The content of today’s lab will make more sense if we can keep in mind some ideas from previous labs. One key idea is that one aspect of climate is temperature, and the global surface temperature is strongly influenced by three factors …
1. The amount of solar radiation reaching the Earth’s surface.
2. … the amount of solar radiation reflected from the Earth’s surface, which is tied to the albedo effect.
3. … and the amount of terrestrial radiation (i.e. radiation emitted by Earth) that gets trapped in the atmosphere, by greenhouse gases for instance.
While all of the labs have discussed these factors to a greater or lesser extent, it is worth revisiting key points related to these factors from two of the recent labs. In the Glacial-Interglacial Cycles lab, we looked at how decreases in solar radiation at the upper latitudes of the Northern Hemisphere (due to changes in the eccentricity, tilt, and precession of the Earth) can cause ice sheets to extend. This results in a positive feedback mechanism in which the ice sheets reflect more sunlight (the albedo effect), which cools the planet, which causes the oceans to absorb more carbon dioxide, which further cools the planet – the net effect of which is to push our planet into a glacial period. In the Temperature Changes over the Past Millennium lab, we re-examined that set of three factors again, and discovered that: (1) the 20th Century was the warmest century on record; (2) within that century, temperatures increased from 1920-1940 and from 1980-2000; but (3) temperatures did not increase from the mid-1940s to the mid-1970s. The lack of warming until the late 1970s was due to an increase in concentrations of sulfate aerosols (i.e., an increase in albedo). This lab focuses on changes in temperature since the late 1970s, which also happens to be the “satellite era” of temperature measurements.
By the end of this lab, you should be able to answer the following research questions:
How do climates and the controls of climate variability vary between the tropics and the middle and high latitudes?
How have Earth’s surface and tropospheric temperatures changed over the past several decades and what region of Earth has experienced the greatest change?
What is the most likely cause of the changes in global temperature over the past several decades and what evidence is there that this is the cause?
Entering with the right mindset
Throughout this lab you will be asked to answer some questions. Those questions will come in three different varieties:
Fact based question →This will be a question with a rather clear-cut answer. That answer will be based on information (1) presented by your instructor, (2) found in background sections, or (3) determined by you from data, graphs, pictures, etc. There is more of an expectation of you providing a certain answer for a question of this type as compared to questions of the other types.
Synthesis based question → This will be a question that will require you to pull together ideas from different places in order to give a complete answer. There is still an expectation that your answer will match up to a certain response, but you should feel comfortable in expressing your understanding of how these different ideas fit together.
Hypothesis based question → This will be a question which will require you to stretch your mind little bit. A question like this will ask you to speculate about why something is the way it is, for instance. There is not one certain answer to a question of this type. This is a more open- ended question where we will be more interested in the ideas that you propose and the justification (‘I think this because . . .’) that you provide.
It is essential at this juncture in the climate-literacy labs to clarify the differences between weather and climate. Weather is the short-term (e.g., daily) condition of the atmosphere, and hopefully you have noticed that very few labs have had any mention of weather. If you watch the local news in the United States, then you are well aware of weather! The National Weather Service issues weather maps each day. The climate-literacy labs have focused on climate, which is the average atmospheric conditions of a location or region over at least several decades. The two most common atmospheric variables that are shown in the context of climate are temperature and precipitation. One way you can visualize the different climates on Earth is to look at a map of climate types. The Köppen climate classification is based on the concept that native vegetation is the best expression of climate; thus, climate-type boundaries have been selected with vegetation distribution in mind. It combines mean annual and monthly temperatures and precipitation along with the seasonality of precipitation. As noted earlier, decades of temperature and precipitation data are needed to determine the climate — and thus climate type — of a location. Click on the Earth image below to view the Köppen climate classification in Google Earth. The image to the right of the Google Earth image is the climate-types legend; it is recommended that you open this on another monitor so you can quickly see to what climate type the colors in the Google Earth image correspond.
Q1. What is the climate type of your location and what city in another country has the same climate type as your location?
