At the end of this lab, you should be able to answer the following questions:
- What factors appear to have been the main drivers of temperature changes over the past millenium?
- How did global temperatures change during the 20th century?
- What appears to be the main cause of the warming of the globe over the last several decades of the 20th century?
Introduction of lab by instructor
1. Trees develop annual rings of different properties depending on temperature, precipitation, solar radiation, etc. in different years; thus, variations in ring characteristics may be used to infer past climatic variables (e.g., temperature). Click Douglas-fir to view a cross section of a Douglas-fir (Pseudotsuga menziesii) from the Zuni Mountains of New Mexico. Since tree growth is controlled by other factors beside temperature, there is considerable debate about the usefulness of tree growth as an indicator of temperature. Click Proxy_Temperatures to view reconstructions of Northern Hemisphere temperatures over the past 1,000 years using mostly treering records. The graph comes from the Fourth Assessment Report by the International Governmental Panel on Climate Change (IPCC). These are rough estimates of temperature, and the most recent temperatures are the most accurate.
- Draw a line best-fit line (i.e. an average line) from 1000 A.D. to 2000 A.D. using the 11 temperature lines.
Q1: Based on temperature estimates from proxy records, what was the warmest century?
Q2: Based on temperature estimates from proxy records, what was the second-warmest century?
Q3: Based on temperature estimates from proxy records, what were the coldest centuries?
2. Click Simulated_Temperatures to view reconstructions of Northern Hemisphere temperature over the past 1,000 years using models that take into account volcanic activity, solar irradiance, and concentrations of greenhouse gases and tropospheric sulphate aerosols. The graph comes from the IPCC’s Fourth Assessment Report. These are rough estimates of temperature, and the most recent temperatures are the most accurate.
- Draw a line best-fit line (i.e. an average line) from 1000 A.D. to 2000 A.D. using the 12 temperature lines.
Q4: Based on temperature estimates from climate models what was the warmest century?
Q5: Based on temperature estimates from climate models, what was the second-warmest century?
Q6: Based on temperature estimates from climate models, what were the coldest centuries?
Background Information
There were three distinct climatic periods associated with changes in solar irradiance during the past millennium: the Medieval Warm Period (approximately 900 years ago), the Little Ice Age (1500s to 1700s), and the modern period (1900 to present). Some scholars argue that the Medieval Warm Period and the Little Ice Age occurred only in the Northern Hemisphere and possibly only in Europe. In the absence of human activities, changes in solar radiation would be the primary driver of climate change over the past millennium. Click Solar_Irradiance to see changes in solar irradiance (i.e. Earth’s solar constant) from 1000 A.D. to 2000 A.D. Earth’s solar constant increases with an increase in sunspots (i.e., temporary phenomena on the surface of the Sun (the photosphere) that appear visibly as dark spots compared to surrounding regions). Click Sunspots to view a NASA image of the Sun taken 30 March 2001; a huge sunspot group is visible. Click Sunspot_Numbers to view changes in the number of sunspots over the past 400 years; notice the 11-year cycle in sunspot frequency as shown by the blue line. Click Solar_Events to view periods with high and low numbers of sunspots during the past 1,200 years. During the Maunder Minimum (1645-1715), which coincided with the Little Ice Age, Earth’s solar constant may have been up to 2.7 W m-2 less than the present-day value.
Human activities have caused major increases in emissions of greenhouse gases, especially carbon dioxide (CO2), and sulphur dioxide (SO2). Click Carbon_Dioxide to view changes in the global atmospheric CO2. Historical concentrations of CO2 are preserved in bubbles in ice cores, such as the Law Dome ice core. Click ice_core to see an example of an ice core from the Law Dome station in Antarctica. CO2 concentrations have been measured at Mauna Loa since 1958. An increase in concentrations of greenhouse gas, such as CO2, enhances the Greenhouse Effect. Click Greenhouse_Effect to view an animation of the Greenhouse Effect. Click Sulphate_Aerosols to view changes in global anthropogenic sulphur dioxide (SO2) emissions from 1000 A.D. to 2000 A.D. Chemical reactions in the atmosphere involving SO2 can produce sulphate aerosols. Sulphate aerosols reduce the amount of absorbed of solar radiation absorbed by Earth’s surface in two ways: (1) the aerosols are highly reflective; and (2) the aerosols make clouds more reflective. Fossil-fuel combustion, and the subsequent release of greenhouse gases and SO2 into the atmosphere, began increasing rapidly after 1850 (i.e. during the Industrial Revolution). Therefore, fossil-fuel combustion was negligible during the Little Ice Age and, of course, during the Medieval Warm Period.
