid circle that indicates the predicted number of species remaining after habitat loss.
However, Brown was able to go even further.
Figure 14 shows a table for boreal mammals, ordered from the one present on the largest number of mountain ranges to the rarest. He could then predict which species would go extinct on particular ranges, or even globally extinct in the Great Basin. E in the table indicates local extinction. The rarest three species are likely to go globally extinct, and two others will become dangerously rare, persisting on only a single range (note that there is an error in the last column of the table). Only two species are predicted to remain present on all ranges where they currently occur.
2) Range shifts in temperate, deciduous trees of the Great Lakes region
There are predictions for individual species in Brown's table, but these are 'unique' small mammals isolated on mountaintops. What changes are predicted for the distributions of species that are not so isolated? The means here are "climate spaces" for the species. Climate space is the range of climate conditions (usually single surrogate variables to represent them) tolerated by the species. For example, Zapinski and Davis (1989, described in Brown 1998) determined that a number of Great Lakes area tree species had northern limits corresponding to the -15ºC January isotherm. They also used the last post-glacial period to estimate the rate at which tree species could migrate, using an artificially high estimate of 100 km/century. For each of the four species shown the current distribution is on the left, and the predicted distribution for the end of this century on the right.The gray area above the predicted distribution indicates the long-term potential distribution given sufficient time for dispersal into new areas. That will take far longer than the next century. Note the compression of each distribution from its current southern limits.
3) Range shifts in European butterflies and Monarch Butterflies
Another specific example of evident effects of global warming on a taxon (i.e. a group of related species, rather than just a single species or genus) is Parmesan et al’s study of range effects on butterflies of Europe (Parmesan, et al. 1999). Her group studied changes in the ranges of non-migratory butterfly species over the last century. Each species had the northern limit of its range in northern Europe, and the southern limit in southern Europe or northern Africa. Limitations in available data forced them to perform three separate analyses: 1) changes in the northern limits of species for all species with sufficient data to evaluate changes in northern limits (which could, of course, be a northward or a southward movement), 2) a parallel analysis for southern limits for those species with sufficient data to evaluate that limit (and not all the same species as for northern limits), and, finally, evaluation of changes in both limits where data permitted.
Northern boundaries have moved northward in 65% of 52 species evaluated, remained stable in 34%, and moved southward in one species (2%). This is a highly significant result (P << 0.001). Southern boundaries have retracted northwards in 22% of 40 species that could be evaluated, remained stable for most (72%) and moved southward for two species (5%). This is not a significant northward movement. Changes in northern and southern boundaries could be evaluated together for 35 species. Of these, 63% shifted northwards, 29% were stable at both boundaries, 6% shifted southwards, and 3% extended range both northward and southward. This, again, is a highly significant result. The distance these species moved northward ranged from 35-240 km. This is a far larger distance than expected in a single new colonization event; in fact it is 5-50x expected dispersal distances for these species. Thus, it appears that these range changes represent slow movement by new colonizations, population growth at the site of colonization, then a further dispersal founding yet another new population.
How closely do these movements correlate with climatic changes in Europe during the last century? Climate records show that annual mean temperatures have warmed by about 0.8 degrees C during the 20th century. Temperature isotherms have, as a result, moved northward by an average of 120 km in Europe. That is also approximately the average northward movement of range limits. Does this mean that species can adapt to climate change? Yes and no. When climate changes slowly enough, many species can keep up. However, the projected change in temperature during the 21st century is far larger, estimated as 2.1 – 4.6°C. Can butterflies (or other species with limited dispersal capabilities) keep up with changes of that magnitude? Sadly, we’re likely to find out soon.
Monarch butterflies (Danaus plexippus) are one of the fascinating characteristics of southern Ontario’s biota. Monarch’s make their annual spring and autumn migrations across Lake Erie, filling trees where they stopover in Point Pelee National Park. During autumn, the butterflies fly to southern locations, including a major sites in the mountains of central Mexico. They spend the winter in the fir (Abies religiosa) trees in high-altitude montane ‘islands’ (the fir stands occupy <0.5% of Mexico’s area) for approximately 135 days (Oberhauser and Peterson 2003). The butterfly overwintering period occurs during the dry season, with very rare snow/freezing conditions. A combination of colder and/or wetter conditions can prove lethal to the overwintering butterflies (less flying, greater exposure to predators). Fully 70-80% of the two largest populations died during a cold, wet period during 2002. With knowledge of the butterfly’s sensitivity to temperature conditions and precipitation, Oberhauser and Peterson (2003) modeled the likely outcome of climate change on butterfly habitat suitability. They found that the butterfly is likely to encounter inadequate conditions across its entire Mexican range if Hadley Centre climate model forecasts are realized, mainly due to enhanced exposure to cool-weather precipitation. If so, we could lose Canadian populations of butterflies even of we were to protect their habitat and food source (milkweed) in Canada.