Click Climographs to open a file in Google Earth that has climographs for 20 cities in the Western Hemisphere extending from 3° S to 71° N. It may take a few minutes for all the climographs to load in Google Earth, so please be patient. A climograph is a chart showing both the average monthly temperature and precipitation of a place. Examine the climographs for the 20 cities and notice that the climographs change when you move both latitudinally (e.g., south to north) and longitudinally (e.g., east to west). For example, Atlanta and Los Angeles are located at approximately 34° N, but Atlanta receives much more precipitation, especially during summer, and has a much larger difference in temperature between summer and winter, than does Los Angeles. Click on the Earth image below to see the terrestrial biomes for each of the locations and notice how that the biomes are generally related to the climate types.
The biomes are distinguished mainly by the predominant vegetation, which is determined by the climate (i.e., average temperature and precipitation) of the location. Some things you may notice when looking at the climate types or biomes or both are as follows:
- The tropical (A) climates receive the most rainfall during the months when the Sun elevation is highest (i.e., the Sun is most directly overhead).
- The arid (B) climates, which have annual potential evapotranspiration exceeding annual precipitation (i.e., there is a deficit of water), have the deserts and xeric shrublands biome.
- Atlanta and Dallas both have the Cfa (humid subtropical) climate type, but Atlanta is in the temperate broadleaf and mixed forest biome and Dallas, which is drier and hotter in the summer, is on the eastern edge of the temperature grasslands, savannas, and shrublands biome.
- There is both a climate type and biome named tundra, but almost always equatorward of tundra are the Dfc (humid continental with cool summers) climate type and the boreal forest biome.
Q2: How does the temperature range (i.e., difference in temperature between hottest month and coldest month) generally change when moving from the Equator to the Arctic Circle and what is the primary cause of this change?
Q3: During what season do Ciudad Bolivar (Aw), Belize City (Am), Mexico City (Cwb), Winnipeg (Dfb), Fairbanks (Dfc), and Barrow (ET) receive a large proportion of their precipitation?
Q4: During what season do Los Angeles (Csa), San Francisco (Csb), and Vancouver (Cfb) receive a large proportion of their precipitation?
The circulation of Earth’s atmosphere is the major control of both temperature and precipitation for the climate types, and the latitude — and thus the quantity of incoming solar radiation — is a major control of temperature. Two important precipitation-producing features on Earth are the intertropical convergence zone (ITCZ) and mid-latitude wave cyclones (i.e., extratropical cyclones). The ITCZ, which is huge and generally doesn’t go away, stays near the equator and is responsible for the precipitation in the A climate types. Mid-latitude wave cyclones, which only appear for up to a week or so, are outside the tropics (i.e., extratropical) and travel west to east across the globe. All climate types except for tropical climates are affected by mid-latitude wave cyclones.
To see the ITCZ and mid-latitude wave cyclones in action, watch the animation below created with the Community Climate System Model (CCSM) and the National Center for Atmospheric Research (NCAR). It has an hourly time step during a typical year. Cloud cover is shown in white and areas of precipitation are shown in orange. The month and hour are shown in the upper right of the animation. Focus on June-August and December-February.
The NASA rainfall animation below also shows the seasonal movements of the ITCZ and atmospheric features to the north and south of the ITCZ known as subtropical high pressure cells. Hot desert and steppe climates usually exist under these cells, and the air originating from these cells — which eventually becomes moist after traveling over warm oceans — converges at the ITCZ.
Q5: What feature brings a lot of rainfall to the Caribbean in the summer (June-August)?
Q6: What features were more likely to traverse across the Canada in the summer (June-August) — and produce rainfall — as opposed to traversing across Canada in the winter (December-February)?
Other atmospheric features that are play an important part of the climate at locations mostly on the western side of ocean basins are tropical cyclones (e.g., hurricanes). The graphics below shows how these systems form in tropical waters and move westward across ocean basins (e.g., the North Atlantic Ocean) and around subtropical high presssure cells.
Q7: Between Cuidad Bolivar and Belize City, which location has its precipitation totals more influenced by tropical systems and how do you know this?
Q8: Which climate types would you expect to impacted minimally by tropical systems?