Q7: What are two possible reasons for why the 20th century was the warmest century from 1000 A.D. to 2000 A.D.?
a.
b.
3. Click NASA_Global_Temperatures to open the file in Microsoft® Excel. This file contains annual global temperatures from 1900-2000. The data were extracted from the NASA GISS dataset.
- Select cells in rows 1 through 102 of columns A and B.
- Under the Insert tab, select Line and then choose one of the 2-D lines.
- The resulting chart shows the annual global temperature from 1900-2000.
- Right-click the line and “Add trendline …” — Select Polynomial and change the order to 6
- Now you should be able to the raw temperatures and be able to determine the general trends in temperatures by examining the trendline
Q8: During what two long periods (i.e. decades) did temperatures increase?
Q9: During what long period did temperatures not increase?
4. Click Temperature_&_SolarIrradiance, Temperature_&_CO2, and Temperature_&_SO2 to view changes in temperature along with changes in solar irradiance, CO2 concentrations, and anthropogenic SO2 emissions from 1900 to 2000.
Q10: Why would someone think that changes in solar irradiance from 1920 to 1940 may have played a direct role in the global warming during that period?
Q11: Why would someone think that changes in solar irradiance from 1980 to 2000 may not have played a direct role in global warming during that period?
Q12: What are the two most plausible reasons for why 1980-2000 was a warmer period than 1900-1920?
a.
b.
Q13: During what period does it appear that increasing concentrations of sulphate aerosols may have played a role in eliminating global warming?
5. The period from 1947-1976 is unique in that both solar irradiance and CO2 concentrations increased, yet global warming did not occur. One possible explanation was that an increase concentrations of sulphate aerosols over this time period counteracted the warming. Click here to see a NASA article on this subject.
a. Click Sulphate_Aerosols to view the typical atmospheric concentration of sulphate aerosols from 2000 to 2007. The red areas have the highest concentrations, and the blue areas have the lowest concentrations.
Q14: Over what countries/continents do you find the highest concentrations of sulphate aerosols?
Q15: In what hemisphere are the above countries and continents?
b. Click SO2Emissions_1947-1976 to view changes in SO2 emissions from 1947 to 1976.
Q16: Which hemisphere had substantially more SO2 emissions from 1947 to 1976?
Q17: Based on the above findings for SO2 emissions and the resulting concentrations of sulphate aerosols, which hemisphere is most likely to not have experienced warming from 1947-1976?
c. Click Temperatures_1947-1976 to open the file in Microsoft® Excel. This file contains annual values of temperatures anomalies for the Northern Hemisphere and Southern Hemisphere. An anomaly is just the difference from the average value.
- Select cells in rows 1 through 31 in columns A, B, and C
- Under the Insert tab, select Line and then choose one of the 2-D line
- The resulting chart shows hemispheric temperature anomalies from 1947-1976
- Right-click the each line and “Add trendline …” — Select Linear for each line
Q18: How did the two hemispheres differ with respect to changes in temperature from 1947-1976?
Q19: Do you think the increase in concentrations of sulphate aerosols from 1947-1976 is a valid reason for the lack of global warming over that period? Explain why.
6. Revisit the initial questions and answer each question using the information gathered in this exercise.
Q20: What factors appear to have been the main drivers of global temperature changes over the past millennium? a. urban heat islands and volcanic activity b. solar irradiance (i.e. the solar constant) and volcanic activity c. greenhouse-gas concentrations and solar irradiance (i.e. sunspots) d. greenhouse-gas concentrations and urban heat islandsQ21: How did global temperatures change during the 20th century? a. global temperatures generally increased during the 20th century, with the most warming occurring from 1945 to 1979 b. global temperatures generally decreased during the 20th century, with the most cooling occurring from 1945 to 1979 c. global temperatures generally increased during the 20th century, with the most warming occurring from 1979 to 2000 d. global temperatures generally decreased during the 20th century, with the most cooling occurring from 1979 to 2000 Q22: What appears to be the main cause of the warming of the globe over the last several decades of the 20th century? a. an increase in solar irradiance b. an increase in concentrations of greenhouse gases (e.g., CO2) c. an increase in concentrations of sulphate aerosols d. the expansion of urban heat islands