{"id":666,"date":"2018-08-26T17:39:23","date_gmt":"2018-08-26T21:39:23","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/16-2-causes-of-climate-change\/"},"modified":"2023-07-04T13:11:05","modified_gmt":"2023-07-04T17:11:05","slug":"causes-of-climate-change","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/causes-of-climate-change\/","title":{"raw":"16.2 Causes of Climate Change","rendered":"16.2 Causes of Climate Change"},"content":{"raw":"

What Is Climate?<\/h1>\r\nOur day-to-day experience of the Earth system is in the form of the conditions we experience at Earth's surface. The daily conditions that we think of as weather<\/strong>\u2014the temperature, presence or absence of precipitation, winds, humidity, and so on\u2014are a snapshot of the state of the Earth system at a particular instant in time and in a particular location. The weather that we get is variable, but in Saskatchewan most people would not be surprised to experience summer days with temperatures of 20 \u00b0C to 30 \u00b0C, and winter days with temperatures between \u201320 \u00b0C to \u201330 \u00b0C. Our notion of what summers and winters are generally like reflects our understanding of Saskatchewan's climate<\/strong>. If we get a day in July with a daytime high of 10 \u00b0C, that would seem like unusually cold weather<\/em> because we know it is uncharacteristic of the climate<\/em> over all.\r\n\r\nWe characterize the climate by collecting data about the weather every day, and then calculating the average conditions over a period of decades. The Government of Canada provides averages for the periods 1961 to 1990, 1971 to 2000, and 1981 to 2010 in an online database that is searchable by geographic location or station<\/a>. Data measured at Saskatoon's Diefenbaker International Airport show that the average annual temperature from 1981 to 2010 is 0.6 \u00b0C higher than the annual average from 1961 to 1990, due warmer conditions in the winter and early spring (Figure 16.3).\r\n\r\n[caption id=\"attachment_646\" align=\"aligncenter\" width=\"720\"]\"\" Figure 16.3<\/strong> Average temperatures for the periods 1961 to 1990, and 1981 to 2010 measured at Saskatoon's Diefenbaker International Airport (YXE). Source: Karla Panchuk (2018), CC BY 4.0. View YXE data for 1961 to 1990<\/a>. View YXE data for 1981 to 2010<\/a>.[\/caption]\r\n\r\nThe climate<\/em> as represented by the 1961 to 1990 interval was slightly cooler than the climate<\/em> represented by the 1981 to 2010 interval. People who lived in Saskatoon between 1961 and 2010 may or may not have a sense that the weather<\/em> they experienced from day to day was different for those intervals. In fact, some may have the record high of 35.3 \u00b0C on September 4, 1978 seared into their memory, and feel that Septembers just aren't as hot as they used to be. They would be correct that as of 2017, there are no September temperatures recorded at the Diefenbaker International Airport weather station with a daytime high greater than 35.3 \u00b0C. But if that gave them the impression that Septembers are cooler on average<\/em> today than in the past, that would not be consistent with the data.\r\n

Climate-Forcing Mechanisms<\/h1>\r\nA climate-forcing mechanism<\/strong> is a process that causes climate to change. Climate forcings work by initiating changes in how heat energy moves into, through, and out of the Earth system. When we discuss a particular climate change event, the climate-forcing mechanism is what initiated the change. Feedbacks also alter climate, but we want to know what triggered the feedbacks in the first place.\r\n

Climate Forcing by Changes in Insolation<\/h2>\r\nInsolation<\/strong>, or in<\/span>cident sol<\/span>ar radiation<\/span><\/strong>, refers to how much of the sun\u2019s energy reaches Earth\u2019s surface in a given period of time. Insolation is measured in Watts per square meter (W\/m2<\/sup>).\r\n