It is important to understand a distinction that only involves climate (and not weather) : the difference between climate variability and climate change. Climate variability is the year-to-year swings in a climate variable, such as temperature. Therefore, the term interannual variability is often used in place of climate variability. We have already looked at how some volcanic eruptions (e.g. Mount Pinatubo in 1991) are explosive enough to inject SO2 and particulates into the lower stratosphere. Since the materials can stay in the stratosphere for years – reflecting sunlight away from the Earth – these eruptions can lead to anomalously low surface temperatures one to two years after they have taken place. The picture below shows data for optical density from the year after Pinatubo erupted (1992) until two years later. The dark blue following the eruption showed just how much light the materials ejected from Pinatubo was blocking.
Another source of climate variability are the El Niño and La Niña events that we hear about in the news some times. An El Niño is characterized by warming of the east-central tropical Pacific Ocean (through a mechanism partly illustrated by the picture on the left below), which can cause anomalously high surface temperatures at the global scale – such as happened in 1997 – 1998 (depicted in the satellite image on the left below), while a La Niña is characterized by cooling of that same region (through a mechanism partly illustrated on the right below), which can cause anomalously low surface temperatures at the global scale — such as happened in 2010.
Many other factors besides volcanic eruptions and ENSO affect climate variability; however, those are two mechanismns that can have global effects on climate. Regional climate variability can be affected by such things as changes in location of the ITCZ, mid-latitude storm tracks, and occurrences of tropical storms (e.g., hurricanes), and some of these changes have been connected to ENSO. Click on the image below to view a time series of summer rainfall for Atlanta, which you might remember has a humid subtropical climate. The year-to-year fluctuations in rainfall represent climate variability.
The total summer rainfall in 1993 was just 192 mm and then rainfall during the following summer was 560 mm … that is nearly three time as much rainfall from one summer to the next summer. The high rainfall totals in 1994 were due partially to the passing of two tropical storms, Alberto and Beryl, over the region; those two storms contributed more than 140 mm to the summer rainfall total. The major control of the interannual rainfall variability of rainfall in the Atlanta region is the Bermuda High, the subtropical high-pressure cell in North Atlantic Ocean.
Climate change, as defined by the Intergovernmental Panel on Climate Change (IPPC), is a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period – typically decades. The key parts of that definition are that it is a statistical variation over an extended period. Stock markets go up and down, but show trends over time; over different periods of time, those trends change (from the upward trend of a bull market to the downward trend of a bear market). Likewise, weather changes over short periods of time, but demonstrates patterns called climate; and climates will shift over longer priods of time resulting in climate change. Usually, there must be a consistent shift over three or more decades in a variable such as temperature to label what is happening as a climate change. We already have witnessed a dramatic example of climate change that occurred over thousands of years when you saw a major increase in temperature of approximately 8 °C from the Last Glacial maximum 21,000 years ago to the beginning of the present interglacial period 10,000 years ago.
Click on the image below view in Google™ Earth the extent of ice and other types of land cover during the Last Glacial Maximum. Also click Climographs so you can view the 20 locations on the globe.
Q9: Based on the climographs for the near-present situation, what do you think was the climate type and biome for the location of Atlanta 21,000 years ago?
Q10: Why would you assume that Atlanta would have received much less precipitation 21,000 years ago compared to today?
In this section, we are going to explore the climate change that has occurred from 1979 to the present by looking at temperature data vertically through space. We will begin by examining changes in the global surface temperature. Click SurfaceTemperature to open the file in Microsoft® Excel. The data, which are annual averages of temperatures measured at stations across the globe, were obtained from NASA’s Goddard Institute of Space Studies. Since our focus is on relating the surface temperature data to a possible change in climate, you are first going to convert that data into a graph using the following steps:
- Select cells in rows 1 through 39 of columns A and C.
- Under the Insert tab, select Line
- Under the 2-D line options, click on first choice.
The resulting graph shows the pattern in the yearly values of the global surface temperature. Feel free to make the graph as large as you desire.