4) Thermal stress in Intertidal Marine Species
Most investigators of coastal marine ecosystems have assumed that climate change (warming) will adversely affect intertidal communities at southern latitudes more than at higher ones. However, a paper by Helmuth et al. (2002) suggests that the exact opposite will occur. Species like mussels that live in intertidal areas must be able to withstand aerial exposure (emersion from water) during low tides, placing these organisms in thermal stress. These investigators examined the intertidal mussel Mytilus californianus along a latitudinal gradient across the western USA from California to Washington state. Their study showed that midday exposure of mussels to high temperature will be greater at higher latitudes than at lower ones, in part because variation in tide height will be more pronounced at the higher latitudes. Thus, for the next few years, the northern sites will experience maximum midday exposure for intertidal mussels, while those in southern California will have minimum exposure. Moreover, areas that sustain higher water temperatures may also experience higher feeding rates by predators (sea stars) whose metabolic activity is positively linked to water temperature. Finally, changes in water temperature could affect species distributions through dispersal or survival of larvae broadcast into the water column (e.g. mussel veliger larvae).
5) Pending Global extinctions associated with climate change (see Required Reading by Thomas et al. (2004)(hotlink above).
In this paper, Thomas et al. (2004) cover 20% of the world’s surface areas (i.e. they have pretty comprehensive data for different areas of the globe) and project what is expected to happen to biodiversity between now and 2050 if the world warms according to reasonable projections. As we shall soon see, there is a very nice and predictable pattern to the relationship between number of species in a habitat and habitat area: larger habitats contain progressively more species. If, however, we assume that climate warming will reduce suitable habitat areas, then we can flip this equation around and predict how many species will be lost. Using 3 different permutations of this concept, Thomas et al. determined average extinction risk probabilities for 3 warming scenarios:
0.8 to 1.7° increase: 18% species loss
1.8 to 2.0° increase: 24% species loss
>2.0° increase: 35% species loss
These loss rates would occur across the planet and, considering that an estimated 5-10 million species occur on the planet, would mean the extinction of millions of species belonging to a wide assortment of plant and animal phyla. The authors note that this risk is at least as great as, if not greater than other perceived extinction threats, and, in fact, interacts strongly with these other threats (e.g. climate change and habitat destruction).
References and Readings:
Brown, J.H. 1995. Macroecology. Univ. Chicago Press, Chicago, Ill.
Brown, J.H. and M.V. Lomolino. 1998. Biogeography 2nd ed. Sinauer, Sunderland, MA. P.567-9, 601-12.
Helmuth, B., C. Harley, P. halpin, M. O’Donnell, G. Hofmann, and C. Blanchette. 2002. Climate change and latitudinal patterns of intertidal thermal stress. Science 298: 1015-1017.
Hughes, L. 2000. Biological consequences of global warming: is the signal already apparent? TREE 15:56-61.
Lorius, C., J. Jouzel, C. Ritz, L. Merlivat, N.I. Barkov, Y.S. Korotkevich and V.M. Kotlyakov. 1985. A 150,000-year climatic record from Antarctic ice. Nature 316:591-96.
MacArthur, R.H. 1972. Geographical Ecology: patterns in the Distribution of Species. Harper & Row, New York. Chapter 1 - Climates on a Rotating Earth.
Oberhauser, K. and T. Peterson. 2003. Modeling current and future potential wintering distributions of eastern North American monarch butterflies. Proc. Nat. Acad. Sci. (USA): 100: 14063-14068.
Parmesan, C. and G. Yohe. 2003. A globally coherent fingerprint of climate change impacts across natural systems. Nature 421:37-42.
Parmesan, C. et al. 1999. Poleward shifts in geographical ranges of butterfly species associated with regional warming. Nature 399:579-583.
Post, W.M., T.-H. Peng, W.R. Emanuel, A.W. King, V.H. Dale and D.L. DeAngeles. 1990. The global carbon cycle. American Scientist 78:310-26.
Root, T.L. et al. 2003. Fingerprints of global warming on wild animals and plants. Nature 421:57-60.
Smith, R. 1990. Ecology and Field Biology 4th ed. Harper & Row, NY. Ch. 4. Climate.
Thomas, C.D. et al. 2004. Extinction risk from climate change. Nature 427: 145-148.
Zabinski, C. and M.B. Davis. 1989. Hard times ahead for Great Lakes forests: A climate threshold model predicts responses to CO2-induced climate change. In J.B. Smith and D. Tirpak, eds. The Potential Effects of Global Climate Change on the United States. Appendix D, U.S. E.P.A. Washington, D.C.