Long-term Solar Evolution<\/h3>\r\nOver the long term (billions of years), stars like our sun become larger, brighter, and hotter (Figure 16.4). Earth receives 40% more heat from the sun today than it did 4.5 billion years ago. In Figure 16.4, the blue Now <\/em>arrow shows the sun\u2019s current point in its life history. Although the blue arrow appears to indicate an instant in time, the time interval reflecting the duration of human existence on Earth is but a tiny fraction of the width of the line. As far as human experience is concerned, the long-term evolution of the sun is so slow that it has made no difference at all on insolation for the entire time humans have existed.\r\n\r\n[caption id=\"attachment_647\" align=\"aligncenter\" width=\"720\"]\"\" Figure 16.4<\/strong> The life history of a star, from condensation of a nebula, to expansion to a red giant, and ending as a white dwarf. Source: Karla Panchuk (2017), CC BY 4.0. Modified after Oliver Beatson (2009), Public Domain. Image source.<\/a>[\/caption]\r\n

Orbital Cycles<\/h3>\r\nInsolation is also affected by cyclical changes in Earth\u2019s orbit and rotation. Over intervals of approximately 100,000 years, the eccentricity <\/strong>of Earth\u2019s orbit changes. Eccentricity is a measure of how elliptical a circle is. Higher eccentricity means that the orbit is more elliptical (Figure 16.5, left, blue orbit), whereas lower eccentricity means the orbit is more circular (Figure 16.5, left, red orbit). Eccentricity<\/strong> is important because when it is high, the Earth-sun distance varies more from season to season than it does when eccentricity is low.\r\n\r\n[caption id=\"attachment_648\" align=\"aligncenter\" width=\"650\"]\"\"<\/a> Figure 16.5<\/strong> Cycles in Earth\u2019s orbit. Left: The shape of Earth\u2019s orbit (its eccentricity) changes over 100,000 year cycles from more circular to more elliptical. Middle: Over 41,000 year periods, Earth\u2019s axis of rotation nods toward and away from the sun. Right: Over 21,000 year cycles, Earth wobbles on its axis of rotation. Source: Karla Panchuk (2017), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0 Image source.<\/a> Click for more attributions.[\/caption]\r\n\r\nOver intervals of approximately 41,000 years, the obliquity <\/strong>of Earth\u2019s axis of rotation changes (Figure 16.5, middle). This results in a nodding motion that alters how directly the sun shines on Earth\u2019s poles. When the angle is at its maximum (24.5\u00b0), Earth\u2019s seasonal differences are accentuated. When the angle is at its minimum (22.1\u00b0), seasonal differences are minimized.\r\n\r\nCycles of precession <\/strong>happen over intervals of approximately 20,000 years, causing Earth\u2019s axis of rotation to wobble (Figure 16.5, right). This means that although the North Pole is presently pointing to the star Polaris (the pole star), in 10,000 years it will point to the star Vega.\r\n\r\nThe importance of eccentricity, tilt, and precession to Earth\u2019s climate cycles (now known as\u00a0Milankovi\u0107 Cycles<\/strong>) was first pointed out by Yugoslavian engineer and mathematician Milutin Milankovi\u0107 in the early 1900s. Milankovi\u0107 recognized that although the variations in the orbital cycles did not affect the total<\/em> amount of insolation that Earth received, it did affect where<\/em> on Earth that energy was strongest. Glaciations are most sensitive to the insolation received at latitudes of approximately 65\u00b0. As continents are configured today, this is most significant at 65\u00b0 N, because there is almost no land at 65\u00b0 S.\r\n\r\nThe most important factors are whether the northern hemisphere is pointing toward or away from the sun at its closest or farthest approach, and how eccentric the sun\u2019s position is in Earth\u2019s orbit. For example, if the northern hemisphere is at it farthest distance from the sun during summer (Figure 16.6, top), this means cooler summers. If the northern hemisphere is at its closest distance to the sun during summer (Figure 16.6, bottom), this means hotter summers. Cool summers \u2014 as opposed to cold winters \u2014 are the key factor in the accumulation of glacial ice, so the upper scenario in Figure 16.6 is the one that promotes glaciation. This factor is greatest when eccentricity is high.\r\n\r\n[caption id=\"attachment_649\" align=\"aligncenter\" width=\"650\"]\"\" Figure 16.6<\/strong> Effect of precession on insolation in the northern hemisphere summers. In (a) the northern hemisphere summer takes place at greatest Earth-sun distance, so summers are cooler. In (b) (10,000 years or one-half precession cycle later) the opposite is the case, so summers are hotter. The red dashed line represents Earth\u2019s path around the Sun. Source: Steven Earle (2015), CC BY 4.0 Image source.