With climate change, we are interested in trends, so you now want to add a trend line. This will let you better see if there is a trend in temperature across these decades and to determine what years ‘bucked’ the overall trend (had anomalously high or low temperatures).
- Right-click the blue temperature line and select Add Trendline….
- A linear trend line has been added to your graph. Close the window.
Q11: What was the general trend in the surface temperature from 1979 to the present?
Q12: What do you think caused the Earth’s temperature to trend in that direction?
There are three years (1985, 1992, and 1993) with extremely low temperatures and one year (1998) with extremely high temperatures based on the distance between the blue temperature line and the trendline. As you answer the questions below about those anomalies, consider the material discussed so far in the lab … and don’t be afraid to explore other sources (e.g. the internet).
Q13: What caused the surface temperature to be relatively cool in 1992 and 1993?
Q14: What caused the surface to be relatively warm in 1998?
Beginning in late 1978, satellite-borne instruments known as Microwave Sounding Units (MSUs) have been making measurements of the temperature of the troposphere and lower stratosphere. The picture below is of the NOAA-18 satellite, which has an Advanced Microwave Sounding Unit (AMSU) on it. This means that scientists have measurements made from a different place (space) and through a different process (microwave detection) than those made on the surface. This allows them to determine if the same changes seen in one place can be observed elsewhere. The AMSUs are used to estimate temperatures for various levels of the atmosphere, and three common levels are the lower troposphere, the middle troposphere, and the lower stratosphere. Data for the lower troposphere are weighted the most at approximately 2.5 km above sea level (a.s.l.), with the measurement layer extending downwards to the surface and upwards into the tropopause. Data for the middle troposphere are weighted the most at approximately 4 km a.s.l., with the measurement layer extending downwards to the surface and upwards into the middle stratosphere. Data for the lower stratosphere are weighted the most at approximately 24 km a.sl., with the measurement layer extending downwards to the tropopause and upwards to the middle stratosphere.
The image below shows that the same trend in temperature at the Earth’s surface exists for the entire lower troposphere (based on satellite data). As noted earlier, the data for the lower troposphere are weighted the most at approximately 2.5 km above sea level (a.s.l.). The lower-troposphere data also appear to more sensitive to ENSO events. The rate of warming of the surface and lower troposphere are approximately 0.4 to 0.5 °C per decade. This is three times the rate of warming that occurred from the Last Glacial Maximum to the start of the Holocene (i.e., an 8 °C warming over approximately 6,000 years).
We are going to consider whether the same trend you saw in the temperature at the Earth’s surface can be seen in the middle troposphere. Click Surface&MiddleTroposphere to open the file in Microsoft® Excel. In the first column of the spreadsheet the year is given. The values in the second column are annual averages of surface temperatures, which you examined earlier. Finally, the values in the third column are the differences between the average temperatures in the middle troposphere during the period 1979 – 1998 and the temperature values for each year.
You are going to convert that data into a graph using the following steps:
- Select cells in rows 1 through 38 of columns A, B, and C.
- Under the Insert tab, select Line
- Under the 2-D line options, click on first choice.
The resulting graph shows yearly values of surface temperature (upper in blue) and the yearly temperature differences for the middle troposphere (lower in red).
- In order to better see the red line (middle troposphere), right-click on that line, select Format Data Series, and then change the axis to the Secondary Axis.
- Right-click the red line and select Add Trendline….
- A linear trend line for the middle troposphere has been added to your graph. Close the window.
Q15: How does the trend in the temperatures in the middle troposphere compare to the trend in the surface temperatures?
Q16: Why was 1998 the hottest year in the middle troposphere?
In addition to the temperature data they have provided for the middle troposphere (seen in the previous graph) Microwaves Sounding Units (MSUs) have made available such data for the lower stratosphere since late 1978. Again, we are going to take advantage of the availability of data from different places to make a comparison: This time it will be a comparison between the Earth’s surface and lower stratosphere. Click Surface&LowerStratosphere to open the file in Microsoft® Excel. The values are annual averages of surface temperatures, which you examined earlier, and the differences between the average temperatures in the lower stratosphere during the period 1979 – 1998 and the temperature values for each year. Create a dual-axis graph like you did above, with the values lower-stratosphere temperatures on the secondary y axis.