<\/a>[\/caption]\r\n\r\nThe effects of all three cycles are evident in geochemical climate data. Figure 16.7 shows the \u201csignals\u201d for obliquity (A), eccentricity (B), and precession (C) over a period from 800,000 years in the past, to 800,000 years in the future. The vertical black line running down the middle of the diagram marks the present day. When the insolation from all three signals is determined, the result is a more complex waveform (D) with times of low variation in insolation, and times with higher variation in insolation.\r\n\r\n[caption id=\"attachment_650\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 16.7<\/strong> Comparison of orbital cycles, insolation, and climate data for a 1.6 million year period. Source: Karla Panchuk (2017), CC BY-SA 4.0. Modified after Incredio (2009), CC BY 3.0. Image source.<\/a> Click for more attributions.[\/caption]\r\n\r\nThe graphs E and F are climate information measured in microfossils dwelling at the ocean floor (E) and in water from ice cores (F). Peaks in temperature in F correspond to peaks in the oxygen isotope record in E, which indicate that it was warmer, there was less ice, and sea level was higher. Troughs are times when Earth was deep within an ice age. It was cooler, there was ice on land, and sea level was lower.\r\n\r\nThe vertical dashed lines on the left-hand side of Figure 16.7 mark the times of peak warm temperatures and allow for comparison of the timing of the temperature peaks with the timing of the orbital cycles. Peak temperature events are approximately 100,000 years apart, suggesting that the eccentricity cycle might be the most important contributor. Indeed, in B most (but not all) of the peak temperature events correspond to a time when Earth\u2019s orbit was at or near peak eccentricity for that cycle.\r\n\r\nIt is tempting to conclude that eccentricity is the most important orbital cycle for climate change over all. However, this pattern only began a little over 1 million years ago. For 1.5 million years before that, the 41,000-year obliquity cycle seems to dominate insolation cycles.\r\n\r\nIn general, times of warmest or coolest temperatures don\u2019t line up perfectly with orbital cycles. There is no one orbital cycle that is most important for all of Earth history. It is also the case that changes in insolation due to orbital cycles are not sufficient to cause temperatures to change as much as the geological record says they have; feedbacks must be factored in to explain the observed temperature changes.\r\n

Sunspot Cycles<\/h3>\r\nSunspots<\/strong> are dark patches that appear on the surface of the sun as a result of intense local disturbances in the sun\u2019s magnetic field (Figure 16.8, left). Loops of plasma (gas with electrical charge, Figure 16.8, right) follow along magnetic field lines from one sunspot to another.\r\n\r\n[caption id=\"attachment_651\" align=\"aligncenter\" width=\"550\"]\"\" Figure 16.8<\/strong> Sunspots. Left: Photograph of sunspots with dots representing the size of Earth and Jupiter for scale. Right: Plasma loops viewed in x-ray wavelengths jumping from one sunspot to another on the sun\u2019s surface. Source: Left- NASA\/Solar Dynamics Observatory (2012), Public Domain. Image source.<\/a>. Right- NASA\/Solar Dynamics Observatory (2015), Public Domain. Image source.<\/a>.[\/caption]\r\n\r\nSunspots appear dark because they are lower-temperature regions on the sun\u2019s surface. For that reason you might think that more sunspots means a reduction in insolation. In fact, just the opposite is true, because sunspots are a side-effect of increased solar activity. Peaks in the number of sunspots counted annually since approximately 1870 (Figure 16.9, blue), coincide with peaks in measurements of solar energy output from the same time period (Figure 16.9, pink).\r\n\r\n[caption id=\"attachment_652\" align=\"aligncenter\" width=\"650\"]\"\" Figure 16.9<\/strong> Sunspot cycles. Peaks in the number of sunspots (blue) occur approximately every 11 years, and these correspond to peaks in solar energy output (pink). The influence of sunspot cycles is too small to have a clear impact on global average temperatures (grey). Source: Karla Panchuk (2017) CC BY-SA 4.0. Sunspot records modified after D. Bice (n.d.) CC BY-SA 3.0 view source<\/a>. Global average temperature modified after Met Office (2015)\u00a0Contains public sector information licensed under the Open Government Licence v1.0.<\/a> view source<\/a>[\/caption]\r\n\r\nSunspot cycles happen over approximately 11 year intervals, and the changes in insolation that occur during these cycles are relatively small. In the end the effect of sunspot cycles on climate can be lost amidst other factors. In Figure 16.9 there is no clear relationship between the sunspot cycles and the global average temperatures (in grey) reported for the same period.\r\n
\r\n\r\nBe Aware of Graph Scales<\/strong>\r\n\r\nFigure 16.9 shows three kinds of data: temperatures, sunspot numbers, and solar energy flow. Each of these data sets is a different type of information, so each needs its own vertical axis. The vertical axes are scaled so that the data fill the area of the graph as much as possible. Stretching the vertical scale to fit the full plotting area makes it easier to see how well the peaks and troughs in each record line up with each other. Unfortunately, this can also skew our impression of the data. For example, in the period from 1880 to 1920, all three records have a similar vertical distance from peak to trough. In other words, all three records have approximately the same size of wiggles. This does not mean that the change in insolation from sunspot cycles was big enough to cause all of the variation in the temperature record. From this graph alone, there's no way to tell how much the change in insolation due to sunspot cycles mattered to global temperatures during the period 1880 to 1920.\r\n\r\n<\/div>\r\n
\r\n\r\n<\/a>Concept Check: Climate Forcing by Insolation<\/strong>\r\n\r\n
\r\n\r\nFill in the missing words to complete this summary.<\/strong>\r\n\r\nInsolation, or \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> (hint:<\/strong> three words, beginning in in<\/strong>, sol<\/strong>, and ending in ation<\/strong>), refers to how much of the sun\u2019s energy reaches Earth\u2019s surface in a given period of time.\r\n\r\nOver the long term, the sun has \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> (hint:<\/strong> increased or decreased?) brightness, but this process is so slow as to be insignificant over timescales of less than a \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> (hint:<\/strong> thousand, million, or billion?)years.\r\n\r\nOrbital cycles include changes in \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> (hint:<\/strong> precession, eccentricity, or obliquity?) (how elliptical Earth's orbit is), \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> (hint:<\/strong> precession, eccentricity, or obliquity?) (how Earth nods back and forth on its axis), and \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> (hint:<\/strong> precession, eccentricity, or obliquity?) (how Earth's axis wobbles in circles like a top). They are important because they control how directly the sun's energy hits Earth's high latitudes.\r\n\r\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> are dark marks on the sun's surface caused by disturbances in the sun's magnetic field, and are more numerous when solar activity is more intense. These affect climate on \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span>(hint:<\/strong> decade, century, or millenium?)-long cycles.\r\n\r\nTo check your answers, navigate to the below link to view the interactive version of this activity.<\/strong>\r\n\r\n<\/div>\r\n[h5p id=\"170\"]\r\n<\/div>\r\n