Based on the distance from the red temperature line to the trendline, there are four years (1982, 1983, 1991, and 1992) with relatively high temperatures in the lower stratosphere.
Q17: What caused the lower stratosphere to be relatively warm in 1982/1983 and 1991/1992, while the surface was relatively cool those years?
Q18: How does the trend in the temperatures in the lower stratosphere compare to the trend in the surface temperatures (and the trend in the middle troposphere)?
To better visualize changes in lower-stratosphere temperatures, examine below the maps of trends in lower-stratosphere temperatures. The top map was produced from satellite data analyzed by researchers at Remote Sensing Systems (RSS) and the bottom map was produced from satellite data analyzed by researchers at the University of Alabama – Huntsville (UAH).
Q19: What do you think caused the cooling trend in the lower stratosphere? Hint: Consider what you learned in previous labs about the stratosphere and its composition.
The objective in Part 3 was to look at trends in global temperature over the last several decades vertically through space – starting at the Earth’s surface, and then comparing that to both the middle troposphere and the lower stratosphere. Looking at data from multiple sources, we found the following: there is (1) global warming at the Earth’s surface, in the lower troposphere, and in the middle troposphere; and (2) cooling in the lower stratosphere. Focusing on (1) (global warming at the surface and in the troposphere), it is important to next try to determine the cause for this effect. This leads us to ask the question, “To what can we attribute the warming?” Therefore, we will totally ignore climate variability in this section and focus only on what led to the warming.
Researchers have estimated radiative forcings for atmospheric drivers from 1980 to 2011 just like data you examined in Lab 7 (Temperature Changes over the Past Millennium). As you may recall, a radiative forcing is the change in energy flux from a beginning year (e.g., 1980) to an ending year (e.g., 2011) caused by changes in an atmospheric driver. The unit of a radiative forcing is the familiar W m-2. The radiative forcings for 1980 to 2011 are shown in the image below. You should ignore the forcings for aerosol-radiation interactions and aerosol-cloud interactions, since the uncertainties (i.e., the “whiskers” in the plot) for those forcings are much larger than the actual forcings.
Q20: Which forcing agent had the largest impact on the energy balance, and was thus the largest contributor to global warming, over recent decades?
Q21: How did solar radiation change over the period and what impact did it have on global warming?
Click CO2&TSI to open the file in Microsoft® Excel. In the first column of the spreadsheet the year is given. The values in the second column are annual averages of ambient CO2 concentrations measured at Mauna Loa. The values in the third column are anomalies of total solar irradiance during the period 1979 – 2013.
You are going to convert that data into a graph using the following steps:
- Select cells in rows 1 through 36 of columns A, B, and C.
- Under the Insert tab, select Line
- Under the 2-D line options, click on first choice.
The resulting graph shows yearly values of CO2 concentrations (upper in blue) and the yearly TSI difference from the average TSI (lower in red).
- In order to better see the red line (TSI), right-click on that line, select Format Data Series, and then change the axis to the Secondary Axis. In addition, add trend lines to both lines.
Q22: How much have CO2 concentrations increased (in ppm) since 1979 and how much has TSI decreased (in W m-2) since 1979?
Q23: How do these changes in CO2 and TSI compare to the radiative forcings you saw in the radiative-forcing figure, which is shown again below?
And to take you back to the Carbon Cycle lab, the reason why CO2 concentrations are increasing is due to the following: (1) the annual gain of four petagrams from anthropogenic carbon by the atmosphere (see figure below); and (2) a typical atmospheric lifetime of 100 years for a CO2 molecule.
Q25: Besides the atmosphere, where else is anthropogenic carbon (i.e., CO2) being taken up and absorbed? You learned this in Lab 4 (The Carbon Cycle) and you can see the information needed to answer the question is in the figure above.