Climate Forcing by Changes in Heat Transport<\/h2>\r\nThe ocean transports large amounts of heat around the Earth through a conveyor-belt-like system of currents. The ocean has surface currents that are driven by wind, but it also has deeper currents that are not wind-driven. The deeper currents behave like stacked rivers because they are different temperatures and have different salt contents, and therefore different densities. The differences in density between these water masses are what drive circulation. Circulation that is driven by density is called thermohaline circulation<\/strong>; thermo<\/em> refers to heat and haline<\/em> refers to salt.\r\n\r\nTo see how this works, consider the warm and saline Gulf Stream current (Figure 16.10, top). It flows northward past Britain and Iceland into the Norwegian Sea, and cools as it moves north, becoming denser. Its high salinity contributes to its density, and it sinks, or downwells<\/strong>, deep beneath the surrounding water, forming the North Atlantic Deep Water<\/strong> (NADW) current that flows south. Meanwhile, at the southern extreme of the Atlantic, very cold water adjacent to Antarctica also sinks to the bottom to become the Antarctic Bottom Water<\/strong> (AABW) current. The AABW flows north, beneath the NADW.\r\n\r\n[caption id=\"attachment_653\" align=\"aligncenter\" width=\"550\"]\"\" Figure 16.10<\/strong> A simplified north-south cross-section through the Atlantic Ocean basin showing the different current layers. Source: Steven Earle (2015), CC BY 4.0. Image source.<\/a>[\/caption]\r\n\r\nThe water that sinks in the areas of deep water formation in the Norwegian Sea and adjacent to Antarctica moves very slowly at depth. It eventually resurfaces, or upwells<\/strong>, in the Indian Ocean between Africa and India, and in the Pacific Ocean, north of the equator (Figure 16.11).\r\n\r\n[caption id=\"attachment_654\" align=\"aligncenter\" width=\"550\"]\"\" Figure 16.11<\/strong> Global thermohaline circulation patterns. Red lines are surface currents, and blue lines are deep currents. Source: NASA Earth Observatory (2008), Public Domain. Image source.<\/a>[\/caption]\r\n\r\nSome ocean currents move warm water from the equator toward the poles. As in the example of the Drake Passage, the path of warm currents can have a significant impact on the climate of a region, and potentially of the planet as a whole.\u00a0Processes that disrupt the density of seawater can slow or stop currents, preventing warm water from reaching higher latitudes. The recovery from the last ice age is characterized by sudden returns to glacial conditions over as little as 3 years. This is thought to be the result of enormous glacial lakes forming on continents as the glaciers melted, then being suddenly released into the ocean by a burst ice dam. The glacial water would be very cold, but it would also be fresh, making it less dense than the ocean water. The fresh glacial water would form a cap and slow the downwelling conveyor belt at high latitudes.\r\n\r\nScientists are trying to determine the current and past state of the Atlantic-basin system of circulation, called the Atlantic Meridional Overturning Circulation<\/strong> (AMOC<\/strong>), to tell whether it is changing in response to warming and adding fresh water from melting ice sheets. The AMOC varies considerably on decadal cycles because of cycles in the wind patterns in the Atlantic, so it is important to distinguish these cyclical changes from any longer-term underlying changes.\r\n\r\nBecause of these studies, we have an idea of what the physical properties of the Atlantic Ocean look like when circulation is stronger or weaker. When circulation slows, the density is lower in the downwelling regions (Figure 16.12, blue patch in the Labrador Sea). The density is higher south of this region, along the eastern coast of the United States and southern Canada (Figure 16.12, orange patch). Model simulations are used to confirm that the changes in density we observe are consistent with how we understand the circulation system to work.\r\n\r\nMeasurements of the actual flow rate at depth in the Atlantic Ocean (Figure 16.12, right, blue dots) confirm that density decreases in downwelling regions (black and red lines) when circulation slows. A more recent study (Caesar et al., 2018) has shown that observed and model temperatures follow the same pattern, with cooling where the blue patches are in Figure 16.12, and warming in the region of the orange patches. This is to be expected because the slowdown in circulation affects how heat is moved northward.\r\n\r\n[caption id=\"attachment_655\" align=\"aligncenter\" width=\"650\"]\"\" Figure 16.12<\/strong> Using changing density to track circulation in the Atlantic Ocean. Left- Density calculated from measurements of temperature and salinity in a layer between 1,000 m and 2,500 m depth in the Labrador Sea (black box). Middle- Model results used to see what change in density can be expected. The model shows the same general relationship, with lower density in the Labrador Sea, and higher density to the south. Note that density units are in kg\/m2<\/sup> rather than kg\/m3<\/sup> because they are integrated over the layer. Right- Changing density in the Labrador Sea over time. Red and black lines show changes in density from two different data sets. In general, the peaks and troughs of these data sets match up. Blue dots are measurements of the rate of circulation from a project that began collecting data in 2004. See the References section for more information about the relevant studies. Source: Karla Panchuk (2018), CC BY-SA 4.0. Modified after Jon Robson (2013), CC BY-SA 4.0. Image source.<\/a>[\/caption]\r\n\r\n
\r\n\r\nIs Atlantic Circulation Slowing Down More than Usual?<\/strong>\r\n\r\nChanges in the Atlantic meridional overturning circulation (AMOC) happen from decade to decade. To know whether circulation is changing compared to what is normal, it is necessary to get information about what circulation looked like in the past. This is difficult to do, because measurements of circulation rates, temperature, and salinity don't go back as far as we need them to.\r\n\r\nIn a new study, Thornalley et al. (2018) have used geochemical analyses of microfossils to build a longer-term record of temperatures, then used that record to look for the temperature \"fingerprint\" of slowing AMOC, an increasing difference in the temperatures of surface waters compared to deeper waters in the downwelling zone (Figure 16.13, top). They observe a longer-term cooling trend beginning at the close of the Little Ice Age, suggesting that less heat is being moved toward the downwelling zone (labelled A on the globe in Figure 16.13).\r\n\r\n[caption id=\"attachment_656\" align=\"aligncenter\" width=\"1024\"]\"\" Figure 16.13<\/strong> Long-term record of Atlantic Meridional Overturning Circulation (AMOC). Top- Temperature fingerprint of circulation determined using geochemical analyses of marine microfossil shells. Results show the difference between temperatures measured at depth at A on the globe, and temperatures measured near the surface at B. Cooling at A relative to B is indicative of weakening AMOC. Bottom- Changes in silt grain-size used to show changes in the velocity of currents at depth at C on the globe. A smaller average grain size means a slower current, and weaker AMOC. Source: Karla Panchuk (2018) CC BY 4.0. Modified after Thornalley et al. (2018). Locator globe modified after Reisio (n.d.), Public Domain. Image source.<\/a>[\/caption]\r\n\r\n \r\n\r\nThey also measured the size of silt grains on the sea floor, above which a southward-moving component of the AMOC system flows (location labelled C on the globe in Figure 16.13). Grain size is used as a substitute for a direct measure of ocean current velocity because the velocity determines what grain size can be carried. They identified a lower average grain size over the past ~150 years compared to the average grain size from earlier (Figure 16.13, bottom, dashed lines). The authors of the study note that the average grain size changes more during cold events in the Northern Hemisphere (the Dark Ages Cold Period and the Little Ice Age).\r\n\r\nThornalley et al (2018) conclude that the AMOC has been weaker on average during the past ~150 years than during the previous ~1,500 years. However, they cannot say for sure how much of that change is from melting that occurred at the close of the Little Ice Age, from melting triggered by warming since the Industrial Revolution, or some combination of the two. Direct measurements of density and current velocity tell us that as of 2017, the AMOC continues to weaken.\r\n\r\n<\/div>\r\n