The relatively large anthropogenic radiative forcing, due almost entirely to increasing concentrations of greenhouse gases, over the past several decades you just explored has resulted in excessive amounts energy in the climate system. This increased energy has resulted in an energy imbalance for Earth as can be seen in image at the far left below. The energy imbalance shown in the figure is 0.6 W m-2, which means that less energy exists than enters the top of the atmosphere. The excess energy has been accumulating in various components of Earth’s system, including the upper ocean, the deep ocean, the melting of ice, the warming of land, and the warming of the atmosphere; see the middle figure below. The Earth has gained an enormous amount of energy from 1971 to 2010: the estimated energy increase is approximately 274 ZJ (zettajoules or 121 Joules). Only about 1% of that excess energy has gone into warming of the atmosphere (i.e., global warming). The ocean, on the other hand, has taken up about 93% of the excess energy, and it has accumulated such a large percentage of the energy due to the following reasons: (1) it has a lot of mass; and (2) it has a much higher specific heat than the land and the atmosphere (i.e., a lot of heat can be added to the ocean without it increasing in temperature as much as the land and the atmosphere do).
Q26: What effect of the increased accumulation of energy by the oceans is seen in the image above to the far right?
The image below shows changes in specific humidity over land areas from 1972-2012.
Q27. What has been the trend in specific humidity and how what process resulting from energy accumulation in the oceans is responsible for the trend?
For a summary of much of what we have covered in this and other labs until this point, watch this brief clip from Cosmos: A Spacetime Odyssey, a 2014 science documentary television series starring Neil deGrasse Tyson.
We will be exploring further the effects of the increased energy accumulation by the oceans in Part 2 of the lab.
In previous sections we found that there has indeed been global warming and we examined the likely causes of the warming which is a result of an imbalance in Earth’s energy budget. In this section, we will begin this part by looking at temperature changes across the globe and thus see just how global the global warming is. The graph below shows changes in surface temperature from 1979 to the present for the Northern Hemisphere and Southern Hemisphere. This graph shows that the Northern Hemisphere has experienced much more warming than the Southern Hemisphere. One explanation for the slower rise in temperatures in the Southern Hemisphere is the greater percentage of water at the surface of that hemisphere: oceans cover approximately 80% of the Southern Hemisphere. And as you learned in the previous section, oceans are huge accumulators of energy and that energy doesn’t necessarily translate into increased air temperatures.
Since the Northern Hemisphere has experienced faster warming – and since those experiencing this lab live in the Northern Hemisphere – we will be focusing on how temperature trends vary by latitude in this hemisphere.
Q28: Which latitudinal zone – subtropics → 24°-44°, mid latitudes →44°-64°, or high latitudes → 64°-90° — do you think has experienced the most warming since 1979?
Click SurfaceTemperature_LatitudinalZones to open this file in Microsoft® Excel. The values are differences in temperature from the average value for the period 1979-2016 for each of the three latitudinal zones. Create a graph that contains a trendline for each latitudinal zone.
Q29: Which latitudinal zone has actually experienced the most warming?
Q30: Why do you think that latitudinal zone has warmed the most? The reasons will be explored in Part 2 of this lab.
Below are maps showing trends in lower-troposphere temperature (left figures) from 1979-2012 and surface temperature (right figure ) from 1981-2012. These data show more detail than the latitudinal analyses you just conducted, but at the same time can be harder to interpret. To simply your examination fo the maps, try to only focus on temperature trends over land. You should notice that nearly all the grid cells on land have increased in temperature, and many of those increases are statistically significant. Remember from an earlier figure that the surface and lower troposphere for the entire globe warmed approximately 0.4 to 0.5 °C per decade from 1979-2013.
Q31: Based on the trends in the lower-troposphere temperature and the surface temperature, what three places that you examined earlier probably experienced the most warming over recent decades?
Q32: What two climate types and biomes should have been the most affected by the recent warming?
Before the next lab, write for yourself a one-sentence response to each of the following big questions of this lab.
How have Earth’s surface and tropospheric temperatures changed over the past several decades and what region of Earth has experienced the most change?
QWhat is the most likely cause of the changes in global temperature over the past several decades what evidence is there that this is the cause?