Plate Tectonics and Heat Transport<\/h3>\r\nThe opening of the Drake Passage is one example of how plate tectonic changes can affect ocean heat transport, and therefore climate. Plate tectonic changes that build or break up continents also play a role. When continents become large, ocean currents warm their margins, but the interiors can be much cooler. Anyone living on the Canadian prairies who has shivered through -40 \u00b0C temperatures in the winter, while watching news reports of rain in Vancouver will be familiar with this effect. When the supercontinent Gondwana was over the south pole approximately 300 million years ago (Figure 16.14), this triggered an ice age. The build-up of ice was hastened by the ice albedo feedback effect.\r\n\r\n[caption id=\"attachment_657\" align=\"aligncenter\" width=\"550\"]\"\"<\/a> Figure 16.14<\/strong> Glaciation on the supercontinent Gondwana. Paleogeographic reconstruction for 306 million years ago. Source: C. R. Scotese, PALEOMAP Project (www.scotese.com). Image source.<\/a> Click for terms of use.[\/caption]\r\n

Short-Term Cycles in Heat Transport: El Ni\u00f1o Southern Oscillation<\/h3>\r\nThe El Ni\u00f1o Southern Oscillation<\/strong> (ENSO<\/strong>) operates on a much shorter timescale than climate forcings driven by plate tectonics or orbital cycles, alternating between\u00a0El Ni\u00f1o<\/strong> and La Ni\u00f1a<\/strong> events on timescales of between two and seven years (Figure 16.15).\r\n\r\n[caption id=\"attachment_658\" align=\"aligncenter\" width=\"550\"]\"\" Figure 16.15<\/strong> Variations in the ENSO index from 1950 to 2015. Source: Steven Earle (2015), CC BY 4.0. Image source.<\/a>. Modified after Klaus Wolter\/ NOAA (n.d.), Public Domain. Image source.<\/a>[\/caption]\r\n\r\nUnder normal conditions, strong winds blowing westward across the Pacific cause water to pile up in the western Pacific. This forces deeper colder water to the surface in the eastern Pacific (Figure 16.16, left). During La Ni\u00f1a events, further intensification of winds causes even more cold water to upwell. During an El Ni\u00f1o event, the winds weaken, allowing water to flow back to the east (Figure 16.16, right). The cold water settles deeper once again, meaning that warmer water is present along the eastern margin of the Pacific Ocean.\r\n\r\n[caption id=\"attachment_659\" align=\"aligncenter\" width=\"650\"]\"\" Figure 16.16<\/strong> El Ni\u00f1o Southern Oscillation (ENSO) cycles are driven by changes in wind patterns that affect the distribution of warm and cold water in the Pacific Ocean. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Fred the Oyster and NOAA\/PMEL\/TAO Project Office. View Normal Conditions<\/a>\/ El Ni\u00f1o<\/a>[\/caption]\r\n\r\nENSO events affect weather on a global scale (Figures 16.17 and 16.18). In western Canada, El Ni\u00f1o years have warmer than average winters, whereas La Ni\u00f1a years have cooler than average winters.\r\n\r\n[caption id=\"attachment_660\" align=\"aligncenter\" width=\"550\"]\"\" Figure 16.17<\/strong> El Ni\u00f1o climate impacts. Source: NOAA Climate.gov (n.d.), Public Domain. Image source.<\/a>[\/caption]\r\n\r\n[caption id=\"attachment_661\" align=\"aligncenter\" width=\"550\"]\"\" Figure 16.18<\/strong> La Ni\u00f1a climate impacts. Source: NOAA Climate.gov (n.d.), Public Domain. Image source.<\/a>[\/caption]\r\n\r\n
\r\n\r\n<\/a>Concept Check: Climate Forcing by Changes in Heat Transport<\/strong>\r\n\r\n
\r\n\r\nFill in the missing words to complete the summary.<\/strong>\r\n\r\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> circulation is ocean circulation driven by differences in the \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> of seawater. Ocean currents transport heat from the \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span>. Changing the rate or path of currents affects changes how much heat reaches the \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span>.\r\n\r\nWarming and cooling cycles due to El Ni\u00f1o Southern Oscillation events are related to \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> upwelling in the eastern \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span>.\r\n\r\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> changes can both alter both the paths of currents, and make it harder to warm the \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/span> of continents.\r\n\r\nFill-in-the-blank:\r\n